『『Original by HaiChuan Translation Team』The long-standing debate on design pressure for pipes/ductwork

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English title: The Perennial Debate On Piping / Pipeline Design Pressure. Original link: https://www.cheresources.com/invision/blog/4/entry-553-the-perennial-debate-on-piping-pipeline-design-pressure/. Original author: ankur2061. Translator ID: @shuangmu_DTQO. Proofreader ID: @zhdw0906. I’ve been involved in debates and discussions on the topic of piping/pipeline design pressure countless times. ““Cheresources” itself has dozens of posts related to this topic. I now firmly believe that for most problems, there is no single solution in process design. There are various design options, each with its own advantages and disadvantages; process engineers need to choose a design based on the preferences and requirements of the end users. However, whenever a process design engineer proposes a design solution, he or she should be aware of all the advantages and disadvantages of that solution, and evaluate various aspects of it from the perspective of end-users or operators, such as safety, ease of operation, design life, and cost-effectiveness. Let’s return to the topic of piping/piping design pressure. There are mainly two methods for determining (or finalizing) the design pressure. 1. Design problem of takeoff pressure based on centrifugal equipment. This situation is more common in centrifugal pumps. However, this is somewhat tricky for centrifugal compressors, as the design pressure needs to be determined based on the surge limit curve across the entire compressor curve. A simple approach is to take into account the maximum suction pressure of the centrifugal pump or the normal pressure difference at the compressor’s rated flow rate during the preliminary design stage, and to include a margin based on this value (such as 1.25 or 1.3 times). This method for determining the design pressure of the outlet pipeline in centrifugal equipment is often used in the design processes of many engineering companies, and it is usually quite effective. So, how should positive displacement machines (PD) be designed? In constant-volume devices, even if the outlet pipe is blocked, the device will maintain its flow rate and continue to discharge fluid into the connected pipe. This causes pressure buildup in the pipeline, ultimately leading to the shutdown of the PD motor and/or damage to the pipeline (cracking/deformation). Therefore, for PD devices, there is no real concept of a starting head or pressure, as the starting pressure here is the pressure that increases until the motor stops rotating and/or the pipeline and flange gaskets start leaking. Therefore, the aforementioned method for selecting design pressure for centrifugal equipment cannot be applied to PD equipment. Safety valves (Pressure Safety Valves, PSVs) are installed at the outlet of PD equipment, and a certain pressure level is set to protect the entire outlet system, including the pipes and other components within the system, from destructive overpressure. There are various methods for setting the pressure of PSV. It is possible to install multiple PSVs to protect different parts of a discharge system operating at the same design pressure, with the same pressure set for them. If there are devices in the system with a lower design pressure rating, a separate PSV can be installed for that device, with its set pressure set to the device’s design pressure. Another approach is to consider the system as a whole, that is, to assign the same design pressure to the entire system, including the pipes and connecting equipment. By conducting a comprehensive evaluation of the system and minimizing the use of stop valves or isolation valves within it, it is possible to determine the minimum number of pressure relief devices required for the system. It is important to note that if blockage occurs due to liquid filling between two isolation valves or globe valves, a pressure relief device for thermal expansion must be provided for that section. The key to this sentence is that the space inside the tube is filled with liquid. And this method is not suitable for dealing with blockages in gas-filled pipes. 2. Designed based on the imperial pipe grades specified in ASME B16.5 (formerly ANSI B16.5) and the force-temperature grades. For young engineers, ASME B16.5 is the American standard for \"pipe flanges and flanged pipe fittings\". Although this is the American standard, it has been widely accepted in pipeline design and engineering. ASME B16.5 establishes the basic premise for determining design pressures, and it is a widely accepted fact that flanges are the weakest link in any piping system; the reliability of any system depends on its weakest components. Personally, I think this is a more appropriate method for determining the design pressure value of any piping system based on the given design temperature. ASME B16.5 provides a comprehensive table of pipe pressure ratings (including various temperatures, pressures, and materials). The pipe pressure rating ranges from 150# up to 2500#. Materials are classified by number, such as from 1.10 to 1.18, 2.1 to 2.12, and 3.1 to 3.19. Interpolation can be used to find other parameters corresponding to temperature between two given temperature values. Let’s take an example: Pipe rating grade: 300#. Material category: 1.1 (this material category generally refers to carbon steel). Design temperature: 80°C. Design pressure: 47.5 barg (calculated according to ASME B16.5). The pressure-temperature class for material category 1.1 in B16.5 is listed in the first table. The pressure corresponding to 50℃ is 50.1 barg, while the pressure at 100℃ is 46.6 barg. For 80°C, the pressure corresponding to the calculated difference is 47.6 barg. If I must select a PSV (commonly referred to as a \"thermal safety valve\" or \"thermal expansion safety valve\") for 300# pipes with a material category of 1.1, in order to protect the system against blockages and hydraulic expansion caused by liquid trapped within the pipes, I recommend setting the pressure value at 47.6 barg. Because it offers flexible operation, this method for determining design pressure is also very suitable for centrifugal pumps. For example, most centrifugal pump manufacturers offer 3 different impellers corresponding to three different characteristic curves. If, due to operational flexibility, replacing the impeller gives the pump a higher head, the pressure relief device set initially based on the design pressure calculated from the rated impeller will be useless, and it becomes necessary to set a new, higher pressure value for the PSV after replacing the pump’s impeller with one that provides a higher head. I believe that it is a good engineering practice to determine the design pressure by conducting a comprehensive evaluation of the pump outlet piping system (including pipes, fittings, and equipment), meaning that the design pressure should be consistent with the pressure in all pipes or conduits at the pump outlet. There may be debate regarding the high costs associated with a full system rating, but it is important to remember that any failure comes at a cost in terms of personnel safety, environmental harm, and production losses, and in most cases the cost of failure is greater than the cost of conducting a rating. Systems without a full rating also require detailed failure mode and effects analysis to determine whether additional protective measures are needed to safeguard the system’s integrity. Although I have clearly stated my preference for evaluating the entire piping system, in industrial practice regarding the flow patterns of oil/gas at the wellhead, the area downstream of the throttle valve is not evaluated. The comprehensive evaluation cost for long-distance flow pipelines based on upstream throttle valves would be astronomical. A \"High Integrity Pressure Protection System\" (HIPPS) with a SIL rating is used to protect the flow pipeline downstream of the wellhead throttle valve, preventing high pressures in the event of a valve failure. API 14C provides recommendations for safety systems in upstream oil and gas facilities, including wellhead flow lines.