We have calculated EII for a long period and we have observed it was very clear that capacity and EII directly related to each other.
If you go on details to decrease bad effect of low on EII the best way to use VSD for every system. (One pump or fan for each system.)
But at this point we have two main doubts:
1. Reliability of the VSDs
2. VSD effect on control system.
Are we correct to have these concerns?
Answers
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31/05/2009
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A:
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keith bowers, B and B Consulting, kebowers47@gmail.com
You have not yet completed your investigation. You should ask vendors of VSDs for reference users to contact and question. Some VSDs are extremely reliable (controllers on electric trains, subways, trolleys for instance) The selection of appropriate VDSs for the particular service is not trivial. The total operating characteristics of the service are necessary for the Vendor to recommend the best device.
VSDs are not justified for all services, particularly if normal operations are near/at maximum efficiency point of the equipment.
Traditional systems engineering resulted in many 'oversized' pieces of equipment. For instance, the Normal process requirement may be 100 GPM (for simplicity say 10 Hp, 10 psig loop pressure drop.), The Process Engineer will likely specify 120 GPM as required, 100 as normal. 50 as minimum (most process operations , particularly reactors and distillation columns) are unstable at even lower turndown. The mechanical engineer will specify a pump capable of 140 GPM (25 Hp required because of increased loop pressure drop at higher flow rate) The motor will be specified as 30 Hp to ensure adequacy. The coupling will be sized for 50 Hp to ensure it does not fail. The electrical circuit will be sized for 40 Hp to ensure adequate capacity. The circuit breaker will be sized for 50 Hp to ensure it does not trip unnecessarily,etc,etc..
Now things get real interesting. Under 'normal' operations at 100GPM process demand, the control valve is screaming because it has to absorb 40 psig instead of desired 5 psig because the pump impeller was selected for developed head needed at 140 GPM with 5 psig pressure drop across the control valve.
While a VDS WILL reduce energy waste across the control valve, you still have a pump and motor 40% oversized for normal operations. Reducing motor size and pump impeller diameter (if feasible) so the motor is 90% of rated speed and power at normal operations, with NO control valve (VSD installed), it should do fine, and be well capable of long term operation at 115% of rated speed and power (125% of normal).
Control of processes using VSDs is not fundamentally different than with standard control valves.
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