Sunday, March 9, 2008

How to Design a Process Tank and Vessel

Table of Contents


Background of process equipment
History of the Pressure Vessel Code
Additional Codes and Standards

Vessel Design Basis Design Basis
ASME Designed, Stamped, and Registered
ASME Designed, No Stamp
ASME Constructed (Material and Welding Only)

Design Specifications
Information Needed for tank and vessel design
Information Needed for Heat Transfer Jackets
Equipment Data Sheets

Material Selection
Comparative Cost of materials
Material Finish
Weld Finish
Comparative Cost of Material and Weld Finishes

Vessel Selection
Selecting a Vessel
Vessel Geometry
Recommended Vessel Dimensions
Head Configuration

Vessel Thickness
Equations for Internal Pressure Calculations
External Pressure Consideration
How to Calculate for External Pressure
Sample Exercise - Calculating Vessel Thickness
Additional Considerations

Heat Transfer Jacket
Heat Transfer Jackets
Selecting the Heat Transfer Jacket
Open Jacket
Half Pipe Jacket
Dimpled Jacket
Why Choose One Jacket Over Another?

Internal Heat Transfer
Squirrel Cage

Installation Details

General Information
Impact of Agitation on Vessel
Agitator Nozzles
Calculating Local Stress Due to External Loadings

Determining the Need For Baffles
Standard Design
Baffle Width
Baffle Offset

Vessel Supports
Ring Girder
Portable Vessels with Casters
Pad Mounted

Insulation Materials

Outer Sheathing

Pressure Relief Device
Vessel Lights

CIP/SIP Considerations

Causes for Fatigue

Vessel Inspection Procedures
Job Review
Final Job Review




Background in Process Equipment

Tanks and vessels are made in all shapes and sizes. The smallest ones may be only a few inches in diameter, whereas the largest vessels may be several hundred feet in diameter. Some may be buried in the ground or deep in the ocean. However, most are supported on the ground or on platforms.

The internal pressure to which process equipment is designed is as varied as the shape and size. Internal pressure may be as little as 1 inch water gage to as much as several thousand psi. However, the usual range of pressure is about 15 psi to 3000 psi. The ASME Boiler and Pressure Vessel Code, Section VIII, Division I specifies a range of internal pressure from 15 psi at the bottom to no upper limit: however, at an internal pressure above 3000psi, the ASME Code requires special design considerations on some components. Any pressure vessel that meets all the requirements of the ASME Code, regardless of the internal or external design pressure, may still be accepted and stamped by the manufacturer with the ASME Code symbol. Other process equipment, such as API storage tanks, may be designed to contain only the internal pressure that is generated by the static head of the fluid contained in the tank.

History of the Pressure Vessel Code

In 1905, a destructive explosion of a firetube in a shoe factory in Brockton, Massachusetts, killed 58 people, injured 117 others and did $400,000 in property damage. In 1906, another explosion in a shoe factory in Lynn, Massachusetts, resulted in death, injury, and extensive property damage. After this accident, the Massachusetts govenor directed the formation of a Board of Boiler Rules.This first set of rules for the design and construction of boilers was approved in Massachusetts August 30, 1907. In 1915, the first ASME Boiler Code was issued. It was entitled "Boiler Construction Code, 1914 Edition." This was the beginning of the various sections of the ASME Boiler and Pressure Vessel Code.

Additional Codes and Standards

In addition to the ANSI/ASME Boiler and Pressure Vessel Code, many other codes and standards are uses through out the United States. Some of the codes include the following:

ANSI/API Standard 620

"Recommended Rules for Design and Construction of Large, Welded, Low -Pressure Storage Tanks," American Petroleum Institute (API)

ANSI/API Standard 650

"Welded Steel Tanks for Oil Storage," American Petroleum Institute (API)

ANSI-AWWA Standard D100

"Water Steel Tanks for Water Storage," American Water Works Association (AWWA)


"Steel Above Ground Tanks for Flammable and Combustible Liquids," Underwriters Laboratories, Inc.


"Steel Underground Tanks for Flammable and Combustible Liquids," Underwriters Laboratories, Inc.

Vessel Design Basis


Design Basis

The first item to be considered in a vessel design specification is the code or standard that best suits the requirements. The design, fabrication, and inspection standard that is most frequently used for pressure vessels is the ASME Boiler and Pressure Vessel Code, Section VIII, Division I.

However ASME rules may not be required in states that do not have a pressure vessel law, or when the pressure vessel is below 15 psig. These vessels may be designed and fabricated to a manufacturer's standard, which uses the code as a guideline.

Many fabricators offer the following standards: (1) ASME Designed, Stamped and Registered; (2) ASME Designed, No Stamp; or (3) ASME Construction, Material and Welding Only.

The following table shows the features offered with each of the three standards.

Design Basis

ASME Design ASME Design ASME

And Stamp No Stamp Construction

U-Stamped Yes No No

Certified Material Yes Yes Yes

Certified Welders Yes Yes Yes

Mill Material Certification Yes Yes Yes

Heat Numbers Yes Yes Yes

Weld Procedures Yes Yes Yes

Calculations Yes Yes No

Material Welding I.D. Yes Yes Yes

ASME Heads Yes Yes No

ASME Designed, Stamped and Registered

The ASME Designed, Stamped, and Registered standard is used when the desing pressure is in excess of 15 psig. THe vessel is stamped with U-Symbol and is assigned a National Board Number.

ASME Designed, No Stamp

THe vessel is designed in accordance with the ASME Code; however, the vessel is not inspected by the authorized inspector and is not stamped with the U-Symbol or assigned a National Board Number. Inspection criterial is generally the sole responsiblity of the fabricator. It is not uncommon to inspect per ASME Code Standards.

ASME Constructed (Material and Welding Only)

THe vessel is not designed in accordance with code rules and published stress values. However, the vessel is desinged in accordance with the fabricator's engineering standards, the material is supplied to Code specifications, and ASME certified welders are used to construct the vessel. This specification offers a customer a quality vessel - at a lower cost - when the pressure is 15 psig or less. Inspection criteria is generally the sole responsibbility of the fabricator.


The API-650 standard for welded steel tanks addresses material, design, fabrication and testing requirements for vertical, cylindrical, above ground, closed and open top, welede steel storage tanks. This standard covers only tanks whose entire bottom is uniformly supported. The API-650 standard for carbon steel vessels which specifies a minimum of 3/16" plate for the top head and shell and 1/4" plate for the bottom head. This standard is often specified for large storage tanks operating at atmospheric pressure. However, if the purchaser is specifying stainless steel and using this standard, the vessel will be more costly than one that is fabricated to API rules but not designed or tested in accordance with API-650.


THe API-620 standard is for the design and construction of large, welded, low pressure storage tanks whose pressure exceeds those specifiedin API-650 but does not exceed 15 psig. This standard also specifies the minimum material thickness for carbon steel. When stainless steel is specified, the cost may be reduced if the API rules are not used for design and testing of the tank.


THe UL-142 Specification is for horizontal or vertical, welded, carbon steel or stainless steel tanks, for outside, above ground storage of flammable and combustible liquids at pressures between atmospheric and o.5 psig. The capacity shall not exceed 50,000 gallons.


The UL-58 Specification covers horizontal, atmospheric type steel or stainless steel, tanks for underground storage of flammable and combustible liquids. The capacity for the UL-58 specification includes tanks up to and including 50,000 gallons with a maximum diameter of 144 inches.

Design Specifications


Information Needed for Vessel Design

THe design information for a process vessel must be given in a design specification. THis is usually done by mutual agreement between the purchaser and the manufacturer. If the purchaser can furnish adequate information about the required vessels in the initial design phase, the fabricator will be able to supply a competitive quote that will be satisfactory to both parties.

The following items must be considered when designing/specifying a pressure vessel.

  1. Design Pressure - A. Internal Design Pressure B. External Design Pressure
  2. Maximum Dsign Temperature
  3. Minimum Design Metal Temperature (MDMT) and Coincident Maximum Design Pressure
  4. Corrosion ALlowance
  5. Specific Gravity of Product
  6. Wind Loading
  7. Seismic Zone
  8. Special Loadings - A. Agitator Specifications - B. Load Cell Information - when required - C. Piping Loads on Nozzles - when required
  9. Type of Material and Finish
  10. Interior and Exterior Weld Finish
  11. Type of heads (ASME, 80/10, 2:1 Elliptical, Hemispherical, Conical, etc)
  12. Type of Material of Supports
  13. X-ray Requirements
  14. Number and Size of Fittings
  15. Primer and Paint Required
  16. Insulation
  17. Sheathing

Information Needed for Heat Transfer Jackets

The following items must be considered when designing/specifying a heat transfer jacket.

  1. Style of Jacket
  2. Design Pressure - A. Internal Design Pressure - B. External Design Pressure
  3. Maximum Design Temperature
  4. Minimum Design Metal Temperature (MDMT) and Coincident Maximum Design Pressure
  5. Material Specifications
  6. Heat Transfer Medium
  7. Heating and/or Cooling Requirements for the HEat Transfer Jacket or specify the amount of heat transfer surface that is required
  8. Design Flow Rate
  9. Maximum Allowable Pressure Drop
  10. Fittings (Type, Size, Location) - A. Flange - B. Ferrule - C. Pipe - D. Couplings

See the equipment data sheets for a complet list of items that should be provided to the fabricator.