How to Control the Flow of Centrifugal Pump System

Centrifugal pumps become one of the most widely used in the industrial world. Here are some reasons:

  • Strong construction
  • Simple design
  • Low fabrication costs
  • Easy operation
  • Easy-to-manage system controls

The working principle of a centrifugal pump is by transfer of motion energy from the rotation of the driving shaft to the pump impeller thus creating the centrifugal energy transferred to the fluid flowing within it. The flow of centrifugal pump output can be varied both head and discharge values, according to system requirements. Because sometimes the system does not always want the fluid flow always constant at a certain value. This is not possible with positive displacement pumps.

One example is the Boiler Feed Water Pump used in steam power plants. This pump supplies the amount of discharge water flow adjusted to the existing electrical load.

Here are ways to adjust the discharge flow and head output of a centrifugal pump:

Controlling Flow Discharge With Discharge Control Valve

The simplest way to vary the discharge of the pump fluid flow is to use a control valve that can be adjusted for the amount of openings and mounted on the output side of the pump.

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The purpose of using the control valve at the pump outlet is to increase the restriction of the existing fluid flow, so that there is a shift in the system characteristic curve upwards. If the pump operates at a constant rotation, the pump operational point shifts to the pump characteristics curve line toward the lower flow rate.

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The sliding of the system characteristic curve results in the decrease of the system discharge requirement as desired. But on the other hand the need for system head (downstream control valve) actually does not change lower. This results in an excess head or leftover head compensated by a throttling valve system that creates pressure drop.

Advantages:

  • Cheap price
  • Good use on system with load often 100%
  • Good to use on short time operational control
  • It is suitable for pumps with flat characteristic curves

Disadvantages:

  • The pump output pressure is too high
  • The pump efficiency becomes low if it is throttling position
  • Not energy efficient if being throttling position
  • The control system is not good if the excess head is high
  • There is a mechanical load on the valve when throttling position
  • There is a risk of making noise when it is high throttling position

Controlling Flow With Minimum Flow Control Valve

The minimum flow is a line attached parallel to the pump and connects directly or indirectly between the output side of the pump and the inlet side of the pump. In this system the fluid flow of the pump output is divided into two, one direction fixed to the system while the other returns to the inlet side of the pump. In this way we can adjust the flow of fluid into the system by regulating the amount of fluid that passes through the minimum flow, of course with the help of the control valve.

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Through the pump and system characteristics curve we can note that in this way, we can adjust the discharge pump output to the system without having change the pump head value at its operational point. So the excess head value is not as big as if the system only uses the throttling system on the pump output side.

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Advantages:

  • There is no increase in head even if the pump works on partial load.
  • The value of the pump’s output pressure is fixed even if the flow discharge varies.
  • Suitable for use in systems that require low head but high flow.
  • Easy to control if full pump load is required.

Disadvantages:

  • The cost of system construction is more expensive.
  • There is no decrease in power requirements at partial load.
  • There is still an excess head at partial load.
  • In terms of energy needs, this system is not economical.

Controlling Flow With Variation of Speed Rotation

One way to get variation of the flow discharge of centrifugal pump is by varying the speed of the pump rotation. If the pump rotation is changed, the pump characteristic curve will shifted. When the rotation is faster, the curve will shift to the right. Whereas if the rotation is slower, then the curve will shift to the left. The curve shift is parallel to the initial position, so the head and flow rate at each curve point may vary according to the variation of the speed rotation used.

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There are several ways we can use to control the pump in order to have a speed rotation variation:

  1. Using an electric motor that can vary its rotational speed. AC electric motors can be varied in rotational speed by using more magnetic poles on the rotor side. This will increase the cost of production. While on the DC electric motor, simply by changing the volume of the supply voltage so that it can vary the magnitude of rotation.
  2. Using a gearbox system.
  3. Using a belt transmission system with a variable pitch diameter.
  4. Using a hydraulic transmission system.
  5. Using a steam turbine as a rotating driver can be adjusted by adjusting the amount of steam that enters the turbine to move the blades.


Advantages:

  • Can avoid excess head.
  • Smooth pump ignition due to speed inverter.
  • The pump components will last longer.
  • Reduce the effect of hydraulic feed-back.
  • Energy saving.
  • Low electrical load (if using an electric motor) due to the large current low when the pump is turned on.
  • Reduce maintenance costs.

Disadvantages:

  • The cost of the control system is high.

Controlling Flow By Installing Multiple Pumps In Parallel

If several centrifugal pumps are installed in parallel, then the total discharge flow is the sum of the flow rate of all pumps at work. In this way, we can adjust the fluid flow by running a number of pumps simultaneously in accordance with system requirements. The characteristic curve of the pump and the system becomes the reference work for each pump.

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Parallel pump characteristic curve is obtained by summing the fluid flow from several pumps at the same head value. In practice, higher flow rate will also increase the resistance of the system. So, to compensate these obstacles, the operating point of the pump becomes a higher practical pressure value than its theoretical pressure value.

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Advantages:

  • It is suitable for system curve characteristic which has high static head component.
  • Good adaptation on partial load.
  • High system efficiency.
  • Low cost control on the operating system.
  • Reliable operational system.

Disadvantages:

  • Construction costs are high.
  • The operational switching frequency of the pump is high if the system design is not appropriate.
  • Problem with fluctuation of pump inlet pressure.