By-pass valve are generally used for the following basic reasons.
There is a significant differential pressure, by-pass valve are balance the pipeline differential pressure lowing the torque requirement of the valve and permit one man operation.
With the main valve closed and the by-pass valve a slow controlled system warm-up before main valve opening. It reduces the risk of demage to componets and equalise down stream pressure.
Delayed filling of pipelines.
By-pass valve are used lot in steam system, oil, gas, mechanical and chemical field.
By-pass valve are used to fill/pressure the downstream side of an isolation valve before opening it; An open by-pass around a control valve gives a minimum flow when the control valve is closed. Stops having to use the larger valve at small opening.
