Why Redundant NRV in Series within a Line ?

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One of my blog reader, DV post a simple question to me recently.

"There are two non return valves (NRVs) in an oil transfer line. 1st NRV on transfer pump discharge and 2nd NRV located some distance away before it tie in to another oil pipeline. Why two NRVs in a single line ?"

Quick response on this question is

NRV internal e.g. flapper, piston, etc is prompt to failed and damage cause by corrosion, erosion, etc. Thus, to increase reliability and reduce probability of reverse flow, redundant NRV is provided on same lime. This is pretty inline with API Std 521 recommendation. In additional, API Std 521 allow credit (reduction) on flowrate in back flow and minimize relief load. This commonly apply in compressor discharge Non-Return Valves (NRV).

Thus provision of additional NRV :

• Redundant NRV increases reliability and reduce probability of back flow
• Credit on flow reduction in back flow

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Will discuss further of "Potential Problem associate with Redundant NRV in Series within a Line"

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