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

 

[GroovyGuy]

Hi Cry,
1) I'm sure that any modern VFD today has built-in PI(D) control. Alternately, if you have a PLC or DCS, you can do the controls there if you wish. Is there a Plant standard?
2) Depends, do you have an installed spare? I would think that you should be able to swap out the VFD quicker than the entire pump & VFD.
3) Nobody offers VFDs with bypass in 2017.

GG

ps I can't think of a single situation where a pump c/w integral VFD would be a good idea.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[PEDARRIN2]

Not sure if this is what you are looking for, but domestic water booster pumps have VFD that are mounted on the pump.

http://www.grundfos.com/products/find-product/hydro-muliti-e.html

 

[EnergyProfessional]

GroovyGuy: most (if not all) external VFD offer bypass option that also acts as fused disconnect. Unless you were referring strictly to in-pump-VFD

For larger pumps like the Grundfos TP I found the cost with built in VFD is similar to just the pump and 3rd party VFD (I use Danfoss, so no crap). In that case i opt for external since i have better chance of repair. If i recall correctly the rep told me the entire unit needs to be replaced in case of failure. If that is true it would be costly.
For smaller pumps you have ECM pumps, so no VFD, but an inverter control (i.e. Grundfos Magna and Alpha). They are single-phase and you don't really find single speed pumps and VFDs (at least not that I know of good ones)

 

[Cry24]

Wow, thanks for the pitch guys.

I run into a VA hospital situation (doing a facility assessment) and they had 17 domestic water pumps with built-in VFD (Grundfos brand). They had a power outage that resulted into 16 of their 17 VFD's failure.
They had to replace all 16 pumps. a by-pass would have done the trick.

What I don't see in these built-in VFD's (and here I agree with groovy guy) - I do not see a true pay-pack for something that is so much subject to failure, especially for domestic water use (where they tend to be implemented), not much pressure build-up in system to justify the VFD to begin with.

Thanks all

 

[PEDARRIN2]

Constant speed pumps have not been commonly used for domestic water systems for awhile - since VFD became affordable.

The problem with domestic water is the wide variability of flow rate. There is the chance you could have a lavatory be used at 0.35 gpm or you could flush a few toilets and need 75 gpm - all at the same pressure requirement. But at lower flows, the pressure from a constant speed pump could be excessive.

So a PRV could be used. But this is like driving your car with your foot on the accelerator and the brake at the same time.

A pneumatic tank could be used, but it would have to be fairly large to minimize pump cycling.

Using a VFD saves energy. In fact it is one of two items which plumbing can contribute to meeting ASHRAE 90.1.

If a power outage took out 16 of the 17 VFD - it sounds like there was something other than the VFD at fault.

 

[GroovyGuy]

Howdy EnergyProf.: My comment was concerned with the fact that VFDs are so reliable these days, nobody bothers with a bypass anymore. (unless you are using Grundfos VFDs.)

The other issue I have with an integral VFD is "Who made the VFD?" I'd much rather spec a reliable make of VFD (ie (Mitsubishi, Toshiba, Siemens? etc) than end up with who knows what, made who knows where.
GG
ps Yeah I know... I'm a drive snob.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[Cry24]

Pedarrin2

Speaking of ASHRAE 90.1, 5 HP or less motors are not required to have VFD's, the savings being questionable. Most domestic water pumps have a much lower motor HP. Say a 2HP pump will hardly save you any money at all. i.e. no pay back can be achieved.

The other thing is: Booster pumps needs to maintain a certain water pressure anyway, pump cannot be slowed down to less than minimum water pressure, which makes the VFD fairly useless.

Then again, ASHRAE 90.1 calls for Pressure differentials to be remotely installed, at the end of the run of each major zone, and it is proven that delta P across the pumps main headers do not work, which is what built-in VFD's use.

So, in my opinion, this energy savings is all hype, built-in VFD's save pennies, add maintenance and less reliability (no by-pass).

What do you guys see as pay-back, are they really worth the money on anything 3 HP motor or less?

 

[PEDARRIN2]

Cry24

I do not see any minimum in the Service Water Booster Pump section 10.4.2. Granted, the lower the horse power, the less savings that would be seen.

I agree that the pumps need to maintain a pressure, but pump curves decrease pressure as flow increases. So a pump with a design point of say 100 gpm and 30 psi TDH, if only flowing 5 gpm, could be at 45 psi, which could exceed the 80 psi maximum for plumbing fixtures allowed by plumbing code. That is why constant speed pumps typically had a PRV to combat this situation.

ASHRAE 90.1 also states "logic shall be employed that adjusts the setpoint to simulate operation of remote sensors.

And since horse power is proportional to the third power of the speed which is directly proportional to the flow, if I can lower the flowrate at lower than peak demand, I decrease horse power and thus energy consumption by a third root. There are published studies that indicate there definitely is a cost savings with using VFD on domestic booster pump systems.

 

[Compositepro]

The logic of arguing the third power relationship is not really correct because that assumes there is no flow control valve. The alternative to a vfd is not nothing, it is a throttling valve. While throttling valves do consume some energy it is often far less than made out to be, because a small increase in pressure drop can have a large effect on flow rate when a centrifugal pump has a flat pump curve.
"And since horse power is proportional to the third power of the speed which is directly proportional to the flow" is simply not correct. Power is equal to flow times differential pressure across the pump.

 

[itsmoked]

Cry24; You appear to be completely ignoring the major PITA of water logged pressure tanks and galloping pressure cycles granted by said systems. Most people could care less about a dollars difference in a month's bill to have steady instant quiet water supplied. Around here no one bothers with a bunch of space consuming eye-sore pressure tanks in new installs. No one does this based on energy savings or even cost.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[EnergyProfessional]

On bypass: they still are needed (nothing is 100% reliable) and don't really cost more since when you have a VFD-manufacturer disconnect you also get the required disconnect and fuses. If you opted for not having that bypass you would need to field-install a fused disconnect. Certainly costly too and clutters up the installation.
also for testing (if you just need to know if the motor works...) a bypass is valuable. i don't know other manufacturers since we use Danfoss only, but all the VFDs offer bypass.

Probably moot for this discussion since the built-in VFDs don't have bypass. but if you use external VFD, I'd opt for them.

for smaller pumps (where you can't get good 3-phase pumps) the Magna, Ecocirc with ECM and built-in control probably are OK. but for larger pumps I think having a pump and separate VFD is more substantial. not sure where the cutoff is. smallest 3-phase pumps we use are 0.5 or 0.75 hp. so anything below is a candidate for built-in inverter (it is not a VFD for ECM)

 

[GroovyGuy]

Howdy EnergyProf; For the last several years I have installed hundreds of VFDs, with a TCL of > 50,000hp, and have never once installed a drive with a bypass. Where downtime cannot be tolerated, you always have an installed spare. This includes not only the drive / motor, but the mechanical / process equipment as well.
This is typical for the industry I am currently in, as well as every industry that I have worked in for the last 25 years. However, I do remember a time when VFDs were first introduced to the market, when reliability was not what it is today, that every drive would be equipped with a bypass, but that was many years ago. I would suggest that if you are experiencing lotsa drive failures today, you need to change your drive OEM, or look at your installation methods / standards.

BTW, a bypass requires more than a fused-switch; it requires a complete starter.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[EnergyProfessional]

True, the starter is big part of the cost. But VFD manufacturer buy thousands of starters at low price and has it installed by robots or efficient or cheap labor. A field-installed disconnect doesn't need the starter part. But the EC buys only one at a time, and uses very expensive labor to install, add conduit, fasten to a wall etc. At least for the small drives we use (typically up to 3 hp, sometimes up to 25 hp) it really doesn't make much difference in cost. I deal with HVAC pumps and fans. For fans we typically don't have redundancy. For pumps we mostly do have lead/lag.
You make a good point, though.

 

[PEDARRIN2]

Compositepro,

I was trying to state the Affinity Laws which state S1/S2 = Q1/Q2 = H1^.5/H2^.5 and BHP1/BHP2 = S1^3/S2^3 where S is the impeller speed, Q is the flow rate, H is the pressure head, and BHP is the brake horsepower. If I miswrote this in my text, I apologize.

The way I understand it, the lower pump speed required for less than design flows will reduce the BHP of the pump at that point, thus reducing the energy being used.

Using a flow control valve, or a PRV is, like I said, analogous to having your foot on the accelerator and the brake at the same time. which is a waste of energy. In this analogy, slowing the RPM of the engine to go 20 mph is better than holding the brake while the engine wants to go 60 mph. That might be a simplistic analogy, but that is how I understand it.

 

[EnergyProfessional]

Pump laws can't be applied like that. They assume no added throttling and equal efficiency at any point of the pump curve and speed. You can use them in a loop that increases or decreases flow and pressure accordingly. In a throttled system (valves) pressure often doesn't decrease as much. It gets a bit better when you use static pressure reset, though.

For domestic water I'd think another advantage of VFD is less cycling, which also increases efficiency and reduces wear.

 

[PEDARRIN2]

EnergyProfessional,

I understand the affinity laws are fairly constrained to "perfect" cases, but even with efficiency changes on the pump curve, a 1%-2% or so loss of efficiency is not going to counteract a third power relationship much.

The amount of static head requirements on the piping system does play a part, but i don't know how significant a part it plays.

Also, the use of throttling valves is a necessary evil that had to be used to minimize the pressure from the constant speed pump being utilized at lower flows than the design point, which is a good part of the argument to use VFD.

Another way they used to get around this was to use a large expansion tank which allowed the constant speed pump to cycle less and maintain system pressure. But these take up a lot of real estate. When the VFD controls became more economical and available about 10-15 years ago, their use proliferated on the domestic system design.

 

[QualityTime]

I agree with the points EnergyProfessional has made about VFD's bypasses. It all depends on how much redundancy you have and how long you can tolerate a disruption.

I would caution against using VFD's without good reason. Harmonics can be a big issue, especially if you do not really understand what harmonics is.

 

[Compositepro]

Pedarrin2, have we not had this conversation before? The analogy between an electric motor on a pump and the speed of a car is completely inappropriate and explains your confusion.

By design electric motors run at nearly constant speed, but the speed has no bearing on how much energy the motor uses. If there is no load on the motor it will use almost no energy running at full speed. The important and unchanging relationship is that pump power is equal to flow rate (Q) times pump differential pressure (DP). The way that motor speed affects Q and DP can be complicated and depends on the pump curve and the system curve, which is variable when there is throttling valve. In a well designed system the throttling valve will increase the DP a little bit and result in a relatively large drop in Q. This causes a large drop in the energy that the motor must provide and thus a large energy savings even though there is some energy loss at the valve. This loss may even be less than the constant energy loss in a VFD (about 4% of full load).

So, please abandon the car analogy because it is wrong and will lead to wrong decisions about where using a vfd is appropriate.

 

[QualityTime]

It is incorrect to say no one offers bypasses on VFD's in 2017. It is just not true. Also as you can see from the discussion above there is a lot of misunderstanding of VFD's as being an energy saver.

In the example above in a VA hospital you know that something is not quite right with the design concept if you need to have 17 domestic water pumps on 17 VFD's. In this case, 16 VFD's had to be replaced in one shot. This is the perfect example of people throwing in VFD's willy nilly without understanding what they are doing and what the implications are. I hope there was no lives lost because a lot of pacemakers and medical equipment could have malfunctioned because of the harmonics that was created

When there is a problem with a VFD a maintenance person does not know what to do. He must bring in a specialist and most often the VFD will have to be replaced lock stock and barrel and then it has to be reprogrammed. Good luck on reprogramming the VFD if there is no past documentation on how it was supposed to be programmed. I again say this....do you really need a VFD or is it a lazy man's way to try to quickly solve a problem you have not attempted to even try to understand? It is my belief that a lot of engineers are weak in the basics of engineering if they blindly try to explain that VFD's are being used because they save energy