The issue of how to determine the design pressure for pipelines is complex; please refer to the post.
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As shown in the figure below, this is a schematic diagram of a commonly used process in our company. Explanation is as follows: 1. Device A and Device B heat the fluid in the flow path using saturated steam in the shell side, turning it into a gaseous state. The operating pressure of the tube train is 0.28 MPa, and the design pressure is 0.35 MPa. 2. A safety valve is installed on the (pipe-side) connection pipeline between Devices A and B, with a set pressure of 0.35 MPa for the safety valve. 3. Devices C1 and C2 – each of these devices operates by alternating between positive and negative pressures. The operating pressure range is from -0.093 MPa to 0.26 MPa, while the design pressure for these devices is 0.5 MPa. Remarks: ① Since the equipment operates alternately under positive and negative pressures, fatigue analysis is required. However, our company does not have the necessary software and hardware resources; therefore, the design of this equipment has been outsourced. The outsourcing team classified the equipment as SAD grade, with a design pressure of 0.5/FV MPa. ②Recently, following consultations and expert advice, the equipment was designed to operate at a pressure equal to 5 times the maximum pressure difference, namely 1.8 MPa, which eliminates the need for fatigue analysis in the design process. The problem now is that our original pipeline design parameters were based on a design pressure of 0.5 MPa, with the nominal pressure of fittings and valves being PN1.6 MPa. After the design pressure of the equipment is increased, in accordance with the provisions of 4.1.1.2.3 on page 7 of \"Part 3: Design and Calculation\" in the \"Code for Pressure Piping – Industrial Piping\" (GB/T 20801.3-2020), it is stipulated that \"when the piping is directly connected to the equipment as one pressure system, the design pressure of the piping shall be no less than the design pressure of the equipment.\" I will adjust the design pressure of the pipes directly connected to devices C1 and C2 (note: those shown as thick lines in the diagram) to 1.8 MPa; meanwhile, the nominal pressure of the 8 pneumatic on/off valves located above and below devices C1 and C2 will also be changed to PN2.5 MPa. For the remaining pipes that are not directly connected, I determined the design pressure for these pipes to be 0.4 MPa, following the principle of using a pressure that is not lower than the piping design pressure of devices A and B. Recently, the client requested that I lower the design parameters for the pipes connecting devices C1 and C2 directly ahead of and behind them; they believe these parameters should be set at 0.4 MPa instead of 1.8 MPa ； Moreover, there is a safety valve in place at the source of pressure to provide protection; the setting pressure of this safety valve is only 0.35 MPa, while it is designed for a pressure of 0.4 MPa. Therefore, there are no issues with the pipeline system at all. As the pipeline has an increased design pressure, the nominal pressure of the connected valves also rises from PN 1.6 MPa to PN 2.5 MPa, which in turn increases the construction cost of the piping system. I propose that, in accordance with the requirements of 4.1.1.2.3 in GB/T 20801.3-2020, the design pressure of the pipeline must be no less than the design pressure of the equipment that is directly connected and functions as a pressure system; this is a strict requirement that must not be violated. The pipes that directly connect devices C1 and C2 must be designed to a minimum pressure of 1.8 MPa, in line with the design pressure of the devices. Teacher, am I doing it correctly?
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In other words, the work pressure and the design pressure are quite different; I’ve never encountered such a situation before. I hope experts with design experience can provide an answer
The fundamental reason is that you, as the design agency, do not possess the capability for fatigue analysis in design; since you outsource the equipment design, why don’t you require the outsourcing agency to carry out fatigue analysis? If a fatigue analysis has been conducted, is it still necessary to raise the design pressure of the pipes to 1.8 MPa, thereby increasing the cost of pipe construction? An analysis and comparison of the costs associated with fatigue analysis consultations and those required to increase the design pressure of pipelines are provided for customers to make a decision
What does fatigue analysis have to do with pressure difference? The design pressure of a safety valve should be no less than its set pressure, so that the safety valve can serve its protective function. You haven’t understood the purpose of the rules; it’s for safety
If fatigue analysis is to be conducted, the drawing design must be outsourced, and the company will have to pay for it; it’s said to cost around 7,000 yuan. If it’s just about increasing the design pressure and we handle the design ourselves, it’s a matter of paying the technical department a few hundred yuan based on the design hours required. Between the two, companies tend to design it themselves.
When the equipment is in operation, it operates on a cycle of approximately 440 seconds, with its working pressure cycling between -0.093 megapascals and a positive pressure of 0.26 megapascals. The equipment operates continuously 24 hours a day; throughout the year, it is generally in operation for around 330 days, with a minimum of 300 days. Therefore, since the equipment operates frequently between positive and negative pressures, fatigue analysis is necessary.