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Controlling Centrifugal Pump Flow

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I've been using relatively cheap centrifugal pumps that I didn't really worry about abusing. But I just upgraded to a nice (and expensive) explosion proof model. I know there is really detailed information about operating pumps, complete with all kinds of performance charts and graphs, but I'm just looking for some practical real-world tips.

Can I just plug it in and control flow rate using an outlet ball-valve? Or should I invest in a variable frequency drive so I can control the flow by adjusting the rpm? I do already know it's not good to run it fully closed, with no flow. But I don't want to burn it up with a lot of low flow use either.

Also, what about not running it dry? Does that mean stopping my fluid transfer before the tank is fully drained, or does that mean transfer all the fluid and then turn the pump off real quick as soon as it starts to run dry?

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In my experience, I'd say that you want a VFD. Throttling back the out-flow will work, but in time the seals will wear out (i've got two pumps that are suffering from that condition with less than 6 months of use).

You shouldn't run a centrifugal pump dry but it's absolutely going to happen from time to time. It's okay to finish your fluid transfer — when you can hear the pump finishing up you'll want to cut off the power. A few seconds won't hurt anything.

I've worked with two suppliers that I'd recommend:

CPE Systems (Don Byres) - I *just* ordered a 1.5hp centrifugal w/washdown motor and vfd from them.

TriState Pumps (Bill Souzzo)

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In my past one never used a VFD on a centrifugal pump, only on PD pumps. However, that notion is changing some, especially in very large applications. To help with this answer I looked to a few sources and overall they say it can be done, but it has some drawbacks. http://www.lawrencepumps.com/Newsletter/news_v03_i12_dec06.html

Pretty sure their concerns are for centrifugal pumps much larger that our typical pumps, but thought you'd be interested in the following bullet.

  • Explosion proof motors must be rated as certified for VFD operation before they can be used with a VFD. Qualified motors will state on the nameplate that they are certified as explosion proof for VFD operation, as well as any operational limitations
  • The motor manufacturer should be consulted for operational limitations. Most variable frequency drives restrict the minimum continuous speed to some percentage of the nameplate speed. Below this minimum speed, temperature rise may damage the motor. Every 10oC rise above the rated temperature in a motor reduces the insulation life by half.limitations.


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Generally it is quite safe to use a valve (on the outlet side) to slow the flow from a centrifugal pump as long as there is enough flow to keep the liquid from overheating in the pump. I have been doing it for 50 years.

Reducing the flow rate of a centrifugal pump with a valve also reduces the energy use. As a rough guide, a centrifugal pump with valve almost closed will use half the power compared with outlet fully open.

For a pump that is running all day it would be more energy efficient to use a variable speed drive, but if you look at the performance curves of your new pump you will find it is not as efficient, (power use),at lower revs.

I don't know why Lenny's pump seals failed so quickly. Maybe there are some very modern pumps that I have not experienced.

Warning, I know of pumps that have been run for too long with no flow. The water actually boils and one burnt a friends face with steam when he took the bung out. The pump was still fine.

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  • 1 month later...

I doubt the payback for a vfd on the pump would be worth it, but if you have the controls to support it, go for it.

Don't use the vfd directly with a chiller, however, a chiller operates with a constant flow once set up.

Discharge ball valve most common. Make sure you have pressure gauge piped in and pump curve so you can see what your output is.

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I am with John, and my pump guy is too. Use VFD on PD pump and output restriction on Cetrifugal pumps.. My pump guy also travels the country and picks me up small batch spirits ;-)

He can put together special pumps/carts/drives, and give advice for pumps you already have, he has really helped out the breweries in our area...

Jeff Allen

Depco Pumps



(727) 446-1656

2145 Calumet St.

Clearwater, Fl USA 33765

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