To pump or not to pump?

By Timothy Allinson, P.E.,
Murray Co., Long Beach, Calif.

Recently, my firm bid on a design-build project, a medical office building on a fairly large medical campus. The bridging documents (the drawings and other materials used for bidding the project) did not indicate domestic water pumps nor did they indicate a backflow preventer on the water service.

The building will be four stories in height, but the absence of a backflow preventer (BFP) was not surprising, since the entire campus has central backflow preventers on the main water services. The absence of domestic water pumps was also not surprising, since the water pressure was indicated to be 90 – 92 psi. In bidding the job, my estimating department figured a central pressure regulating station to protect the lower floor fixtures from exceeding the code maximum of 80 psi.

After a lengthy proposal process, we were finally awarded the job. As the details began to unfold and we started communicating with the utility providers, we learned that the street water pressure had a range of 80 – 90 psi, which, on the low end, is quite a bit less than the 90 – 92 psi indicated on the bridging documents. We checked the water pressure with a gauge downstream of the central backflow preventer; it read 75 psi, which would be consistent with the water utility’s figures after a 10 –12 psi drop through the BFP.

Further investigations into the details of the job revealed that there was an RFI issued during the bid by one of our competitors, questioning the need for a BFP serving the building; the response indicated that a BFP should be provided. Now, I’m sensing a water pressure problem.

If the city water pressure ranges from 80 to 90 psi, we have to design around the worst case scenario. We have to assume low pressure for design purposes but also safeguard the building against the higher static pressure.

With 80 psi in the street, if the central BFP loses 12 psi, and the building BFP loses another 12 psi, the dynamic low pressure to the building will be 56 psi or less, once you account for friction, water meter loss and the like. The building is only 45’ to the top floor slab, which is just under 20 psi, so the dynamic pressure at the top floor would be 36 psi or lower. This is not even accounting for friction within the building.

Since most flush valve fixtures today require 35 psi — which closely approximates our available pressure — there is no pressure left over to account for friction, and there is no basis for sizing piping without a friction factor. Further, since the static pressure condition to the building is in excess of 80 psi, a pressure-regulating station will be required to avoid static over-pressurization, which will add even more pressure loss to the system.

One can make the argument that, since there are central backflow preventers, the utility main is already protected, so no BFP should be required for the building. If the building BFP were deleted, that would allow an additional 12 psi for friction, which is a manageable scenario.

If the building BFP cannot be deleted, then pumps will have to be added. Since the pumps will be low pressure and commensurately small horsepower (5HP duplex in this case), a VFD pump set can be provided at a reasonable cost. VFD control is desirable for this scenario, since the pump pressure demand will be marginal and variable in nature.

If the booster pumps are utilized, the building still requires protection against static over-pressurization; the most economical means of achieving this is to fit the pumps with pressure-regulating style discharge check valves set to 80 psi. This is unconventional for a VFD controlled pumps set, since pressure is controlled with pump speed and not PRVs. When the pumps aren’t running and the static pressure rises to 90 psi, this pressure would travel through the pumps and to the fixtures if PRVs are not provided.

So, right now we are grappling with two scenarios. The first is a street pressure system with no BFP on the water service and a regulating station to prevent over-pressurization. My preferred method of piping a building with this pressure sensitive condition is to run express to the top to a PRV set and water heaters in the roof mechanical room and then have a down-fed supply to take advantage of static gain in pipe sizing. Refer to the schematic diagram (below).

The second scenario, if the BFP is required, is a pumped system with VFD control and PRV check valves on the pumps. In this scheme I also prefer an express riser to the roof with a down-fed system, since we don’t have a great deal of pressure for friction due to the 80 psi limit at the first floor. Refer to the associated schematic diagram (right).

The two scenarios are not dramatically different, but it is essential to determine early in the program which arrangement the project requires. The architect is not pleased about having to sacrifice program space on the 1st floor for the pumped scenario, but the required pump room is not large, so it is not that big of a deal. The key is to catch potential problems such as this early in the design, as, the sooner you catch them the easier they are to deal with.

 

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.