School of hard knocks

By Timothy Allinson, P.E.

Murray Company, Long Beach, Calif.

Some people hate to admit their errors. This is usually because of insecurity or ego. On rare occasion I think it is the result of a unique form of intelligence — an unshakable belief that an error is not possible. I had an old boss who never made a mistake, because even when he did, he could convince an entire room full of very bright people that he had not — something or someone else was to blame. Unfortunately, I don’t play that game very well. In fact, I never even try to play it. I tend to wear my mistakes proudly like war wounds or badges of honor. Call me stupid, but there it is.

 

Often when one has made a mistake designing a plumbing system, the symptom of the mistake shows up as noise. Water hammer, cavitation, pipe vibration — all of these produce the symptom of noise. Left uncorrected, that noise, which is usually produced by unfriendly pressure conditions, can lead to an even worse problem such as a pipe rupture and flood, pump failure, valve failure, etc. Where there’s smoke there’s fire, and in plumbing, where there’s noise there’s pressure problems.

 

I’m a big fan of pilot-operated pressure reducing valves (PRVs), such as those made by ClaVal, Watts and Wilkins. They are not cheap, but I find the fact that they maintain a constant downstream pressure regardless of inlet pressure and flow fluctuations invaluable and well worth the premium, especially where pressure is sensitive to the fall-off associated with direct-acting PRVs. One thing you do have to be careful about with these valves — as with any PRV — is cavitation. If you ask the valve to reduce pressure to an excessive degree you enter the cavitation zone — first the cavitation noise zone followed by the cavitation damage zone as pressure differential increases.

 

The other day I got a call from the foreman on one of our projects — a recently completed building that was already in operation. He was busy balancing the hot water return system when he heard an alarming sound coming from the PRV closet. He opened the closet and found that at least one of two PRVs on the cold water PRV station making a lot of noise. As soon as he told me this I knew it was cavitating but found it strange that it was only occurring on the cold water rig and not the hot water rig.

 

A check of the design drawings revealed my mistake. The hot water rig was reducing pressure down-fed from a rooftop PRV rig that delivered a steady 84 psi to the hot water PRVs. In contrast, the cold water PRV rig was being supplied by an unregulated pump discharge riser. At shut-off head, this floating riser could supply as much as 160 psi to the cold water rig. Outlet pressure for both rigs was set to about 37 psi. Immediately I realized that the CW rig was in the cavitation zone during low flow conditions as the pumps approached shut-off head. Oops.

 

The first thing I did was solicit the help of the manufacturer, in this case ClaVal, which is made locally here in Newport Beach. The engineer I spoke with immediately confirmed that the valves were cavitating, and just as quickly shocked me by saying, “You’ll need an anti-cavitation kit for both valves.” Anti-cavitation kit? I never knew such a thing existed. I feared the remedy was going to be much more complicated and expensive that an anti-cavitation kit for a few hundred dollars. Live and learn. I really dodged the bullet on that one.

 

In a nearby building — another recently completed project — one of the condo owners was complaining of plumbing noise and water flow fluctuations from his fixtures. This particular building had street pressure water supply to ground floor duplex condos with a pumping plant that sent water to the high-rise condominium units above. I suspected the source of the problem was associated with the main water supply, so the first place I looked was at the incoming water service.

This building had a very typical water supply configuration with a 6" backflow preventer (BFP) immediately inside the building, street-pressure take-off downstream of that, followed by the domestic water pumping plant. The pumps were operating normally, so I went to the homeowner’s unit to witness the issue about which he was complaining.

 

The condo owner, a young lawyer with his own practice, turned out to be much nicer than I feared, and not at all threatening (from a litigious perspective). He was thrilled that I was concerned about his problem. He demonstrated the flow and pressure fluctuations at various fixtures. I called my project manager via cell phone who was positioned at the pumping plant. As he turned the pumps on and off, I witnessed the flow fluctuations and knocking noises that correlated with the pump operation. I soon realized that the knocking sound was the reduced pressure zone (RPZ) check valves opening and closing as the pumps cycled, and the flow and pressure fluctuations were the result of the BFP fall-off. 

The easiest fix to this problem would be to take the street pressure zone off the water service ahead of the backflow preventer with a braided flex to dampen the BFP check valve noise. If the units were stand-alone two story structures this would not be an issue, so in theory the presence of the building above shouldn’t matter, but this is yet to be confirmed with the City. The alternative is to repipe the street pressure supply to the pump discharge with a PRV rig. I will soon know the required solution.

 

Speaking of backflow preventers, another interesting problem came up on a project that is worth sharing. Another building recently occupied — a hospital — had its BFPs tested by a registered testing agent and he found them to be discharging to an unusual degree. He promptly informed the facility’s personnel that “the building had major plumbing problems”! This, of course, created a panic and I was sent in to find the problem.

 

Since the BFPs supplied the domestic water pumps directly, this was the obvious place to start my forensics. After watching the pumps operate for a while, turning each of the three pumps on and off, I realized that when the lead pump shut off there was a sudden increase in suction-side pressure. This of course raised the pressure to the BFP outlet higher than the street pressure inlet, causing them to discharge substantially with each cycle of the lead pump.

The lead pump was piped to the pneumatic tank through a ¾" check valve. This check was intended to prevent the pumps and suction piping from being exposed to the elevated pressure in the tank after lead pump shut-off. Since the suction piping was clearly being subjected to tank pressure after pump shut-off, the obvious conclusion was that the ¾" check had failed. This proved to be the case, and a minor fix resolved the “major plumbing problems in the building.”

 

One last note about the same building and backflow preventers: This building also had a ¾" BFP serving a steam gun. The pipe serving the BFP failed one evening causing a minor flood. After fixing the pipe we witnessed the operation of the steam gun and associated BFP. Whenever the steam gun was turned off (via a quick-closing valve) the BFP was subjected to water hammer. This water hammer sent the BFP into a harmonic oscillation with the check valves alternating frantically, spitting like crazy, and the supply pipe whipping around in the basement ceiling like a 100-foot long angry snake. This had been occurring unobserved over previous weeks, so it’s no wonder the supply pipe eventually failed.

Investigation into the BFP literature revealed something I had never known before. The installation manual states (probably so for all BFP manufacturers) that for a dead-end installation serving equipment with periodic flow requirements, a check valve may be required ahead of the backflow preventer. Well, installation of a check valve ahead of the BFP did resolve the harmonic vibration so I recommend doing so whenever you have a small BFP serving a boiler, cooling tower, or any equipment with a quick closing valve. Note that a water hammer arrester after the BFP will not necessarily do the trick.

 

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.