Benefits of Installing Variable Speed Drive on Centrifugal Pump

The benefits depend upon the electricity tariff.

If the tariff is based on peak demand, then a VSD will pay for itself in weeks as a result of shaving the starting power required.

If the tariff is based on kWh consumed, then it is possible to program the VSD to back the pump speed off and lower the terminal voltage based on some flow or pressure criterion, and use less kWh to push the fluid around.

I heard the motor starting kw, while significantly high, is of such a short duration as to not impact the 15 minute (or 30 minute) averaged kw. Is this true?

Take a look at your load profile and duty cycle – if you have frequent stop/starts and substantial operation at partial load conditions then the VSD's can payback fairly quickly. On the other hand, if the pumps are at a constant or near-full load the majority of the time then VSD's do not necessarily make sense, especially since there are energy losses associated with the drives (typically 2-5%). For this size of drive, the energy loss will probably be at least 5 kW (per drive) which may be rejected to the space as heat or directly vented depending on the design, so you will likely need a supplemental HVAC system (requiring more energy & capital) to maintain a proper operating environment for the drives.

Many people assume that VSD's will automatically save energy, but it really requires a comprehensive energy study and payback analysis to determine if they are beneficial. There could be more cost-effective options, such as trimming the impeller to the optimum duty point.

Good Answer.

My experience has been if you are mechanically throttling the flow with a valve then a VSD males a lot of sense. You can then vary the speed of the pump instead of wasting the energy in a valve.

If the pump runs consistently at rated speed it makes no sense.

Hi tlefefa,

You are talking about a large pump load and therefore depending on the annual hours run a potentially significant energy saving. Your process operating regime is therefore the starting point plus the obvious question of where on the pump curve the fixed speed operation puts the motor load.

Installing a VSD will save you energy even if you merely set its speed anywhere below maximum, BUT the real savings will accrue if you employ a dynamic control loop to continuously match the pump speed to the needs of the process. Two options are frequently employed , constant pressure or constant volume. Both these options are frequently employed in the HVAC industry where energy demands fluctuate widely due to climate related stimulii and therefore a fixed speed approach will result in a waste of energy.

In the former you will be monitoring delta P at some appropriate point and in the latter the actual flow through some device; either way there will need to be a control instrument involved to produce a 4-20mA signal for the VSD to operate from.

If your process has a range of demand and operates on a continuous basis there will be substantial savings to be made,and therefore a decent return on investment can be obtained which could be enhanced by introducing some form of monitoring with a feed-back loop as suggested.

Best wishes,

Massey