1000 hp pumps (4160 volt each with itÆs own VFD) 1

[Andy32821]

We have three 1000 hp pumps (4160 volt each with it’s own VFD) to control the clear well level of a water discharge station. There is one PID level controller giving an output to the three pumps. The third pump is seldom used and the first two pumps have to run at 100% for ten minutes before the third pump is allow to start. The pumps are rotated monthly as to lead, lag 1, and lag 2.

The biggest concern is that the third pump not start unnecessarily to keep the month’s power demand as low as possible. None of the pumps will run below 50% because they can’t open the check valve against the head pressure at low RPM’s.

Most of the time it takes 1 ½ pumps to maintain setpoint.

My question is:

Is it better to let both pumps run all the time and ramp them between 50% and 100% or is better that the lead pump runs at 100% and lag 1 pump runs from “off” to 100% as needed (this results in the second pump shutting down completely about twice an hour)?

 

[d23]

Andy32821

My opinion for what it’s worth.

Drives are harmonic generators. The harmonics must be dissipated as heat somewhere in your system. If you run one unit at 90 percent you will allow additional cooling (more iron than required) that will result in longer run life for your motors, cable and drive. You can control flow with the second unit. Based on your post the second unit should not need to run at 100 percent to keep up with your demand.

This is just my opinion and I’m sure some of the “EE” guru’s will smoke me on this.

Good Luck!
D23

 

[jbartos]

Suggestion to the original posting marked ///\\\:
Is it better to let both pumps run all the time and ramp them between 50% and 100%
///This operation will guarantee that both sets will run at least at 50% load or more. This will be better in a sense that both set will get the same wear. However, the efficiency will have to be analyzed with respect to the following item below.\\ or is better that the lead pump runs at 100% and lag 1 pump runs from “off” to 100% as needed (this results in the second pump shutting down completely about twice an hour)?
///The second set will experience bigger electrical and mechanical stresses. However, efficiency of this operation is to be compared to the above mentioned operation. Then, a break even analysis of these two operations is suggested to select the better operation on the overall assessment of each operation.\\

 

[jOmega]

d23:

Just out of curiosity, how much additional heat (percentage) do you think that VFDs output harmonics contribute to the motor;

a) 0-5% ?
b) 6-10% ?
c) 11-15% ?
d) 16-20% ?
e) >20% ?

... and your answer is based upon what type of modulation:

a) Sine weighted (sine coded) PWM
b) SVPWM (Space Vector PWM)

 

[jOmega]

JB:

Perhaps you will enlighten us as to what "bigger electrical and mechanical stresses" set 2 will experience, since Andy32821 stated that all three are on VFDs.

 

[jOmega]

Andy,

How are you controlling the three pumps ?

Are you using a Pump Controller ? PLC ?


And are you controlling level utilizing a level sensor in the well.... or are you controlling discharge flow or pressure ?


What is the set-point..... level ?.... flow ?.....pressure ?


Are all three pumps matched; i.e., same pump characteristics/rating ....? and do you have access to the pump curves...?

 

[dadfap]

jomega

the company I work for has run heat rise tests in motors that run on VSDs. The ususl heat rise was less than 5%, for both PWM and VVI styles.

d23

since the water flow past the motors is sufficient, we assume, for cooling at 100% load the heat rise in the motors as demonstrated by tests conducted by the companies we both work for is under 5% why the concern about the heat rise? the other assumption, and that is such a loaded word, is that the motors were designed with VSD application in mind.

andy32821

my preference would be to allow the two pumps to run 100% of the time with the PID loop controlling the speed. I would rather run them both than start and stop large units like these. that is a personal preference but I believe it is preferable to continual stops and starts on large equipment.

all the best

dadfap

 

[rbulsara]

Since you need more than one pump and not less than 2 pumps at any given time, simplest and logical way is to run both the pumps at equal capacity.

This will ensure even wear and tear, an important issue with lead/lag units. You still need to keep the rotation between pumps 1-2, 2-3 and 3-1.

 

[jOmega]

Dadfap,

Maraton Electric Motors has run heat rise tests on most major VFDs with their inverters.

VVIs or six-step type inverters produced the most heat rise (highest harmonic content). Next would be PWM sine-weighted/sine coded modulation. The space vector PWM types were the least offensive and indeed, were typically less than 5% rise.

BUT.... and here's the kicker..... rotor bar design has a lot to do with it too.

The testing led to the policy that motors (in general) with Class F insulation, were a class B rise and 1.15 s.f. on sine wave power.... and Class F rise and 1.0 s.f. when operated on inverter power.

That's a broad range for inverter user... but it covers the gamut of drives ... of which it can be said that all are not created equal.

The last drive I tested there employed voltage vector PWM and was less than 3% heat rise over sine wave.

By-the-by... would the company you work for be Reliance Electric ?

Not too many companies referred to six-step inverters as VVI which is what Reliance called them back in the late 60's.... I seem to recall also that the Reliance motors they provided for use with their VVIs were special design, heat being only one consideration.

jO

 

[gordonl]

I would suggest that the answer to this question is purely in the efficiencies of the two configurations, and of course the demand charges.

If the process is unstable and at some points during the billing period high load will be imposed on the pumps then it is purely efficiency. (ie rain, peak water load, etc) If properly setup the drives should have impose virtually no mechanical stress on the system (accel & decel curves), and the VFD drives should very little electrical impact on the system or motors from starting and stopping.

I would guess the outcome would be to run one pump full out and cycle the second to allow water level to fluctuate. And share roles for all three motors.

 

[electricpete]

You have already expressed some concern for the no-flow scenario (discharge valve forced closed and no fluid pumping) which can result in damage very quickly.

A condition not as severe but certainly potentially destructive is operating a pump very far to left of it’s curve. You have the potential to enter that situation with one pump running at 100% speed and 1 pump running at 50% speed, depending on how the 100% speed pump influences the system.discharge pressure seen by the 50% spped pump.

Operating far to the left of the curve (flow far below BEP) causes high vibration.

 

[CB2]

andy32821,

Your question can be answered with a generality as to energy consumption, "best to run multiple pumps at the same speed".

To get a better answer you need a system curve, the pump curves, and an energy analysis software porgram.

The system curve will provide the relationship of static head vs. friction head (you save energy in the friction head). If you were designing a pump station from scratch you would compare pumps with steep vs. flat performance curves with the goal of staying as close to the pump's BEP as possible during the majority of the operating time.

It sounds to me that you might like a suggestion on how to prevent the third pump from starting when your system needs a little more flow than is avaliable with two pumps running at full speed.

Check your pump curves and amp load (two pumps at full speed). If you have enough head room (BHP vs. nameplate HP), consider increasing the max frequency (speed) of the pumps. Suggest no more than 66 Hz, unless the motors are substantially oversized you may not even get that high in speed.

One caution, you may need to set the controls/programming to limit "one" VFD operation to 60 Hz, otherwise it may overload.

CB2

 

[HarryDampers]

""Just out of curiosity, how much additional heat (percentage) do you think that VFDs output harmonics contribute to the motor; ""

Depending on your VVVF drive design , the Motor may run cooler with the VVVF supply than on the mains .

Differences of a degree or so in favour of the VVVF have been recorded on large MV motors.

 

[dadfap]

jOmega

Nup, it ain't Reliance and I still refer to 6 step as VVI, purely out of a bad habit.

I work in the oilfield and the company I work for has tested Toshiba, Robicon, ABB et al and with each of the various waveforms in the rather unique motors that go into oil wells we saw around 5% heat rise.

all the best

dadfap

 

[BRIS]

I can't comment on the electrical side - in fact the above experts have been giving me some very helpful advice on the benefits of VFD.

From a hydraulic point of view running two pumps at 75% is clearly more efficient than running one pump for 100% of the time and the second pump on and off for 50% of the time.

Pump power is proportional to QH and the head loss in the pipe is proportional to Q^2. Thus if you keep the flow at 75% over 24 hours you use less power than cycling between 100 and 50% flow.

brian

 

[gordonl]

When I suggested one pump full out and the second intermitent my thinking was one pump close to 100% all of the time, and the second pump at close to 100% intermitently starting and stopping based on the level in the pit. Maybe this doesn't apply to the situation at hand.

 

[dpc]

Unless I was really obsessed with energy costs, I'd just run two pumps at equal speed and forget about it.

But I do want to mention that your concern about starting and stopping the pumps is somewhat reduced because of the Adjustable Frequency Drives. These will provide a very smooth start without the the high torque levels and heating associated with full voltage starting.

 

[BRIS]

Gordon

I think you have it right he is comparing one pump running 100% for 100% of the time and the second running at 100% for 50% of the time against the alternative of them both running at 75% for 100% of the time.

However since Andy32821 has only ever once logged into the forum and doesn't appear to be logging in to read the answers I don't suppose that we will ever know.

brian

 

[Andy32821]

Thanks for the help guys. I am going to control the two pumps together as one big pump from 70% to 100% RPM.

 

[jOmega]

dadfap

For my own curiosity,

what was the methodology used the measure the temperature rise.......?

Thanks.

jO