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.