Control Valves - To Bypass or Not?

While developing P&I D’s one question that often stares squarely in the eyes of a Process Engineer is ‘to bypass or not?’ I am referring to the provision of bypasses around the control valves. Control valves like any other piece of hardware can fail or malfunction and might have to be taken out for maintenance. Would the entire plant need to be shut down for that resulting in loss of production? Not if the intended control can be achieved by operation of a manual valve. When is manual control advisable? Here are some thoughts:

1) When manual operation is safe and would not lead to catastrophic damage of people and property due to operator error

2) Pressure is most difficult to control manually and Flow is least difficult. Level and Temperature would fall in between depending on the process dynamics.

3) Large valves are difficult to operate manually than smaller ones. There is no exact cut-off point, but it could be in the range of 8 to 12 inches.

So bypasses can be ruled out for

1) Control valves performing complex and critical functions that can never be entrusted to an operator.

2) Large size Control valves, say greater than 12 inches. (it should be noted that the size of the bypass valve is one or two sizes lower)

What is the option when bypass cannot be provided due to any of the above reasons? Providing two control valves in parallel could be a clumsy and expensive solution. Another way of looking at this is to procure a top quality control valve and ensure it’s all round availability through periodic preventive maintenance.

While searching the Internet for references on this topic, I came across these Guidelines for Application of Control Valves. Though it comes with a bold ‘Not for Commercial Use’ warning, the guidelines are a treasure trove of information about Control Valves. It is a Process Industry Practices (PIP) initiative with the objective of minimizing the cost of process industry facilities. More on PIP later.