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.

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