3.6 Steam traps

Common Steam traps types

Thermodynamic
Application

• Steam tracer lines or small steam headers
• Low flow rates
• Good for startup air venting, 
• Compact in size

Common problems

• Clogging
• Disc stuck or worn
• Seat worn
• Unreliable, requires frequent PM

Troubleshooting

• Check for cycling noise to verify functionality. No cycle indicates Failed in open, fast cycle indicates flow too high or faulty. 
• Open blowdown to remove particles, if any.
• Remove cover to lap seat and replace disc if blow down does not works, 

Problem faced during maintenance

• Steam cuts on seat, as it may not be made from harder material like the disc.
• Some welded to pipe models where lapping needs to be done, has to be performed in the field.
• Old isolation valves to these welded traps could be passing steam, as such it is either difficult or in some cases dangerous to perform maintenance.
• Old isolation valves could be leaking steam at packing and could only be repaired during shut down. 
• Mix up of steam trap models, ie. Spirax Sarco thermodynamic traps TD42 comes in various models for high pressure rating, high condensate flow, low condensate flow rate and they may use different disc types and these could be mismatched by maintenance operators.

Thermostatic bimetallic

Application

• For low to mid flow rates such as large or HP steam distribution headers
• Long life span

Common problems

• Clogging at strainer, ball/seat area or bimetallic plates
• Valve setting runs out.

Troubleshooting

• Particles trapped on bimetallic plates n valve assembly will impede on operation, blowdown if possible, clean assembly and adjust valve setting. 
• Servicing will usually fix this steam trap, internal assemblies can be rather costly to replace. Costs usually about 40-50% of a new bimetallic steam trap, especially for high pressure versions.

Problems faced with maintenance

• Open blowdown to remove particles, if any.
• Expensive spare parts, difficult to justify for PM budget until major system breaks down eg. pipeline puncture.
• Steam cuts at the valve assembly, if none blow to remove dirt with compress air. 
• Need training for maintenance operator to adjust bimetallic valve properly

Balanced pressure

Application

• Venting air during start up
• LP Steam headers and small steam equipment
• Long life span

Common problems

• Stuck capsule
• Punctured capsule

Troubleshooting

• Open blowdown to remove particles, if any.
• Clean or replace capsule

Problems faced with maintenance

• None so far.

Ball Float
Application

• For High condensate flow rates, such as condensate discharge of steam heaters.

Common problems

• Does not vents trapped air in system, will requires a air vent mechanism.
• Air vent mechanism stuck
• Float mechanism stuck 

Troubleshooting

• Open blowdown to remove particles, if any.
• Particles trapped on valve assembly will impede on operation. Remove and clean assembly if stuck.
• Steam cuts at the valve, if none blow to remove dirt with compress air. 

Problems faced with maintenance 

• Thin and small width graphite gaskets used, prone to damage followed by leakage during start up. Ensure installation is monitored by competent supervisor.
• Provide IOM to maintenance operator, assembly can be confusingly installed in wrong direction and etc at times.
• Bolts may be stuck if it was never serviced throughout, prepare additional bolts.

Inverted bucket
Application

• Vents trapped air very well
• Moderate condensate loads,

Common problems

• Bucket assembly stuck
• Loss of prime

Troubleshooting

• Open blowdown to remove particles, if any.
• Check for cycling noise to verify functionality. Close downstream valve to prime the trap with condensate to restore functionality.
•  Remove for service or repair if priming does not work.

Problems faced with maintenance

• Provide IOM to maintenance operator, assembly can be confusingly installed in wrong direction and etc at times.
• Bolts may be stuck if it was never serviced throughout, prepare additional bolts.

Tutorials
Visit Spirax Sarco tutorials for more information regarding sizing, application, maintenance of steam traps. Its a brilliant webpage.
http://www2.spiraxsarco.com/resources/steam-engineering-tutorials/steam-traps-and-steam-trapping/why-steam-traps.asp


General Maintenance Guidelines
Steam traps are automatic valves meant to eliminate condensate out of steam systems to improve system efficiency, as presence of condensate reduces heat transfer. It is one of the most neglected item in a process plant, as such it poses a risk to steam system blow outs.

But this device appears to be harmless and runs on its own. Why bother? The amount of discharge is insignificant to cause any damage. No!

When steam trap maintenance is absent, wear and tear tend to cause it to pass steam. Over time, as it gets excessive, significant two phase flow within the system will take over and cause erosion and pitting internally (Imagine sand flowing at high velocity thru the pipes which peens and grinds down on the surface), these effects enhanced with CUI in corrosive and humid environment, which eventually leads to pipe puncture.  (Some photo of the leaks to be attached.)

This is a well known problem in the industry, as steam operate at much higher velocity about 30 -60m/s, when it enters condensate stream heavily, it increases velocity of the condensate, therefore:

• The flowrates increases beyond the design for condensate flow,
• Condensate droplet impingment prevails which can be detrimental to tees and elbows over time.

Good design practices

1.  keep traps as close to condensate lines as possible and keep the return lines as insulated.
2. Size the condensate line for flash steam flow, assuming 15 to 20% traps are passing
3. Make use of flash drums closer to point of condensate generation to minimize effect of flashing steam in long condensate pipes.
4. Assign separate condensate lines for the various steam pressures ie. LP, MP, IP, HP.
5. Minimize sharp bends and injection points by using swept tees, 45EL or diffusers respectively.

Schedule & Recording keeping
A schedule should usually be available to determine steam trap survey on regular intervals and these can vary depending on willingness to invest or past operating experience basis, for example. If otherwise please develop such a schedule to ensure steam traps maintain in good condition.

Regular schedule

• A Check on all steam traps in the plant every 1 year
• Blow down on all steam trap line blowdown valve every 3 months

Risk based schedule

• A Check on all thermodynamic traps every 6 months
• A Check on all mechanical traps every 2 years
• Blow down on all high particulate lines every 1 month
• Blow down on others every 6 months

Survey and Repair Records

• These records will allow future generations to make decisions whether to replace a steam trap or not, as some could be costly.
• Survey records shall be kept as there could be thousands of steam traps in a plant and it will not be easy to manage the repair and maintenance effectively without one.
• Repair records shall be kept so recurring problems could be identified for redesign, decisions could be made whether to invest in new steam traps or spare part replacement.
• Repair records could also help to develop spare pares strategy as we now know the usage pattern. This strategy will also be effective to help cost saving intiative in spare parts purchase during construction of new plants

Troubleshooting & Survey
Simple methods to identify faulty steam traps
1) Temperature method
Run a temperature gun on the pipe before and after the steam trap. As pressure difference exist due to resistance and separation, there will be a temperature difference across the trap. This is the most suitable test on higher pressure steam lines where dT across the trap is relatively higher. If there are no temperature difference across the trap, it is likely passing steam.

Run the temperature gun on the trap. Cold could mean its failed close/clogged. Hot could means its working or passing steam.

2) Visual method
Open vent/drain valves before the steam trap (live steam will release), after the steam trap (flash steam will release).  If nothing is released after a long wait the steam trap has failed in close position, if all you see is steam blowing out in a jet then it has failed in the open position.

3) Listening - audible noises
This method involves holding a long metal rod, one end touching the trap and one end touching your ears. The effect of condensate passing can be confirmed when cyclic action of the steam trap is heard.

4) Listening - Ultrasonic testing
This is the most reliable method and will probably require engaging and external vendor/operator to perform the task. Steamtrap vendors such as Spirax Sarco & armstrong intl would normally offer complete steam trap survey services for an entire plant at a nominal fee.

Watch this video for a detailed guide with illustration to troubleshooting the traps by Armstrong intl.
Guidelines for Steam Trap Troubleshooting: http://youtu.be/smjuAEZOrlk

3.5 Stud bolts / Hex nuts and their relation to Flanges

Stud bolts and nuts

Stud bolts and nuts are the most commonly used fixtures for joining. 

They are most commonly used to join pipe flanges, heat exchanger shell girth flanges and many more. 

Reasons for replacement ?

This topic came to my mind, as I am required to purchase replacement Stud bolt and nuts for several heat exchangers girth flanges. The more common reasons for replacement in the process plant and specifying them correctly are due to the following:

1) Corroded stud bolts and hex nuts, 

a) Corrosion sometimes can be so severe that the diameter of the bolt is below minimum required diameter handle the bearing stresses. 

b) Corrosion is common occurance at the crevice between the flange bolting hole and bolt surface. 

c) Due to the recessed surface of the pitch on a bolt, they tend to catch dirt and corrosive particles more easily.

- The norm is to just replace the studbolt/nuts with same material/spec on wear and tear

- An upgrade can be performed to reduce maintenance cost, but remember take care of galvanic corrosion when using bolts of a seemingly better material on susceptible pipe lines, to avoid ill effects of "fixing problems to cause more problems".

2) "Frozen" bolts

a) This can be the resulting combinations of either corrosion, thermal expansion, over torque, loss of lubricant, crossed threaded bolt-nut, etc etc etc.

b) this is very common on very hot services approx above 80degC

 - These are usually broken free by cutting the bolt due to ease, which means replacement. 

- The better & toughy fitter may attempt to break free for you after applying anti-seizure lubricant but will take up much time.

3) Needs to be specifically defined to the correct vendor 

a) Contractors may not check or calculate for you, if your drawing does not show the exact length or details at all.

b) Flange/Pipe stockist may not carry stud bolts and nuts.

c) Different equipment uses different flange standards, which means different size and lengths of bolts and nuts. We shall discuss this later.

Standards for Bolts/Nuts

The most commonly used standards for defining Stud bolt and Hex nuts in the process industry are the following, due to the wide range design temperature it can handles and its superior market availability.

ASME B193 Grade B7 - Steel Stud bolts

ASME B194 Grade 2H - Steel Hex nuts 

For ASME B193 B7, it will handle from -29 to 427degC.

For higher temperatures, we will need to move up to grade B16, grade B8C1 and so on.

For cryogenic temperatures, use A320 L7, grade A320 B8C2 and so on.

For ASME B194 Grade 2H. it can handle -29 to 537degC.

The grades of the stud bolts and hex nuts will be marked on their body, provided they have not corroded.

Standards for its threads
Most commonly used standard for stud bolt threads is the Unified Thread Standard to ASME B1.1:
UNC - UN Course thread, common for up to 1", where bolt diameter size increases with decreasing thread pitch.
Others include:
8UN - 8 pitch thread, common for bolt/nut larger than 1")
UNF - UN Fine thread
UNEF - UN Extra Fine thread
UNS - UN Special thread

Thread pitch refers to the number of turns per inch length.

Standards for Flanges

Most commonly used standards for defining pipe flanges in the process industry are the following:

ASME B16.5 - Flanges from 24" and down

ASME B16.47 - Flanges from 26" - 60"

Read these standards and they will have tables to guide you on the number of bolts, length of bolts and size of bolts required for each type of flanges/pressure rating. You could also request a vendor version of these references from your Flange stockist.

Take note for ASME B16.47, there are Series A and Series B flange which do not share the same bolt hole sizes. Series A is adapted from MSS SP-44, and type B is adapted from API605 as part of ASME effort to standardize the dimension of flanges. 

Reference: StandardNE B16.5 Bolt Chart

Calculation for bolt length

L = 2(n+f+rf+s) + g + etc

Where

n = Nut thickness

f = Flange thickness

rf = Raised face thickness

s = Free length (1/3 diameter of bolt)

g = Gasket thickness

etc = additional fixtures such as tube sheets, orifices, additional gaskets, etc.

Additional Information

Collar /Jack bolts

These are additional bolts which sometimes or rather most of the times comes with heat exchangers.

The purpose of these bolts is to secure the tubesheet on the shell, should maintenance be only carried out on the channel heads.

But these bolts are usually more corroded or "frozen" than the regular stud bolts due to the irregular shapes it may have. Some of the irregular collar bolts may not be available off the shelf, as such do pre-fabricate some spares in times of need.  Check out the reference below for pictures which may better explain things.

Reference: Wermac Collar Bolt Information Page

Torquing the Flange bolts

Why torque the bolt to the recommended setting? You will tend to hear these from the contractors first:

- "I did not torque any bolts/nuts for the last blah blah years and they worked fine"

- "You don't need to torque, just go for the tightest"

- "It will not explode, dont worry"

My opinion goes to say that I agree to their advice for most services operating under ambient temperatures with exception to what to be mentioned next. One shall start obeying the rules of torquing the flange bolts when things starts going extreme ie.

- Very high temperature & pressure (HP boiler steam drums and reactors)

- Very large variation in operating temperature (Cryogenic loading/vaporizer lines)

- Very large variation in ambient temperature (Temperate location experiencing 4 seasons)

- Very large variation in operating pressure (Pulsating line, swing adsorption, etc)

Be very worried, unless you are prepared to seal the leaks on-line which can be very costly. Costlier than a 3ft torque wrench. 

Theory underlying the need for torquing:

- The forces from the bolt torque will affect the way gasket seats on the flange faces

- Uneven seating faces due to random tightening will cause leaks

- Over tightening will cause some solid material gaskets to yield or crack.

Reference: Bolt torquing tables & bolt tightening sequence

4.3 Weld & Fabrication: Non-Destructive Testing Basics

Basic forms of Non Destructive Testing (NDT)

The below methods are usually used for pipe and pressure vessel inspection during construction for material defects especially on welds.

Some of these techniques can also be used for maintenance inspection of an operational plant such as UT for thickness measurement, RT for flaw imaging, thermography for leak detection behind insulation and Eddy current for heat exchange tube inspections.

-Visual inspection
-PT
-MT
-UT
-RT
-Eddy current
-Thermography

Watch this youtube video from TWI for a clear picture of the physical equipment used and their operating principles.

4.6 Weld & Fabrication: Reviewing PV Elite Datasheets

PV Elite is a pressure vessel and heat exchanger design software set to summarize ASME Section VIII and other international code rules into software making the design process convenient and easier to interpret as compared to reading sections after sections of the code book to skim for information often misinterpreting and confusing it with the involved parties. For using this, you will only see what you want to see based on what you want to produce without reading the 700 over pages of ASME Section VIII over and over again to confirm the confusing interpretations.

Some code sections which requires selection are also available as option for the designer to choose from based on client specification or following vendor standard design.

Analysis models can be generated for user check if nozzle or irregular shape designs exceed allowable stress and allows for further optimization of materials usage.

Design parameters and material properties are pre-loaded in the software such as tensile strength of materials at various temperatures, weld efficiency, etc.

* My job at the Company requires review of the output from this software from our Contractors, therefore I cannot give review and comments on how well functional this software is. But my personal opinion says that this is a good software to optimize work flow process and improve productivity through reducing the metal fabrication design lead time. Don't waste time on MS Excel spreadsheets, how much can you save from there as compared to:
i) Being awarded the entire pressure vessel contract for an EPC project because of the shorter design lead time?
ii) Submitting a no-nonsense and clear cut calculation output which is easy to interpret, reducing the design review time.


DESIGN CALCULATIONS
Below are typical of what design calculations will be generated by PV elite output for a typical ellipsoidal dish head pressure vessel.

Defined design pressure of the Pressure vessel internally and externally by the user.

Defined the MAWP (Maximum allowable working pressure) by the user

Defined the joints and radiography requirements. Followed by the material UNS number.

The pressure vessel is divided into nodes for analysis, mainly on the dish end, shell, followed by nozzles. Other stress calculations defined by the code will also be performed, calculations not defined by code can be or not be produced on the output datasheet.


1) Initially it will calculate for internally pressured requirements (DISH END)
a) The require thickness based on the specified design pressure will be calculated. [tr]
if the thickness is less than code UG-16 minimum of 0.0938", the code minimum thickness will be applied.

b) The maximum allowable working pressure (MAWP) based on the thickness after corrosion will be calculated. This is less the value of static head at the top of the vessel for hydro test purpose per UG99c.

We are safe at this moment, as long as this MAWP after corrosion is higher than user specified MAWP, which means that minimum thickness required can support user defined MAWP.

c) The Maximum allowable pressure (MAWP), based on New and cold thickness will be calculated.

d) The actual stress at given pressure and thickness after corrosion will be calculated. [Sact]

e) The head straight flange section require thickness will be calculated.

f) The head straight flange section MAWP will be calculated. Explanation is same as 1(b)

g) Factor K for corroded condition allowance will be calculated [Kcor]

h) Extreme Fiber Elongation to UCS-79 will be calculated.
If this is more than 5% after head forming, heat treatment will be required. Unless the 5 conditions exist which will allow fiber elongation to allowably go up to 40%.

i) MDMT calculation to UCS-66(w/o impact) and UCS-66(1) at dish knuckle portion

j) MDMT calculation to UCS-66(w/o impact) and UCS-66(1) at head straight flange section


2) Next it will calculate for internally pressured requirements (CYLINDRICAL SHELL)
a) The require thickness based on the specified design pressure will be calculated. [tr]
if the thickness is less than code UG-16 minimum of 0.0938", the code minimum thickness will be applied.

b) The maximum allowable working pressure (MAWP) based on the thickness after corrosion will be calculated. This is less the value of static head at the top of the vessel for hydro test purpose per UG99c.

We are safe at this moment, as long as this MAWP after corrosion is higher than user specified MAWP, which means that minimum thickness required can support user defined MAWP.

c) The Maximum allowable pressure (MAWP), based on New and cold will be calculated.

d) The actual stress at given pressure and thickness after corrosion will be calculated. [Sact]

e) MDMT calculation to UCS-66(w/o impact) and UCS-66(1)


3) Hydrostatic test pressure requirements 

This needs to be specified by the user:
Hydro test Pressure per UG99b = 1.3 x MAWP x St/Sd
Hydro test Pressure per UG99b[34] = 1.3 x Design Pressure x St/Sd
Hydro test Pressure per UG99c = 1.3 x MAWP x St/Sd - Static Head
Pneumatic test Pressure per UG100 = 1.1 x MAWP x St/Sd
Pressure per PED = 1.43 x MAWP


4) Externally pressured requirements (DISH HEAD)

Defined Elastic modulus based on material used.
Defined Material UNS number.

a) Calculation for Maximum Allowable External Pressure (MAEP) based on minimum thickness

b) Backward calculation for minimum thickness based on design pressure

c) Calculate required thickness due to internal pressure [tr]
being P = 1.67 * External design pressure per UG-33(a)(1)

d) The maximum allowable working pressure (MAWP) based on the thickness after corrosion will be calculated.

e) The final Maximum Allowable External Pressure (MAEP) will be the minimum of the calculated MAEP and MAWP.


5) Externally pressured requirements (CYLINDRICAL SHELL)

Defined Elastic modulus based on material used.
Defined Material UNS number.

a) Calculation for Maximum Allowable External Pressure (MAEP) based on minimum thickness

b) Backward calculation for minimum thickness based on design pressure

c) Calculate allowable pressure based on Maximum Stiffened Length (Slen)


6) Allowable stresses
Check allowable compressive and tensile stress for all elements based on strength of the material. This is what was done in college with Mohr's circle.


7) Longitudinal stress check
This checks for also the compressive and tensile stress based on live loading, weights and pressure


8) Nozzle check
Performs all the pressure, MAWP, allowable stress checks on the nozzle.


9) ANSI B16.5 Flange calculation
This is standard to check if flange meets requirement of pressure and temperature rating.


10) Vessel support check
Check that the legs, skirting, lugs and etc are adequate.


11) Rigging  check
Check that the vessel body and the lug locations are adequately designed for lifting and transfer.
The others below are straight forward and are part of college or technical school syllabus which assume that most reading this post should understand.


12) Vessel CG and weight data calculation
13) Vessel cross sectional area and moment of intertia
14) Natural Frequency
15) Vortex shredding 
16) Wind & Seismic loading



References:
PV Elite Design code and analysis capabilities
PV Elite Quick Start (pdf)

4.5 Weld & Fabrication: Pressure vessel basics

Pressure Vessel Fabrication Basics

- Dish head

Usually by hot forming which will offer the metal less stress than cold forming. 

These are made from circular flat plate, heated to red hot, and then pressed through a ring using a die. The result is the dish head which can be of the following shapes:

a) Hemispherical - requires most work to be done

b) Ellipsoidal  

c) Torispherical  

Referring to UCS-79, Stress relief heat treatment is recommended if extreme fiber elongation exceeds 5% after cold forming. Stress relief can be avoided if
- The fiber elongation is less than 40%
- The material is P1 or P2 and
- The following exists
1) This vessel is not used for hazardous fluid.
2) The material is exempted from impact testing or impact testing is not required by material specs.
3) Thickness of the part before forming is less than 16mm
4) The reduction by cold forming shall be less than 10%
5) the temperature of the material during forming is outside of the range of 120degC to 480degC

Double curvature (dish heads) = 75t/Rf * (1- Rf/Ro)
Single curvature (Shell course) = 50t/Rf * (1- Rf/Ro)

where
t= thickness of plate
Rf = Final centerline radius
Ro = Original centerline radius (Infinity)

Note that extreme fiber elongation for ellipsiodal heads shall be calculated based on two Rf it possess namely the knuckle radius(=0.17D) and spherical radius (=0.9D). D is original diameter of plate. Read UG-32 for more information.

Dish head making by hot forming

Dish head making by cold forming

- Shell course

These are usually made from flat metal plates and rolled. Plate reduction needs to be considered in the final thickness when purchasing the plates initially.

Plates are usually rolled cold, unless material is unusually thick <3".

ASME Section VIII, Div 1&2  requires roundness of the rolled shell ring course to be within 1% of the required nominal diameter.

Fiber elongation applies for shell course as well. See calculation for Single curvature above.

-Nozzles

Nozzles are pipes for the purpose of connecting the pressure vessel to the systems, vent, drain or for man entry purposes. These are usually welded to a cut hole on the pressure vessel body.

The thickness of the nozzle and its connecting joints such as flanges shall meet or exceed the MAWP of the pressure vessel body calculated.

These are usually welded directly to the pressure vessel stub-in, stub-out or via a reinforcement pad (repad) for more strength.

Reinforcement pads are usually installed for added strength due to pressure or weight issues.
Davits are usually installed on manhole nozzles for the purpose of carrying the heavy blind flange attached to it.

-Skirting and legs
Skirting are installed to raise the pressure vessel above ground should it come with a bottom nozzle, skirting are stronger and more reliable than legs which is more susceptible to failure by buckling in areas of high environmental corrosion. These are welded to the pressure vessel body and are not pressurized,

However skirting presents a obstacle to repair works and inspection due to it being classified as a confined space, additional control procedures needs to be in place for entry into such confined space such as additional permits to work, attendant, assessors, gas monitoring, lighting, ventilation, communication devices, etc.

- Stiffeners, demisters and other internal reinforcements
Stiffeners are usually added, should the weight, live loading or pressure of the vessel cannot ensure its structural stability, this is especially so for long vertical vessels such as columns and towers.

Demisters are installed to prevent unwanted dispersant of vapor to outgoing streams and splash plates are installed to prevent incoming fluid turbulence from causing unintended erosion to other vessel components

Classification

The following are usually classified as pressure vessels under ASME section VIII :

Storage/holding drum

Liquid-Gas separator

Heat exchanger

Reactor

Distillation column

Extraction column

What to take note of during construction stage

- Proper welding and inspection procedures of the weld process will play a part in ensuring integrity of the weldments

- Selection of proper mills with decent origins, minimum with 3.1 mill cert on pressure retaining conponents which ensures a third party endorsement on their material quality.

Material selection goes beyond just matching the process fluid, but also matching the process type. For example cyclical pressure process would prefer a vessel which is less hard, to elimiate possibility of cracking. Cryogenic process would prefer a vessel which will not crack upon reaching -40degC minimum design metal temp(MDMT) threshold of common carbon steels.

For materials which are susceptible to stress corrosion cracking under thermal, chemical and physical stresses, Post weld heat treatment (PWHT) is recommended. 

- Proper thickness, corrosion allowance, design temperature, design pressure and design material is selected for maximum operating life.

The recommended corrosion allowance for each service should be established from company best practice guides or by a metallurgy specialist.

The correct type of insulation is applied after construction, at design thickness.

-By applying ASME U-stamp for each pressure vessel, additional assurance on compliance to ASME codes is ensured.

Thru ASME U stamp program, all design drawings, calculations, non destructive test process and documents will be reviewed by an ASME authorized inspector before approval. ASME U stamp is mandatory in the USA. For other countries it depends on local jurisdiction.

Any additional hot works done to a U-stamp vessel after being stamped shall require R-stamp endorsement by the Authorized Inspector. Regardless of it being transported, prior to installation, addition to the name plate, as long as an arc is strike, R-stamp needs to be in place to keep the U-stamped vessel valid.

*Disclaimer
Videos in the post belongs to the rightful owners found on the youtube.com links and are thankful to them for the production. This site is for the author's personal knowledge, therefore videos embedded are for convenience of viewing and none of the videos in this post belongs the Author.

5.2 Plant maintenance: Pressure Vessel Repair

Operation stage

• Operator: Ensure these vessels operates within their specified design limits and measures shall be taken to ensure no overpressure beyond the specified allowable times. Not more than 500h/yr for 20% overpressure and not more than 100h/yr for 33% overpressure. 

• Mechanical Integrity: Ensure these vessels are constantly monitored on-stream visually, check all vessel body paint, nozzles, stud bolts, supports and anchors are in good condition without distortion or corrosion, inspect under insulation if any suspected damage to insulation are observed and apply suitable non-destructive testing over time time of operation to observe any signs of material failure, especially in processes susceptible to such failure modes. Read API 570 for the recommended inspection intervals and API 571 damage mechanisms for some examples of damages with photos. 

Repair stage

• What constitutes to a Repair? 

A method to restore the equipment condition to where it can operate safely within the Maximum Allowable Operating Pressure (MAWP) and Allowable operating temperatures.

Adding of nozzles can also constitute to a repair, if
- It does not require a reinforcement pad
- It is smaller than any existing nozzles on the vessel

Adding of any other nozzles are termed an "Alteration", which usually comes with rerating as the vessel strength could be significantly reduced.

• Avoiding Repairs 

First thing when problems are identified, management point of view may be safety, cost, operational we don't know. But these are all the potential problems to avoid a repair, can we actually avoid?
The answer is yes, and how do we do it?

•  Rerating
• Rerating to a lower MAWP and operating pressure for thinned vessel walls for continued operations until minimum thickness is hit again. This is usually the easiest way out.

• Rerating shall be performed by an Engineer or Manufacturer.

• Fit For Service Assessments (FFS)

• In reference to API 510, Chapter 7, it shows how to assess the pressure vessel in damaged condition to extend its life. Not all scenarios are covered, they cover only the usual problems for example pitting, localized wall thinning and wall thinning near by a weld seam.

• If the problem cannot be found in API 510 Chapter 7, move on to API 579-1 or ASME FFS-1 for the full assessment. They come in level 1 to 3, for 1 is simplest and 3 is detailed. 

• Level 1 are usually performed by End User Engineers, and level 2 and 3 usually outsourced to software or consultants. 

Nevertheless, basic Risk Assessments should at least be carried out to evaluate the Consequence or Probability of a leak before putting the damaged equipment back to service.

• Is it hazardous to working persons?
• Is it environmentally harmful ?
• Would it affect operations and customers adversely? 

• When are Repairs required?

The number one reason for a repair would be corrective maintenance. This is usually carried out as follows:

• [Unplanned] As leakage or damage has been initiated.  
• [Unplanned] Thinning beyond acceptable thickness, damage is unsafe for continued operation, rerating is not possible and FFS assessment fails,
• [Planned] Equipment suffers from damages which can last till the next outage 

• Repairs in USA and Repairs elsewhere

National Board U-stamp vessels: In the USA, the repair shall be performed by a certified NB R-Stamp repair shop under approval of Authorized Inspector in compliance to NB-23.

• If outside of USA, check local jurisdiction or company best practice for R stamp requirements. If required is usually for Quality Assurance or Company requirements 

• Generally pressure vessel repairs without R stamp are commonly done. Alternative codes and guidelines can also be used for repairs such as ASME PCC-2 or API 510. Check Company guidelines, if none to consult a pressure vessel engineer.

• API 510 Inspection, Repair, Rerating & Alterations: 
• Alternatively an API 510 repair procedure can be used also under the approval of the Authorized Inspector. Refer to Appendix D of API 510 document for the check list

• Typical repairs based on API 510 includes Temporary and Permanent Repairs.

• Ensure that all minor repairs are authorized by the pressure vessel inspector before commencement. Approval for major repair may not be given for an ASME Section VIII designed vessel until approved by an experienced Pressure Vessel Engineer.
• Crack repairs shall be consulted with the pressure vessel engineer before proceeding, as cracks may propagate even after the repair. 
• Pressure tests may not be required after the repair. Unless it is believed to be necessary by the inspector. Pressure test if required, may be waived if suitable Volumetric NDE are in place to ensure integrity of the repair.

• Temporary Repairs includes:
• Lap patch - This is also commonly known as doubler, sometimes could be done on-stream as emergency repairs, if the process fluid is not hazardous or flammable, and welding process permits so.  A plate of usually similar material, properties and profile is welded to the external wall of the pressure vessel over the leaking spot. There are size/thickness restrictions where you need to calculate and avoid welding over to or near existing weld seams, the edges needs to be rounded to minimum 1" radius.

• Pipe Cap/Nozzle - Welding a pipe cap or nozzle to seal in the leaking spot. These are non-penetrating.

• Leak sealing - Possible for very small pressure vessels, usually below 8" diameter. The leaking spot could be box in or wrap sealed by epoxy-metal binders.
• Lap band repair - A huge band is attached around the vessel body, this is not recommended unless the repair needs to remain in place for a longer period of time.

• Permanent repairs includes:
• Shell course replacement - cutting out the damaged section for replacement. This is usually an expensive job, especially if done on sections of a huge vertical column, high costs largely attributed to lifting and manpower.
  
  
  

• Strip lining - a thin layer of sheet metal, usually of a superior material is lined internally. Cheaper than the other methods.

• Insert Plate - A piece of the pressure vessel is cut out and replaced with a new piece of metal plate of the same profile over the spot of leakage. The edges should always be grounded unless the side falls on an existing weld seam. In contrary to the sketch, the corners of the insert plates shall be rounded to eliminate residual stresses.
  

• Weld overlay - Adding a layer of weld filler metal over the thinned down sections of the vessel to restore the wall thickness. Very tedious and time consuming. Watch the manhours.  Take note that the repair thickness shall not be more than 50% of the minimum required thickness of the vessel (exclude Corrosion Allowance)
  

• Groove Repair - This method usually used for cracks and pinhole leakages. The section where leakage is present is grind down to a U or V groove. Weld metal is then deposited to restore the surface.

• Additional Considerations
• Materials 
• SS Cladding/Plate lining to P3, P4, P5 materials tends to crack when excessive heats are applied during welding. Check for delayed cracking with UT 24h after job completion.
• Hydrogen service vessels shall be outgassed and checked for its hardness after welding

• Post Weld Heat Treatment
• Check whether if vessel to be repaired require PWHT during fabrication. If yes, PWHT shall also be applied during repair in compliance to latest edition of ASME VIII Div 1 requirements (or refer to MDR). PWHT by oven is unlikely for in-service vessels however the following are usually used
• Local banding PWHT
• Preheat in place of PWHT
• Controlled Deposition Welding in place of PWHT

Check API 510 Section 8 for the full requirements of PWHT by alternative methods, as there are certain material tests which needs to be performed, certain requirements to welding procedures on top of ASME IX WPS/PQR requirements, certain requirements for temperature, certain requirements for welding electrode type and not forgetting material considerations.

For additional repair procedures and guidelines on top of API 510, refer to ASME PCC-2.

3.4 Insulation

Insulations for process piping

Insulations are installed for the very obvious:

1) Heat conservation for economic purposes
-These are such as, preserving heat within a hot steam pipe during transfer from a boiler to the steam appliance
-or preserving cold coolant during transfer from air con condenser to the room evaporating unit.

2) Personnel protection purposes
- These insulation are usually thinner, to prevent direct contact between man and hot surface
- Insulation material can be replaced with a suitable cage, if heat needs to be dissipated for some reasons, or corrosion under insulation(CUI) is prevalent on the piping circuit.

3) Types of insulation
Mineral wool/Rock wool
This material is commonly used for hot service.
Operating range: 0 to 250(glass)/760(stone)/1200°C(ceramic)
It is usually white/yellow and sold in rolled bundles of wool. The process fluid used with this is usually non flammable, as mineral wool can induce fire, despite it being "fire resistant". Mineral wool can catalyse oil, breaking down it down to lighter and easily combustible components which makes it more dangerous. Note that it also absorbs moisture easily. Always wear gloves when handling, it irritates and makes the skin itchy.

Source: Wikipedia

Calcium silicate
This material is usually used for hot service.
Operating range: -18 to 650°C
It is usually whitish, pre fabricated in blocks and appears like chalk. Good for use with hot oil service, as it does not catalyse oil. Also commonly used for fire insulation, see photo below for an example of a ductwork requiring fire protection rating.

Source: Wikipedia

Foam glass
This material is commonly used for cold or hot service.
Operating range: -260 to 480°C
It is usually black color, pre fabricated in blocks, has the texture and characteristic of somewhat a harder styrofoam and can be cut to fit.

 Source: Wikipedia

Polyurethane
This material is usually used for cold service.
Operating range: -210 to 120°C
It is usually yellow or white color, requires injection of PU foam into preinstalled cladding put in place around the piping/equipment.

 Source: Wikipedia

Perlite
This material is usually used for cold cryogenic service.
Comes in sacks of white loose powder or compressed blocks. Very good for cold service, however needs to be kept dry for optimum performance. Usually used in N2 purged spaces or vacuum spaces.

 Source: Wikipedia

Vacuum jacketed insulation
This is usually used for cold service in cryogenic applications or hot service in thermoflasks.
It is very good method for preserving energy and vacuum level needs to be checked on intervals, if lost, needs to be replenished by hooking up a vacuum pump to "pull vacuum". Tell tale signs of losing vacuum are sweating on the exterior of vessel or algae formation.

Source: technifab.com




Installation of Insulation
 The sizing of thickness and materials are very much dependent on application, process fluid and heat transfer rates to achieve less than 50°C on the external cladding surface for personnel safety reasons, as 50°C is usually the rule of thumb "ouch" threshold for humans.

During actual installation, we have no time to calculate the above, therefore tables are usually prepared before hand by the engineers, to advise the right insulation material and thickness. These information can usually be found on the piping specification which will specify different thickness for each application and the different temperature ranges.

eg. on the P&ID, isometic dwg or line list, the insulation fitter will have to interpret the following:
2"-HOTOIL-2330-1DC1A-1P    ----> 1P usually refers to 1" of personel protection insulation.
2"-HOTOIL-2331-1DC1A-2H   ----> 2P usually refers to 2" of heat conservation insulation.


Maintenance of Insulation
Insulation material when installed bare will disintegrate when exposed to effects of the environment, as such they are usually protected by cladding of aluminium or stainless steel material.
A sealant is then applied to the gaps where the cladding overlay each other.

Perlite insulation is different, it is usually dumped between the double skin section of the vacuum jacketed piping or vessel, as such it is easier to maintain.

 Source: Wikipedia

1) Avoid stepping on insulation cladding during maintenance work
2) Check yearly to ensure sealant between cladding are in place
3) Install inspection ports for checking corrosion and thickness monitoring on the pipelines/vessels, these pockets will also be good for inspection the insulation condition beneath the cladding
4) Top up insulation material when they deplete under environmental deterioration
5) Avoid re-routing insulated piping to where they will be exposed to moisture, vents, drains and process mists areas

5.1 Plant Maintenance: Static Equipment and typical maintenance jobs

What are Static Equipment?
Basically these are the equipment that do not have a prime mover driven rotating component will be considered a static equipment, and here's some example with what sort of Offline/On-line preventive maintenance which are usually carried out on the static equipment. Corrective maintenance will be discussed at the end of this post, as it is what we wish to avoid.

Piping & Typical PM jobs: 
- Thickness monitoring(5y or less, depending on code used API570:Piping or API580:RBI)
- Visual Inspection (1y or less)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)

Source: Wikipedia


Pressure Vessel & Typical PM jobs: 
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Above ground tanks & Typical PM jobs: 
- Thickness monitoring(10y or less depending on code used API653:AGT or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (12y or less, unless with internal lining or leak detection installed)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Heat exchangers & Typical PM jobs: 
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- NDT Inspection, WFMPI, eddy current, ultrasonic leak test (when leaks are suspected, or when service is critical, in corrosive media)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Distillation/Absorption columns & Typical PM Jobs
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Tray/Packing replacement (Usually 2y or more, depending on manufacturer design or efficiency)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Reactors & Typical PM Jobs
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Catalyst replacement (Usually 2y or more, depending on manufacturer design or efficiency)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia

Separators vessels & Typical PM Jobs
- See Pressure vessel above.

Source: Wikipedia


Steam traps & Typical PM Jobs
- Downstream steam check (1y or less, Live steam (faulty), lazy steam(working))
- Ultrasonic testing (1y or less, functionality check)

Source: Spirax Sarco

Manual Valves/Pressure relief valves & Typical PM Jobs
- These are usually done offline during shutdowns, unless the valves can be positively isolated for removal.
- Refacing of valve seat/trim
- Repacking of seal
- Replacement of corroded/ parts
- Overhaul of soft kit/spring/actuator/etc
- Overpressure test (MV), Set pressure test (PRV)

 

Source: Wikipedia

Note: About positive isolation - Positive isolation is done for safety reasons, usually either by closing two valves both up/downstream OR closing one valve with a blind installed both up/downsteam, to prevent any passing process media from escaping when maintenance works are carried out.

Valves are usually neglected by operators, they operate these valves using pipe wrench of various sizes. The huge torque applied with this leverage tool usually will speed up damage of the seats and as a result we experience passing of process media. Therefore, positive isolation is good practice to protect maintenance workers.


Filters/Strainers
- Differential pressure monitoring (Frequently)
- Backflushing or cartridge replacement(See manufacturer's recommended interval and action)

Source: Spirax Sarco


What sort of Plant maintenance should be carried out?
Preventive Maintenance
These type of maintenance are pre-scheduled on the plant's CMMS (Computerized Maintenance Management Systems).  The benefits of having a preventive maintenance program is that, there will always be some work for the contractors to carry out (be it to clock time sheet or earn earn extra cash??) Okay, the real benefit is that the manufacturer/code/national recommended maintenance intervals can be practised and followed up, so as to prolong the equipment life.

Types of preventive maintenance for Static equipment
- As discussed above at the top of this post.
- This type of maintenance are carried out offline or on-line, depending on the type of plant operations adopted or code requirements which may or may not permit carrying it out on-line.



Corrective Maintenance
These type of maintenance are usually carried out because preventive maintenance is not well practised, due to natural causes, design flaws, operating not according to manufacturer's prescribed conditions and many more reasons.

Types of corrective maintenance for Static equipment
- Box-in/Leak-sealing of pinhole leakage on pipe/vessel/weldments
- Pipe Spool replacement of piping due to the above
- Vessel/tank repair by cladding, shell course replacement, lap patch, plate lining due to thinning
- Steam trap replacement due to malfunction
- Heat exchange tube plugging due to tube leakage
- Unplugging of catalyst/packing/trays/filters by manual intervention


*Source: All images are from Wikipedia.org, except steam traps from Spirax Sarco. All photos are taken from open sources educational sites which allows for their material to be shared. Credits are given to the rightful image owner in this statement. Please drop me a comment should you be the rightful owner of the image and would like to request compliance for credit to be given to you directly.

4.2 Welding rod, filler wires and materials

Different welding process uses different types of filler material to aid the mechanism. A brief description to the types of filler rod/wire they use, to have a visualization of how they are fitted for the actual welding process, watch the videos from post 4.1 Welding terminologies.

SMAW - Uses a generally thicker rod with flux. Flux help produce shielding gas, adds alloying element to strengthen weld, reduces cooling rates when doing each pass. Manual feeding of weld stick is required.

Electrodes are usually classified in 4 numbers E-XXXX or E-6015
First two numbers refers to the yield strength in kpsi, 60 refers to 60k psi.
Third number refers to the welding position --> 0, All position. 1, Horizontal & flat
Fourth number refers to the electrode type

 There are 3 main types of electrodes based on the Fourth number:
a) Cellulose - No. 0, 1. High moisture content, good for joining thick sheet metal
b) Rutile - No. 2,3,4. Good for thin sheet metal.
c) Basic  - No. 5,6,8. Low hydrogen type, requires oven or hot box. Good for eliminated hydrogen embrittle cracking, when moisture is broken down by the high temperatures.

FCAW - Uses a hollow filler wire, filled with flux at the core. Basically same properties as above, however it comes in a wire coil, filler wire is automatically dispensed, no need for manual feeding.

GMAW - Same automatic feeding of filler wire like FCAW, but uses a shielding gas and no flux wires are required.

GTAW - Uses a thin and long filler rod without flux. The stick is hand held and melted with the tungsten torch.

Electrode sizes

These rods & wires comes in different sizes as well, usually depending on the application of weld:

1) Weld build up, usually use a thicker rod to speed up the process

2) Welding small pipes, use a small rod for better control 

3) Welding larger pipes, use a thicker rod to reduce weld time.

Benefits of thick filler rod

- Improve speed of material deposition

- Reduce number of passes (improves speed, good for weld build up jobs)

Benefits of thin filler rod

- Better control of welding process

- Increased number of passes 

Filler Rod Materials

Filler wire should preferably match the material of the base metal, in cases where dissimilar metals needs to be joint there are special application filler wires available. Here are some examples but not all listed for common stainless steels:

Base Metal A	Base Metal B	Recommended Filler material
SS301/302/303/304/305	SS301/302/303/304/305	SS308L
SS304/SS316/L	A106/A53, CS	SS309L
SS301/302/303/304/305	SS409	SS309L
SS301/302/303/304/305	SS316L	SS309L
SS316L	SS316L	SS316L

SS309L, a very versatile filler material, good for joining dissimilar stainless steels. However it contains no molybdenum which SS316L requires.

Stainless steel 316 has high carbon content and is not recommended as SS316L for welding. Due to the high carbon content, carbon tends to combine with chromium at grain boundary and the depletion of elemental chromium leads to localized corrosion. Strongly recommend SS316L, if the component needs to be welded and 316 material needs to be used.

Hardfacing, whenever repairs are required to a surface which requires increased hardness for erosion resistance, for example metal valve trims or injector tips, hardfacing electrodes such as Stellite is recommended. Cobalt is the main ingredient to give the extra hardness.

4.1 Welding Terminologies

Welding is very important in the industry. It's process is often taken for granted, as not many people know them well, especially for those who need to work with pipes but not on the fabrication floor. By extensively understanding these concepts, it improves communication between the various teams which in turn improves weld quality. Welding is the most commonly used joint in all process piping connections around the world. It can join similar materials, ferrous or non ferrous(brazing) or even dissimilar materials.

In this post, welding terminologies would be run through to familiarize with welding before proceeding further and more in dept into welding processes, discontinuities and other information.

Why weld pipes?
- Superior strength compared to other
- Proven and documented methods of joining which controls welder quality, process control and material quality.
- Welding is required to move forward to using other joints such as union joint and flange joint.

Different ways of specifying welds by different trades (Design Engineer vs Welding engineer)
Types of pipe weld normally specified by Design Engineers on Weld Map drawings
Butt weld - direct butt to butt join
Socket weld - fillet weld to a pipe connected to a recessed fitting
Field Weld - pre tack welded, allowance for field modification usually based on tolerance usually of about -+100mm
Tack Weld - pre weld, to join pipes before fully welded on the field.

Types of pipe weld normally specified by Welding Engineers on Welding Procedure Specification (WPS)
Bevel-groove weld
V-groove weld
J-groove weld
U-groove weld
Square groove weld
Fillet weld



Welding terminologies

Discontinuities
A flaw on the weld not desired, may be acceptable or unacceptable by inspectors depending on welding code specifications. Discontinuities are allowed, as long as they meet code requirements. The types of discontinuities described in AWS codes will be described in another post.

Defects
A flaw on the weld unaccepted by welding code specifications. Defects shall be rejected.

Base metal
The material which the welder is expected to join.

Filler metal
The material which the welder applies to the base metal.

Weld metal
- A new metal formed by melting of base + filler metal.
- The reason why PQR is required to verify strength. Different material batch produces different weld strength.

Flux
- Provides the following to the weld process: shielding gas, interpass cooling temperature control by slag, reinforcement alloy to weld metal
- Present in SMAW & FCAW.
- Can cause inclusion discontinuity

Slag
By-product of burning flux, needs to be remove before adding additional weld passes.

Weld Passes
The number of times the weld stick needs to pass between the gap to be welded. One who has never welded a pipe would think that one weld would close the gap, but it all depends on pipe size and thickness. For thicker pipes, more passes would be required usually in this manner root pass --> hot pass --> fill pass --> cover pass. On smaller pipes, less passes would be required.

Bevel size & angle
Size of slope for bevel, V, J, U groove welds and its angle. The lesser the angle, the less filler material is consumed.

Steeper angles for Bevel/V, usually around 37deg (Usually on process pipes)
Less Steep angles for J/U, usually around 20deg. (Usually on long distribution pipelines to save time and cost)

Groove face
Face of the bevel.

Root face
Face of the root.

Root gap
The gap between the base metal. Root gap depends on thickness of weld metal.

Weld toe
The top side of weld where filler metal and base metal touches.

Weld root
The bottom side of weld where filler metal and base metal touches.

Weld reinforcement
The height of the weld protrusion on top side of weld. Excessive reinforcement is a discontinuity, as it weakens the joint at the weld toe, as sharp corner induced stress.

Weld Face
The face of the weld looking from top side of the weld.

Root reinforcement
The protrusion on bottom side of weld. Excessive reinforcement is a discontinuity, as it weakens the joint at the weld root, as sharp corner induced stress.

Weld interface
The surface on base metal where filler metal are deposited.

Heat Affected Zone
A section on the base metal parallel to the fusion zone. This section is slightly harder which increases likelihood of embrittement cracking, the reason why pre-heating and PWHT are required, to slow down the annealing(cool down) which reduces hardness.

Fusion Zone
The area where new metal is situated after welding.

Fusion Face/Fusion Line
Same as weld interface, but a name used for when welding has complete.

Source: Weldguru.com

Hydrogen induced cracking
When moisture content are present during welding, they break down under high temperature of approximately 3000 - 3500degC at the arc, hydrogen is produced and entrapped in the weldment. This will induce porosity and subsequently cracking.

Filler rod/Electrode
Commonly known as welding stick, consumable and many more, it transmit electricity to heat up the tip of electrode which melts it along with the base metal.

Weld cup
Commonly for gas welding, purpose of the cup is to reduce welder handshake during weld and is a nozzle for shielding gas. Filler wire is fed from the cup for GMAW/FCAW, no filler wire for GTAW.

SMAW
Shielded Metal Arc Welding or Stick welding, a type of manual welding which uses electrical arc current to melt the filler stick(electrode) and base metal.

Here's a clear visual explanation from ChuckE2009 of how it is done.

Source: ChuckE2009


GMAW
Gas Metal Arc Welding/Metal Inert Gas welding is a type of automatic fed welding process which uses a flexible wire electrode. During welding, electrode is fed from the feeder along with shielding gas.

Here's a clear visual explanation from Weldingtipsandtricks of how it is done.

Source: Weldingtipsandtricks


FCAW
Flux Core Arc Welding is a type of automatic fed welding process which uses a flexible tubular electrode with flux within the tube core. It is similar to GMAW, except with a flux core and that that shielding gas may or may not be used.

SAW
Submerged Arc Welding is a type of automatic fed welding process, where flux in powder form is laid before filler rod passes through the joint.

Source: Redrockautomation

GTAW/TIG
Gas Tungsten Arc Welding or Tungsten Inert Gas welding is a type of automatic fed welding process which uses a tungsten non-consumable electrode on the cup. A separate filler rod (without flux) is manually fed to the joint together while the tungsten electrode is fired.

Here's a clear visual explanation from Weldingtipsandtricks of how it is done.

Source: Weldingtipsandtricks



X-RAY TEST
X-ray tests may or may not be conducted after welding to verify weld size for integrity and soundness, it usually depends on design parameter Ew (Welding efficiency) specified by design engineer.

PQR
Procedure Qualification Record. This is a record of the welders ability to perform certain type of welding process and the type of materials joined. Various tests are performed on the test coupon to verify the integrity and soundness of the joint
- Bend test
- Tensile test
- Ductility
- Hardness (Rockwell)
- Toughness (Charpy V-notch)

WPS
Welding Procedure Specification describes how the welding process will be performed. This specification needs to be backed up and referenced from the PQR. Welding inspectors will refer to these two documents to verify the weld preparation, interpass preparation and post welding inspection activities.

3.3 Valve types, parts, accessories and application,

Gate valve
Application: on/off. Usually use for complete isolation.
Typical serviceable parts: Stem, seat, wedge
Types: Solid wedge, flexible wedge, split wedge.
What to specify when buying: Pressure rating, temperature, flow rates, process medium, end connection type, bonnet connection type, design/material of the body, disc, seat, stem, packing.

Image Source: profmaster.blogspot.com

Globe valve
Application: throttling. Usually used for throttling liquid flow. Large pressure drop across due to significant change in direction in design.
Typical serviceable parts: Stem, disc, seat, packing.
Types: Angle, Y-type, straight type.
What to specify when buying: Pressure rating, temperature, flow rates, process medium, end connection type, bonnet connection type, design/material of the body, disc, seat, stem, packing.

Image Source: www.globalspec.com  /  Mcgraw-hill Publishing

Needle valve
Application: throttling. This valve is exactly like globe valve, except with a tapered seat. For more info, see globe valve above.
Typical serviceable parts: Stem, disc, seat, packing.
What to specify when buying: Pressure rating, temperature, flow rates, process medium, end connection type, bonnet connection type, design/material of the body, disc, seat, stem.


Diaphragm valve
Application: on/off. Usually used for very corrosive chemicals to protect valve trim.
Typical serviceable parts: Stem, disc, diaphragm.
What to specify when buying: Pressure rating, temperature, flow rates, process medium, end connection type, bonnet connection type, design/material of the body, disc, seat, stem, diaphragm

Image Source: www.globalspec.com

Ball valve
Application: throttling or on/off. Usually use for quick opening with less torque requirement.
Typical serviceable parts: stem, handle, ball, seat.
Types: straight, angle, L-port, T-port
What to specify when buying: Pressure rating, temperature, flow rates, process medium, end connection type, body connection type, L/T port arrangement, design/material of the body, ball, seat, stem.

Image Source: www.kitz.co.jp

Plug valve
Application: See ball valve, except that a turnable plug with a bore is used instead of a ball.


Butterfly
Application: throttling or on/off. Not suitable when opposite sides of fluid have large differential pressure, makes the valve hard to open, not tight sealing.
Typical serviceable parts: Stem, disc, seat, packing.
Types: wafer type, lug type.
What to specify when buying: Pressure rating, temperature, flow rates, process medium, end connection type, bonnet connection type, design/material of the body, disc, seat, stem, packing.

Image Source: www.kitz.co.jp

Check valve
Application: Non-return flow.
Typical serviceable parts: Disc/ball, seat
Types: Swing Disc, lifting disc, butterfly, ball,
What to specify when buying: Pressure rating, temperature, flow rates, process medium, end connection type, bonnet connection type, swing/lift/flap arrangement, design/material of the body, disc, seat.

Swing type check valve

Image source: www.kitz.co.jp

Lift Type Check Valve (Similar to globe, but w/o a stem)

Image source: www.jdvalves.com

Safety valve 
Note: Usually referred to many as PSV (Pressure Safety valve for gas systems, opens when overpressure detected) or PRV (Pressure relief valve for liquid systems, opens based on proportion of overpressure)
Application: Pressure relief or thermal relief application. Selection based on pressure and flowrates usually sized according to API 520 PSV Sizing. ASME Section I & Section VIII also denotes their required settings for allowable tolerance on set pressures and blow down(closing back pressure)
Typical serviceable parts: Lever, spring, adjusting ring, disc, seat, bellows.
Types: Conventional, Balanced pressure, Pilot operated, Power operated,
What to specify when buying: ASME Section I (w/ Lever, open bonnet, 2 adjustment ring configuration with U stamped for boilers), ASME Section VIII (Any configuration with UV Stamped for general process), Other design (with or without lever, open or closed bonnet, with or without bellows(back pressure compensation).) , Pressure rating, temperature, flow rates, process medium, end connection type, design/material of the body, disc, seat, stem,

Typical ASME Type I PSV.

Image source: Consolidated Catalogue Type 1900.

Image Source: spiraxsarco.com

Detail of PSV with Bellow/diapgragm

Image Source: spiraxsarco.com

Pneumatic Actuator - variable control

Pneumatic Actuator - on/off control
Air being a more powerful actuating media suitable for explosive environment, is also used for on off control of a valve by an piston actuator.
It comes with a solenoid valve which allows air into itself, the electrical component(solenoid valve) is smaller and less costly to own, as compared to having a huge electrical component with hazardous area protection which could cost significantly more.

Solenoid Actuator - Usually used for on/off control, for where quick opening and closing of the valve is required. Used on small systems, or when used for saving cost on pipelines which requires explosion proof set up(see Pneumatic Actuator above).

Motorized actuator - Usually used for on/off control, for where more torque is required to actuate the valve, motor can be powered by electrical power, hydraulic power or pneumatic.

Handwheel, handle, disc, seat, bonnet - See above photos on valve description for a clearer picture.
The bonnet-body of these valves could be integral or separable. Advantage to be separable if the valve is costly to purchase as replacement. ie. Exotic material or CTE/BAM tested are very costly to purchase, disadvantage is that the leak sealing will not be as robust.

Spoke wheel pulley - Used where valves are installed in high location, where access is inconvenient.

Extended spindle - Used on cryogenic systems, where high humidity leads to icing which eventually ices up the entire handwheel, as such denying user from operating the valve. A long handle alleviates this problem.

Pressure rating - American systems, 150#, 300#, 600#.... sometimes it may denote 600WOG instead which is equivalent. WOG stands for water, oil gas. Read the post on Piping Specs for details on how pressure rating come about.


Valve Sizing for pressure loss (liquid)
dP = SG * (41720Q/Cv)^2

dP = Pressure loss (kPa)
SG =  Specific gravity (dimensionless)
Q = flowrates (m^3/s)
Cv = Coefficient of flow (check valve specification from manufacturer)

For gas/steam, use its specific formula for Cv.


What to specify when buying
The above will help you the understand a valve. With regards on how to specify a purchase to the procurement or vendor when sourcing for valves, refer to the short write up on this section:
3.2 Valve Spec & Purchase

For each of these valves, the essential details to be knowledgeable to select the right valve for the right application. If in doubt, consult manufacturer's application engineer for advice.