Centrifugal pump protection using flow controllers

A correctly-sized flow controller close to the pump discharge provides tamper-resistant protection for centrifugal pumps.

Running at below their minimum operating head is a common cause of submersible centrifugal pump failure. (This is the same as allowing them to deliver too high a flow rate.) Flow rate and head should be kept within the manufacturers specifications for a long trouble-free life, .

A typical pump performance curve is shown below.

The system curve is governed by friction and the effects of valves, fittings, orifices etc.

Gate valves and pressure sustaining valves are often used to prevent out of curve operation, but they have disadvantages:

  • Prone to unauthorized adjustment.
  • Can fail due to gate vibrating loose.
  • Require maintenance.

The also impose a head loss penalty. Head loss at lower flows remains high with “fixed orifice” gate valves. The head loss across a pressure sustaining valve will not change at all, resulting in a significant energy loss at the duty point.

This head loss increases pumping costs and may even force a larger pump size.

Flow control valves offer protection without these disadvantages.

The Maric flow control valves cause less head loss than the common gate valve, fixed orifice or pressure-sustaining valve.

This is because the head loss drops off significantly as the flow rate drops below the rated flow. The duty flow rate is usually well in from the right hand side of curve. The flow controllers’ orifii actually open up as the pressure differential across it reduces in an attempt to maintain the same flow. The Maric valve will impose whatever resistance (head) is required to maintain the rated flow rate.

For example, when flow rate through valve is 70% of its rated flow, the head loss is only around 4 metres.

The actual head loss in your installation depends on the flow rate. eg At full rated flow, head loss will be between 140 and 1000 kPa*. At a lower flow rate, i.e., duty point, head loss will be less. e.g., 60% of flow = 30 kpa only. (*For standard “Precision” spec 140 – 1000 kPa flow controllers.)

Submersible pumps are at risk from damage from:

  • People can unwittingly open up the bores’ gate valve in an attempt to increase flow.
  • A high water level at start up that draws down to a much lower level during normal operation. At start up the pump faces very little static head.
  • Empty pipework at start up. Usually caused by a no or damaged check valve or draining of surface pipework. It takes time to fill pipes sufficiently to obtain the required head.
  • Over-pumping beyond the refill rate, to point of drawing in air or sand, leading to unstable conditions.
  • A burst in the pipework may allow uncontrolled flow and upthrust or cavitation.
  • Separate duties; eg one tank elevated 50m up a hill, and the other, to feed a dam at the same elevation as the pump. (Without a flow controller here, pump damage may result, due to lack of head).
  • Rising water tables. Limiting pump peak flow rate can prevent electric motors from overloading as operating head reduces.

Key features of Maric flow controllers include:

  • Tamperproof. The valves are non-adjustable, which prevents owners from trying to “get more from their bore”.
  • Maintenance free. There are no wearing parts so the valves will perform reliably without maintenance or adjustment during their 20+ year life span.
  • Self cleaning.

Pressure reducing systems for housing complexes

Housing complexes need pressure reducing valves for several reasons:

  • Higher supply pressure (from municipalities and waterboards) to meet growing demands.
  • Lower required pressures to homes to reduce any potential leaks.

The solution is simple and inexpensive for traditional free-standing houses. A line size (40mm, 32mm) direct acting pressure reducing valve costs less than R1500.

It’s more complicated in housing complexes where water to individual homes comes from a common supply.

Ineffective pressure reducing solutions often lead to catastrophic results: broken pipes, huge water losses and homes without water for extended periods.

This article provides guidelines for the correct selection and installation of pressure reducing systems for housing complexes.

There are 3 types of pressure reducing valve:

  1. Direct acting valve. A very simple device with an adjustable handle to set the pressure to the required value.
  2. Pilot operated valve. Usually used for large systems and often the only solution for housing complexes.
  3. Ratio reducing valve. Reduces pressure in a ratio between upstream and downstream pressure.

The direct acting valve.

These are limited in size range and capacity. Often a 100mm direct acting PRV will be too small to handle the required capacity for a typical 100mm pipe feeding a housing complex.

The pilot operated valve.

This is the most commonly used valve for housing complexes as they have a much larger capacity.

Issues to be aware of with these valves are as follows:

Sizing. These valves have to be carefully sized to ensure that they operate within the design flow rate because they can become unstable at higher or lower flow rates leading to wild fluctuations in pressure and the possibility of broken pipes.

Minimum and maximum inlet and downstream pressures must be considered to ensure that the valve is stable under both extremes.

The biggest problem is the minimum flow rate (which occurs at night). The solution is to install a small direct acting PRV as a bypass to the main valve. This bypass valve will be set at approx. 1/2bar higher than the main valve. At night the main valve will automatically close and all the low flow will then go through the bypass PRV.

Dead–end service. The valve must maintain the set downstream pressure under zero flow demand conditions eg in the middle of the night.

Pilot operated control valves take time to react. The valve might not be able to react in time if the demand ceases suddenly. This leads to an increase in downstream pressure and the risk of broken pipes.

The solution is to install a relief valve immediately downstream of the PRV to blow off for short periods during demand closure.

The ratio reducing valve.

This valve has the great benefit of instant reaction time and also being able to handle very low flows which does away with the need for a small bypass PRV.

It is the most simple PRV and is becoming more popular as users discover its enormous benefits.

Its only disadvantage is that it is not adjustable. This is not always a disadvantage because it means it is tamperproof. Tampering is a significant cause of incorrect operation and expensive problems.

All pressure reducing installations should include:

  • A strainer upstream. Foreign objects often get into potable water piping due to pipe breaks and other causes. A strainer will protect the installation from these.
  • Isolation valves either side of the installation to ease repair and maintenance without interrupting supply for too long.

Reputable control valve suppliers will assist with sizing and usually provide all the components made up as a single unit. The supplier should be involved in commissioning after installation.

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