VSD energy savings 18

[mfqd13]

Hi,

I would like to perform some detailed calculations to determine the energy saving in the appliance of a VSD (variable speed drive) in electric motors mainly for pumps.
I searched in many references and there are some explanations, but i didn't find yet one that suits for my intentios. Complete enough...
So, my base point is that i can only measure in site the power consumption of the motor and with this i would like to perform some calculations to estimate the energy saving after apply a VSD.

Can anyone help me?

 

[stanier]

Hi All,

Attached is the presentation delivered in Sydney Australia to ASME/IMechE & Engineers Australia. Also below some other links that are of interest.

http://www.energy-online.net/storie...verters/lies_damn_lies_and_vsd_efficiency__2/

http://www.reliance.com/mtr/b7087_5/b7087_5_3.htm

http://www1.eere.energy.gov/industr...estimate_motor_efficiency_motor_systemts2.pdf

http://www.itrc.org/reports/vfd/r06004.pdf

http://ieeexplore.ieee.org/Xplore/l...286616.pdf?arnumber=1286616&authDecision=-203

http://www1.cetim.fr/eemods09/pages/programme/006-Viholainen-final.pdf

http://www.p2pays.org/ref/40/39569.pdf

http://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html

http://texasiof.ces.utexas.edu/texasshowcase/pdfs/presentations/d5/rschiferl.pdf

 

[ScottyUK]

Thanks Stanier, that little lot will take some reading!


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If we learn from our mistakes I'm getting a great education!

 

[mrspcs]

To marcosdias :
The following article could be of help in answering the questions you initially posed.

http://www.mty.itesm.mx/etie/deptos..._Sistemas_bombeo/Energy_savings_ASD(IEEE).pdf

The author is a senior IEEE member with ample practical experience on this subject.
The article starts by covering the correct use of Affinity Laws. In this thread the comment has been made that “Affinity Laws overestimate energy savings with VSDs”. A more accurate statement would be that the incorrect use of Affinity Laws often leads to overestimation of energy savings. The same could be said of any law or equation when it is misapplied, be it Affinity, Bernoulli or the First Law of Thermodynamics.
Affinity laws are often used without adequate understanding of their origin, the assumptions behind them and their limitations (Mr. Stepanoff’s postulates from back in 1948, I think). This can result in over-estimations and, therefore, in bad engineering and economic decisions. When properly used however, these are helpful engineering tools.
The article also presents sample calculations for energy savings (the ones you asked about) and a case study.

I have some additional material on the subject in connection with different applications that may also be of help and which I would share when possible. Contact information may be found in my web page

http://www.manuelsuarez.com

For those in the Houston area in May (5th to 7th), there will be a seminar where the subject of VSDs will be covered to some extent within the general title of “Energy Efficiency in Refineries”. Information may be found in

http://www.petroleumrefining.com

(Refining Process Services).

I would also like to add some comments on the subject based on my own experience
(chemical engineer who arrived at AC VSDs through Process Control & Automation and Energy Use Optimization).

VSD: Final Element of Control or Energy Savings device ??
Probably a final element with good energy savings potential in some cases.
Proper analysis of its applicability, configuration and implementation requires significant joint work of ME / EE / ChE to produce optimized - improved - better processes and equipment.

Not always the only justification for its use is energy savings, although that is perhaps the easiest to quantify and therefore the one that gets more attention. To quantify the others requires more inter-disciplinary work and analysis.
There are cases where process control reasons will outweigh energy savings. Decisions on this may require, for instance, the calculation of Speed (rpm) Control loops Response Time (e.g. response time of flow rate for the control of pressure, temp. etc). The VSD’s Frequency switching (micro-seconds) is only part of the story. Enter Moment of Inertia of the Load (rotational equipment) and the need for proper VSD and Motor power ratings specs to obtain the required acceleration / deceleration for the specified loop response time.

For Engineers, using a VSD should be very much like using a computer. Ignorance about the internal details of the computer's CPU architecture does not preclude its use. As long as the user has sufficient understanding of its functionality, limitations and constrains, it is possible to make use of its applications. The same goes for the VSD and its 'internal details' vs the use of its applications.

It has been said that nothing beats the energy efficiency of properly sized pump running at the BEP at 100 % of rated flow all the time. That is true but it probably is not always possible to operate in that mode. If it were, control valves would not exist.
There is always some amount of DP across the control valve (even when 100 % open) which requires some amount of Work (e.g. Energy) to overcome. If an alternative exists to eliminate that resistance and still comply with Flow and Head constrains, then an objective comparison should be made based on rigorous Energy Balances. However, that would only account for the Energy Savings issue. For the comparison to be meaningful (e.g. choice between two final elements), other aspects of their characteristics and performance should be included in the evaluation.

To say that engineers using VSDs are simply too lazy to work out the math to develop proper equipment specs and sizing is simply not true. I assume the engineering personnel in Aker Kvaerner, Shell, Ebara Elliot, Statoil-Hydro and many others could take exception to that and to the notion that they get carried away with ‘myths’. In the Ormen Lange project they spec 40 MW AC VSDs to drive 3 compression units and some 7 MW re-compression units for this world largest Gas pipeline between Norway and Great Britain running at approx. 70 MM Sm3/day. And in the water cooling system (sea water) at Ras Laffan industrial city it was 250 MW of water pumps total capacity (27 units at 7 MW each). Both cases, on the high-end of power rating, are good examples of quadratic, variable loads very good for VSD application (aside from other issues like motor start, compressor or pump enhanced operation / control etc.).
Whenever a system involves a variable load, it may be worthwhile to examine the use of speed variation compared to fixed speed and restricted flow.

By comparison, Burckhardt Compressors is delivering 23 MW units for Ethylene service where they only contemplate ‘soft starters’, not full VSD capability.
The reason: process requirements do not merit a VSD. No special process control advantage and the energy savings are not significant in the intended operation regime.
Axens on the other hand has a process license that includes Hydrogen recirculation compressors. The spec clearly indicates “Speed Variation”. This was accomplished via Steam Turbines years ago. Today, the same spec is fulfilled with Electric Motors + AC VSDs (more efficient and avoids implementing a high pressure steam system for the turbine).

One comment specifically about Steam Boilers (presently working on an application for this type of unit). Without a doubt, the Damper in the Forced Draft Fan is an excellent candidate for replacement with a VSD, specially in swing boilers. More accurate combustion control, increased turn-down ratio, reduced fuel, electricity and emissions. In the case of the Feed Water Pump yes, the speed control range is narrow. However, we are looking at the design power rating of the pump and motor. They must be rated for peak conditions but will probably operate at approx. 80 % of top loads most of the time. A potential 20 to 40 % margin of loss that could be avoided with the VSD. We are looking at VSD providing full torque at -0- rpm / soft start of motor / no Current peak at the start / constant power factor over the speed range, maybe help with water hammer reduction (although this is taken care of by proper hydraulic design), avoid resonances (VSD programmed to “jump’ over critical frequencies) etc etc. (It’s a simple one pump / one boiler case. Even so, there may be some departure from the BEP so a low L3/D4 spec is considered).

One comment that came up is that with the VSD we can run the motor at over 100 % (nominal) rpm for limited time (or even on permanent regime if properly sized) but could not, for instance, open the control valve 110 % for any amount of time for instance.....

In closing (and kindly excuse the length ..):

Nothing beats good design based on solid engineering principles and good economic analysis and evaluation.

There is good and bad sales engineering just like there is good and bad process or mech. or electrical engineering. They each have roles and responsibilities, complementary and not interchangeable, in producing good, solid and safe processes and systems.

Some times it will be VSD and others it will be Control Valve.

It is possible to miscalculate a VSD system just like it is possible to miscalculate a pump and valve combination.

If possible, resort to installed base references. In most cases and applications there is plenty of solid historical data and information to use as basis for judgment and decisions.
The “myth” seems to have convinced a lot of very capable professionals all over the world... and now they have the operational data and information to back up their decisions.

VSDs use technology (power electronics) that is rapidly evolving, it takes time to stay abreast of developments to be able to apply it correctly and in its full potential. Advanced Vector Control (not PWM), regenerative equipment and 4-quadrant operation, IGCTs, Sine Filters, adaptive motor control models etc., result in a wider spectrum of applications that require ME / EE / ChE combined work to put to practical uses.

Thanks to all, picked some very good comments and advice in this thread.

MS

http://www.3rps.com

 

[mrspcs]

Correction / clarification on my previous post:
Ras Laffan cooling sea water units: 27 units is for stages 1 and 2 of the project. Additional stages will be added for the total capacity requirement of 256 MW. Single unit capacity is 7 MW.
Thanks

MS

http://www.3rps.com

 

[BigInch]

Not a bad article, but I'm not sure I believe the 66% efficiency number in Figure 8.

And I have this feeling that I could have saved Saudi Aramco even more money by replacing the pumps and looping the line after 10 years.

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"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/