Switching VFD 3

[BRIS]

We have a potable water pumping station which will be equipped with 13 identical pumps. For the majority of the time only 3 of the pumps operate. The remaining pumps are for emergency transfer of water. The 3 pumps used in normal operation are required to meet variable flow conditions and will have VFD. In emergency conditions all 13 pumps must operate at maximum flow and hence do not need to have VFD.

For O&M reasons it is intended that the 3 pumps are selected from the 13 installed pumps on rotation.

My question is do we need to install 13 VFDs. Must each pumps set have its own dedicated VFD or can we install only 3 VFDs and connect the VFDs to any 3 of the 13 pumps.

 

[BigInch]

I think it will be impossible for me to understand why anyone would think that they need VFD on 10 pumps that will hardly never operate. You could bring on those 10 pumps 1 at a time and theoretically control flowrate to any 10% step within 0 to maximum capacity. It is EXTREMELY doubtful that you will ever need to control flowrate during an emergency situation (or any situation) to any finer tolerance than 10% of max capacity. With that fine a resolution available with NO VFD, how will you do better with VFDs??? Certainly any gain in flowrate resolution will NOT be worth the extra headaches, never mind the extra expenses, capital or otherwise.

Independent events are seldomly independent.

 

[BRIS]

BigInch - thanks for your response - true we have no need to control the 10 pumps, in an emergency condition we need to push trough the absolute maximum possible. (We are pumping from reservoir to reservoir). I was looking for a method of cycling the pumps on the distribution supply so that we do not have 3 pumps with VFD running 365 days per year 24/7 and 10 pumps standing idle for years on end. i.e I need 3 pumps running out of 13 so we could cycle the 3 operating pumps every 4 or 5 days. The 3 pumps that are operating need VFD the rest do not.

 

[BigInch]

Control valves on a common header. You could cycle all 13 ... every day if you like.

Independent events are seldomly independent.

 

[BigInch]

What exactly are the partial flowrates and % of total time spent at each flowrate?


Independent events are seldomly independent.

 

[Artisi]

It seems your internal client, (operations department) don't live in the real world. When prices come in for 10 VFD's or a switching system to rotate pumps thru a smaller number of VFD's you might see a change in attitude.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

Going back to the original question.

Some users switch between a soft start and a VFD, where a soft start is being used as an emergency backup to the VFD.

The only consideration to keep in mind is that the two output power systems must be isolated from each other when operating the motor. Two contactors, one being fed from the VFD output and one fed from the softstarter output, tie together to feed either output to the motor, but never both. If not, damage to both the VFD and the softstarter would result.

This works for a single motor but will be too complex for 13 motors. Probably would cost more for the switching equipment than for all the VFD's.

 

[itdepends]

Keep in mind with newer VSD's the VSD needs to be tuned to the motor. We've just had a bunch of ABB VSD's installed and if you change the motor out on a pump the VSD needs to learn the characteristics of the new motor (even if it's the same size/specs) before you can use it. This would make a switching system less simple than you may think.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[Artisi]

Can not really see the need to rotate the 13 pumps, in theory it sounds good but in a practical sense not very practical or necessary.

Guess we can assume these are custom build pumps from a reliable pump manufacturer build to very high standards in high quality materials.

My experience with custom built pumps of this size is that you could expect a life of 20 maybe 30 years + of constant use, that is of course providing they are being run within their design capabilities etc. Based on this it seems that the suggestion of itdepends of 3 units + 1 standby unit rotating with the other three units is a valid arrangement. You can always consider changing-out these 4 units in years to come with 4 of the other 10 units which as you say may never be run in earnest. This could well be a much cheaper option than the complicated switching arrangement and no doubt expensive VFD drives on all units.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[ScottyUK]

Medium voltage switchgear for 13x 2MW motors is not a trivial installation, it's a decent sized substation in its own right just to have DOL control of the motors.

I agree with Artisi's assessment that the client's requirement is not reasonable, even if it is technically possible. The additional cost and operational complexity of the switchgear required to achieve what you are considering would likely buy you a number of replacement motors or pay the electricity bill for a long time.