Sizing inlet piping losses

When designing the inlet piping to a relief valve, what pressure losses are recommended See API 520, Sizing, Selection, and Installation (1994). 

It is essential when designing the pipe layout for gas distribution that unavoidable pressure losses are not incurred. For low-pressure gas, the pressure available at the meter inlet will be only 21 mbar, and the allowable pressure loss to the point of use only 1 mbar, although higher pressures may be available in some circumstances. If such a low-pressure loss is not to be exceeded it is essential that the pipework be sized correctly. It is preferable to oversize pipework rather than undersize, particularly as this allows... 

Compute the size of the connection pipe. In usual vacuum-pump practice, the pressure drop in pipes serving mechanical pumps is not allowed to exceed 20 percent of the inlet pressure prevailing under steady operating conditions. A correctly designed vacuum system, where this pressure loss is not exceeded, will have a pump-down time which closely approximates that obtained under ideal conditions. 

The pressure drop allowed through the inlet and discharge lines is unlimited as long as the capacity of the line is adequate for the relief requirements. That is, at the required flow rate the vessel pressure must not exceed the maximum allowable accumulated pressure. In sizing a safety disc, it is usually assumed that the entrance loss at the nozzle is the governing restriction insofar as capacity is concerned. Thus the effective orifice area is considerably larger than the effective orifice area of a safety valve of the same pipe size. Consequently,... 

As with the layout of components, the component sizes depend on many factors. The power requirements, number of operating components, pressure drop through the system, and heat balance requirements all affect the dimensions of components by varying the heat transfer area, inlet and outlet ducts, and structural support area. It is also necessary to refine metrics, such as pressure drop, to include component specific values as they become available. Not only are the component dimensions dependent on these metrics, but they are dependent on each other. For example, operational component pressure losses and the pipe diameter and routing affects pressure drop, which affects system efficiency. System efficiency, then, affects the size of the reactor, which also determines the size of the shield and the need for shield caps. Thus, all components, especially the reactor, shield, and primary PCS components would require re-evaluation to both continuously update the individual dimensions and promote the individual design optimizations as well. 

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