Correct Sizing of a Pressure Reducing Valve
A question that All Valve Industries receives regularly throughout the plumbing industry is ‘how do I correctly size a pressure reducing valve?’ Control valves such as pressure reducing valves (PRVs) need to be sized correctly in order for optimum control of the water; whether the control valve is made for flow control, or in the case of this article, a pressure reducing valve for pressure control. Undersized or oversized valves can mean poor control accuracy, under flows, noise, cavitation or premature wear of components.
The first step that needs to be considered for selecting the correct size of pressure reducing valve is;
Do not size only for a maximum flow requirement. During low flow demand, an oversized valve will operate in a nearly closed position causing premature wear and an undesirable noise may occur.
If normal flow requires a line size PRV, a smaller PRV, piped parallel to the main PRV should be considered. Adjusting the smaller bypass PRV at approximately 50-70kPa higher than the main PRV will ensure that during periods of low flow demand, these lower flows are handled by the smaller bypass PRV and will prevent premature wear and possible noise of the larger valve.
Once the minimum and maximum flow rates required in the system have been considered, then the valve size may be selected. The correct parameter to base the valve size selection is by velocity.
Size the pressure reducing valve based on a velocity of 1-2 m/s. This range is advisable for good pressure control within the valve’s optimum flow rate range.
With the calculated nominal flow rate and using the relevant valve water velocity flow chart, select a size that intersects its flow rate within 1-2 m/s on the chart. Sometimes there is a choice in terms of valve size within the acceptable water velocities; at that point you can decide if you want a better control at reduced flow rates with higher velocity and higher pressure loss at nominal flow rates (selecting the smaller size) or vice versa. The corresponding pressure loss graph may then be used at the same flow rate and valve size to obtain the pressure loss.
If the minimum flow rate expected is less than approximately 0.3 m/s on the main PRV, then install a bypass line with a smaller PRV which is suited to the required minimum flow rate.
Although sizing a valve based on a pressure loss curve can sometimes achieve a similar size selection as one based on velocity, it is not the ideal method. Pressure loss charts for automatic type control valves, like a pressure reducing valve, are based on certain parameters and conditions at time of testing, and should therefore only be used as a guide to the pressure losses expected when installed on site. A different supply pressure and set pressure to that used during testing will produce a different curve on the pressure loss chart. Therefore, basing the size on the pressure loss chart will likely yield a different pressure loss than expected and possibly a different size selection than one based on the valve’s ideal velocity range.
In addition, a well-designed control valve will have a lower pressure loss at the same velocity and flow rate than a similar, poorly designed valve; whether this is due to a rough internal casting, or an inefficient internal configuration. The internal fluid-dynamic shape and smooth surfaces of a well-designed pressure reducing valve makes it possible to attain low pressure losses, even when a large number of draw off outlets are open.
There are several methods for calculating the design flow, however, for the purpose of this example they are not detailed in this article.
Referring to AS3500.1, the probable simultaneous demand (PSD) flow rate is based on the number of dwellings. For example, Table 3.2 suggests the nominal flow for 9 dwellings should be 1.64 l/s (98.4l/m).
It might also be expected in this application that only one basin or shower would be used during low demand, therefore the minimum flow may be as low as 6 l/m.
EXAMPLE BASED ON THE CALEFFI 535 SERIES PRESSURE REDUCING VALVE
The size of the pressure reducing valve is determined by means of Graph 1, starting with the design flow figure and bearing in mind that the ideal velocity is between 1 and 2 m/s (blue band).
The 32mm pressure reducing valve is selected for a nominal flow rate of 98.4 l/m, which relates to a velocity of 1.5 m/s. With consideration of the 6 l/m minimum flow, the 32mm valve will be oversized and so the only option is a 15mm valve. Refer to the red line on the chart. The smaller valve will need to be installed on a bypass line around the 32mm valve.
Using Graph 2, still starting with the design flow figure, identify the pressure drop intersecting the curve relating to the size already selected (the downstream pressure falls by a value equal to the pressure drop in relation to the zero flow calibration pressure). Note that the pressure loss from the chart is used as a guide only and actual pressure loss will vary when installed on site depending on the supply and set pressure conditions.
Note that this chart shows curves at a 800kPa inlet pressure and 300kPa set pressure; curves for other settings are similar. The curve shifts slightly to the left for a
smaller differential and to the right for a greater differential.
The set pressure of the smaller bypass pressure reducing valve can also be estimated using the velocity and pressure loss charts. First, identify the maximum flow rate of the smaller PRV on the water velocity chart; generally around 2 m/s (see yellow line). Follow the flow rate down from the smaller PRV and ensure that the flow rate intersects the curve of the larger PRV. This will ensure that the maximum flow of the smaller valve is sufficient to meet the minimum flow requirement of the larger valve. Then, using the pressure loss chart (Graph 2), identify the approximate pressure loss of the smaller PRV at its maximum flow rate. The example shows a 70kPa pressure loss at 30l/min. Therefore, if the main pressure reducing valve is set to 400kPa, the smaller valve should be set 70kPa higher at 470kPa in order to handle the lower flow demands, in this case up to 30 l/min, of the system.
In addition to providing the ability to regulate pressure during low demand, a bypass setup will also save on maintenance costs as the larger (more costly valve to service) will only operate during peak demand, thereby extending the service life of the large valve and achieving a greater return on investment.
The final consideration when selecting a pressure reducing valve is the amount of pressure reduction required. Optimal performance is achieved at a 2:1 ratio, and a maximum of no more than 3:1. Example: 600kPa supply pressure, 300kPa static downstream pressure.
Where large pressure drops are required, for example in multi-storey buildings where inlet pressures are likely to exceed 1,000 kPa, this may be achieved through staged pressure reduction measures. A situation with low flow and a high differential pressure forces the valve to operate in a near closed position, potentially resulting in cavitation and possible noise.
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