VFD's and pumping 3

[cdale]

Wondering if anyone has experience using variable frequency drives on centrifugal pumps for pumping out of a tank?

I have what is essentially an above ground tank with dimensions of 4 ft I.D. and a water level of 5 ft. I want to pump from this tank with a single centrifugal pump, while maintaining that water level. My plan is to use a VFD, but I don't know how well a pressure transducer will work for this particualar set up? In my experience transducers have a bit of a lag with reacting to changes in water level and I am not sure if there is a transducer, or any other similar control, I can use in this size tank that will have a sufficeient reaction time?

Any input would be helpful, I have really only been involved with implementing one VFD on a pump similar to this setup and it has been kind of finicky. Thanks.

 

[quark]

If the water is not hot, you can use ultrasonic level transmitters and they are not too costly(check at Endress+Hauser). Another option would be to use pressure transmitter installed at the bottom of the tank. You should carefully place it so that suction effect is not being influenced(better to install it at 180degrees to pump suction)

A simple on/off controller is better unless your process requirement specifies a variable speed pumping system.

Regards,

 

[HEC]

cdale,

There should not be any problems associated with a simple control loop for the flow from the tank for level control. What is the flow rates you are dealing with? Stability of control will be dependant upon the rate of change in level. This is related to the inflow and diameter of the tank.

What is the nominal discharge head for the system? Siphoning is an issue that you may need to consider. If there is a chance the system is going to siphon past the pump either a shut off valve should be used in conjunction with the pump or a fixed speed pump and control valve used.

In short you need to do a full design analysis of the system. There are certainlly advantages to usiing a pump and VFD especially with reduced operating cost (power consumption).

Good luck
Mark Hutton
hutton4eng@picknowl.com.au

 

[maytag]

We use a 0-2psi pressure transducer made by Pressure Systems for our level indication in our hydraulic and lubrication oil tanks. The original floats are left intact as a backup. These transmitters have been in use now for about 3 yrs and have been pretty trouble free. A different pressure transducer moniters our water systems and a couple of these systems use the VFD on the pumps-not much trouble and pretty hostile environment. As with the hydraulic and lube oil tanks there are floats for a backup. Maytag