Why bypass Non-Return Valve (NRV) ?

Someone raised a question :

Non-Return Valve (NRV) is provided on centrifugal pump discharge to avoid pump damage due to backflow. In some special case, designer provide a manual block valve across the NRV. Why ? Isn't it defeat the purpose of NRV ?

Infact there are several reasons behind this application...

Art Montemayor :

" 1. Prime the pump with the discharge fluid that is on the downstream side of the check valve

2. Drain the discharge fluid that is on the downstream side of the check valve."

JoeWong :

"If the pump is started-up automatically whenever the duty pump failed, then the standby pump discharge isolation valve shall always open. The bypass gate shall always open to allow small amount of hot fluid from discharge manifold back to suction manifold. Allowing hot fluid passing the standby pump will always keep the pump hot. Whenever the standby pump kick in, it is ready to receive hot fluid avoid pump crack due to uneven thermal expansion. Sometime designer drill a hole at check valve to perform similar function..."

So got it ?

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Generalised model to predict pressure drop and capacity of countercurrent gas/liquid packed column

How to calculate pressure drop and capacity of countercurrent gas/liquid packed column ?

J. Stichlmajr et al has published an article may answer above question :

<< Generalised model to predict pressure drop and capacity of countercurrent gas/liquid packed column >>, Gas Separation 8 Purification 1989 Vol 3 March

A generalized model has been developed for the prediction of pressure drop and flooding in packed columns in which gas and liquid flow countercurrently. The model has been validated for a wide variety of packings, both random and structured. A single mathematical expression is used to describe all flow regimes: dry gas, irrigated gas flow below the load point, loading region, and flooding. The approach to the model development is fundamental in character and is an improvement over models published.

The model can be used with

- Structured packing : Montz, Gempack, Sulzer

- Random packing (ceramic) : Raschig, Pall, Reflux, Hiflow, Berl Saddles, Intalox Saddles, Torus Saddles

- Random packing (metal) : Raschig, Pall, Bialecki, Nutter, Cascade Mini, Super Saddles

- Random packing (plastic) : Pall, NSW, Leva

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