Piping Versus Pressure Vessels

Frequently it is difficult to decide if a unit being incorporated into a piping system should be classed as piping or as a pressure vessel. No precise differentiation exists; hence the classification must be left to engineering judgment.

The unit should be called piping, however, if:

1. As a part of the piping system, its primary function is to transport fluid from one location to another within the system. A header or manifold for the distribution of fluid would fall into this category. Special design features or accessories added to permit secondary functions would not change its classification. For example, enlargement of any part or all of a header to provide a degree of pulsation dampening, or to accumulate and remove liquid from a gas in connection with its primary function, would not, in itself, classify the piping system or any part of it as a pressure vessel.

2. The element under consideration is available from, and is classified by recognized piping equipment suppliers as a piping component or accessory. Examples would include certain types of strainers, filters, steam traps, steam separators, expansion joints, and metering devices. Units which are normally constructed in accordance with the Code, however, should not be included in this category, but classified as pressure vessels.

Even if fabricated exclusively from pipe and fittings, a unit other than a commercial piping accessory should be classified as a pressure vessel, if:

1. Its primary purpose is not to transport fluid, but to process fluids by distillation, heat exchange, separation of fluids, or removal of solids.
2. Its primary function is to store fluids under pressure.

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