Tank evel conrol (powerflex 525 pump feeding tank)
- Thread starter [email protected]
- Start date
I'm often a fan of this method. It effectively solves the issue of overfilling the tank from a bad tune or changing process as the pump will be commanded to stop well before it gets full. And if the tank is low, the pump is commanded told to go to 100% immediately without waiting for the PID to respond.
If you were asking the question - I'll provide an answer ....
Because the PV will not suffer rapid changes, there will not be any need for Derivative gain.
However, the application appears to be for a buffer tank that is drawn from by multiple processes, so there will be a variable out-take flow. A small amount of Derivative gain may be necessary for peak out-flow periods, to give the pumping capability a chance to "catch up" (I am assuming it will have a ramp to CV, as described in the OP).
On most of the VFD driven pumping systems I have worked on over the years, default ramps in the VFD's have caught me out, making PID control loops hard to stabilise.
My first train of thought on any commissioning job was "Do we need a ramp on this pump drive ?". If the answer was no, and it invariably was no, I took it out.
Peter Nachtwey
Member
A PI controller should respond instantly too.
If you guys want I can go very deep in to tank level control
This is overkill for this application but it shows how to 'engineer' a problem
https://www.youtube.com/watch?v=d_twlM8VDuM
If you guys want I can go very deep in to tank level control
This is overkill for this application but it shows how to 'engineer' a problem
https://www.youtube.com/watch?v=d_twlM8VDuM
[email protected]
Member
OP
Thanks for your response@ Parky.
I was under the same impression to leave it up to the PID to ramp up and ramp down but thought of an emergency situation and planning to set the CV to 0 % if needed.
I was under the same impression to leave it up to the PID to ramp up and ramp down but thought of an emergency situation and planning to set the CV to 0 % if needed.
[email protected]
Member
OP
Thanks peter. I will try the P controller and SCP Block once I get to the field.
Thanks
There are a few other factors to consider in the design of your controls.....
1. Unless money has been spent, and the pump has been spec'd for it, there will be a minimum speed at which the pump will comfortably operate, typically about 5% of it's rated speed.
2. Depending on the design of the pump, the pipework layout, number of bends, head to the tank, head from the supply etc., you will likely find that the pump produces zero flow below a certain speed. I have seen this as much as 20 to 30% of its rated speed.
3. If the pump is being fed from a pressurised supply (e.g. mains water) then it is likely that you will get flow through the pump, even when it is not turning.
4. You might want to set an upper limit on the speed of the pump. Often the point of maximum efficiency will not be at maximum speed. This speed will be given in the pump data as "peak efficiency or Best Efficiency Point (BEP)", and could be as low as 60 - 70% of maximum speed.
I am no expert at all, however I know enough that these factor exist, and should be addressed, both at the plant design stage (is it too late?), and when developing any speed controls.
1. Unless money has been spent, and the pump has been spec'd for it, there will be a minimum speed at which the pump will comfortably operate, typically about 5% of it's rated speed.
2. Depending on the design of the pump, the pipework layout, number of bends, head to the tank, head from the supply etc., you will likely find that the pump produces zero flow below a certain speed. I have seen this as much as 20 to 30% of its rated speed.
3. If the pump is being fed from a pressurised supply (e.g. mains water) then it is likely that you will get flow through the pump, even when it is not turning.
4. You might want to set an upper limit on the speed of the pump. Often the point of maximum efficiency will not be at maximum speed. This speed will be given in the pump data as "peak efficiency or Best Efficiency Point (BEP)", and could be as low as 60 - 70% of maximum speed.
I am no expert at all, however I know enough that these factor exist, and should be addressed, both at the plant design stage (is it too late?), and when developing any speed controls.
[email protected]
Member
OP
Thanks Daba. Pump is rated for a max 800 RPM. with max 250psi pressure rating, Inlet 1 inch , Outlet 11/2 inch.
Based on the RPM I could go only upto a Max 13HZ(Approx) or even less if I consider 70% of max rated Frequency.
Peter Nachtwey
Member
NO! Only one gain is required for every pole in the system. However, the integrator doesn't count because it adds a pole.
So we are assuming the fluid drain rate is a function of level as if it is flow out an orifice at the bottom of the tank.
So far the OP hasn't told us how the fluid is exiting the tank. If there is a flow meter or some control of how the fluid exits the tank then a feed forward can be used instead of an integrator.
Everyone is taught PID is a childish way. It will get you by but the truth is much deeper. Closed loop gains place close loop poles. It is the location of the closed loop poles that determine the response of the system. So a better way is to know what response you want and where the poles must be get that response. Then adjust the closed loop gains to move the close loop poles to the new location.
BTW, if the pump takes a significant amount of time to ramp up and down then it is adding its own pole so now a derivative gain may be required. It depends on the pump motor response. Hopefully the pump response is relatively quick so the pump motor response time can be ignored.
Similar Topics
I want to measure the tank level and get the sensor output to the PLC. Below are the details :
Tank Height =0 - 3m, the sensor is stalled 0,2m...
Does anyone have RSLogix 5000 ladder diagram program of tank leveling (factory IO). Fill valve, discharge valve, set point, level, etc?
I looked...
I seem to remember a number of posts regarding calculation the actual level or volume in a horizontal tank, here is the formula written in ST...
I'm coming from a background of PLC ladderlogic/rockwell products. I am new to blocks. Can you point me in right right direction?
What I would...
