Do pipes not have a pressure rating?

Generally, the pipeline grade is determined according to a grading table, based on the highest pressure grade in the piping system. However, in special cases, such as when the client requests valves of a higher grade, or when the client insists on using PN40 valves for pipes that are supposed to be PN25, the pipe grade can be PN25, but the valves can be PN40. As long as the pressure rating of the valve fittings above is higher than that of the pipeline, it’s not a problem.

You are right. This is exactly the situation we’re facing now: the client insists on using valves of a higher grade, and the inspections require that the pipeline grade also be set at a higher level. However, the cost of inspecting pipelines of a higher grade is high, and the client is not willing to pay that amount

Then communicate with the owner and have them issue a document specifying that the pipe grade should remain at the original design level, while the valves should be designed at a higher grade. Some factories choose valves of a higher grade based on their production experience, as they are concerned that the quality of the valves purchased may be poor.

The quality of valves cannot be ensured simply by using higher-end models. Selecting a valve of one higher grade based on experience (including other control components) is done by choosing a higher grade according to the design pressure of the pipeline, rather than the pressure rating. This is for the sake of safety redundancy in the process pipelines, rather than product quality. For example, pressure fluctuations in the pipeline (such as short-term overpressure), as well as temperature fluctuations, can cause the moving parts in the pipeline to exceed the specified parameter ranges, leading to abnormal operation of valves or other operating equipment.

Generally, the pressure ratings of valves and flanges are higher than those of pipes, primarily to provide a safety margin. If the pressure rating of the pipeline is determined based on the valves or flanges, it not only increases the costs associated with inspections but may also raise the expenses for purchasing the pipeline as well as the management costs. Generally, experienced owners will require that the valves and flanges be designed in accordance with the requirements of the pipeline, and they will purchase valves and flanges of a higher quality on their own; this way, they ensure safety while also reducing inspection costs.

There is a standard for this, right? Generally, it should be 1.5 times the design pressure

For safety, the maximum allowable operating pressure of the flange at the design temperature and with the corresponding material must be higher than the design pressure

According to TSG D0001, the scope of pipelines to which these regulations apply is as follows: (1) Pipeline components, including pipeline assemblies (Note 2) and pipeline supports (Note 3)； (II) Connection joints between pipeline components, the first connection joint between the pipeline and equipment or devices (such as welds, flanges, seals, and fasteners), and connection joints between the pipeline and non-compressed components ； (III) Safety protection devices used in pipelines, such as safety valves, rupture disc devices, flame arresters, and emergency shut-off devices. Note 1: The nominal diameter refers to the nominal bore or nominal size, and it is generally denoted by DN. Note 2: Pipeline components are elements used to connect or assemble into pressure-bearing and sealed pipeline systems, including pipes, pipe fittings, flanges, seals, fasteners, valves, safety devices, as well as items such as expansion joints, flexible connectors, pressure-resistant hoses, filters (such as Y-type, T-type, etc.), throttling devices in pipelines (such as orifice plates), and separators. Note 3: Pipe supports, including hangers, spring-mounted supports, tie rods, counterweights, adjusting bolts, support rods, chains, guide rails, saddles, rollers, brackets, sliding supports, lugs, pipe hangers, clamps, pipe clips, U-clips, and splints, etc. Therefore, it cannot be said that just because valves and flanges are of grade 4.0, the system is P2B; all pipe fittings and supports must also meet the P2B standards in order for the system to be considered P2B level. This is my understanding of the regulations.

The thickness of the pipe corresponds to its pressure rating; the actual installation shall comply with the pipe material grade table provided by the design institute