Volume XXX: ASME Pressure Vessels

ASME PRESSURE VESSELS

The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.

DATA SHEETS – Do they tell your story?

Fabricators rely on information contained in the Customer’s Data Sheet and drawings to design and fabricate a pressure vessel that meets the requirements the Customer needs.  In this edition, we are going to look at the Data Sheet and review the information furnished with the information required.  How does your Data Sheet stack up?

Our industry uses the term “data sheet” to refer to a description of what equipment is supposed to do. Yet, such sheets often list specifications or estimates rather than hard data. Nevertheless, the term “data sheet” leads many engineers to automatically assume the numbers on it accurately represent what the equipment should do. It’s difficult to avoid jumping to this conclusion even after years of experience — indeed; some people never learn to question entries. Unfortunately, this leads to confusion and errors that can undermine equipment troubleshooting and plant modifications.

A more-correct term is “specification sheet” because that conveys the idea that some doubt exists about equipment performance. Regardless of what the sheet is called, it’s crucial not to blindly accept what appears. You must evaluate the nature and accuracy of the numbers listed.

vendor-quote

Let’s start by defining what actually constitutes data. They are values resulting from direct measurement or counting. A temperature reading taken from a thermometer is a datum. If you count the tubes in a shell-and-tube heat exchanger, that number is a datum.

If that heat exchanger had thousands of tubes, you might not want to count them one-by-one. Instead, you might weigh all the tubes and then calculate the number of tubes by dividing that value by the average weight of an individual tube. However, the result is not a datum because it isn’t a direct measurement. (The datum is the overall weight of tubes.) Instead, your calculation provides an estimate; some calculations may be very precise, others not so much.

Figure 1 shows part of a specification sheet for a shell-and-tube heat exchanger. The tube count is a datum. However, the duty isn’t a datum but the result of a calculation. Actual exchanger performance will vary.

shell-and-tube heat exchanger

When troubleshooting or modifying a plant, far too many conversations include the remark: “The data sheet shows the heat exchanger will do so many BTUs per hour.” You can substitute tower capacity, pump head, filtration rate, NOX release and lots of other parameters for equipment performance. However, unless the equipment has been put through a test, the performance values shown on the data sheet only are estimates. Modern pumps, compressors and burners tend to have reasonably good estimates. Heat exchangers, distillation equipment, separators, filters, centrifuges and many other types of equipment often perform much differently than “expected.”

So why does performance vary from that listed on the specification sheet? Figure 1 highlights some issues:

Age: The sheet was filled out in 1977. We do have better methods for estimating exchanger performance today — especially for complex systems with phase changes.

Guesstimates:  At the specification stage, estimating the fouling factor is guesswork for many heat exchangers. For troubleshooting, you must know the real fouling factor; it may be much higher or lower.

Other potential issues are less apparent:

Fitting the bid. Many equipment items only come in standard models. So sometimes, to minimize risk of losing the job, a vendor will make the specification match what’s expected rather than what the standard equipment really delivers.

Maintenance. The equipment now may differ from what was delivered. Wear, use of non-OEM parts and cumulative “insignificant changes” may markedly affect equipment operation.

Safety margins. Some calculations have enough uncertainty that engineers, vendors and operators all may add safety margins. This may result in equipment that’s far from a best fit to the process requirements. Vastly over-sized equipment may operate completely differently than expected.

Many other factors may come into play as well. In any case, never forget that “data sheets” often show numerous things that aren’t data. Don’t fall into the trap of thinking that equipment operation won’t deviate much from what’s on paper.

So – what information is required for a Vessel Fabricator to design and fabricate your process equipment?

Following is a list of the information which needs to be shown on your Pressure Vessel Data Sheet:

  • ASME Design Code
  • Joint Efficiency
  • Materials – head, shell, internals, gaskets and bolting
  • Paint and coatings
  • Design temperature and pressure
  • External pressure design
  • MDMT (minimum design metal temperature)
  • Non Destructive Testing required (for process reasons)
  • Process fluid specific gravity and liquid height (operating)
  • Material requirements – impact testing, UT, chemical testing, etc.
  • Orientation (vertical or horizontal)
  •  Skirt height, saddle height, leg length, or support lug information
  • Service (lethal, wet H2S, amine, etc.)
  •  Internals
    • Packed bed (height, type of packing, density of packing, liquid holdup, differential pressure)
    • Trays (quantity, type – single pass, two pass, etc., material type, liquid holdup, differential pressure)
    • Linings required
  • Nozzles – size and elevations (on head or shell)
  • Nozzle loads required
  • Insulation or refractory required
  • Ladders and platforms required
  •  Wind and Seismic design information
  • Any cyclic operation
  • Steam out conditions and affected nozzles
  • Lifting and tailing lug information

Even if items are provided by others or the owner when it gets to the field, the vessel engineer needs to account for the loads.  All of these items (provided by others) affect the foundation, the wind loading, the skirt and bolt stresses.  So please include everything on the data sheet.

Take a moment and look at your pressure vessel data sheet.  Are they completely filled out?  Are there any blocks missing?  An incomplete data sheet only delays the design of your equipment.  If it is not initially provided, correspondence and phone calls are required to get all the information.  If someone who knows the answers is out of town or on vacation, this can delay the design by a week or more.  So, please use this information to make sure that all the required information is provided with your request.

Material selection is always an important factor in designing your pressure vessel.  Temperature and pressure affect the thickness of your vessel and the flange rating for your nozzles.  By using the correct materials for your process temperature requirement, the optimum thickness and nozzle thickness can be utilized.  The following chart is a quick reference for material types used at different design temperatures.  There are some overlap of material types for a given temperature range.  If you have a question on which of the materials would be the most economical for your design, please call us at Boardman and we will explain the methodology used to determine the correct material.

material-types-design-temps

This is presented to you as a service from BOARDMAN INC. located in Oklahoma City, Oklahoma.

Since 1910, Boardman has been a respected custom fabricator. We take pride in our ability to take the most stringent specifications and requirements to provide a high quality solution to our customers.With more than 75 years of ASME Section VIII, Division I engineering experience, we have the unique ability to provide custom solutions to our customers.

Fabricated Projects Include:

  • Trayed Towers & Columns
  • ASME Pressure Vessels
  • Molecular Sieves
  • Rotary Dryers & Kilns
  • API Tanks
  • Acid settlers
  • Stacks, Scrubbers
  • Thermal Oxidizers
  • Accumulators, Condensers
  • Crystallizers
  • Ducting
  • Bins
  • Large Diameter Piping

The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick. BOARDMAN INC. is available for shop tours and Pressure Vessel and Static Equipment Fabrication Classes.

BOARDMAN INC. is available for shop tours and Pressure Vessel and Static Equipment Fabrication Classes.

 

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