Why throttle pumping systems?

UNIDO International Pump Expert, Harry Rosen, relates his experiences about throttling and makes compelling arguments for trimming impellers or installing VSDs instead.

When I first started TAS over 20 years ago, we specialised in developing pump selection software for the pump manufacturers. Over the years we developed various software modules that could handle anything from submersible, vertical line-shaft, multi-stage and positive displacement pumps through to the pumping of slurries and viscous fluids.

At that stage, I naively thought that if you selected the optimum pump for the application – taking into account the system requirements, type of fluid, etc – then the pump would operate efficiently and reliably over its lifetime and everyone would be happy. Little did I know.

The more involved I got with actual users of pumps, the more apparent it became that most pumps were not operating anywhere near their original design duty. This was due to a number of reasons going right back to when the system was designed.

Pumps are often selected very early in the design process when insufficient detailed information about the system is known – static heights, pipe materials, types of valves, etc. As in any case where assumptions have to be made by engineers, safety factors are added to the design. Plants are also designed with a view to increased throughput in the not-too-distant future; so maximum long-term flow requirements are used in the selection.

No consulting engineer wants to commission a plant where the pumps cannot satisfy the required duty. Rather over design than be caught short? Wrong!

If you were designing rolling stock for the railways or a bearing housing for a large mill, overdesign using safety factors will ensure a longer life for the components. In the case of pumps, however, overdesign or selecting the pump for a much higher flow/head requirement will reduce the reliability of the mechanical components over the life of the pump – as well as dramatically increasing the energy required to pump the required fluid.

The traditional solution to the problem – a control valve to reduce the flow back to the original requirement – would have been acceptable in the distant past before Eskom load shedding and the increasingly expensive cost of electricity. Nowadays, throwing away energy through a control valve is no longer acceptable.

A typical gate valve in the pulp and paper industry, clearly showing the valve is approximately 90% closed. The energy implications of throttling pumps. The area under the curve represents the fluid power required. When throttling, the red area shows wasted power while the green is useful fluid power.

Traditional throttling

Here is a typical scenario from a paper mill with a requirement to pump final paper stock to a header box in the paper machine:

  • The plant hopes to expand plant capacity in the next five years, when the flow might increase by 50%.
  • The exact piping and configuration is not known so a safety factor of 10% is added to the head.
  • The pump supplier selects a pump to give slightly more flow than required.
  • When tested, the pump over performs on flow and head, but still within the test tolerance.

During on site commissioning, the pump is found to deliver twice the flow rate that is required. The solution? Throttle the pump using the gate valve as a control valve until the required flow is achieved.

I have hundreds of pictures from countless plants showing gate valves more than 70% closed, having been told that they have been like that for years.

If we look at the pump curve, the area under the curve represents the fluid power required (red and green blocks in the example shown). If we trimmed the impeller or reduced the speed of the pump, the actual power required is represented in green. The portion in red that is being wasted through the control (gate) valve amounts to more than 75% of the power absorbed by the pump.

What is the solution?

Trimming the impeller costs very little money and could be implemented immediately. A full size impeller could be then be kept in the store in case future demand escalates inline with the designer’s expectations. The cost of installing a VSD or downsizing the pump would be a more expensive solution, but it would still be easy to justify based on energy cost savings.

The cost of throttling

If there are gauges before and after the control valve, it is very easy to estimate the energy losses through the valve. Divide the pressure drop across the valve by the discharge pressure of the pump, and multiply by the motor rated power to get an estimate of the wasted kWs. Multiply by the number of hours the pump operates and the cost of power and you now know just how much money you are wasting by using a control valve.

More importantly, you now know how much money you have available – on a return-on-investment basis – to fix the problem! Table 1 demonstrates just how quickly these costs accumulate.

When I observe pumps being throttled and ask why, the answer often given by operators is that it is due to the maximum amps on the motor. This applies from rural pump stations next to a farmer's dam to high-tech plants where SCADA systems control every machine in the plant. Sometimes there is a red mark on the dial meter showing the maximum amps for the motor. In most cases the operator has been told something like "keep the amps below 70 A" and nobody questions why.

The value quoted by the operator or marked on the meter very often has no bearing on the actual maximum rated current the motor can handle. The maximum value specified on the motor nameplate is often higher, or on checking with the control room, nobody knows of any reason for the current to be kept below 70 A. “This is just the way it has always been done.”

Maybe years ago they had a very hot summer and due to insufficient cooling, they de-rated the motor power for a specific period. There are generally no flow meters in pump stations, so the maximum current setting could have been related to a flow rate required for a specific duty. Or someone might have just read the maximum value from the technical spec sheet incorrectly.

The solution? Open the valve and see if there are any adverse effects on the pump or motor. The energy savings can be quite stupendous.

In a parallel pumping case study for a steel mill cooling water system, for example, six pumps were operating in parallel, providing cooling water into a steel mill. All the pumps were throttled to between 25 and 50% open to ensure the current never went above 73 A, even though the rated current for the motors was found to be 85 A.

When opening the valve on one pump to 100%, we observed that the current never went higher than 79 A, well within the rated capacity. We were able to achieve the same flow rate (actually slightly higher) with only five pumps running. And shutting down one pump reduced the total power drawn by 258 kW.

These pumps have since been running in this state for over a year with no adverse effects on the motors, resulting in saving of over R1.5-million/year in electricity costs and 2 050 000 kWh per year in energy savings.

Identifying energy-saving opportunities from throttled pumps is actually easy. All that is needed is a stroll through the plant with your eyes open.

  • Look for control valves with pressure gauges upstream and downstream of the valve and use the formulas outlined here to estimate the wastage through the valve.
  • Look for ammeters showing pumps are drawing maximum amps – these pumps are, most likely, being throttled.
  • Question every answer you get and never assume the original reasons are still valid.

If you discover that any pump is being throttled, the first question is, does it need to be? If it is only being done to limit the motor current, then it is often unnecessary. Open the valve and see what happens.

If there is a valid reason for throttling the pump, then two far better and more economical options are available to pump operators. The first is to trim the pump impeller. This will return the pump’s operating point to the duty point of system, saving significant amounts of energy and money.

The second solution, which is slightly more expensive but still highly cost effective, is to reduce the pump speed by using a variable speed drive (VSD).

There are no good reasons to continuously throttle pumps. Not only are you throwing energy away, but also the reliability of the pumps always suffers if they are operated too far away from their duty points.