Boosters may have varying suction value. Typical cases:
- The suction is connected to a tank, whose water level varies.
- The booster is supplied by the municipal network, that may have different pressure levels at various hours and / or seasons. If we want to maintain fixed flow rate downstream the booster, or limit its flow to a specific maximum value regardless of suction pressure , we should control the discharge flow by a valve. As the duty point of a pumping system is not determined by the pump only, but by the combined characteristics of the pump and the network, it is impossible to control the flow rate using pressure as a reference: Increase of suction pressure at fixed network characteristics (fixed number of open demand valves, for example) will increase the flow. And in the other hand- constant suction pressure will generate varying flow when the demand vary (more valves are open, as example).
2. Flow Rate control valve (FR)
This valve may control the flow rate quite accurately and efficiently. It uses the pressure differential created by the water flow through an orifice plate and sensed by a differential pilot valve. This pilot valve will maintain fixed valve position as long the differential is at the preset value (hence the flow rate at the requested value). It will open the main valve in case the differential drops (= too low flow) or close the main valve when the differential rises (= too high flow).
The FR valve has two basic limitations:
- The pressure differential created by the orifice can not be too high, since we do not want to limit the flow initially by this device. Such small differential (1-3m) requires high sensitivity of the pilot valve- consequently large, expensive pilot that requires frequent maintenance and cleaning to ensure reliable, accurate regulation.
- The differential vary at exponential ratio to the flow: Increasing the flow by 20% causes 44% rise of pressure differential, for example. Therefore we are limited in the range of flow rates, that can be controlled by a specific pilot / orifice configuration
3. Differential Sustaining valve (DI)
This valve is using the pump data as a reference to flow. When we know the booster performance curve- we can easily define the TDH that corresponds to the requested flow. We can install a valve that will maintain a minimal value for the added dynamic head of the pump, thus a maximal value of the flow rate. This valve, also, is controlled by a differential pilot,
that senses the pressure differential (= TDH) between the suction and discharge side of the booster. Should the flow increase, due to rise in suction pressure or drop of network resistance, the pump duty point moves to the right side of the pump curve, and its head is now smaller. The pilot valve senses too-low differential, closes the main valve, until the added restriction caused by it reduces the flow- and increases the pump head to the preset point.
The advantage of this solution is that the pilot is activated by a large pressure differential (the booster TDH) rather then the very small differential generated by an orifice plate. This factor enables troubles-free operation and simpler, less expensive pilot valve. The accuracy, obviously, is determined by the characteristics of the pump. The flow control of flat- curved pumps will less accurate than the control of steep- curve pumps.
It is not recommended to install any automatic control valve upstream of a pump. Such configuration may risk the pump by cavitation conditions, that may develop by the valve restriction (lower NPSHa), or reduce efficiency by inserting air (in case of a negative suction). The valve should be located, therefore, downstream the Booster. The DI valve should be sized to a maximal velocity of 5m/s, but in case the operating conditions may require minimal pressure loss of the valve- it must be sized according to the allowed headloss (and, obviously, will be larger).
A sensing port should be prepared at the pump upstream pipe, not on the reduced diameter section that is normally assembled at the inlet flange. The upstream control bores of the valve itself can be used to provide the pilot activation water (which must be filtered) and for the sensor of the pump's discharge pressure.