Flow Meter Placement

[Casimo5]

When designing a flow control run in a pipe, where would be the ideal place for the flow meter? Upstream or downstream of the control valve?

Do the flow rates make a difference? What about the fluid?

In this case I am controlling the flow of steam to a reboiler that is currently on temperature control. The addition of a flow meter will allow us to switch between temperature control, flow control, or cascade temperature-flow.

 

[schnipp]

Why measure steam if you want to control temperature?

I would have the flow meter on the steam line for information (and accounting) purposes, but wouldn't control off of it. Steam is tough to get a reliable (for control purposes) flow reading. It probably won't go to zero properly, either. All of this introduces error in your control loop that can be avoided.

 

[schnipp]

Forgot, put it upstream of your control valve at least 5 pipe diameters, 10 if possible.

 

[Casimo5]

Thanks. We'll probably end up running it in cascade control. You're right, the flow meter will be for information as well.

 

[sshep]

The flow meter is generally upstream of the valve in all cases where practical, but in the case of heating steam it is essential because the chest pressure (aka control valve downstream pressure) changes based on the process situation.

Use the normal meter run location guidelines. Include the weep hole at bottom of plate if using an orifice plate in horizontal run. Include a condensate removal upstream of the orifice to get better metering and control.

best wishes,
sshep

 

[dcasto]

see thread thread698-185758

 

[rzrbk]

If this is still in design, consider controlling condensate level on steam side as a stable way to control reboiler temperature. This would avoid the need for a control valve on the steam inlet.

 

[Casimo5]

All good advice, thanks!

This is in the troubleshooting stage. We are also controlling condensate in the reboiler by putting two steam traps in parallel (instead of just one) going to a condensate pot with level control.