Under pressure

Over the years, I have learned that there is a significant amount of confusion about the application of pressure reducing valves, and which valve to use for various applications. I will try herein to clarify any uncertainties. (Notice how fancy I sound when I use the word “herein” – don’t let that fool you.)

Most of us in the industry know that the code dictated maximum pressure to plumbing fixtures is 80 psi. So, if you have a house such as mine, where the utility (street) water pressure exceeds 80 psi (130 psi in my case), a pressure reducing valve will be required.

In the case of my home, a direct acting PRV is the appropriate choice, because:

1. Delivery pressure is not critical. The intent of the PRV is to stay below code maximum.

2. Minimum pressure is not an issue. As long as the PRV protects the fixtures from the 80 psi maximum, the, low-end pressure is not of concern since static and friction losses are minimal.

3. Money is an object. The valve for this application should be economical.

For the sake of clarity, a direct acting PRV is a simple mechanical device with proportional inlet and outlet pressure using a spring and diaphragm. The bonnet of the valve contains the spring and provides access to the diaphragm. The top of the valve has a nut and lock bolt. As the nut is tightened (screwed into the valve), the outlet pressure increases. As it is loosened, the pressure is reduced.

Most direct acting PRVs have a preset pressure range, depending on the spring inside the valve. Typically the range is 25-75 psi outlet pressure (as in the case of my house). But, they can be ordered for different pressure ranges, depending on the application.

These valves are typically of bronze construction. The material of the diaphragm can be critical depending on water quality. Here in Southern California, where we have issues with chloramines in the water, PRVs with Buna-N diaphragms tend to only last a few years before the diaphragm must be replaced. The chloramines, it seems, attack the Buna-N rubber.

I have refurbished my share of Buna-N PRVs, and after a few years the black rubber of the diaphragm disintegrates and leaves oily smudges all over your fingers when you handle it. Some valves are available with EPDM diaphragms, and I believe those might stand up better to certain water qualities such as ours. Other valves, I believe, are made with a material called Viton. This might also be a better material for chloramine infested water. I am by no means a chemist, so talk with your local sales representative or the manufacturer if you have need to explore this subject further, and provide them with a copy of the water analysis for use in selecting the appropriate material.

Where low-end delivery pressure is critical, the alternative to a direct acting PRV is a pilot operated PRV. These valves are made by a well-known company called Cla-Val (manufactured right here in beautiful Newport Beach), as well as Zurn, Watts, and probably others.

The concept behind a pilot operated PRV is that it uses downstream pressure and a small pilot valve to control the action of the pressure reducing diaphragm to keep downstream pressure constant regardless of fluctuating inlet pressure. These valves are an order of magnitude more expensive than direct acting PRVs, but their cost premium is warranted when delivery pressure is critical. Direct acting PRVs have a significant fall-off pressure that increases with flow. If a direct acting PRV is set for 50 psi under static conditions, as flow initiates the delivery pressure will immediately fall by 5 to 10 psi, and the fall off will increase as flow increases. Pilot operated valves do not have this fall-off pressure. When the valve is set for 50 psi, it will maintain 50 psi regardless of flow (within the limitations of the valve size). The only caveat is that the inlet pressure has to be at least 10 psi greater than the set outlet pressure.

So, if you have a PRV rig controlling the pressure to a 10-story zone of a residential high-rise with 9-foot, 6-inch floor-to-floor, or 95-foot static, and you want to maintain 35 psi minimum on the top floor of the zone without exceeding 80 psi at the bottom of the zone, then a pilot operated valve would be the best choice. Actually, two pilot operated valves in parallel, one smaller to handle low flows, and one larger to handle higher flows. Small valves are better at controlling pressure for low flows, while large valves are better at controlling pressure for higher flows. Typically, in this scenario you should set the smaller valve two psi higher than the larger valve. This encourages the water to flow through the smaller valve during low flow conditions. As flow increases and pressure falls off due to friction, the water will be directed hydraulically to flow through the larger valve.

If one were to use direct acting PRVs for the example, the valves would be set for 80 psi on the bottom floor, producing about 38 psi on the top floor. As flow initiates, the pressure on the top floor would immediately fall off to 30 psi or so. As flow increases, the top floor pressure would be further reduced to 25 psi or less, and this is not an acceptable pressure for pressure balanced shower valves, or most of today’s low flow water closets.

One warning of caution: while direct acting PRVs are typically of bronze construction, pilot operated PRVs are ductile iron. They are also available in bronze and stainless steel at a significant cost premium. Normally, these valves should be specified as ductile iron with NSF 61 epoxy coating. It is also wise to spec the trim and pilot system as stainless steel, since these small components don’t stand up well under poor water quality if they are standard bronze.

Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He holds a bsme from Tufts University and an mba from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed accredited professional. Allinson is a past-president of aspe, both the New York and Orange County Chapters. He can be reached at laguna_tim@yahoo.com.
 

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