Required Pump Speed And Rate

Hi All,

 

I want to ask information on how I can use a pump with 1755 rpm and impeller size 180 mm for tank wash operation.

 

The rate for the tank are 25 gpm @ 25 psig and 11 gpm @ 20 psig respectively. 

 

What effect would it have on the speed if used for the operation? 

 

The existing pump curve is attached.

 

I look forward to your advise.

 

Regards

 

  

 

Nothing attached...!

Hi All,

 

I want to ask information on how I can use a pump with 1755 rpm and impeller size 180 mm for tank wash operation.

 

The rate for the tank are 25 gpm @ 25 psig and 11 gpm @ 20 psig respectively. 

 

What effect would it have on the speed if used for the operation? 

 

The existing pump curve is attached. 

 

I look forward to your advise.

 

Regards

 

Well, if you don't have a variable speed drive and its just operated as a full speed pump, then its rather negligible impact on the actual shaft speed.

You can either throttle the discharge valve to achieve your desired flow, or install a variable speed drive to actually reduce the pump speed to achieve the flow you want.

Edited by thorium90, 20 March 2019 - 08:24 AM.

The pump has a variable speed drive. 

 

How can the value for required speed and frequency be determined to input into the drive?

If it is just direct input of speed or frequency, then there is normally a button pad interface on the VFD itself. You can navigate the menu interface to look for a manual speed or frequency input. Since each VFD vendor has a different interface, I would suggest to download the VFD operation manual from the vendor's website to get the specifics on how to do it for that particular manufacturer.

 

If you want to control the speed based on some specific closed loop control, some VFD's have an option to take in an analog input on the terminals and then you can configure the VFD to do a PID control on that analog input. This analog input can be a flow transmitter or even pressure transmitter. The VFD will then automatically adjust the speed of the pump to the desired setpoint that you have input into the VFD.

Edited by thorium90, 21 March 2019 - 06:34 AM.

You will need to contact the pump vendor to get the pump curve at different impeller speeds since the one you have is at 1750 RPM.

 

Generally running at a lower speed will shift the pump curve downward (less discharge head at a given flow), but as far as I know there isn't a way to predict this without getting it from the vendor directly.

 

Another good reason to go to the vendor too -- many pumps and motors have a minimum operating point that varies with speed.  Your pump will have a Minimum Continuous Stable Flow (MCSF), and your motor may have a minimum speed if it has a self cooling fan installed.

 

Once you get that information though, you should be able to adjust your VFD output to obtain a certain speed -- if if the 1750 RPM is based on 60 Hz (here in the US), you will be able to cut the speed back to 875 RPM (50%) at 30 Hz and so on...but again, I would not recommend doing this without consulting with the pump vendor first.

Use the available manufacturer pump curve for getting the flowrate for which the pump was actually procured and then use the pump affinity laws for reaching at the required speed to be fed into VFD.

I hope this helps.

Edited by Che#pet, 28 April 2019 - 09:44 AM.