Switching VFD
Switching VFD
(OP)
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.
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.





RE: Switching VFD
Independent events are seldomly independent.
RE: Switching VFD
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.)
RE: Switching VFD
In emergency conditions we may run up to 5 pumps at full speed into the distribution system or 10 pumps at full speed into the transmission system. In my worst case scenario I have 13 pumps operating with 10 at full speed into transmission and at variable 3 into distribution. I have many other scenarios. The client insists that the distribution pumps operate at variable speed. Thus I need 3 pumps with VFD into distribution and 10 fixed speed pumps into transfer. Because the transfer pumps operate very rarely (may be 2 to 3 days per year - they are there for a catastrophic event) whilst the distribution pumps operate 24/7 we wish to utilise all 13 pumps on rotation on the distribution system. What I do not want to do is to provide 13 VFDs when only 3 pumps ever need to work on VFD at any one time. The hydraulics are good it is the electrical end that I have problems with.
RE: Switching VFD
When a system includes multiple motors, it is the usual practise to provide a dedicated VFD for each motor. A motor starter is typically dedicated to a pump. You do not mention the size of the pumps, but it is not usually feasible to switch VFD's and motor starters.
If it is necessary to rotate the operation of the 13 pumps, then you can equip each VFD with a bypass. A bypass allows a motor to be run directly from the incoming line, bypassing the VFD. Have an electrical engineer evaluate the cost for this option.
RE: Switching VFD
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/
RE: Switching VFD
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.)
RE: Switching VFD
Thanks for the responses, I will let you know if we come up with any interesting solutions. By the way the motors are each rated at 2 MW.
RE: Switching VFD
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.)
RE: Switching VFD
A good thing that your client did not listen to you or the pundits above for that matter. There is no single control strategy that is optimal for all pumping systems.
One generic best practice is to minimize the use of valve throttling losses in system control. Throttled valves convert hydraulic energy that the pump has imparted to the fluid into frictional heat, thus wasting a portion of the pump's energy.
In addition, most power companies will not allow a motor of that size to start across the line. A soft start of some kind is usually required because of the large starting power demand of an on/off motor on the power grid. A VFD by the way is considered to be a soft start. It might also be noted that the transient protection considerations would need to factor into the nature of the system, including the static head as well as the general piping distribution.
For a 2 MW motor, you may be talking about a power cost of over $500,000 per year. Energy usage is a critical factor in determining the lifecycle costs of pumps and their motors. Energy accounts for over 85% of the ownership cost for the pump and motor.
There is no single control strategy that is optimal for all pumping systems.
Here are links for further information:
http://w3.usa.siemens.com/us/internet-dms/dt/Drive...
http://www.ciras.iastate.edu/publications/EnergyBP...
However, there are many systems for which the choice is not so clear or in which two or more different control schemes would work equally well. And there are some systems that merit a combination of controls, such as multiple parallel pumps with adjustable speed drives for each pump.
Each system must be evaluated on its own terms. The nature of the system curve, the
performance characteristics of the installed pumps, the nature of the load variability, and other factors influence the decision process.
Note that you have not provided information on the system head. Understanding of the system head requirements is also a critical best practice. Note that it is not an equipment choice or simple rule of thumb. It is recognized that knowledge and understanding are essential to proper equipment selection.
You need to contract with an electrical engineer with experience in control systems and medium voltage switchgear. This should be part of the project since the switchgear will be valued at hundreds of thousands of dollars.
RE: Switching VFD
Independent events are seldomly independent.
RE: Switching VFD
Adding an MV drive is not a trivial task. Usually the motor will be designed for inverter duty, which basically means reinforced insulation in vulnerable areas. Adding one to an existing drive is possible, but the additional protection required for the motor will knock the efficiency down.
In terms of switching you are looking at a fairly complex arrangement to say the least. I would expect a switching scheme of this complexity to be interlocked via Castell keys or similar. Suggest you post a link to this thread in forum237: Electric motors, generators & controls engineering and see what the opinions are from a couple of the guys there: two or three of them work for the major drives manufacturers, so their input would be worthwhile. I must say that my instinct is to dedicate a small number of drives to VFD control if your client insists and to use automation via a PLC / DCS / SCADA to equalise usage on the remaining drives. The switching plan as-proposed sounds like an operational nightmare.
Or just use an FCV on the pump discharge.
RE: Switching VFD
Without information on the head requirements, proposed pumping equipment, equipment run times, utility reuirements, etc., nothing can be evaluated. It is not that simple.
RE: Switching VFD
A control valve will have a pressure drop but it could well push the operating point of the pump to its BEP.
The VFD has a great deal of energy losses not only including that in the VFD itself. This has been covered in many postings on this site. Refer attached. Also check the notes in the presentation and download the reference papers.
This whole topic is way beyond a simple question posted here. A design review by specialists is required. As BIMR has said the final design is dependent upon a number of issues not annunciated in the OP.
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/
RE: Switching VFD
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.)
RE: Switching VFD
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: Switching VFD
Independent events are seldomly independent.
RE: Switching VFD
RE: Switching VFD
RE: Switching VFD
RE: Switching VFD
Independent events are seldomly independent.
RE: Switching VFD
RE: Switching VFD
Independent events are seldomly independent.
RE: Switching VFD
Independent events are seldomly independent.
RE: Switching VFD
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.)
RE: Switching VFD
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.
RE: Switching VFD
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: Switching VFD
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.)
RE: Switching VFD
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.