A bypass line is recommended in terms of process control to prevent a shutdown or lost product from a failure of the control valve or to add a fine (trim) valve.

To provide a manual or duplicate control valve in a bypass line to keep the plant running until the control valve in the main line is fixed or replaced, you basically need a bypass line about the same size as the main line. If there is going to be no attempt at regulating the flow in the bypass line, it could be sized smaller to provide a minimal flow but the risk of shutdown or poor control is significant. These bypass lines are justified if the cost of the loss in product from the failure is significant. If the failure of the control valve can cause the plant to shutdown and then go through a startup, there are additional safety, quality and environmental issues. While we normally think of this requirement for continuous processes, it can be justified for a very expensive or hazardous batch operation.

To add a fine (trim) control valve to obtain additional rangeability or sensitivity, the bypass line would generally be about 2 to 3 in. for an 8-in. main line. The additional small fine valve is traditionally split ranged with the big valve to improve rangeability or manipulated by a valve position controller to keep it at a mid throttle position by slowly adjusting the coarse valve position in the main line to improve sensitivity. The article in Control magazine in November 2005 titled “A Fine Time to Break Away from Old Control Valve Problems” discussed the use of model predictive controller to do a better job of simultaneously manipulating both the fine and coarse valves.