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By Ron Astall, United Pumps Australia

Of course it depends on what you pray for.  If you are a Variable Speed Drive vendor, this is the new religion.  Electric variable frequency drives (VFDs) have become very affordable and are being touted as the “Saviour” for centrifugal pump energy saving and process control and the Drive Vendors are the new evangelists.

Why the excitement?
Most centrifugal pumps are driven by simple, rugged and reliable squirrel cage induction electric motors, which run at essentially a constant speed. Their speed depends on the number of poles in the motor and the electric supply frequency; usually 50Hz or perhaps 60Hz as in the Americas and some parts of Asia. For Electric motors we are stuck with the typical fixed speed options as shown below:

Available Motor Speeds

Pump Running Speed – RPM
(assuming 20 RPM slip)

Number of Poles

50Hz supply frequency

60Hz supply frequency

2

2980

3580

4

1480

1780

6

980

1180

8

730

880

10

580

700

VFDs use inverter technology to vary the supply frequency to the electric motor.

In principle we can have any pump speed we want, just by the turn of a knob or by the graphic user interface on the VFD. Not only can we have any speed we want, we can change it at will while the unit is running.

Now, before the suppliers of other technologies become upset, I must point out that the most if not the same benefits are available from many other Variable Speed Drive (VSD) technologies such as fluid couplings, hydrostatic drives, continuously variable transmissions, DC drives and slip ring motors. The VFD evangelists are, however, the ones currently leading the push for much more affordable speed control in pumping installations.

To be even handed, from this point onwards I will use the all encompassing term ‘VSD’ except where specifically referring to VFDs.
The Gospel message from our evangelist friends is that VSDs allow flexibility, energy saving, accurate flow control and ideal process control.
Let us examine whether this could be true.

Systems change
A centrifugal pump can only operate where its curve intersects with the actual system curve (Fig 1). Normally the pump curve does not change except through damage or wear.
Systems, however, change all the time. The designer may have over-specified the pump in the first place, the piping may corrode, filters and heat exchangers may clog, tank levels may change and plant demand may change. So the systems change, but the pump curve stays the same (Fig 2).

A centrifugal pump is a slave to the process. System changes can play havoc with the pump operating point and the pump may end up running well away from the rated flow and the system may need to be throttled to keep the pump on its curve or to control the flow to suit the process.

Throttling – Modifies the System Curve
Throttling varies the system curve by increasing the friction losses (Fig 3). The head or pressure loss across the throttling valve is obviously an energy loss.
Throttling is a highly effective but often energy wasteful method of flow control.

What if we could change the pump curve to match the changes in the system curve?

Well, yes, this is the whole point of VSD. Changing the pump speed changes the pump curve. So how well does this work and how can we calculate what will happen?

To figure this out, we need to understand the centrifugal pump “Affinity Laws”

Affinity Laws – Modify the Pump Curve
The performance of a centrifugal pump will vary with speed according to the formulae below (Fig 4).

These calculations result in a set of curves called “Affinity Curves”, with imaginary lines (Affinity Lines) that pass through equivalent points on each speed curve.

We can use the above to predict the change in performance of a centrifugal pump with a change in driver speed quite accurately.

We can thus change the pump curve to match the system.

We can thus change the pump curve to control system flow instead of throttling (Fig 5).

When we throttle, the energy lost across the control valve must still be covered by the pump driver. If we instead slow the pump speed to match the different system and flow requirements, we can reap dramatic power savings.

Variable Speed Process Control
Variable speed technology offers major benefits (Fig 6).

Instead of throttling, run the pump slower and save on power, save on control valve wear, eliminate throttling noise and use driver speed to control the process.

Savings
The saving in power consumption can be calculated by comparing the full speed power with the power at the required operating point assuming reduced speed. The inefficiency of the drive must be allowed for in the power calculation. Electric VFDs are slightly less efficient than a basic direct on line motor. This is because every VFD will have some unavoidable electrical losses and due to additional eddy current losses in the motor which stem from the normally less than perfectly sinusoidal wave form that a VFD produces. Other types of VSDs also have losses which must be factored in. In general, these losses are small compared to the overall power saving.

In comparison with a control valve, there may also be significant maintenance savings due to the elimination of valve wear.

Pump life between overhauls is also potentially improved when a VSD system allows the pump to operate closer to its Best Efficiency Point (BEP) at the various system flows.

Costs
The capital cost of the VSD. If this is a VFD retrofit to an existing unit, the existing driver will need to be evaluated for suitability for variable frequency service. Usually an electric motor has to be de-rated to take into account the small but sometimes significant eddy current losses, and this may mean that a new, higher rated motor will be needed. If the speed is being increased, then almost certainly a new, larger motor and perhaps a new shaft coupling will be necessary.

Mechanical Considerations
In most VSD retrofit applications, the idea is to use the VSD to slow the running speed down from the previously fixed maximum speed. Normally this will not present any mechanical problems for the equipment because running stresses reduce dramatically when the speed is decreased. Some aspects to watch however include hydrodynamic bearings and mechanical seals which will have a minimum speed requirement to maintain a fluid film and to ensure adequate lubricant and seal flush flow. This minimum mechanical speed is typically a few hundred rpm and is normally not an issue but should be considered.

Another factor that is often queried is less effective motor cooling due to lower cooling fan speed. Because the power required by a centrifugal pump reduces with the cube of the speed change, motor power is dramatically reduced and the reduced cooling capacity ought not be an issue. Motors in other VSD applications such as constant torque service may need auxiliary cooling at low speeds.

When the pump itself has a shaft driven cooling fan in a hot service application, the reduced cooling capacity on the pump may need to be addressed.

It is now also routinely possible to increase the pumpset speed above the standard supply frequency. For a new installation, it is expected that all aspects would be engineered at the time to ensure the equipment is correctly rated. For an existing installation, the pump and driver combination will need to be assessed to ensure that the pump itself has been engineered for the higher speed, which implies higher pressures, higher shaft power, higher bearing loads and almost certainly poorer suction performance (higher NPSHR).

Hydraulic Considerations
The system hydraulics will often dictate the minimum running speed, rather than mechanical considerations. If there is a significant static differential head in the system, the danger is that as speeds reduce, the pump head will drop below the system static head and the pump will be running at zero flow or will experience reverse flow if there are no check valves. The pump speed must always be high enough to ensure that pump developed head exceeds the system static head sufficiently to maintain minimum pump flow.

Hazardous Areas
In hazardous areas, the motor will also need to be certified for use with a variable frequency drive. This is normally available routinely for a new motor, but an existing unit may not have suitable certification.

Will it Work for You?
The VSD gospel is that you will generate energy savings, improved reliability and enhanced system control.

Will it work for you? It depends on your system. In our previous discussion on “Pumps in Parallel” we discussed “Flat” and “Steep” system curves (Fig 7).
A mostly frictional “Steep” system such as a closed loop system or a long pipeline is ideal for VSD control. In this sort of system, the ratio of flow versus speed will be very linear and flow control will be straightforward.

Conversely, with a “Flat” system where the head is mostly static differential, VSD flow control may be difficult. If the pump curve is also reasonably flat, a small speed change may result in a dramatic change in system flow. A steeper pump curve may help, but control of “Flat” systems is often problematic; particularly at low system flows. For “Flat” systems it is valuable to draw the pump curves at various speeds against the system curve and calculate the Gain or {87a03eb4327cd2ba79570dbcca4066c6d479b8f7279bafdb318e7183d82771cf} flow change vs {87a03eb4327cd2ba79570dbcca4066c6d479b8f7279bafdb318e7183d82771cf} speed change and plot this against flow (Fig 8). This will allow a prediction of where VSD flow control, may become impractical. In these instances, throttling with a control valve may be the best solution.

The VSD gospel is that you will generate significant energy savings, improved reliability and enhanced system control.

Will it work for you? It depends on your system. Plot your pump curves against your system curves. This is the window to understanding how it will work.

In some cases, it may be a complete disaster. In most cases, however, VSD offers wonderful advantages and is an ideal solution to generate energy savings and easy process control. It may indeed be the answer to your prayers.

The moral of the story is, as always, understand your system.

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