4.4 Welding discontinuties

Why you should be very careful with the choice of words:

Welding discontinuities are conditions that exist in the weldment which may or may not be accepted. Meaning these little "defects" on the welds may still be accepted, if it falls beyond the code acceptance criterias, so be very careful with using the terms discontinuities and defects.

There are also many other words in use on the market such as Flaws, Imperfection, Indications.

Welding defects are the discontinuities which are not acceptable by the inspector in reference to Company design requirements, ASME code requirements or AWS code requirements, whichever takes precedence.

For pressure vessels, you could find the defect acceptance criteria on latest edition of ASME Section VIII Div 1 in the Appendixes and for pipes in ASME Section B31.3 in the Appendixes. Most of these defects will be measured based on the result of NDE testing such as RT, UT, MT and PT. However an inspector may reject a weld based on visual inspection if it obviously contains reject-able discontinuities.

According to API Inspection codes, NDE shall be carried out by the Examiner which is basically the certified NDE technician. Results of the NDE shall be reviewed by the Inspector for acceptance at his discretion.

Types of discontinuities

Excessive Overlap

- When weld bead extends too much beyond the weld toe. 

Excessive Undercut

- When weld bead do not fully cover the beads

Excessive Weld Reinforcement

- Height of weld bead is too high, presents unnecessary stresses to the join. 

Inclusion 

- Inclusion of impurities

Porosity

- Inclusion of porous air sacks/bubbles

Arc Strike

- Spatter like effect caused by electrode scratching when it does not starts due to cold weather.

Cracking 

- Usually caused by presence of water when welding, water breaks down to its elemental form causing causes hydrogen induced cracking. 

Crater crack

- Always pull the weld away from direction of weld when completing, this prevents crater cracks from forming. 

Lack of fusion

- Temperature is too low during passes, the binding between the metals is incomplete. Can occur between passes, or between base/filler metal.

Lack of penetration

- Too fast welding,  and low temperature resulting in inability to penetrate into the root gap,as such not meeting desired weld size. 

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.

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.