Control Valve Bypasses

When a pump has a motor drive, the process engineer must verify that the motor will not overload from extreme process changes. The horsepower for a centrifugal pump increases with flow. If the control valve in the discharge line fully opens or an operator opens the control valve bypass, the pump will tend to run out on its curve, giving more flow and requiring more... 

Put the vacuum system control valves at the highest point of a horizontal run and the control valve bypass in the same horizontal plane. This is in compliance with item 8. 

Close minimum-flow loop or control valve bypass valve. 

Control valve bypasses will not be used unless the control valve is bloeked out... 

The control valve bypass valve should be a globe valve of the same size as the control valve and should meet line specifications. 

When a process-flow control valve is opened past a point, it may start to vibrate. The flow through the control valve becomes erratically low, and the pressure drop through the valve becomes erratically high. The vibrating sound coming from the valve indicates that the seat inside the control valve is loose. To prove this, leave the valve on automatic FRC and open up the control valve bypass a few turns. If the vibration stops, you can be sure the control-valve seat is loose. 

Operating with a control valve bypass open is not permitted by many operating companies. In theory, I agree that control valve bypasses should be kept closed. In practice, I frequently have opened bypasses. For example, we have a fired heater that is limited by the unit charge pump. Opening the bypasses around the heater pass control valves seems reasonable and safe. But suppose there is a leak in a heater tube. The panel operator closes the pass control valve remotely to stop the fire, but the flow continues through the bypass valve. The reader can imagine how I became so smart on this subject. 

Running for a long period of time with a control valve bypass open in hydrocarbon service should be considered a change in operation requiring a HAZOP review. For me, as a field troubleshooter. I ll always close all control valve bypasses I opened during my investigations before I leave the plant in the evening. 

Light each pilot or burner in the same manner as the first one. Never light one burner from another to do so invites explosion. Increase the fuel gas flow with the control valve or control valve bypass to maintain steady gas pressure and stable flames. Inspect the firebox frequently. A lighted burner may go out in a cold firebox. 

With the control valve wide-open, open the control valve bypass. 

The control valve bypass valve will be considered fully open when it is required to be open, e.g., startup or shutdown situation. 

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