Key insights into lowering system pressure will increase efficiency.
by Mark Gimson

If pressure feeding into a zone needs to be lowered, the traditional method employed is to install a pressure reducing valve. This is a valve that will limit the outlet pressure of a valve to a preset pressure regardless of the flow rate and the inlet pressure.

The location of this valve needs to be in an accessible area—typically in a valve vault, as it will require occasional maintenance.

Q: What do I need to know to choose the right size valve?

You will need to know normal inlet and outlet pressure required and normal flow rates, as well as minimum and maximum flow rates. These numbers will be used to select the correct valve size based on manufacturer recommendations.

The valve engineer will then use those numbers to calculate the right valve size based on the manufacturer’s guidelines and calculations.

Q: What if the inlet pressure to outlet pressure ratio is greater than a 3:1 range?

There is a real possibility that cavitation will occur. That will require one of two things:

a) Two control valves in series and pressure lowered gradually over two stages

b) A control valve with anti-cavitation trim

Q: What if there are two valves feeding into the same zone?

The valves will certainly work but the issue will be ensuring that they both open together. This is something that must be done during commissioning and will require two people, one at each valve. They should communicate with each other to ensure that both valves open together. This will typically involve a slight adjustment to one of the reducing pilots.

Q: Can a valve be installed where the chamber is sometimes underwater?

While this is not ideal, in a pressure-reducing valve situation the valve will continue to operate underwater. If this is a regular occurrence, try using a full stainless steel pilot system.

Q: Why is my valve size smaller than the main line size?

Valves are sized for 20 feet/second (6 meters/second) velocity through the seat area for continuous service. I like to see these valves operating in the 20 to 80 percent range of their stroke, so you can also increase velocity through the very short distance of the control valve than you would normally size your distribution lines for. For this reason, it is common to have a control valve one size smaller than the actual line it sits in. The reduced port valve is a great option for this, as it includes connecting flanges the size of the main line but with the internals of a smaller valve.

Q: Will my valve handle the entire flow range?

Typically not. This is why you will frequently see a smaller reducing valve installed in parallel to the larger valve. This is there to handle the low flows that the larger valve cannot handle. This low flow rate will not be an issue if you use a control valve with the rolling diaphragm technology, as that type of control valve can regulate down to extremely low flows. Bypass valves are not required.

Q: Why do I have different pressure set points on larger and smaller reducing valves in a station?

In order for the smaller valve to take control during times of low flow, this valve will be set about five psi higher than the larger valve.

This ensures that when the pressure downstream starts to drop, this is the first valve that will open.

Remember, it is downstream pressure operating on the reducing pilot that keeps it closed. When downstream pressure drops lower than spring setting pressure, the pilot will open, in turn allowing the main valve
to open.

If the small valve cannot maintain the pressure because flow demand increases, the pressure will drop in the downstream system, thereby allowing the larger main valve pilot system to open. This allows the main valve to
now take flow.

Q: What if the upstream pressure drops below the pressure setting of the pilot?

The pressure-reducing pilot is set by the force of a spring in the top of the reducing pilot. As a normally open pilot, this means the spring is always trying to keep the pilot open.

As the reducing pilot senses the downstream pressure, the pilot will close if the pressure is greater than the spring setting. The pilot opens when the pressure is lower than the set point, releasing the pressure from the bonnet of the main valve and opening the valve. Should the upstream pressure rise, the pilot will start to regulate again to maintain the set point.

Concluding Tips & Recommendations

Installing air release valves ahead of reducing valves is always recommended to eliminate any chance of air in the pilot system. In-line strainers will also help to eliminate the items that frequently get flushed down water lines and eventually get trapped in the control valves. It is much easier to remove a strainer lid and clean out debris than it is to remove a pilot system and control valve bonnet.

If somebody downstream of the reducing valve has the ability to shut off the entire flow quickly (maybe feeding into an industrial complex, line ends with a fire hydrant, etc.), then there is the risk that the valve could be in an open position when somebody closes the flow. This results in high pressure passing through the valve and getting trapped downstream.

A solution to combat this is to have the reducing valve installed with downstream surge control. This assists the valve in closing quickly if the valve pressure downstream raises a few psi higher than the normal set point.

An alternative to the downstream surge control would be to install a pressure relief control valve, but that will depend on the location and the ability to discharge the water safely if the relief valve opens.

In order to set this valve in the field, it would be advantageous to have at least an isolating valve on the outlet side of the valve. An outlet pressure gauge will also be required to set the valve.