Flow control centrifugal pump with frequency drive or control valve? 9

[CARF]

Dear all,

I want to control to a constant fluid flow of 20 ton/hr with a centrifugal pump motor attached to Danfoss frequency converter. My collegue says this is NOT a good way of controlling flow and this should ALWAYS be done with a control valve.

We have a magnetic flowmeter to measure the flow.

Who of us is wrong? Anybody experience with both methods?

Many thanks for helping and sharing knowledge,
MVD

 

[00098]

One thing that has not been mentioned with a VFD; if for some reason the pump will be required to pump significantly less flow (i.e. slow RPM) the motor could overheat and burn out due to insufficient cooling from its fan. Special precautions may be required in motor selection.

 

[MortenA]

jmw thank you. Compared toyour delivery i certainly seem to have a more easy task

Best regards

Morten

 

[Karlando]

In Finnish paper industry people use more and more Vacon and ABB VFDs. If you go more than 30 kW power, the price difference is quite high.
At least in Finland, Allweiler is out of market 'cause the prices for spare parts almost double comparing Bredel and Wangen.
In Finland is saying: flow control with fixed speed pump and control valve = stupid boy control method. Always use VFD if it's possible.

Best slurry in town

http://www.chemec.fi/

 

[hydrae]

MVD
To quote you on the 9th "VFDs do not require any maintenance".
I disagree
They require cooling therefor they contain fans that stop working or get clogged, they also contain electronics which are vulnerable to power surges. As such, I have found VFD's failure rate erratic and unpredictable, but about equal to a control valve in rate of occurance. Hopefully soon VFD's will have a MTBF (mean time between failures) like PLC's. were a valve's erosion you can schedule an inspection.

I still specify the installation of more VFD than PRV's in pump station due to their benefits.

Hydrae

 

[CARF]

Hi all,

It's me from hot sunny Greece. We just started the VSD with the centrifugal pump. It's all running very smooth even at very low speed (10 Hz.) Thank you all. If I have some time next week I'll put some datasets on my website for the "ultimate" evidence.

I'll keep you informed,
MVD

PS. No problems so far with temperatures around 40 gradC.

 

[pumpcan]

Another issue to be concerned with when operating a centrifugal pump at variable speed is the critical speed issues that could arise, both lateral and torsional. You should know your critical speed values so that you do not adjust the speed to be on top of a critical. This could lead to mechanical instability, vibration , and possible pump failure. Normally, a centrifugal pump is operated below the first lateral critical speed.

 

[CARF]

Coming back on hydrae's remark: they also contain electronics which are vulnerable to power surges...

We had a least one power surge / failure each day and during a heavy thunder storm we had 6 power surges in half an hour (very spooky at 3 o'clock in the morning).

Of course the process was a mess, but after the power was back the VSDs functioned properly again, without losing any settings.

MVD

 

[ccfowler]

MVD,

You nicely point out that variable frequency drives CAN be reliable when suitably designed and applied as is apparently that case with your system. That is as it should be. They CAN provide significant energy savings and/or control advantages. The important point to keep in mind is the potentially dreadful difference between CAN and WILL.

Mis-applied or inadequately designed applications can hardly be presumed to work well or be wise choices. Similarly, prudently designed and applied variable frequency drives are likely to be an excellent choice in many applications. The casual presumption that a VFD is always the best choice is every bit as absurd as the casual presumption that a VFD should always be avoided.

All too commonly, the "nasty" complication to the issue of using VFD's with pumps is that many do not fully understand how to evaluate the potential energy savings, and as a result, these savings are often greatly over-estimated. This, in turn, can lead to substantial disappointments when operating costs greatly exceed expectations.

 

[PUMPDESIGNER]

ccfowler - Great measured response.

Loved your statement:
"Mis-applied or inadequately designed applications can hardly be presumed to work well or be wise choices."

I would push that point:
These systems are usually costly. Once the team buys into a design, they will often avoid accountability for evaluating whether or not the long term worked out like they predicted. Worst yet, they will actively defend their choices out of fear, fear of accountability for their choices. Any mention of measurement, cost analyses, 2-5 year tracking and they all panic.

Perhaps they did make a good choice, but no one will ever really know for sure. These braggadiacio companies and their marketing departments are never there 5 - 10 years from now. They published their bullshit and are gone. How many of us have seen an abandoned VFD stapled to the wall with little starter box sitting next to it? Ask the maintenance engineer and he gets wrinkles in his forhead and says simply, "don't ask".

PUMPDESIGNER

 

[PUMPDESIGNER]

I need to add one balance.
There are decent knowledgeable men out there properly applying VFDs.

All the hype just gets in their way and makes it hard to do a job because people have been either led to expect too much, or they have already been stung and are reluctant to try again.

PUMPDESIGNER