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.
Note: Readers are advised to make their own engineering judgments on the validity of the design improvements suggested here, and to develop their own conclusions.
Design operations-and-maintenance-friendly pressure vessels—Part 3
Transportation and storage notes
While part of an operating company’s technical support team, the author was asked for a particular grade of mineral oil. It was discovered that the oil was requisitioned to apply on the inside surface of a vessel before it was put back into operation. Further questioning revealed that the vessel GA drawing included the comment, “Mineral oil, grade xxxx, is to be applied inside the vessel.” The mineral oil coating was meant for new vessels as a rust preventive during transportation and prolonged storage. The technicians had unknowingly been following the procedure for vessel maintenance.
The lesson is that transportation notes should not be appended on vessel GA drawings. If appended, the purpose of such notes should be clearly stated to avoid confusion, keeping in mind that maintenance personnel faithfully adhere to all instructions stated on vessel GA drawings. The impression that the role of vessel GA drawings ends once a vessel is commissioned is incorrect. GA drawings are a valuable maintenance aid and necessary throughout the life of a vessel. Providing a set of A-3-size GA drawings and internals, no matter how congested the reduced-size copies look, is suggested. Jumbo-size drawings (A-0, A-1) are difficult to handle in the field; photocopiers are rarely available to make copies of A-0 and A-1, and such drawings usually end up in untraceable custody, albeit with good intentions.
State vessel operating weight
General protocol dictates that vessel GA drawings provide a vessel empty weight and a hydrotest weight simulating a vessel that is full of water. The transporter must know the empty weight. The hydrotest weight is required by the vessel manufacturer to ensure that the shop floor can handle the load, and it is also used to design the vessel foundation onsite. This works well if the fluid is water or lighter than water.
What happens if the vessel is designed for fluids heavier than water—e.g., sulfuric acid (specific gravity = 1.84)? The fluid weight would almost double, but the GA drawing may still state the hydrotest weight. Cases have been reported where the hydrotest weight from a GA drawing was inadvertently used to design a civil foundation for sulfuric acid vessels, resulting in undue foundation settlements. Specifying a vessel weight as full of liquid for heavier fluids, in addition to usual empty steel weight and hydrotest weight, would be a good practice.
Recertification after repairs
Old vessels sometimes require new nozzles when moved and pressed into another service (FIG. 6). New nozzles were added to this 155-t separator vessel. Built in 1977 to ASME Div. 2 standard, the vessel did not have readily available design calculations nor National Board (NB) registration. Nevertheless, the additional nozzles were added and the vessel recertified to Div. 2. The recertification process is not discussed here. The intent is to inform the vessel owners that it is possible to regularize the redundant fit-for-service vessels and recreate the vessel “birth certificate,” or “U” certificate. This defies a prevailing notion that an ASME “R” certification cannot be accorded unless an ASME “U” is produced. A redundant fit-for-service vessel need not be discarded for want of repair modifications and missing documentation.
While oil and gas wells may go dry, a pressure vessel can survive and move to another location. This movement (e.g., state to state) may require different pressure vessel regulations. For this reason, some owners request multiple certifications, which should not pose problems if handled at the design stage. Challenges arise if a vessel is certified for only one state and must subsequently be recertified for use in another. It is advised that design engineers ask vessel end users whether multiple certifications are needed. In this instance, it is better to pay a little extra at the design stage and save undue paperwork, delays and associated expenses later.
The remaining strength of corroded vessels
Despite the existence of ASME B31G, “Manual for determining the remaining strength of corroded pipelines,” since early 1980, there was no such document for pressure vessels. The author’s query to ASME in the 1990s received the following response: “The committee has no plans to develop code criteria providing guidelines for calculating the remaining strength of corroded vessels originally fabricated to the ASME code.” In the absence of guidelines, there was a disconcerting practice to treat a vessel as a large-diameter pipe and apply ASME 31G to calculate the remaining strength of corroded vessels. This approach was erroneous and should not have been used.
Realizing the growing demand for the fitness evaluation of aging vessels, API came out with RP 579, “Recommended practice for fitness-for-service,” in 2000. Not wanting to be left behind this time, ASME joined in collaboration, and a massive (1,128-page) second edition was published in 2007 as API 579-1/ASME FFS-1. The edition’s status was elevated to a “Standard” from the earlier “Recommended Practice.” To illustrate the complex calculations used in the assessment procedures of 579-1/FFS-1, API issued an example manual, 579-2/FFS-2, titled, “Fitness-for-service example problem manual,” in 2009, a very informative 374-page document. It is recommended that practicing engineers consistently reference the example manual to avoid potential errors and conclude the fitness evaluation of corroded vessels with a high degree of accuracy and confidence.
Summary of cost implications
While a majority of items will reduce vessel ownership costs, some items may slightly increase the initial cost (e.g., reinforcing pads and larger size nozzles for relief valves and vessel bottom outlets).
However, project costs (CAPEX) should not be evaluated alone. If the costs incurred by the plant maintenance personnel in maintenance and possible field modifications (OPEX) are added, then all of the suggested measures eventually reduce the ownership cost of the vessel. Traditionally, the OPEX for static equipment, such as a pressure vessel, has been considered to be very low as compared to CAPEX. This is not true. An undersized gravity drain alone can repeatedly make OPEX much higher than CAPEX by way of lost production year after year.
Equipment as Capital Assets
For some, it may be surprising to learn about the enormous impact boilers and pressure vessel equipment have on their daily lives. In contrast, certain plant and facility engineers have had direct, even catastrophic, experiences that have taught them the value of this equipment and the necessity of giving it the care it requires.
Let's face it, plant utilities represent a substantial overhead expense that reduces the bottom line. Time and time again plant engineers are required to do more with less and, consequently, are forced to defer maintenance. Eventually, equipment failures occur with increasing frequency, forcing immediate, unplanned repairs.
The consequence of this approach typically transforms what was once a proactive management policy into a reactive role. However, it is widely recognized that the most costly approach to maintenance is reactive planning. These repair costs, direct and indirect, can be significantly higher than those of a proactive management program. At this point, the concept of treating such equipment as a "capital asset in need of management" is key to financial and operational success.
Treating pressure vessels and equipment as assets involves establishing a mindset that is proactive towards maintenance, upkeep, and repair. Don't wait until the warning signs are flashing before taking action. For example, consider an actual situation involving an extensive district heating system. Leaks in the distribution system went unattended because of cuts to the maintenance budget.
The consequence of this action affected the boiler water treatment program by causing excessive chemical consumption. With the budget unable to support the increased chemical cost, the system was left to its own demise. Years later, instead of having only to repair the distribution loop, the plant faced a major capital project, including possible replacement of the critical pressure equipment assets.
Equipment as a capital asset
In most situations, the life of pressure equipment can be extended through an asset management approach. Boilers, pressure vessels, and production equipment and systems are typically classified as capital and are considered assets because their value diminishes over time and is depreciated according to appropriate accounting practice. Although categorizing major equipment as capital assets is not new, the way it is managed changes significantly when we introduce a combination of condition assessment, altered maintenance practices, and financial planning.
Managing equipment as a capital asset essentially involves three steps:
- Understanding the condition of all major equipment
- Altering maintenance practices and procedures
- Developing a long-term equipment strategy.
Implementing these steps almost always results in more effective use of resources and financial capital.
Understanding your assets
Understanding the life and reliability of an asset is key to this approach. A number of organizations provide clients with the latest in maintenance reliability, reliability-centered maintenance, and preventive/predictive maintenance. These tools are in-tended to increase productivity and reliability while more effectively managing maintenance costs. They are an essential part of any operation and provide the information required to manage the life of pressure equipment.
Before selecting a maintenance strategy that will be integrated into a comprehensive asset management program, a thorough understanding of the equipment condition is essential. An engineering assessment provides a road map to prioritized plan development. Data acquired during an engineering review represent the basis for subsequent decisions regarding maintenance philosophy and asset classification. Once the equipment condition and remaining life are established, it becomes easier to find the appropriate balance between maintenance and financial management.
An alternate perspective
Consider the need to manage the maintenance of boiler and pressure vessel equipment relative to the customary practice of managing daily operations. Operating personnel routinely inspect boilers and pressure vessels during periods of operation. They look for any visual signs of distress or abnormal performance. Leaks, for example, whether liquid or gaseous, indicate a developing problem.
Operations may incorporate either direct or indirect observation methods. For instance, testing water chemistry requires a combination of direct and indirect measurement. It is used to directly indicate whether the system is being adequately maintained to safeguard the boilers and system from scale formation and corrosion. The operator may indirectly conclude from test results showing a chemistry imbalance that a leak creating an excessive demand for makeup water has developed.
Monitoring temperatures, pressures, and flows during operation yields useful data for instantly gauging the operation of any system. Analyzing these data points for trends forewarns the operator of deteriorating conditions and provides a guide to informed decision making.
Decisions about the necessity and timing of maintenance, including repairs and replacement, significantly impact the budget. By implementing a carefully structured inspection program, plant personnel can develop a preventive maintenance (PM) program that ensures a high degree of operational reliability at optimal cost.
Making detailed evaluations
State, jurisdictional, or insurance regulations require nearly all industrial boilers in operation to be inspected annually. Pressure vessels are held to the same requirements albeit to a much lesser degree. The actual inspection made prior to issuing an operating certificate is safety oriented. Although these inspections are a necessity, they are not intended to be an evaluation of equipment reliability.
The reliability and ultimate life expectancy of pressure equipment is directly related to the ability of its components to withstand a given pressure at a specified temperature. A detailed evaluation is required to adequately assess the reliability of pressure equipment. This approach involves three steps.
- Historical review. A review of the equipment history provides critical information about use and repairs that may influence the reliability of the pressure equipment. An awareness of past events guides the next phase of the evaluation: condition assessment.
- Condition assessment. This phase is broader in scope than the jurisdictional inspection discussed previously. It is more quantitative than qualitative. Although visual inspections are fundamental to both inspection programs, a condition assessment includes several other routines. The most common techniques include ultrasonic testing, magnetic particle or liquid penetrant examination, hardness testing, metallurgical evaluation, and material analysis.
- Data analysis. The final step in the process is to analyze and report the collected data. Information is reviewed with consideration toward improving reliability and future operating requirements. A detailed evaluation report should include an assessment of conditions that discusses the probable cause of each deficiency and its effect on equipment reliability.
o Long-term equipment strategy
Once base-line equipment conditions are established, a long-term strategy for its management can be developed. Combining assessment results with future strategic planning enables a plant engineer or facilities manager to make appropriate decisions about long-term equipment management.
Source: Murti, D.G. – The Augustus Group, Montgomery, Texas. Kossik, J., “Draining time for unpumped tanks,” Chemical Engineering, Vol 107, No. 6, June 2000 Loiacono, N.J., “Time to drain a tank with piping,” Chemical Engineering, Vol. 94, No. 11, August 1987.
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