It is important to size a pressure reduction valve (PRV) to suit the flow rate that passes through it. You should not necessarily match the inlet pipe size as this can lead to over-sizing of the PRV and oversizing the PRV has adverse effects on the system such as causing undesired noise and also damaging the PRV seal.
To size the PRV correctly, you need to find the technical data sheet from your preferred manufacturer and find a table that looks similar to below.
You can then plot the peak flow rate on the graph and find the corresponding PRV size. As a rule of thumb, you want to be within the 1-2 m/sec zone on the chart.
As you can see in the example above, for a flow rate of 32 l/min:
– The 1/2″ size is too small (velocity = 2.05 m/sec)
– The 3/4″ size is perfect (velocity = 1.10 m/sec)
– The 1″ size is too large (velocity = 0.75 m/sec)
It is important that PRVs are sized to cater for low flow rates too.
Buildings rarely operate at a peak demand so they are not commonly subject to the peak flow rate. Neglecting the low flow rates will also have adverse effects on the system such as causing undesired noise and damaging the PRV seal.
As a rule of thumb, the PRV bypass should be designed for 20% of the peak flow rate and need to be set at least 50 kPa above the primary PRV.
Setting the bypass PRV to a higher pressure allows the low flow rates to take the path of least resistance and this allows the bypass PRV to be used during the non-peak demand.
When setting the outlet pressure of the PRV, it is important to get the step-down ratio correct.
A 2:1 ratio works well e.g. 1000 kPa inlet pressure to 500 kPa outlet pressure.
If you have a ratio that is higher than 2:1, you risk cavitation which again could have adverse effects on the system such as causing undesired noise and damaging the PRV seal.
The diagram below is useful for checking the inlet and outlet pressure of the PRV design to see if it falls in the ‘normal operating conditions’ area.
If you did need to reduce the pressure at a higher ratio such as from 1200 kPa to 300 kPa (4:1), installing an additional PRV in series is recommended as shown in the example below. This allows a 2:1 ratio to be maintained through each PRV.
It is very important to consider static pressure when setting the PRV outlet pressure.
For example, as shown in the section diagram below, if you have a PRV in the ceiling and set the outlet pressure to 500 kPa, when the water gets to the outlet 2m below, the pressure will be approximately 520 kPa due to the vertical pressure gain. This consequently means that there is too much pressure at the fixture and that could lead to a non-compliant system and damaged tapware.
As water flows through a PRV, pressure loss occurs and needs to be considered in the design.
The table below gives an idea of what pressure loss you can expect under different scenarios.
A flow rate of 32 l/min through a 3/4″ PRV has 80kPa pressure loss.
It is important to be aware of this in the design because 80kPa pressure loss could be the difference between supplying a compliant pressure at the fixture and a non-compliant pressure at the fixture.
Each material has ideal working conditions and not every PRV is suitable for all applications.
For example, if you have hot water passing through a PRV at a high velocity on a regular occasion, it would be prudent to select a stainless steel PRV. Alternate material selections could cause corrosion of the valve.
Always check with the manufacturer that the PRV you have chosen will suit the design it is being used in.
It is recommended to install air valves downstream of PRVs.
Air valves eliminate air pockets from the system which are created under normal working conditions.
Eliminating air from the system minimises the associated consequences of pressure surges, friction loss, corrosion, and vibrations.
When using H2X to design your systems:
1. You receive a warning at fixtures to indicate where PRVs are required:
2. The PRV is sized automatically, including the bypass if it is required:
3. You receive a warning for having a step-down ratio greater than 2:1:
See how H2X works for yourself in this video:
We hope this blog helps with your next design which involves a PRV.
Please leave comments below if you have any feedback or suggestions for future blog posts!
Pressure-control valves are found in practically every pneumatic and hydraulic system. They help in a variety of functions, from keeping system pressures below a desired limit to maintaining a set pressure level in part of a circuit. Different types of pressure control valves include relief, reducing, sequence, counterbalance, safety, and unloading. All of them are typically closed valves, except for reducing valves, which are usually open. For most of these valves, a restriction is necessary to produce the required pressure control. One exception is the externally piloted unloading valve, which depends on an external signal for its actuation, which normally comes from a digital pressure regulator. In certain applications, like ventilators and anesthesia machines, the flow must be consistent at all times. Variations in the flow of gases can lead to serious injury or death. That’s why control valves are so important.
Most pneumatic and hydraulic power systems are designed to operate within a defined pressure range. This range is a function of the forces the actuators in the system must generate to do the required work. Without controlling these forces, the power components and expensive equipment could get damaged. Relief valves make it possible to avoid this hazard. They are the safeguards that limit maximum pressure in a system by diverting excess gases when pressure gets too high. The pressure at which a relief valve first opens to allow fluid to flow through is known as cracking pressure. When the valve is bypassing its full rated flow, it is in a state of full-flow pressure. The difference between full-flow and cracking pressure is sometimes known as the pressure differential, or the pressure override.
In some cases, this pressure override is not objectionable. It can be a disadvantage if it wastes power via gas lost through the valve prior to reaching the maximum setting. This can allow maximum system pressure to exceed the ratings of the other components.
In circuits that have more than one actuator, it’s probably necessary to move the actuators in a defined order or sequence. Limit switches, timers, or other digital control devices working with sequencing valves can do this. Sequencing valves are normally closed two-way valves, and they regulate the sequence that various functions in a circuit will occur. They resemble direct-acting relief valves, except that their spring chambers are generally drained externally instead of internally to the outlet port like a relief valve. A sequencing valve allows pressurized gas and fluid to flow to a second function only after a priority function has been completed and satisfied first. When closed, a sequence valve allows gas to flow freely to the primary circuit, to perform its first function until the pressure setting of the valve is reached.
The desired sequencing can also be achieved by sizing cylinders according to the load they must displace. The cylinder requiring the least pressure to move extends first. At the end of its stroke, system pressure increases and extends the second cylinder and so on. In many applications, space limitations and force requirements will determine the cylinder size. In those instances, sequencing valves are used to actuate the cylinders in the required order. Sequence valves sometimes have check valves, which permit reverse flow from the secondary to the primary circuit. However, sequencing action is provided only when the flow is from the primary to the secondary circuit. In some applications, an interlock can prevent sequencing from occurring until the primary actuator reaches a certain position. This is done with remote operations.
The most practical components for maintaining lower pressure in a pneumatic system are pressure-reducing valves. Pressure-reducing valves are usually open two-way valves that close when subjected to sufficient downstream pressure. There are subcategories of pressure-reducing valves: direct acting and pilot operated. Direct-acting valves are pressure-reducing valves that limit the maximum pressure available in the secondary circuit regardless of pressure changes in the main circuit. This assumes the work load generates no back flow into the reducing valve port, in which case the valve will close. The pressure-sensing signal comes from the secondary circuit. The valve operates in reverse from a relief valve because they are normally closed and sense the pressure from the inlet. When outlet pressure reaches the valve setting, the valve closes except for a small quantity of gas that bleeds from the low-pressure side of the valve, usually through an orifice in the spool. The spool in a pilot-operated, pressure-reducing valve is balanced hydraulically by downstream pressure at both ends. The pilot valve relieves enough gas to position the spool so that flow through the main valve equals the requirements of the reduced-pressure circuit. If no flow is required during the cycle, the main valve closes. Leakage of high-pressure gas into the reduced-pressure section of the valve, then returns to the reservoir through the pilot-operated relief valve. This type of valve generally has a wider range of spring adjustment than direct-acting valves and provides better repetitive accuracy. However, in hydraulic applications, oil contamination can block flow to the pilot valve, and the main valve will fail to close properly.
These valves are typically closed and are often used to maintain a set pressure in a portion of a circuit, usually to counterbalance a weight. The style of valve is ideal for counterbalancing an external force or to counteract a weight like in a press to keep it from free-falling. The valve’s primary port is connected to the rod end of a cylinder, and the secondary port is connected to the directional control valve. The pressure is set slightly higher than what’s required to keep the load from free-falling. When pressurized fluid flows to the cylinder’s cap end, the cylinder extends and increases pressure in the rod end and shifts the main spool in the valve. This creates a path that allows fluid to flow through the secondary port to the directional control valve and reservoir. As the load is raised, the integral check valve opens to allow the cylinder to retract freely. If it is necessary to relieve back pressure at the cylinder and increase the force at the bottom of the stroke, the counterbalance valve can be operated remotely. When the cylinder extends, the valve must open and its secondary port is connected to reservoir. When the cylinder retracts, it doesn’t matter that load pressure is felt in the drain passage because the check valve bypasses the valve’s spool.