Introduction to Pressure Reducing Valve

Purpose:

In the field of water engineering, the Pressure Reducing Valve (PRV) plays a crucial role. The main purpose of the PRV is to have a device that automatically reduces inlet pressure to a steady lower downstream pressure regardless of any changes in flow rate and upstream pressure, as shown in below Figure (1).

Application:

Pressure-reducing valves are typically positioned at entry points to residential zones, especially 1) near or below the elevation of lifting stations or 2) in residential zones with a notably lower elevation compared to the water reservoir in a gravity system as shown in Figure (2). Noting that high-pressure levels for entering these areas could lead to significant line ruptures or bursts.

Benefit:

PRVs are essential in safeguarding systems against the dangers of excessive pressure, which, if unmonitored, can result in equipment breakdowns, and system leaks. By consistently regulating the downstream pressure, PRVs guarantee that systems function within their predetermined pressure limits.

In the context of water infrastructure, PRVs serve a vital purpose by minimizing water loss. They achieve this by overcoming potential overflows and leakages that arise from surges in pressure.

PRV Sizing:

Ensuring the correct sizing of a PRV is paramount for its efficient operation. The flow rate that the valve is expected to handle is the primary determinant of its size. It's a common misconception to believe that the PRV size should directly correspond to the size of the inlet pipe. Such an approach can inadvertently lead to the PRV being larger than necessary.

An oversized PRV can introduce a host of complications to the system. Not only can it produce unwanted noise, but it can also damage PRV's seal. This can result in leaks and reduced efficiency, leading to potential system failures.

It's always a good practice to consult with the PRV manufacturers and ask for their technical data sheets or guidelines when determining the appropriate velocity for sizing. They often provide detailed charts or tables that can guide you in selecting the right PRV size based on the expected flow rate, velocity, and required pressure drop.

The majority of references recommend sizing pressure-reducing valves to accommodate a fluid velocity between 1-2 m/s. This specified range is strategically determined to ensure precise pressure regulation and optimal performance within the valve's ideal flow parameters.

Other manufacturers provide a pressure loss curve to assist in the sizing and selecting pressure-reducing valves (PRVs). This curve illustrates the relationship between the flow rate passing through the PRV and the associated pressure drop experienced across the valve. Within this curve, distinct inclined lines represent different valve sizes. By identifying the intersection point between the anticipated flow rate of your system and the appropriate inclined line, you can accurately determine the most suitable PRV size for your application. This method ensures that the selected PRV will operate efficiently and within its optimal performance range for your system's requirements.

Pressure Reduction Ratio:

The pressure reduction ratio is a critical factor to consider when selecting a pressure-reducing valve (PRV). This ratio is determined by dividing the supply pressure by the desired downstream pressure. For optimal performance, a 2:1 ratio is recommended. However, the multiple PRVs can tolerate a maximum ratio of 3:1, as illustrated by a 12-bar supply pressure being reduced to a 4-bar downstream pressure.

Exceeding a 3:1 ratio increases the risk of cavitation. Cavitation can introduce unwanted noise into the system and potentially damage the PRV seal and the system. Therefore, maintaining an appropriate pressure reduction ratio is essential to ensure the sustainable and efficient operation of the PRV and the system it serves.

Cavitation Index

Most manufacturers and designers refer to a term called the Cavitation Index, this dimensionless index is used to characterize the likelihood of cavitation in fluid flow systems, especially in control valves.

According to the ISA-RP75.23-1995 standard, the cavitation index, denoted as σ, is a critical metric for evaluating the likelihood of cavitation in pressure-reducing valves. While specific design criteria might vary, a general benchmark is that a σ value greater than 1.5 is deemed acceptable and indicative of a low-medium risk for cavitation. Conversely, a σ value below 1.5 suggests a high potential for cavitation damage, leading to further evaluation or preventive measures. See Figure (5).

• P1 =inlet pressure

• P2 =outlet pressure

• Pv = water vapor pressure

Solution for High-Pressure Reduction Ratio:

1. Staged Pressure Reduction by placing two or more PRVs in series. As Figure 6 presents, it is required to decrease the pressure from 20 bar to 4 bar, a direct reduction poses a significant 5:1 ratio, increasing the risk of cavitation. To mitigate this, a staged pressure reduction strategy is recommended. Initially, the pressure is reduced from 20 bar to 10 bar, achieving a 2:1 reduction ratio. Following this, a subsequent reduction from 10 bar to 4 bar is executed, corresponding to a 2.5:1 ratio. This two-step method ensures a safer and more controlled pressure decrement, effectively minimizing the potential for cavitation.

1. Anti-Cavitation Pressure Reducing Valve: This is an alternative solution to address substantial pressure differentials. This specific valve is furnished with an Anti-Cavitation Trim. This feature ensures optimal control over downstream pressure, concurrently minimizing noise and effectively averting potential damage. Figures (7 & 8) show the 90-01KO Valve that is manufactured by Cla-Val Co. for anti-cavitation purposes.

Parallel PRVs Configuration:

Water supply networks seldom operate under peak demand, which means they don't frequently experience peak flow rates. In scenarios where there is considerable fluctuation in flow rates, especially with pronounced low minimum values, the parallel PRVs configuration becomes particularly advantageous.

Accordingly, it is common to encounter a configuration where pressure-reducing valve (PRV) stations consist of a primary, larger valve complemented by a smaller bypass valve in parallel. This bypass valve is specifically designed to manage lower flow requirements, given that larger valves might struggle to efficiently handle minimal flow rates. See Figure (9).

However, it's essential to emphasize that not all systems necessitate a dual PRV setup in parallel such as having constant flow in a continuous supply system or fixed intermittent supply. The need arises primarily when the system experiences broad variations in flow and particularly low minimum values.