Two setpoints to control a pump

I have a VFD driven pump that needs two setpoints. One is level setpoint at which the pump will start and stop. And second is pressure setpoint as we want to maintain a particular discharge pressure in the system (50 psi). So as long as the level is above the setpoint the pump will run but will ramp up or down to maintain a discharge pressure of 50 psi.
can this be done?

sameer said:

I have a VFD driven pump that needs two setpoints. One is level setpoint at which the pump will start and stop. And second is pressure setpoint as we want to maintain a particular discharge pressure in the system (50 psi). So as long as the level is above the setpoint the pump will run but will ramp up or down to maintain a discharge pressure of 50 psi.
can this be done?

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Sure.
What is the application?
Which PLC are you using?

Must the pump run all the time?

The level control potion shoulf function like a start/stop circuit with a latch coil. The pressure control can be obtained with a PID. What controller are you using?

The most common approach is to use the analog pressure to start the pump and ramp it up and down in speed to maintain constant pressure using a feedback loop, PID or other. Float switches are provided in the wet well of the pump as a low level alarm that will stop the pump to prevent damage and annunciate the abnormal condition to the staff. A second high level float switch is provided above the normal operating range to annunciate an alarm.

i would bet sameer wasn't planning to include a plc ???

Few drives offer the option of a pid control and you may use something like a cheap 4-20mA relay to give start and stop but at the end...you will spend more in engeeneering around limitations than using a small plc

Jeff23spl said:

i would bet sameer wasn't planning to include a plc ???

Few drives offer the option of a pid control and you may use something like a cheap 4-20mA relay to give start and stop but at the end...you will spend more in engeeneering around limitations than using a small plc

Click to expand...

I guess at that point we can only wait until the VFD info is given. Sorta strange, why post on a PLC forum if you dont plan on using one..

Edit: Not implying that he wasn't planning to use a plc, just it would be strange if he wasn't

Just take a look at a VLT from Danfoss called Aqua. This vlt has all you need. It was built to be used in this type of application. And yes, it has an input for the pressure transmitter and can control up to 4 pumps I think.

http://www.danfoss.com/BusinessAreas/DrivesSolutions/Frequency+Converters/VLTAQUA++Drive+FC+200.htm

Just curious, what PLC are you using?

I would use a scheme where when the level drops too low the PID is placed in program manual mode and the output set to zero. When the level rises to the start level you return to auto mode.


Another option is to use a common control scheme called a low select, where you have two PIDs, one controlling level and one controlling pressure. Which ever PID has the lowest output is selected and the other non-selected PIDs integral sum is synchronized with the selected PIDs output. This is easy to do with a PLC/5 or a Control-Logix, and can be done with a SLC/Micrologix with some careful hoop jumping. A low select will keep your pump running as your approach minimum but slow it down instead of completely turning it off. You will have to determine if that is suitable for your process.

I is a noob said:

I guess at that point we can only wait until the VFD info is given. Sorta strange, why post on a PLC forum if you dont plan on using one..

Edit: Not implying that he wasn't planning to use a plc, just it would be strange if he wasn't

Click to expand...

I'm looking to see but at some point in my world, customers are always looking for low cost way of doing things. You are right it is a plc forum but some peoples here can answer other related questions about industrial controls like VFD controls and sometimes we request them for it
And to my opinion, this case would became a challenge without a plc

Most modern drives can do this without a PLC.

Yes you could use a level transmitter in you sump to start the pump on high level and stop the pump at low level (or alternatively low and high level switches). Use a PI controller to control discharge pressure via your VFD. Is it possible that you could have a scenario where more liquid is entering your sump at a higher rate than that which can be pumped out resulting in your sump level rising too high? If so how would you want to close off the supply to the sump?

I have some toshiba g9s that will do this. Maybe its my freshness in the industry but I can't really see any sizable company only wanting to control the drive locally but have other operations in or around the area.