Pumps with Built-in VFD's 2

[Cry24]

Hi all

Does anyone have experience with pumps with built-in VFD's?
I suspect a few draw-backs in specifying these.
1. How does the pump react to pressure? does it have built-in PD or is PD provided by others near the pump? can it read multiple PD's far away? ASHRAE 90.1 require remote PD's, not close to pumps.
2. Upon VFD failure, does that mean you need to replace the entire pump?
3. do they have VFD by-pass?

Thanks

 

[ScottyUK]

QualityTime,

Wow, where to start on the myths?

Basic programming of a VFD is within the capability of many maintenance techs, especially reloading of a previously-defined parameter set into a replacement drive. Record keeping and backups of settings are just prudent management of the system. Most techs are willing to learn, so invest in some basic training.

Harmonics should not be a problem on a properly-designed system. There are standards which define limits for harmonic pollution, and when equipment is installed the system should be designed to meet these standards. Incompetent design doesn't mean a technology is bad, just badly applied.

Sixteen failures from a single event suggests either a very poor quality drive, or something very wrong with the installation, or that the electrical system has experienced a major event which has likely affected a lot of other power electronics devices.


I do completely agree that VFDs are often mis-applied, and that their benefits are almost always over-sold by salesmen trying to hit their targets. The most obvious solution to save energy - switch the motor off - is often overlooked. Energy saving is a possible benefit if the VFD is a suitable device to control the process variable, but in many cases there are other options for control elements which give a lower overall cost.

 

[QualityTime]

Hi Scotty:

...Incompetent design doesn't mean a technology is bad, just badly applied...
...Sixteen failures from a single event suggests either a very poor quality drive, or something very wrong with the installation, or that the electrical system has experienced a major event which has likely affected a lot of other power electronics devices....
...I do completely agree that VFDs are often mis-applied, and that their benefits are almost always over-sold by salesmen trying to hit their targets...

I think you just echoed my point about engineers who use VFD's willy nilly .

As far as for basic programming of VFD's by instrument techs my point, in my head, was that I would think that the vast majority of commercial office buildings, smaller sized factories etc have no instrument techs. They would source that expertise to an outside contractor. That was most likely how it was installed in the first place. Unless you are programming EVERYDAY and that IS YOUR CORE BUSINESS there is no point to trying to KEEP UP with the technology because you will just forget it. The drives are changing all of the time and a replacement drive will likely be "another model" which will much likely have different programming subroutines. Then there is the issue of whether you understand what you are programming. An example would be that a programmer may be responsible for programming the operation of a filter in a municipal water treatment plant. He may have programmed the water level in the filter so that it bounces up and down and yet he would think that is normal because he does not understand water treatment. An experienced water treatment engineer would walk by the filter and would know that the water level should be constant all of the time for water quality reasons. He would then talk to the programmer for corrective action. My point is that, for your everyday maintenance and electrical department, it is easier to diagnose and replace a starter, motor and fan belt than a VFD full of electronics, resistors etc .....Apply the KISS principle

 

[ScottyUK]

Differences between industries I guess. In the power industry and O&G many maintenance techs would be capable of doing this type of work. Electricians wouldn't necessarily have the skills, but they're two different competence levels within the trade even though some organisations blur the lines.

Yes, VFD's are an excellent modulating device in the right application, and a very poor one in the wrong application. Good engineering distinguishes between the two and makes an appropriate choice.

 

[PEDARRIN2]

Compositepro

The car analogy may be simplistic, but it makes sense to me.

And please educate me where I am wrong. I want to learn more.

As I understand it, running at constant speed, the pump has a single curve it runs on (higher pressure at lower flow). This is dependent of course on other factors, but let's keep it simple. The system curve is based on piping, fittings, flow rate. Where they intersect is what determines the actual flow conditions which includes hydraulic efficiency. Choosing a pump where this point is maximizing the hydraulic efficiency is the ideal. Total efficiency includes both hydraulic and motor (electric) efficiencies.

So, if my design point is 100 gpm and 50 psig boost, but at times the actual flow required is 25 gpm, the system curve will change, but the pump curve will not. I know such a reduction in flow will likely be a big hit on the efficiency and the affinity laws don't like that, but it is real world (at least in my world). It will follow its curve to the new intersection, which will be a higher pressure. But, if i am required to stay below 80 psig (it is plumbing in my world) and my static head is around 30 psig, then I have to put in a PRV, throttling valve, etc. to keep the total pressure less than 80 psig. But I have now wasted that energy the constant speed motor is inputting to the impeller.

But, with the VFD, the pump slows, changing its curve and the intersection of the two curves is at a lower pressure and less energy being used.

I admit I am not knowledgeable about the electrical aspects of how the VFD works - I deal with the water in the pump. But if I put energy into pushing the water, but then throttle it back, how is that different than having your foot on the accelerator and brake at the same time. Wouldn't it be better to pull off the gas and run at a lower rpm?

Please tell me where I am wrong.

 

[QualityTime]

Hi Pedarrin2:

I have three comments:
[ol 1]
[li]When you speed up or slow down the pump, the intersection of the system head curve and the pump curve will be elsewhere. That means you are on a different part of the pump curve with respect to efficiency. That has to be taken into account[/li]
[li]A VFD will produce about 3-5% reject heat at the electronics. You just lost efficiency right there and that is on top of the motor efficiency loss.[/li]
[li]For large VFD's, HVAC systems MIGHT need to be installed to get rid of the reject heat. More energy issues and maintenance costs to consider[/li]
[/ol]

I guess all of the above has to be weighed off against putting in a modulating valve

 

[PEDARRIN2]

QualityTime,

1. Efficiency does play a part with using the affinity laws, but from what I have read, it is only a factor if you have a significant static lift which makes the system curve start higher so it is more "horizontal" than vertical and crosses more of the efficiency curves.
2. Efficiency for the pump includes both electrical and hydraulic. From my perspective (dealing with the water side only), I am focused more on the hydraulic portion. But it is interesting about the heat loss and effect on the electrical efficiency.
3. The pumps I deal with are typically 5 hp at most - so I typically don't have an issue with requiring extra HVAC for my VFD.

As an aside, my VFD are always on the pump, not remote like a lot of HVAC VFD. Not sure how that plays into the discussion. Maybe the plumbing pumps are VFD light.

 

[lukaiENG]

I agree with an earlier comment by @PEDARRIN2 - if a power failure took out a raft of pump inverters it sounds like they need to review their power infrastructure. I'd suggest they need to do a protection and short circuit study to ensure any fault is cleared by breakers closer to the source, protecting all the other system components.

I also agree with the sentiment of removing VFD bypasses - if it is critical have a spare fed of different power distribution. I have a large MCF client removing VFD bypasses from all their future buildings as they have sufficient redundant components.

 

[Cry24]

Great discussion,
Thank you very much for your inputs