PRV Horror Show
By Timothy Allinson, P.E.
Murray Company, Long Beach, Calif.
This October I will break tradition and deviate from the plumbing horror stories I have told for the past 2 years in the October issue. As much as I hate to break a tradition started, I have not accrued enough foibles over the passed 12 months to produce a complete article, unless I were to use the word "very," very, very, very often.
However, one of the horror-ble plumbing design issues I see on drawings and in the field on a regular basis is the misapplication of pressure reducing valves. It gives me the impression that the issue of PRVs is one that is poorly understood by many in our industry, making a review of the subject warranted.
There are two basic types of PRVs: direct acting and pilot operated.
Direct Acting PRVs
Direct acting PRVs use a spring to apply pressure to the seat of the valve. The seat usually has a 4-to-1 surface area ratio inlet to outlet. The force of the spring on the seat will create a pressure reduction, linear in nature, which can be adjusted in the field on most valves (Figure 1).
Because of the linear 4-to-1 relationship, if the inlet pressure to the valve fluctuates, the outlet pressure will fluctuate 25% of the inlet fluctuation. For example, say the PRV inlet pressure is 100 psi and the outlet pressure is set to 50 psi. If the inlet pressure drops to 80 psi, the outlet pressure will drop to 45 psi. Said differently, a 20 psi drop of inlet pressure will produce a 5 psi drop of outlet pressure. The same holds true if the inlet pressure increases.
Direct acting PRVs also experience a phenomenon called fall-off. The fall-off pressure of the PRV varies by valve size, flow rate, and manufacturer, but it is frequently in the 5 to 20 psi range. Fall-off is the pressure drop that occurs as the valve opens from the static condition. If a direct acting PRV is set for 80 psi in the static condition and has a 10 psi fall-off at 20 gpm, the outlet pressure will drop to 70 psi during flow.
Direct acting valves are typically available in sizes 1/2" to 3" and come with a range of options, such as strainers, internal bypass, and end connection patterns. These valves are considerably less expensive than pilot operated valves.
Pilot Operated PRVs
The alternative to direct acting PRVs are pilot operated PRVs. These valves use small pilot sensing lines to apply or remove inlet water pressure to the diaphragm of the main valve. The valve will maintain a constant outlet pressure regardless of inlet pressure fluctuations (Figure 2). Simply said, these valves are much more precise than direct acting PRVs and that precision comes at a premium.
Pilot operated PRVs have zero fall-off pressure, which is another of their benefits. However, this statement is a little deceiving. In order to operate properly, most of these valves require a 10 psi differential between inlet and outlet, so in a sense they have a 10 psi fall-off. However, that pressure differential does not vary as it does with direct acting PRVs. If the inlet pressure is 100 psi and the outlet is set to 80 psi, the valve will maintain 80 psi regardless of flow from static to the valve's maximum rating. The only exception would be if the inlet pressure were to fall to less than 90 psi, in which case the valve would not operate as accurately since the minimum 10 psi differential had been compromised.
How to Choose?
So when do you specify direct acting PRVs and when do you pay the premium for pilot operation? This is where my "horror show" title reference comes into play.
Say you need a PRV rig to protect a street pressure zone of fixtures against an elevated water supply pressure. All the rig needs to do is reduce pressure from 130 psi to something comfortably below the 80 psi Code maximum. Since the pressure criterion is flexible, direct acting PRVs would be the appropriate choice since the fluctuations inherent to their operation would not be problematic.
In contrast, say you need a PRV rig to control the pressure to a 10-story zone in a high-rise condo. The rig needs to be set to deliver a maximum of 80 psi on the bottom floor of the zone. The top floor of the zone would see 41 psi static since it has a 90-foot (39 psi) static loss. If a direct acting PRV rig were used (which I have seen many times), that 41 psi could easily drop to 25 psi or less during peak flow condition just due to the PRV fall-off, neglecting pipe friction. That would be unacceptable for most water closets and pressure balanced showers. In this case the cost of the pilot operated PRVs would be warranted - either that or the building should be re-zoned with fewer floors per zone, and that would surely be more expensive.
Another Example
Yesterday I went to look at a building that had been experience pressure fluctuation problems for many years (presumably since its construction in 1988). The building had two direct acting PRV rigs, one north and one south. The north PRVs served all of the building's hot water and a portion of the cold water. The balance of the cold water came from the south PRV rig and coupled it with the hot water from the north PRV rig, creating what could be called a mixed pressure zone.
In this arrangement, if the north PRV rig were experiencing maximum flow and fall-off while the south PRV rig was experiencing minimal flow and fall-off, the differential at the mixed pressure zone could be as much as 25 to 30 psi less on the hot side than the cold. As system dynamics reverse, the cold side could suddenly be 25 psi less than the hot side. Such extreme swings would could dramatic flow and temperature fluctuations at the fixtures.
If the PRV rigs had been provided with pilot operated PRVs, the hot and cold pressures at the mixed-zone would only vary minimally due to pipe and water heater friction differences. Our recommendation for this building will be to replace the direct acting PRVs with pilot operated.
Pumps & PRVs
Many if not most pump manufacturers provide their packaged pump sets with pilot operated PRVs standard on the outlet of each pump. This is to ensure the pumps will deliver the intended pressure without subjecting the system to over pressurization from pump shut-off pressure or a rise in suction pressure.
If said pumps were supplying a high-rise building, the pump PRVs would waste energy that could potentially be delivered and stored at the top of the building. In such a case the preferable arrangement would be to have non-slam check valves on the pumps with the system pneumatic tank at the top of the building and a pilot operated PRV rig downstream of the pneumatic tank (Figure 3). Such an arrangement allows the tank to absorb all of the pump energy and store it for delivery during low-flow conditions.
This arrangement carries little cost premium since the PRVs that would have been purchased with the pumps are instead purchased separately for installation at the top of the building. Pneumatic tank storage is maximized at the top of the building where it operates at lower pressures than it would down at the pumps. Lastly, friction is minimized and pipe sizing optimized since the PRVs are much closer to the upper-most fixtures.
Think Before you Spec
So next time you are selecting PRVs, be sure to think about the system requirements before you spend money unnecessarily on pilot operated valves, or save money foolishly on direct acting valves.
Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design-build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim 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. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.