VFD Reliability 2

[BRIS]

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?

 

[itsmoked]

Give a read of this:
thread237-409533

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

 

[BRIS]

Thanks if we adopt DOL starting we would need to also install recirculation and control valves to protect the pipework. VFD will certainly protect/extend the life of all other components. Because of the criticality of the water distribution system the client is asking for standby VFDs for each pump set. He has the money we don't have the footprint space to double up on the VFDs. Searching the literature I cannot find any information on reliability of drives.

 

[jraef]

There are higher reliability options available in MV drives because of the fact that they are almost always used in high value and critical infrastructure assets, although the highest reliability versions are often not the least expensive. Search for MTBF ratings on the different choices, you can get between 11-25 years.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

Could you use three VFDs using one as a "contactored-in" spare so you don't need the room for 4 complete VFDs? BTW I'd automatically cycle thru the three VFDs so they are all tested continuously by service.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

I don't trust any published marketing or reliability numbers. For the most part, they are all best guesses based on the components in the package or some historic data. As a manufacturer, the problem is that by the time you get enough field time on a product to gather any true reliability data you've changed things in the current version and that could change any future reliability numbers. I've seen changes as simple as switching some resistors on a board cause a large number of field failures.

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.

 

[ScottyUK]

No variable speed drive yet invented is as reliable as an MV contactor or circuit breaker. If you want reliability then go DOL / ATL.

 

[BRIS]

Thanks for the response = yes I could design a perfect system with 4 DOL duty pumps that would operate better , more efficiently and have a lower PV cost than what has been designed. But I have inherited the design too far down the road to change. What I do find is that the new breed of Engineers just cannot be bothered to do the work of matching pumps to system curves. Stick in a big pump, put a VSD on it and hope for the best. In this case thee is very little dynamic head and the VSD probably won't do anything anyway.

 

[LionelHutz]

In this case thee is very little dynamic head and the VSD probably won't do anything anyway.

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?

 

[wayne440]

...We need to convince him that the VFD will provide a high degree of reliability (I am not convinced that they will)...

I would have to convince myself before making an attempt to "sell" my customer. I claim zero expertise in this particular field, but I have seen the aftermath of selling an idea that may be viewed as questionable by previous experience. With all due respect, my advice is that until YOU become convinced that a VFD solution is the best option, tread very carefully. If you sell the concept, your name will be the first one to be mentioned when there is a failure, regardless of the actual cause.

 

[GroovyGuy]

I would not have any issues with recommending ASDs in this application. With an installed spare, what is there to be concerned about? How reliable is the power supply, is there a back-up generator?
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)

 

[jraef]

OK, back to your actual question rather than second guessing the entire concept.

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.

Because of the criticality of the water distribution system the client is asking for standby VFDs for each pump set. He has the money we don't have the footprint space to double up on the VFDs.

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

 

[BRIS]

Solution we have gone for is to standardize the drive size (we have a total of 4 pump groups each with two duty and one standby pump: each group as a different power rating) to that of the largest pump. We now have 12 VSD drives all the same. Some drives are therefore oversized. We will provide spare power cells and one complete spare VSD - so we have 4 hot standby VSD on the 4 standby pumps and also one spare VSD - The client wants that also to be a hot standby able to drive any of the 12 pumps - that appears to me to be an over complication of cabling and switchgear - but that is for our electrical engineers to sort out.

Thanks for the comments

 

[GroovyGuy]

Hi BRIS,
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)

 

[oldsparkie]

Hello Bris
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.