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VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/25/2009 11:47 PM

good morning every body and happy new year

please yesterday we was fighting with our consultant as he want to delete the VFD of our 24 pumps and connect direct on line DOL (pumps with 11 KV motor ) and he mentioned that no need to it as VFD not saving any power and maintenance will cost too much and we can change it with dol motor with flow control valve FCV. please i want to know is that opinion right or wrong should we follow him or use VFD drive

happy new year

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#1

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/25/2009 11:58 PM

In some cases it might be correct that a flow control valve would work well enough, but not always. More detail would help, as in design flow and pressure and specific pump selection, as well as the varying conditions the pump might face.

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#2

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 2:18 AM

i think it is good to use VFD n u should have to use it.

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#3
In reply to #2

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 2:49 AM

Many situations will justify the use of a VFD, but no, I do not agree that one should always use this technology. If you think so, please explain why.

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#4

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 10:20 AM

What are motor sizes, run time and age. Some USA programs will pay for VFD in US, under energy efficiency programs.

We are seeing increased efficiency and shorter paybacks using KVAR from GreenPower Management, www.greenpowermgt.com. The best application is for long run times and pumps over 10hp. Far less cost than VFD. I would get a second opinion for sure

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#5

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 12:02 PM

So the consultant is putting his reputation above the that of the contractor that originally installed the 24 VFD pumps! That's gutsy. I'd ask him for a written statement and then compare it to the *original* quote for the 24 drives.

It's possible that the 24 VFD's were sold as a pig in a poke to obtain some tax incentive and it made economic sense at the time because of the skew introduced by the bean counters.

Arguing with you consultant gives us no indication one way or another. Seems like there are a lot more kwestions[sic] you need to ask him before anyone can determine the correctness of the opinion.

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#6

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 11:06 PM

What type of flow control valve?

What type of process is this circuit in.

Will this have any impact on flow surging or hammer (due to poor control programming/or circuit design) with the valve slamming, such as a modulating valve can do.

I was in a situation where to save money they used a flow control valve that was nothing more that a shut off valve with a tapered plug, instead of using a VFD.

p911

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#7

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 11:15 PM

VFD is good if you need to keep your pressure same; this is when you pomp your water to the closed pipe system. If you have a tank to fill it with water, the VFD is useless for you do not need pressure adjustment; it is simply extra, and if this is a case, just make up with your consultant somehow... Your motors will work longer, and it will cost you less.

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#8

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 11:18 PM

Well it all depends on the area of application. VFDs do save energy but if not installed correctly can also be harmful.

Harmonics are inherently present in the network and VFDs can aggravate the problem. So I would advise doing a detailed plant study before taking any decision.

Cheers,

Rathin Vyas

www.econserve.in

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#9

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/26/2009 11:55 PM

Dear friend,

Use of VFD is according to your use, I mean ,load factor. If flow control is possible you can go otherwise.

VFD is suitable for constant / heavy starting loads, to keep the motor safe.

Your consultant is absolutely right in saying this as the VFD should increase maintenance cost as well as downtime.

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#44
In reply to #9

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

01/01/2010 12:04 PM

I actually think using the VFD will reduce down time. When you compare the harshness of across the line starting to the soft start capabilities of the VFD you quickly realize that you eliminate inrush current which builds heat and cause fatigue. Also, you eliminate water hammer in the system. The motor isn't the only thing stressed in a system like this. Can the pump connected to the system handle across the line starting stress? You have to make sure that the system can handle the absense of the VFD.

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#10

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 1:16 AM

Dear Mr. Notenn8,

The details in your message is NOT sufficient. When you refer motor is of 11 KV, I presume the capacity should be large enough. Please provide some data on the working system and how frequent the flow varies and what is the limits where the quantity varies.

On going through, the detais, which I am sure you will furnish, we shall be able to reach logical conclusion. I am sure others also will offer their view. Ultimate aim is minimise the production cost.

DHAYANANDHAN.S,

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#11

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 2:26 AM

Happy New Year same to you...!

Yes, Cost is the only criteria ( both Initial and running ) especially under huge investments ie., 24 VFDs. I didn't get the process industry you are mentioning. But you are having 24 pumps, that means you may have option of rationing and go for it, if it worths. Provide some pumps with VFD and and rest with FCV. According to conditions you can switch more VFDs or more FCVs in running.

However for a pump having a motor rating of 11 KV, the flow control valves are also need proper maintenance regularly. And I am not sure that maintenance cost of VFD is more when compared with costs involved in maintenance and efficiency of FCVs.

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#12

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 4:43 AM

VFD will certainly save power as compared to flow control valve. However, capacity reduction below 30 % may not be possible. Check with VFD manufacturer. Regards,

R. M. Desai

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#13

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 5:07 AM

I seem to remember this question from a few months ago. Are you talking about submersible pumps or surface pumps, do you have the correct voltage, that is 11,000 volts or is it 1,100 volts (one thousand, one hundred volts), I could answer on what you have given, but to me, it seems you have to many variables. or you can contact me on contact@submersiblepumpstandards.com

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#14

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 8:13 AM

Happy new year to all!

I find your question impossible to answer in an intelligent way. We are missing all sorts of parameters that your consultant might have and we don't. There's nothing wrong with flow valves, DOL start or VFD's, it all depends on a specific application. A VFD will do much more than just control speed. It will also for example soft start your pumps. Will your power supply handles all 24 pumps starting at the same time? You might say "that will never happened". Well, that's exactly what I mean. We don't know, and maybe you don't know either.

Want an intelligent answer? Ask an intelligent question.

Wangito.

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#15

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 12:33 PM

In reply to our friend's question and his unfortunate lack of information, I have thought about several ways to answer his question. I'm still not sure if he is using multi-stage centrifugal pumps, or split casing pumps or a single vane surface pump, and I still don't know if he is talking about submersible pumps. However the difference between Variable frequency drives, variable speed drives or what ever you'd like to call them and Direct On Line or FSD Fixed Speed Drives are as follows VSD Advantages: no real need to work out the flow rate or look at the pump curve as the RPM changed by the frequency will change the pump curve. Higher Hz = increase in flow rate, and movement of the OR (Operating Range) to the right. The opposite applies for a decrease in Hz. However you will STILL require either fixed orifice plate or flow control valve to keep the pump within the OR as the OR will change every time the Hz changes. Big advantage when a new well has been drilled and the PI is not known then using a medium size pump (2000-3000 barrels per day) then a VSD will, as the frequency is adjusted, keep the dynamic level constant therefore giving you the well PI.(short version explanation) Soft Start not withstanding and the limited in-rush current on the supply, a big consideration if on standalone generators, the VSD will ramp the motor up to its required speed and running current. Disadvantages: VERY Expensive, difficult to repair, extensive training required to operate and repair. Easy option for many application engineers who try to cover all their bases and in many cases need to make sales. Fixed Speed Drives (DOL) Advantages: inexpensive, easy to operate, easy to fix, motor controllers are easy to use. If the flow rate is known or is constant then no need for a VSD. A standalone FSD can put a strain on a supply due to the in-rush during start-up, however a "Soft Start unit" can be attached. Again a fixed orifice plate or variable flow control valve is a MUST to keep the pump in the operating range. An FSD is a "must use item" if the power supply is constant, as repeated shutdowns and start ups reduce motor life Disadvantages: one speed but the flow rate depends on the flow control valve, lower the flow the higher the pressure and an increase in flow reduces pressure moving the pump BEP up and down. So pump run life could be affected of the pump is operated outside its OR It has been stated in the question that there is no requirement for the VSD, therefore I can only assume that certain factors are known, that is; flow rate, inlet pressure, Pump DeltaP, TDH, and maybe static and dynamic fluid levels. Also we must assume that the supply can handle the re-starting of the motors, but no system I know could ever handle the simultaneous starting of 24 high voltage motors. So we must again assume that they would be started one after the other if there was a complete shutdown. I suggest that you stop fighting with your consultant and start working with him, study his findings and try to understand what he is telling you. Failing that, sack him and get someone else who'll not disagreed with you, but that might cost you in the long term.

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#16

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 3:04 PM

In some cases using VFD's are beneficial rather than using Direct online as energy saving is more bcoz the initial starting current will be high and VFD is inversely proportional to flow More advisable to use VFD's rather by controlling with flow but as per cost wise it is also acceptable to use flow instruments....

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#17

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 4:03 PM

VFDs are NOT always the best choice, neither are valves always the best choice. As others have been saying, it is a complex issue and there are no easy answers. It's true that many times in recent years people ASSUME that VFDs will always save you energy on pump systems, when in fact that is not always true.

Energy use in most centrifugal pumps is based on a reduction of flow. Flow can be reduced by a flow control valve or a change in motor speed. The difference is that there is a pressure drop across the valve that represents an energy loss, and there is a certain amount of inefficiency in a VFD to change the speed that represents an energy loss. Typically, the losses in the VFD are slightly less than the losses in a valve, hence the energy savings. If the application calls for a lot of flow reduction for long periods of time, the slight difference can add up to a lot of money, so much so that VFDs often pay for themselves in energy savings.

But if your flow is at 90-100% most of the time, you will have little of no losses from the valve, yet the VFD efficiency loss continues, making the VFD a less efficient choice. In fact if you are not varying the speed, trimming the impeller is even more efficient than using a valve.

Hopefully your consultant took all these things into consideration before making that recommendation.

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#18
In reply to #17

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 4:47 PM

providing you keep the pump in the curve and operate at the pump's BEP you losses will be small, and the pump/motor will work as per design. as for pressure drop across a control valve, the only control valve one should worry about is the one that keeps the pump in its operating range and close to its BEP, after that, unless the downline pressure equal the pump discharge pressure (on the pump side of the control valve) then you have no worries. As for losses in a VSD, converting AC to DC and back to AC again you will ALWAYS get losses... goes with the territory. Never heard of trimming the impeller, in centrifugal pumps the impeller design is set and is at the most efficient design, may work with split casing pumps(??) VSD's have their advantages & disadvantages as with ALL types of control equipment, the design and selection is down to what is available and the type of pump you want to run and its duty and application. I could spend days writing about pump application and design. If you have trouble sleeping at nights, let me know and i will.

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#34
In reply to #18

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/29/2009 9:32 PM

If you have never heard of "trimming the impeller" then I for one would not be too interested in reading anything that you have to write about centrifugal pump design (insomnia notwhithstanding).

How do you think that pump manufacturers can give a family of curves for a pump model and size?

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#38
In reply to #34

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/30/2009 3:39 AM

Kaisan. Good comments, would love to tell you, what is your email address so i can send you info

mine: contact@submersiblepumpstandards.com

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#19

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/27/2009 5:55 PM

The initial selection of a centrifugal pump should take into account the desired pressure and flow, the suction head available, and the pressure losses from pipe friction. To be conservative, the more adverse possible conditions should be considered.

Once the system is running, it may be found that pipe losses are less than allowed for, resulting in greater flow and brake horsepower, and maybe even overloading the pump motor. This can be controlled in at least three ways: 1) throttling valve on the pump discharge, 2) slowing the pump down (VFD, drive belt sheave) , and 3) trimming the impeller (i.e., lathe-turning a bit off the O.D. of the impeller).

This is both a technical and economic decision. There is no one-size-fits-all answer.

It would be interesting to know the original consultant's rationale for eliminating the VFDs. He/she could well be right--or not--or open to other points of view.

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#24
In reply to #19

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 3:46 AM

Tornado, I've been in the ESP business for neary 20 years and have never heard of trimming impellers on a lathe if you're not happy with the pump output. The relationship between impeller and defuser is critical by design to maximise the efficiency of the pump. Altering the impeller will reduce the pressure balance within the stages of the pump causing wear and shorting the life of pump. The whole purpose of the VSD is to give a pump that is designed to produce for example, 2000 barrels per day at 60Hz, a NEW operating curve at a new frequency, using the "Laws of Affinity". Your comment about trimming impellers leave me wondering if you have ever taken a multi-stage pump apart, it's not practical (and NOT advisable either) to trim EVERY impeller when you can have over 200 impellers. Maybe I need to run a "Master Class" in pumps for you all. However I do agree with you that there is no "Right answer", I have noted that the person who started this off has not added anything, not even answered my first question but hey its Christmas!!

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#28
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Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 6:02 AM

I haven't worked with multistage pumps to speak of, but I'm with you there: it would be weird to try to trim a multitude of stages. I'm just guessing, but it seems that if a multistage pump had been "overselected," you could just take out one or more stages. I have run across maybe six-stage impeller groups, but 200?

My neck of the woods is refrigeration, which typically includes refrigerant, condenser, chiller, and defrost pumps. All of these operate at modest heads and are normally single-stage.

Of the pumps I have used, I would say the most extensive engineering has been done by Cornell (Portland, OR, USA), on their refrigerant pumps. Within one volute size, there is a range of impeller diameters, with corresponding horsepowers. If you wind up overshooting on a selection, you can indeed trim it back down, or have them do it. Although I have seldom had to revisit a selection, I thought this was relatively common, so I am surprised that you haven't encountered it. But this may depend on the internal construction of various manufacturers, as well as the number of stages involved.

It may also be that in recent years there is less usage of this technique. (I am comparatively an old-timer.)

If you produce some kind of Web-based pump tutorial, this could be a valuable service to the industry. I may know my way around some of this world, but there is always lots more to learn.

The OP has just added some more info...off to that next.

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#30
In reply to #28

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 6:38 AM

contact me via my site

www.submersiblepumpstandards.com

regards Bryan

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#35
In reply to #28

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/29/2009 10:39 PM

Tornado

Trimming of impellers is a very normal and common means of adjusting the peformance curve of centrifugal pumps, both single and multi stage. It is done as a normal procedure by the OEM's to correct performance curves.

All the pump design books have a section dedicated to the subject. (Stepanoff, Karrasik, Lobanoff etc.)

It can only be done between the OEM's design minimum and maximum impeller diameters. Most times for volute type casings we trim the vanes and shrouds and for diffuser casing trim the vanes only, but this is not a hard and fast rule.

Just for your interest here is an example from Lobanoff and Ross:

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#36
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Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/29/2009 11:10 PM

Thanks! The formula part of your chart was hazy on my screen, but it looked like calcs based on affinity laws and/or specific speed. I'm familiar with the idea in the single-impeller case. I couldn't see if your example applied to multistage units as well.

I too will pass on the tutorials and/or seminars, unless I decide to conduct some!

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#37
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Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/30/2009 3:21 AM

Hi Tornado. I've learnt something here, however it never happens with multi-stage centrifugal electrical submersible pumps as they are designed & built to perform to a set flow rate, operate within a specific operating range and when you have several pump units each containing (maybe) 100 stages (impeller/defuser) you don't want to pull the pumps (expensive Rig time and time consuming to pull from 6km down) strip down each unit to trim the impellers, as there could be over 300 of the little beauties.

The design of the centrifugal pumps I use do not require that level of engineering. If you need a pump for a certain volume of fluid per hour, you look at the pump curves and select your pump, work out the number of stages required (lift ft/stage) and then calculate the HP required, (of course taking into consideration the losses, TDH etc).

So you either change the pump if you require a different flow rate or power the motor with a VSD which changes the pump curve every time you change the frequency.

Reducing the number of stages does nothing for the flow rate, it only reduces the total lift of the pump, that is, each stage at 60Hz will lift fluid vertically "X feet". Depth of well divided by lift ft/stage = number of stages required.

So when do we start our "Master Classes"?

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#39
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Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/30/2009 3:59 AM

It looks as though we are operating in different segments of the pump industry, addressing different circumstances. One thing that intrigues me about the multistage pumps, where my experience is lacking, is the large number of stages in some cases. Are these primarily in series to attain high pressures, in parallel for greater volume (flow), or various combinations?

Deep-well submersibles, with limited casing diameters, must surely face some interesting design challenges. I doubt that I will ever be involved much in this part of the field, but I enjoy learning about all sorts of principles and techniques.

The large pumping station in this thread is likewise interesting, and I am going to give the whole discussion a few stars.

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#40
In reply to #39

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/30/2009 5:00 AM

Yes and No

In a standard ESP the pump housing as you rightly said due to the size of the well casing, is of a limited diameter, however it can go up to 7 inches OD. The flow rate of a pump is first by design and then after casting and assembly it is tested on a Pump Test Bench. The bench using a VSD and flow meters determines if the pump is within tolerance of it design curve.

For example: We have a pump marked as a "GN2500" the "G" gives the the OD, in this case 5.4 inch. The "N" the type of material the stages are made from, the "2500" the flow rate at the pump at its BEP (Best Efficiency Point) of 2500 barrels of fluid per day. We will also during testing on the bench work out the required HP, Efficiency, discharge pressure (giving lift ft/stage) and its operating range.

In the case of this pump the OR is 2000 bpd to 3100 bpd.

For ease of design and assembly at the well head (on a rig floor) the pump is assembled containing "X" amount of stages, dependent upon the order, the compatibility and several other local requirements for the oil field this pump may go to. So you could have a pump that contains 50 stages or the same size pump (GN2500) that contains 70 stages or more, depending on the factors applied at time of assembly.

I say ease of assembly at the well head, if you require 150 stages to lift fluid from "Y" feet below surface, then having that number of stages in one or two pump units makes assembly quicker and easier, and yes they are all in series.

As for the pressure, yes the more stages you use the greater Pump Delta P becomes and the DP is greater, taking out the losses and TDH, the Well Head Pressure (WHP) therefore is also greater at surface. The flow rate is then dependent on two things at this point, the BEP of the pump and the WHP which is controlled by the "choke" or flow control valve.

Higher the WHP the lower the flow rate and the pump is to the left of the OR curve and vise versa. But you don't want the WHP to be so high or low that you are now operating the pump outside the OR which can happen.

Why can't you operate outside the OR?

If you produce to much fluid you pump off (drain the well). The resivour has a flow rate into the well, if your pump is not matched to that flow rate, you pump the fluid out of the well, causing a motor shut-down due to underload, and second if your motor controller is not set correctly, you burn the motor as it is the fluid passing over the motor that keeps it cool. If you produce to little fluid the same happens, the motor get very hot and will fail, or it will continually trip due to underload and reduce the life of the motor. So matching the pump to the well is VERY important. That's what VSD's do.

As for greater volume, you match the pump to the well, you can't get more out of the well if you put a large pump in. The well PI is what it is and will not change. If the well has a bigger PI, you put in a larger pump. So if the well can produce 3000 bpd you use eg, a GN3000 with the correct number of stages to lift the fluid to surface.

Why not open the "choke" for more flow?

Not a good idea, as you want to keep the pump running for at least two years if not more, and operating at the maximum of its operating range will reduce pump life, upset the efficiency, use way to much power and cost a small fortune to call in a rig to pull the old pump; and replace with a new one before time.

There are also other factors to be considered to, if the choke is opened and the design of the pump is such, any huge increases in flow rate reduces the DP and lowers the lift ft/ stage, and you may not get fluid to surface

Simple really, it must be I can do it

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#41
In reply to #40

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/30/2009 6:06 AM

That all computes. I haven't encountered all of these design issues, but they were well explained and easy to understand.

For pumped refrigerants, ΔP is usually low: the main issue is that the liquid is at or near its boiling point, so the vapor pressure aspect of NPSHr becomes critical. This is similar to boiler feed water situations, but there the ΔP can be much higher. These pumps usually draw from a sizable vessel, so running out of fluid is not so frequent of a problem as in ground water (or oil) applications.

Thanks for this explanation.

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#42
In reply to #41

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/30/2009 6:26 AM

You're welcome

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#43
In reply to #36

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/30/2009 8:17 PM

Calcs are, as you say, based on the affinity laws. Lobanoff also uses an experience based correction factor (Stepanoff's is slightly different) that takes into account the slight changes in exit angle created by trimming of the impeller. Specific speed is looked at to judge by how much you can trim the impellers without getting a dramatic drop in efficiency or causing any other hydraulic problems.

We are indeed talking about different areas of the pump industry, but centrifugal pump theory remains the same. The affinity laws apply equally to single or multi stage pumps as they apply to individual impellers.

As a matter of interest, it is not necessary to trim all the impellers in a multi stage pump, though a lot of people prefer to do this so that the impellers are interchangeable. The total head (differential head) across the pump is the sum of the individual heads of all the impellers, so it is only necessary to trim sufficient of them to get the head that you require for the system. (Small changes to lots of impellers is usually better for efficiency than large changes to a small number.)

Correctly matching pumps to their systems has the potential to save a huge amount of energy as most pump systems are conservatively (over) designed.

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#20

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 2:16 AM

Hi notone8,

You mustn't "fight" with the consultant, instead "discuss" most of the response on your question have all valid point and justification. why don't try to gather the record/information on energy and maintenance cost while the VFD wasn't installed yet, compare to the present, from there you can have an answer.

However if those 24 pumps was installed with the VFD to begin with, then what you can do is take the ampere reading while the VFD is connected, and removed the VFD put direct on line with FCV then you can have the actual basis and comparison which one is better. it would be great if you have of data logger to find the difference in term of power consumptions, in terms of maintenance cost I don't think that with the VFD will hurt you much.

Can you request your consultant to provide you a detailed calculation and justification on how the savings will be soon you replaced the old system against his recommended system.

Kind regards

Roman

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#21

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 3:19 AM

A slightly different take,

the motor winding design is crucial here from an electrical insulation perspective as well as from a mechanical bracing point of view.

Depending on the speed of the machine, the faster the worse, electric motors have their endwindings braced to avoid excessive movement on startup. If the designer had a VFD in mind then there is a chance that less care was taken on the bracing of the endwinding. If this was the case to then alter this to a DOL application could have unwanted implications, i.e. excessive movement in the endwindings leading to premature electrical failure.

Also designers of the insulation system should treat the line coils differently based on the anticipated speed of the wave front they will be subjected to. Poor VFD units can introduce unwanted transients, unless supression is fitted to the DOL motors the same could be exprienced.

It is best to contact the OEM of the motors and find out what parameters were considered in design, there is a more than good chance that DOL was never considered as they have VFD's. If so I would suggest do not change unless approved by the OEM.

Cheers

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#25
In reply to #21

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 4:12 AM

Andy you make a valid point but you are assuming that the motor is a surface type, (Mr Notone8 who started this has not given enough info). VSD's ramp up motors and if the surface motor is secure on it's base plate, that is, no "soft foot" then the vibration will be almost none & that will also stop chassis twist too , and unless the design of the motor is so radical, ALL motors are designed to start with a DOL as VSD's are not always used. Reading what you have written you seem to think that there is a different motor for VSD and DOL, maybe as a "one off" or designed for a specific application, but standard 3 phase 2 pole motors can't tell the difference between DOL & VSD.

As for "unwanted transients" you mean harmonics. Yes they are a BIG problem, and that might be another reason why the "consultant" wants to remove/not use VSD's, I never thought of that until now, Well done Andy!

Mr Notone8, did you ask about your consultant about "harmonics" generated by the VSD and harmonic mitigation?

Thinking about it.....24 VSD's on a flaky power supply, harmonics up to your armpits.... not good!

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#22

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 3:22 AM

IF WE ARE THROTLING THE VALVE TO CONTOL FLOW THEN WE CAN SAVE POWER WITH THE HELP OF VFD BY KEEPING VALVES FULLY OPEN AND REDUCING SPEED OF PUMP FROM VFD TO GET THE REQUIRED FLOW. EVEN START STOP OF PUMP BECOMES SMOOTH.

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#23
In reply to #22

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 3:31 AM

not sure who taught you about pumps but you need a control valve with centrifugal pumps to keep the pump in the operating curve. Regardless of the VSD settings if you have no control valve the pump will run in UPTHRUST destroying the impeller and defuser, PDQ (pretty damn quick)

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#26
In reply to #23

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 5:00 AM

dear sirs, our pumps rate as follow

horizontal split case single stage double suction volute

rate capacity 1650 l/s each pump

rated head 120m (1.1mpa)

efficiency not less than 85 percent

lowest available NPSH 7.7 mtrs at duty point

max speed rotation 750 rpm

suction diameter 800 mm

discharge diameter 600 mm

motor class F

rating 2850 kw

IP55

voltage 11kv

thank you every one

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#27
In reply to #26

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 5:33 AM

if the RPM is 750, then you need a 4 pole motor with a heavy duty gear box, motor controller that will detect under current (low load) and if you have 24 pumps, have some arrangement where each motor is started one after the other. Maybe a VSD would be useful for some of the pumps depending on the required flow rate and distance to pump the fluid.

You might want to think about a diesel prime mover where you can control the RPM without all the fuss of electric motor speed control

I would suggest that you read ALL the comments from our friends as they all have good points for you to follow and maybe you'll work with your consultant

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#29

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 6:22 AM

This is obviously a large pump station. If these pumps are in parallel groups, rather than individual, it would seem that flow could be controlled by how many pumps are running, rather than by altering the speed of individual units. To say nothing of the cost of (24) large medium/high-voltage VFDs.

On the other hand, VFDs can provide smoother starting/stopping of the pumps, as well as adjustment of flow/pressure. Has the consultant given you the rationale for eliminating the VFDs, versus the alternate rationale for keeping them? What does the grid authority say about starting motors of this size?

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#31

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 10:41 AM

dear,

the VFD is very useful to extend your motor life and useful also for power supply. as you know the starting current of an motor very high which lead to cause :

1- overheating and decrease the motor life

2- if you haven't strong power supply to feed this current can cause under-voltage on main bus bar which lead to loss bus voltage "by under-voltage protection"

so, you have to check the cost per penfit and your power feeder

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#32

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/28/2009 10:59 AM

are vfds meant to save power,rather u can save power by running the 11kw pumps on star-delta.

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#33

Re: VFD (VARIABLE FREQUANCY SPEED) GOOD OR BAD

12/29/2009 6:50 AM

[Duplicated from companion thread on same topic]

In a bank of several identical centrifugal pumps, if one pump runs even a bit slower than the others, it is essentially a "dead duck," unless you can control its speed within a very narrow band just slightly less than the other pumps.

A single centrifugal is highly likely to benefit from VFD control, but parallel operation gets tricky. If one pump is under VFD control, the rest should also be under VFD, all of them the same (or very close.)

The situation would be different with positive displacement pumps; in that case a half-speed pump would still produce full pressure.

On this basis, maybe I would keep the consultant and dismiss the dissenters.

How much does an 11 kv 2850 kw VFD cost, anyway? If the consultant can blow $1 million of dubious confetti out of this installation, more power to him/her!

(No, that is not necessarily my final opinion, but I don't mind a little verve. The OP hasn't responded on the grid authority's input as to starting large motors.)

For an installation of this magnitude, I am surprised that the rationales are as yet inadequately presented. If all these pumps discharge into a common header, it would look like about 84 inches or so (7 feet) {[24 (24 exp 2.5)] exp 0.4 ≈ 85.6}. That's just a thumbnail to estimate the overall size of this substantial project.

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