VFD Reliability
VFD Reliability
(OP)
I have inherited a water pump station design that has adopted two duty one stand-by pump sets. The pumps each rated at 1.8 MW are designed with VSD drive. The pumps are a sole source of supply to a distribution zone of a major city. The station must operate 24/7, Reliability is paramount for the client. The client has no recent experience of VFD and his historical experience is not good.
We need to convince him that the VFD will provide a high degree of reliability (I am not convinced that they will).
I am seeking advice on the long term reliability, design life and maintenance down times. Information does not appear to be readily available?
We need to convince him that the VFD will provide a high degree of reliability (I am not convinced that they will).
I am seeking advice on the long term reliability, design life and maintenance down times. Information does not appear to be readily available?





RE: VFD Reliability
thread237-409533: Adjustable Frequency Drive
Very hard to answer your customer's needs question because when you speak of wanting reliability you're not just talking about a VFD, in this case you need to be talking about the system. While a VFD certainly brings more parts to the party that can eventually fail you're also talking about pipes and flows. Say you were to switch to DOL starting, you may stress the rest of the system considerably more causing failures elsewhere.
Further, if VFD's were needed before to ramp flows or maintain pressure control points what other sane option is there?
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Reliability
RE: VFD Reliability
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: VFD Reliability
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Reliability
If you want to convince then find other sites in your area using similar VFD's as you need and then talk to the people there. Things like downtime depend heavily on the manufacturer and location. The decision can depend heavily on being close to a good repair facility which either stocks or has quick access to repair parts for a certain manufacturer. The reliability numbers can look great on paper, but the reality is that when your VFD is working the reliability numbers mean nothing to you. But, if your VFD goes down the only thing that matters is getting it repaired. A manufacturer with great numbers on paper means nothing when they can't fix your problem for a week or two or more.
I do have to ask. Can't you make the system work with ATL starters and motorized control valves? A correct design should be as efficient as a VFD controlling the flow.
RE: VFD Reliability
RE: VFD Reliability
RE: VFD Reliability
Sound typical. "Just make the pump bigger so I'm safe in case I got it wrong and then use a VFD to dial it in." I've seen VFD running above 58Hz many times, just there to correct for a slightly over sized pump.
Anyhow, since you didn't design it, then you shouldn't be responsible for convincing anyone the design is better then DOL. Why tell the client an inferior design is better just because someone else though it was?
RE: VFD Reliability
RE: VFD Reliability
Having said that, I would be particular wrt the technology & make of any MV ASD.
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: VFD Reliability
MV drives are, as I mentioned, somewhat of a different animal from LV drives for many reasons, the least of which is that a "repair facility", although a valid concept, is irrelevant because you cannot pull one out and send it in for repair. The nature of the beast means it MUST be repairable on-site. So while the ideat is valid, what you really want to look for is availability of a good local SERVICE TECHNICIAN to attend to their needs. I was just involved in a huge project like this in California and the Consulting Engineer invited almost every mfr of MV VFDs to propose their solutions. The two MAJOR criteria were the reliability and service organization. In several of the proposals, service people would have to come from the East Coast of the US, two of them would have to come from Japan, one from China. All of those were rejected at the first pass, they insisted that a service tech be available within 3 hours on an emergency basis, something to consider.
Also, MTTR (Mean Time To Repair) is a critical issue, especially for power components. Some require 2 people for the removal of 150 to 400 lb. power cells using a special lift truck assembly, others can be repaired by a single person sliding in a 3lb power "card" from the front without having to disturb the entire assembly or adjacent power cards. That can make a big difference in an emergency repair, because "Murphy's Law" dictates that the first time they will need to repair something will be 3:00AM on Christmas Eve, and getting two people available is a lot harder than getting one.
For this, one thing to think about is what is referred to as "N-1" functionality. This can be thought of in two ways, especially with pumping systems:
1) The VFD can sense a bad power device and "hobble itself" by turning off and bypassing corresponding devices in other phases so that the drive can keep pumping at a reduced rate commensurate with the loss of power handling capability.
2) You can do the same thing, but start with a VFD that is 2X the required current capacity, so even if you lose half of the devices in the drive, you still have full pumping capacity available. So in your case, you buy 3.6MW drives that have N-1 capability for each motor. Might be a little more floor space, but not as much as two drives per motor.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: VFD Reliability
Thanks for the comments
RE: VFD Reliability
Did anyone ever consider a synch-transfer scheme? That will significantly reduce the number of ASDs required for the Plant, and maybe even provide some form of DOL backup.
I have implemented synch-transfer on recip-type compressors (up to 7000hp), albeit w/o DOL backup. One large manufacturer of MV ASDs told me that 40% of their drives are now synch-transfer applications, so the technology is well understood.
Please find attached a 1-line diagram for a typical MV synch-transfer scheme.
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: VFD Reliability
Firstly let me say that there is nothing that can ultimately be considered as 100% reliable for ever.
Reliability is a "WISH LIST" item that needs to be addressed properly and in context with reality.
When things go wrong and/or the VSD won't go how this situation is and can be addressed for the particular VSD will be high on the list of guiding determinants for the overall best choice.
History of working in lots of places and with a variety of machinery will attest to most faults being simplistic in nature and easily resolved with a structured approach to diagnostics. In reality complete failures do occur but are extremely rare compared to simple faults.
Ultimately what needs to needs to considered very highly is how quickly can reliable information about the status of the VSD be accessed and acted upon to get the machine back on line.
A rotatable spare on hand ready to go is a must when considering reliability of critical plant. Additional stocks of peripheral devices that can stop the VSD or make it unusable in its application needs to be on hand also.
Trust this assists. More detailed information is available on request.