Since the late 1980s Variable Frequency Drives (VFDs), also known as Variable Speed Drives (VSDs), have become the de-facto standard for controlling the speed of an induction motor or a synchronous motor. VFDs are used to control the motor speed in various applications including fans and conveyor systems – but one application they are particularly beneficial for is industrial pumps.
In this article, we explain the advantages of using a VFD to regulate pump speed, in comparison to the use of valves which have traditionally been used to throttle the flow from the pump.
Why do we need to regulate the flow rate of industrial pumps?
Most industrial pumps are designed to operate at peak efficiency while at their maximum flow rate. However, for many pumping applications, maximum flow is seldom if ever required. This is because:
- You may want to have dynamic control over flow in order to reduce or increase the water level in a tank and the volume of water being supplied at any one time. When the water demand is low, the pump’s motor needs to slow so supply is reduced or stopped accordingly. When the demand is high the tank would empty quickly so the pump needs to be speeded up accordingly.
- Operating at maximum flow also uses maximum energy, meaning maximum costs. Clearly, when maximum flow is not even needed the majority of the time, this is an inefficient use of resources. And with 90% of the cost of owning a pump coming from energy consumption, energy saving can have a major positive impact on the pump’s lifetime cost.
Traditionally the only method of controlling the level of water in the tank, and in its simplest form, was by the installation of float switches near the top of the tank and near the bottom. The pump motor would typically be controlled by a DOL starter, meaning the pump could only ever be on or off. When the pump was on it would operate at maximum designed flow with the water level rising as quickly as the pump and associated pipework maximum capacity could manage, and also depending on the demand level from the process plant.
This would continue until the water level reached the high-level float switch, activating it and cutting the power to the pump motor. With demand and the water level in the tank now dropping, the motor would remain off until the low-level float switch was activated, at which point the motor would restart and the pump would once again attempt to fill the tank at maximum flow. This method has a major disadvantage in that supply can never match demand.
Why are valves an inefficient way to regulate a pump’s flow rate?
A traditional way of getting around this problem and controlling the flow from an industrial pump is to use a valve situated on the inlet (suction) side or the outlet (delivery) side.
Closing the valve applies a throttling effect thereby reducing the flow, while opening the valve increases the flow rate. The most common reason for installing a valve to control flow is when a pump and consequently its motor have been oversized (i.e. the system head has been incorrectly calculated too high).
In this case throttling will reduce the pump’s flow rate and can shift the duty point on the pump curve to within the designed operating range. Although there are many reasons for installing throttling valves inline this is a very inefficient way to control flow.
When a valve is closing, the pressure drop across it increases friction and the system curve becomes steeper meaning power loss is higher.
Why Variable Frequency Drives are the most efficient method of controlling flow
If we look at the storage tank example mentioned earlier, by replacing a centrifugal pump’s motor starter with an appropriate VFD, the high and low limit float switches can be removed from the tank and replaced with a suitable level sensing transducer system. If this transducer’s analogue output is inversely proportional to the water volume in the tank, the VFD can receive such a signal and control the speed of the motor (and therefore the flow from the pump) smoothly and variably under most process conditions under PID control.
Crucially, this will prevent any frequent short term overloading of the electrical supply each time the pump starts or mechanical shocks such as water hammer which can cause damage to pipelines, joints or seals and can greatly reduce energy consumption.
VFDs also reduce the maintenance costs that a pumping system will incur – with maintenance another major contributor to a pump’s total lifetime cost. Other than significant energy savings one of the main benefits of using a VFD to control the pump is the inherent soft starting and stopping which means pipes and valves are less likely to suffer damage as a result of rapid flow change.
Looking to dimension a drive system for a pump application? Dalroad are on-hand to help
Dalroad will help you choose the best variable frequency drive for your project and give you all the support you need to dimension your drive system. Whatever your project challenge, we provide best-in-class expertise every step of the way – from dimensioning to installation.
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