I have an application whereby two rotary lobe pumps run in parallel (equal size) and have a back pressure controller with a control valve on the discharge of each pump. The the outlet side of each control valve is then piped to a common header. My concern is that the back pressure controllers are now 'coupled' and may require some method of decoupling to minimize process interaction. Can anyone suggest an approach to this problem?

 

I'll need more information in order to help you

>I have an application whereby two rotary lobe pumps <

are you controlling the pumps speeds?

>run in parallel (equal size) and have a back pressure controller with a control valve on the discharge of each pump. The the outlet side of each control valve is then piped to a common header.<

If they are in parallel they have to have the controllers "coupled" i think, or better you only need 1 controller, and the speed of the 2 pumps should be the same. If you modify the lay out, may be you should decouple them, if not, i don´t see the need to separate the 2 controllers.

<My concern is that the back pressure controllers are now 'coupled' and may require some method of decoupling to minimize process interaction.<

What do you mean by process interaction?

<Can anyone suggest an approach to this problem?<

If you explain a bit more we can work it out...

 

Do you have any evidence of this "coupling" between the two backpressure controllers? If each controller is controlling the pressure upstream of the control valve, then they are controlling different things. There would be coupling if both controllers were trying to control the same thing.

However, if your controllers are really trying to control the pressure downstream of the control valve, then they are trying to control the same thing. Due to slight differences in calibration, one controller will close down completely and the other controller will be controlling the common pressure through controlling the flow rate. Unless such a flow rate is beyond the capacity of a single pump. In that case, one controller will open up all the way and the other controller will be doing the controlling.

If what you want to do is balance the flow rates of the two pumps while controlling the common downstream pressure, then you could use a single controller from a single pressure sensor. Then the output of the controller could be paralleled to drive both control values the same.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

The pump speeds are controlled by a flow controller; one for each pump. My thought was that since I'm discharging both pumps into a common header with dedicated back pressure controllers on each pump, it strikes me that the system may be become unstable at times due the back pressure controllers fighting since the developed dp across these valves has a common P2 or outlet pressure.

 

When you say:

<The pump speeds are controlled by a flow controller>

You are still not describing the loop. This "clarification" is no more enlightening than your first posting. To describe a control loop you need to say what is the quantity being measured for comparision against a setpoint and where is that quantity measured. In your case, it could be pressures upstream of the control valves, the common pressure downstream of the control valves, the flow rate from each pump, or the common combined flow out of the header. Until you clarify your control loop or loops, we cannot understand the interaction that you are worried about.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

I'm puzzled as to why you need the back-pressure controllers since they will have no effect on the manifold pressure, For any given pump speed it makes no difference if the back-pressure is 10 or 100#, the flow through the valve to the manifold will be the same.

Rotary Lobe pumps are positive displacement so flow will be proportional to speed irrespective of back-pressure setting.

Send me a sketch if you want any further [email protected]

Regards
Roy