The nominal pressure of a valve is mainly determined by the working pressure and working temperature of the medium, as well as the materials of pressure‑bearing and pressure‑controlling components. Generally speaking, calculation can be performed based on the actual working pressure of the valve under any circumstances. Naturally, the medium temperature that affects material strength shall also be taken into consideration. Nevertheless, it cannot be ignored that medium parameters vary over a wide range for different pipelines.
Obviously, if valves are designed in accordance with the above‑mentioned method — namely calculating, designing and manufacturing valves for every combination of working pressure and working temperature of the medium in each pipeline — it will greatly increase the types and specifications of valves, complicate the production process and raise costs. Although this approach is mechanically reasonable, it is economically unnecessary.
Through continuous summaries from production practice, all commonly‑used working pressures in practical service are divided into several groups or pressure classes at standard temperature. In fact, working pressures for various industrial facilities also exist in graded values. Further taking materials and medium temperature into account, the maximum and minimum working‑pressure ranges for each class are formulated.
As the medium temperature rises, the allowable working pressure decreases correspondingly. This is because material strength (especially yield strength) declines with increasing temperature. Therefore, valve design can be carried out based on pressures at conventionally‑adopted standard temperature rather than the actual working pressure of the medium in the pipeline, i.e., design according to nominal pressure.
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