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Variable Frequency Drive Benefits
Pros and Cons of Variable Frequency Drives vs. Control Valves in the process industry.

I am interested in knowing about how variable frequency driven pumps compare with more conventional pressure/flow control methods in the process industry such as control valves, regulators etc. Any knowledgeable sages out there?

Rgds
B.Wills

By Dick Caro on 10 July, 2003 - 6:24 pm

VFD or speed control for pumping is a great idea to both save money on pumping costs (reduced head requirement) and to improve control, since the VFD is generally more responsive than a pneumatic control valve. However, from a previous study I did when I was at Arthur D. Little, less than 15 percent of the pumps in controlled variables can effectively be used with VFD. The problem is that centrifugal pumps are not designed to run at variable rotational speed. We do know that pump head and volume are reduced as speed is reduced, but in a highly non-linear way. This would be evident if you could find a pump curve with volume plotted against speed with parametric lines for head, but I have never seen such a curve. The usual pump curve is head vs. volume at constant speed.

The other big problem is turn-down ratio. For control loops with flow control valves, we think nothing of using it at 100:1 turn-down ratio. Because of the centrifugal pump, using a VFD limits use to about a 3:1 turn-down ratio. Now, that said, there are a large number of control loops where a 3:1 turn-down ratio is more than adequate during steady-state operation -- these are the 15 percent.

Dick Caro
============================================
Richard H. Caro, CEO
CMC Associates
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +1.978.764.4728
Fax: +1.978.246.1270
E-mail: RCaro@CMC.us
Web: http://www.CMC.us
============================================

By David W Spitzer on 12 July, 2003 - 10:30 am

Dick,

It would not surprise me that only 15 percent (or less) of centrifugal pumps utilize variable speed drives. However, although not designed to run at reduced speed, centrifugal pumps do so quite well... and usually with fewer maintenance problems.

The affinity laws mathematically describe the relationship between pump head, volume and speed. In short, reducing the speed is equivalent to reducing impeller size (in an energy-efficient manner). Impeller curves commonly appear on pump curves and their equivalent speeds can be calculated.

Turndowns of 100:1 may or may not be practical, however centrifugal pumps with variable speed drives will generally operate well in excess of 3:1 turndown, say 20:1 or so, depending upon the hydraulics.

For more information (and the mathematical details), see my book "Variable Speed Drives: Principles and Applications for Energy Cost Savings", available at www.isa.org.

Best Regards,

David W Spitzer 845.623.1830 dwspitzer@aol.com

By Jose R Pabon on 10 July, 2003 - 6:30 pm

The most important advantage is energy savings. When you use a control valve or regulator, you lost energy because the pumps are always operated at high speed. Other advantage is lower cost of pump maintenance because the soft start extends its mechanical life. You can find a lot of examples in the web pages of variable drives manufacturers

Jose R Pabon
rpabon@dextonsa.com

One of the most important items to me is the fact that when a device such as a valve is used you are wasting energy. VFDs actually slow the device, conserving power. They may be more expensive to buy/install but they pay for themselves over time in your hydro bill.

Jon

By Steve Myres on 10 July, 2003 - 9:03 pm

A centrifugal pump pumping a solution having known properties (viscosity, specific gravity, etc.) at a given rpm with a given impeller, will have a characteristic curve of pressure vs. flow. You've probably seen such curves which indicate that a pump will deliver 10000 gph at free discharge, or 100 ft head at shutoff, with intermediate value combinations in between. Obviously, for the pump to work when connected to a given system, it must have sufficient capability in BOTH flow and pressure. Thus, if the pump is not a perfect selection for a given set of operating conditions, either excess pressure must be throttled, or excess flow must be dumped (through a restriction producting as much pressure drop as the system itself). A pump can rarely be selected that perfectly, expecially when flow and pressure requirements may vary significantly due to the needs of the system (and system pressure drop may not be able to be calculated at design time). A VFD enables the pump to run at exactly the required rpm so that the desired flow and the desired pressure can be achieved simultaneously without the need to waste energy by mechanically regulating or dumping excess flow. The desirability of variable speed drive can be seen from the effort which was put into mechanical variable speed drive systems before the advent of VFDs.

--
Steve Myres, PE
Automation Solutions
(480) 813-1145

1 out of 2 members thought this post was helpful...

Steve,

You are correct regarding application criteria.

The very first two applications to use the then new NASA patent (eventually to become VFD or VSD) were a total failure. One caused a refinery fire. The other revealed that reducing the original motor size by 50% would have saved an enormous amount of money... w/o the VFD!

All forum contributors, thus far, have lauded the benefits... but what about the hidden costs! Often overlooked are the impact on the electrical system. How often have you heard... harmonics are not a problem! Just add a filter here! Or, an isolating transformer there! Just "tweak" the VFD parameters! Perhaps the cable is too short! Or too long! Or, not the "right" one. How about "start-up" problems! Harmonics? Must be the motor. Maybe, its the utility! Is it 'P'! Perhaps 'Q'. No, its the tech... Uncanny how often the tech or operator is blamed. But, never, I said, never the fault of the engineer! Bring in the PQ team!

How about those "strange" unexplained events. For example, Plant B experiencing electrical problems about the time plant A, 1 mile away, installed a VFD. Then, there's the panel is too hot! The grand-daddy of all... the dreaded "neutral" current phenomena, or power-factor capacitor resonance. A close third... IGBT failure syndrome. Could it be dv/dt? Or is it di/dt? etc, etc. Don't even get me started on software "glitches"! And, I could go on, and on, and...

Reminds me of the car owner that kept adding fuel-saving additives and devics to his car. Finally, he had them removed, because his fuel tank kept overflowing!

Enough of the rant. Too all... have a good life!

Regards, Phil Corso, PE Boca Raton, FL [tal-2@webtv.net] (Epsiconinc@aol.com) {pcorso@itt-tech.edu}

By David W Spitzer on 11 July, 2003 - 12:16 am

B. Wills,

This subject is is covered in my book "Variable Speed Drives: Principles and Applications for Energy Cost Savings" available at www.isa.org. Properly applied... the benefits can be many.

David W Spitzer
dwspitzer@aol.com
845.623.1830

By The Water Boy on 11 July, 2003 - 12:40 am

Both have their advantages, but overall I like drives better. Fewer moving parts overall means more reliability. Cost is usually an issue though. On bigger systems with large motors and control valves, the cost difference is small but on a small system there's a larger difference. Drives have been coming down in price so be sure and ask... the quote you got last year is likely going to be a lot higher than the one you'd get now.

A drive can give you so much more flexibility and information about the state of the process that it's just silly. It's also better for the system IMHO because of smoother changes in pressure.

By rahul chander on 12 July, 2003 - 12:15 am

vfd:
pros:
1)very accurate flows. the shaft of the motor is normally attached to the pump, if the speed of motor is known, the pumping capacity will be known as well. you can do an open loop flow control without much trouble. using the vfd, the speed of motor is known at all times. a flow meter will not be required in normal cases, you can probably guess-timate the flow by trials.
2)vfd is especially good for low flow rate applications, where the line size is fairly small ~1" (metering pump applications). you would not be able to use a control valve here as the line size is small and you may risk build-ups in the valve. also it is very hard to size flowmeters for low flowrates/small lines.
3) nearly no maintenance as no moving parts are present in a vfd.
4) fairly high linearity in flow vs speed setpoint graph.

cons:
1) the best turndown a vfd can achieve conveniently is around 4:1. beyond that, it is really pushing it and you may need a bigger motor/gear-reducers to achieve 5:1 turndowns or higher.

control valves:
pros:
1) you can shut the valve right up, assuming it is safe to do so.
2) are suitable on boiler applications or other critical applications where the ability for a complete flow stop is required (emergencies)
3)a flow valve's cost varies depending on the fluid flowing thru it. cast iron valves are dead cheap and so are their actuators.
4) suitable for large line sizes ~15"-36" where super accuracy isnt required. example mill-wide water supply etc.
cons:
1)they require annual maintanence and rebuilding.
2)you require a flow meter in all the cases for a successful control of the flow. a magflow tube is expensive and the cost increases with higher line size. oriface plates can be used too but only with water as the fluid.

these are my $0.02.
rahul chander

1 out of 1 members thought this post was helpful...

Dick,

Do you remember how ecstatic the refinery control potentates were when they discovered (late 60's) that NASA' simple patent for controlling motor speed could, potentially, save lots of money... a fact that the chem boys knew from day one! It was also the main reason EXXON bought Reliance Electric!

Ah, the joy of being old, oops, older!

Regards, Phil Corso, PE Boca Raton, FL [tal-2@webtv.net] (Epsiconinc@aol.com) {pcorso@itt-tech.edu}

By Dick Caro on 15 July, 2003 - 9:00 pm

Phil,

I remember that fiasco very well. Exxon thought that VFD would completely replace the need for control valves, and they wanted a piece of that action, so they bought Reliance Electric who actually held the patent. Then the Federal Trade Commission opposed Exxon in court and actually prevented them from exploiting the technology. Exxon allowed Reliance management to do a leveraged buyout a few years later resulting in a huge loss for Exxon.

Reliance had it right....and so did Exxon. Government interference really killed this one. Today VFD is big business, although not where Exxon thought it would be.

Dick Caro

By John Gierich on 14 August, 2003 - 6:31 pm

Go to http://www.joliettech.com/case_studies-variable_frequency_drives.htm
there are several case studies that may be helpful.

John Gierich
Joliet Technologies, L.L.C.

By Blake Nichols on 14 September, 2007 - 12:45 pm

You can find a great article about the benefits of variable frequency drives (VFDs) on our website at http://www.emcsolutions.com/vfdbenefits.html

I would post the entire article if it wasn't so long.

Feel free to browse our website. We've got some great information about electric motors, variable frequency drives, and backup generators.