03 Jul 2019

Water Pressure In Tall Buildings: PRV Sizing

Our blog series on water pressure control in tall buildings has been widely welcomed by interested readers from a range of audiences – from system designers to specifiers and building managers. This third blog focuses on the importance of sizing pressure reducing valves (PRVs) in the design of these systems.

Regardless of whether a PRV is direct acting or pilot operated, it is essential that it is correctly sized. As with any control valve, incorrect sizing can cause bad control, noise and poor performance

To ensure trouble free operation, PRVs should be sized according to the required flow rate not necessarily the nominal pipe size. Velocity should be kept between 1-2 m/s to keep noise to a minimum. Where noise control is not so important e.g. plant rooms, the velocity can be higher - up to 4 m/s.

For large PRVs, the turn down ratio is more critical and should be no more than 3:1 to avoid cavitation. In general, this is particularly important for the larger flanged PRVs, and typically the minimum flow rate through a PRV should not be lower than 10 per cent of the maximum designed flow rate.

If an oversized valve is selected it could mean that the flow rate is so low that the PRV is working in the almost closed position where instability and pressure fluctuations may occur. The Kv value of a valve is always a good guide and may be used to size a valve, given the flow and pressure drop requirements. Additionally, a PRV should ideally be selected to operate in the middle of its pressure and flow range.

The choice of control equipment must be robust and fit for purpose to ensure that the system performs effectively, with safety, hygiene and a comfortable level of noise. As with everything, there is a balance to be achieved between good design and cost to the end user.

The reality is that if corners are cut with sub-standard pressure control equipment, this is sure to have a negative impact on the day-to-day system running and the threat of catastrophic pipe failures following pressure surges.

In the fourth and final blog in this series, we will discuss the issue of surge protection and the dangers of excessive and potentially damaging water flow velocity. 

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