Fabrication of Metallic Pressure Vessels. Maan H. Jawad
before any required post weld heat treatment (PWHT), even if they are required after PWHT as well, so that any needed repairs can be completed prior to final heat treatment. This is because repair of a defect found after PWHT will normally require repair and an additional heat treatment. Such additional heat treatment can be costly as well as have the potential to reduce material mechanical properties.
After PWHT and required final NDE, any final machining that is needed takes place. Intermediate machining processes may already have taken place if thick welds require J‐grooves or if unique machining is required because of special configurations. Also, for vessels such as heat exchangers requiring tubesheets or other special components, machining of these tubesheets and components is accomplished in parallel with other work on the vessel.
Next, the vessel will be pressure tested when inspections and NDE demonstrate compliance with all requirements. Pressure testing is done either by a hydrostatic test, which is preferred for safety reasons, or by a pneumatic test. Although failures are not anticipated, access is usually restricted during such tests due to the potentially high levels of stored energy. This is especially true during pneumatic tests, but even though water is considered an incompressible fluid, the energy stored by compression of water or other liquid and any trapped air and the stretch of the metallic shell can result in a significant hazard during such tests.
Once the pressure test is completed and all other quality requirements are verified, the vessel is ready for final cleaning and application of any required paint, conversion finishes, anodizing, or other surface treatments. A name plate describing various vessel parameters is then attached to the vessel, indicating compliance with the applicable code and other requirements.
Finally, with fabrication, inspection, NDE, and testing completion, and coatings applied, the pressure vessel is readied for shipment. Shipment may include low level pressurization with a clean, dry, inert gas, sometimes referred to as “pad pressure.” It is used to ensure that nothing is sucked into the vessel on cold days and to prevent condensation. Shipment also includes blocking or cribbing, special supports, possible packaging, and tie‐down on the truck, railcar or barge for shipping.
Once the product arrives at the customer’s facility, it will often undergo further inspection to ensure that all of the requirements have been met and that there has been no damage during shipping. The Quality Assurance package, when supplied, will be reviewed in detail and placed on file. Only then can the vessel be installed and put into service.
1.3 Cost Considerations
The cost of a pressure vessel is a function of many parameters. In areas where labor is costly, it is often the biggest single factor, but many decisions by both the designer and fabricator influence overall cost. The most effective design from a cost standpoint will be one in which schedule, cost and availability of materials, cost and capability of labor, inspection options, and available equipment and tooling are all considered. In addition, short versus long term product cost considerations may need to be discussed with the customer.
It follows that the designer will either have some experience in all of these areas or will work closely with people who do. Similarly, the shop management will be familiar with a wide range of production techniques, including means of cutting and machining, forming, fixturing and fit up, welding, heat treatment, inspection and testing, cleaning, painting and other surface treatments, and packaging and shipping options and requirements.
If large numbers of vessels of the same or similar designs are fabricated, design and fabrication choices will be different from those involving fabrication of a single unit.
The particular capabilities of a vessel fabricator often make one variation of a design more cost effective than another, and if the designer is not directly associated with the fabricator, it makes sense for these two parties to discuss design options with an eye on cost reduction.
This book is not about fabrication cost estimating, and this chapter does not address actual product cost. It, however, addresses a number of considerations affecting the cost of an overall pressure vessel fabrication to help the user, designer, and fabricator make judicious choices regarding design and fabrication approaches.
1.3.1 Types of costs
For a business, one way of dividing costs is to separate them into either capital or operating costs. Capital costs are the one‐time expenses such as purchase of land, construction of a plant, and major equipment purchases that are expected to last a long time. A small hand grinder, for example, would not be considered a capital cost, while the costs of constructing a building or purchasing a large forge would be. Operating costs are the other costs of being in business, including wages and salaries, real estate expenses (rent, taxes, etc.), materials, furniture, consumables, maintenance, etc.
This way of looking at expenses is useful in understanding what things cost overall, and it might be enough for a company with a single product line. For calculating and controlling costs of production of individual products in a job shop, it is usually easiest to work with burdened labor rates that represent the hourly cost of performing an operation, plus material and other direct costs of a particular job, plus capital costs. The burdened labor cost includes such items as direct wages, cost of vacations and holidays, social security and other tax cost, sick leave, and pension or 401k plans.
Some companies use a single rate for essentially all personnel whose time is charged to a job, while others charge a rate that varies by function or even by the individual assigned to the job. Sometimes costs are broken down further to identify and charge for specific assets outside of the burdened labor rate. This is most likely to occur in a case in which an asset of particularly high value is used only on some jobs. In such a case, dividing its cost among all jobs would subsidize those jobs that require this equipment at the expense of those that don’t. The result would be extremely competitive prices on the jobs requiring this equipment, but a lack of competitiveness on those that don’t need it.
Companies arrive at burdened labor and equipment rates in different ways, but the intent is to allocate costs in a way that allows bidding jobs, recovering costs, and making a reasonable profit. Because the fabrication environment is competitive, it is important to understand enough about the individual cost elements that (1) wise trade‐offs between design approaches can be made to ensure competitiveness, and (2) accurate total cost of a particular fabrication can be identified for pricing purposes and to ensure a reasonable profit.
1.3.2 Design choices
1.3.2.1 Major cost decisions – long term choices
Some design choices must typically involve the customer because they involve significant product cost differences that can only be amortized over the long run. An example of this occurs with a vessel that will contain a corrosive medium. In this case, material choices may make a significant difference in short term vessel costs. A vessel might be fabricated with a corrosion allowance, anticipating that at the end of some term (approximately five years, for example) the vessel will simple be replaced. Another approach would be to fabricate it entirely of a material that does not undergo corrosion in its particular internal and external environments, or to clad it with such a corrosion‐resistant material. The cost of fabricating a pressure vessel of high alloy steel or other material may be significantly greater – perhaps double or more – than that of a fabrication using steel. If a more expensive product allows essentially unlimited life versus five years for the steel pressure vessel, then amortizing the cost of the single vessel versus initial vessel purchase plus replacements, and downtime and labor for the replacement, can make the farsighted decision attractive. Whichever way this decision goes, all other cost issues still apply.
1.3.2.2 Labor–material trade‐offs
Some choices regarding materials simply minimize material costs. Others have the additional advantage of reducing labor. A third category reduces costs by eliminating whole operations. A fourth category is to increase labor in situations where labor cost