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.