Process Design - Control Valves - Questions about their origin...

 

Hello everyone, I would like to share some notes I made, when I had to design, purchase and install control valves for the plant I was working in.

Just simple questions we should ask ourselves when looking at control valves - A quick read, nothing too taxing as this is the general theme of this blog. 

Figure 1: Parallel control valves

Questions on Sizing of a control valve:

1.       Should the valve be sized for trapped volume, i.e. the volume length between Level Alarm High (LAH) to Level Alarm Low (LAL) set points of a vessel?

2.       How long is the control valve being used for? Is it for 15 seconds to 30 mins?

3.       Is it sized only for a ranges, i.e. usually 10%-90% open and does it account for a range of operating conditions? Typically control valves are sized at 65% open

4.       Is there an extra margin?

5.       Does it allow for “fluid expansion”?

Usually control valves are one pipe size down from source piping and sometimes have to fit into existing piping. Modifications can be made and information about the trim and body should be ascertained form the vendor.

It should be noted, vendors can sell novel products & exaggerate their benefits, whereas clients (plant operators) may seek economical & proven solutions.

 

What is the Basis of the control valve and how is it introduced into a system?

1.       Where does the set point of a control valve come from?

2.       How is introduced... Is it from the trends on measurement devices?

3.       Does equipment efficiency affect this value?

4.       Is it from the Operator or the Distributed Control System (DCS)?

 

Example of a cascade control system of a shell and tube heat exchanger:

1.       The control system must take account of temperature range

2.       Is temperature control critical?

3.       What are the criteria for override control?

a.       If the flow control of heating medium is linked to temperature control of process fluid, then is there a flow override value? Is there a region where the operation must be in to be feasible?

b.       It should be noted override control is used to regulate a process input to maintain one process output at target without violating a constraint on another output

Thank you for reading

Have a great day

Mr C