jethridge
Member
We're implementing a second pump in a piece of equipment that pumps primarily water, or fluids similar to water. This pump is located in a stand alone piece of equipment that will not always be used. This stand alone piece of equipment can be connected to our main piece, depending on which process the operator needs to run.
If we put the pump downstream of the flow meter, and use that flow meter as feedback, can we call our control "closed-loop"? We also have a second pump upstream of the flow-meter that uses the flow meter as feedback for closed-loop control.
System Diagram
First Controlled Pump -> Flow Meter -> Second Controlled Pump
I ask because I'm under the impression that we cannot call it "closed-loop", and must call it "open-loop". Here are the two arguments..
For Calling it Closed-Loop
1. Flow in "equals" flow out for this pump. See #1 for Open-Loop on why equals is in quotes.
For Calling it Open-Loop
1. Flow in "equals" flow out is a conditional statement. Although this is true for most scenarios, the flow meter will read a different flow rate going into the pump than what is coming out of the second pump. This is especially true since we use closed-loop control on the pump upstream of the flow meter. Other variables that cause flow in not to equal flow out include stuffing pressure to the second pump, and the pressure downstream of the second pump.
2. Most of my control experience is from classes taken during my BSEE, but I was taught it is best practice to measure your feedback directly after your control has been applied. Placing a flow meter before a pump and using it as "feedback" for closed-loop control seems to contradict this.
3. Our second pump's manual recommends using the stuffing pressure as the first pump's feedback source. My opinion is that you cannot claim that both pumps operate under "closed-loop" control if the first pump's adjustments cause the feedback for the second pump to vary, before the second pump's control has a chance to influence the feedback.
I should also say that I've advised adding a second flow meter downstream of the second pump, or having "jumpers" that redirect flow so (1) flow meter can be used downstream of either pump, depending on the process.
Let me know if I didn't explain something well enough, or if you have any questions. Thanks!
If we put the pump downstream of the flow meter, and use that flow meter as feedback, can we call our control "closed-loop"? We also have a second pump upstream of the flow-meter that uses the flow meter as feedback for closed-loop control.
System Diagram
First Controlled Pump -> Flow Meter -> Second Controlled Pump
I ask because I'm under the impression that we cannot call it "closed-loop", and must call it "open-loop". Here are the two arguments..
For Calling it Closed-Loop
1. Flow in "equals" flow out for this pump. See #1 for Open-Loop on why equals is in quotes.
For Calling it Open-Loop
1. Flow in "equals" flow out is a conditional statement. Although this is true for most scenarios, the flow meter will read a different flow rate going into the pump than what is coming out of the second pump. This is especially true since we use closed-loop control on the pump upstream of the flow meter. Other variables that cause flow in not to equal flow out include stuffing pressure to the second pump, and the pressure downstream of the second pump.
2. Most of my control experience is from classes taken during my BSEE, but I was taught it is best practice to measure your feedback directly after your control has been applied. Placing a flow meter before a pump and using it as "feedback" for closed-loop control seems to contradict this.
3. Our second pump's manual recommends using the stuffing pressure as the first pump's feedback source. My opinion is that you cannot claim that both pumps operate under "closed-loop" control if the first pump's adjustments cause the feedback for the second pump to vary, before the second pump's control has a chance to influence the feedback.
I should also say that I've advised adding a second flow meter downstream of the second pump, or having "jumpers" that redirect flow so (1) flow meter can be used downstream of either pump, depending on the process.
Let me know if I didn't explain something well enough, or if you have any questions. Thanks!
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