Assessment Of Vent Line Design To And From Relief Devices

1 Assessment Of Vent Line Design To And From Relief Devices 

The assessment of the adequate vent line design to and from the relief device has the task to ensure the required operability especially of the emergency relief valve. For this purpose it must be checked that there is only a limited pressure drop in both vent lines. 

The permitted distinct values for the maximum allowable pressure drop depend, among other things, on the exact design of the valve. It is common practice to limit the maximum permitted pressure drop for the inlet pipe to 10% of the set pressure. For the outlet vent line from a balanced bellows safety relief valve a 30% pressure drop with respect to the set pressure is regarded as permissible. The manufacturers of relief valves provide the exact limit values. Therefore the data given here should be looked upon as recommendations without any mandatory character. 

In the following some equations shall be provided which allow a fast estimation on the pressure drop to be expected in the case of two-phase pipe flow. Exact calculations, especially for very long pipes with changing cross-sectional area, are very extensive and can be performed correctly only by a few experts. However, the equations presented here are sufficient to estimate the pressure drop if the purpose is to compare the calculated values and the given limit values. 

In principle, the classical Farming-equation can be used to calculate the frictional loss in pressure 

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