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Water has the unfortunate quality of being heavier than air. In fact, it weighs 62.4 pounds per cubic foot. This mass requires a pressure of 0.433 psi to lift water one foot (62.4 lbs/144 in in ft). To put it another way, one psi will lift water 2.31 feet (1/0.433). In a single story building with 70 psi in the street, this can be insignificant. In a high-rise building, this factor will drive the design of both the hot and cold water systems.
I don't know why anybody would want to live in the penthouse. The water pressure is dismal up there. To avoid this, the plumbing engineer needs to pay careful attention to zoning the water systems. First, high and low pressures need to be determined. Plumbing codes usually limit the high water pressure to 80 psi. Using 70 psi will result in more manageable flow rates at the fixtures, reduced water hammer and lower velocities. These characteristics will result in lower operating costs and a longer life of the system.
Codes often limit the low water pressure to 20 psi, unless there are fixtures such as flush valves that require greater pressures. Nevertheless, a minimum pressure of 40 psi is recommended for the comfort of the end users. With a pressure differential of 30 psi, a zone can be no more than 69 feet in height (30 ft x2.31 ft/psi). Using a typical floor to floor height, for a hotel, of 11 feet, no more than six floors can be served by a single zone.
The next step is to determine the system pressure. The suction pressure can be determined by adding the street pressure and the elevation gain (assuming your booster pump is in the basement). Adding the anticipated losses including friction, elevation and PRV falloff to the minimum pressure results in the system pressure. Subtracting this from the street pressure yields the boost pressure. The manufacturer will also need to account for internal losses in the booster pump system.
Booster pumps today can be configured in any number of ways. With advancements in pumping technology, vented roof tanks are a thing of the past. A constant speed pump, carefully calculated, could operate without PRVs. If so, PRVs might be required at the top floor, and shutoff head must be checked. Shutoff head is the system pressure resulting from the demand approaching zero. It can be determined by adding the suction pressure to the pressure indicated on the far left end of the pump curve. In some cases, this pressure can exceed the capacity of the piping system. If PRVs are provided on the pump discharge, problems with shutoff head can be eliminated outside of the booster pump package but must still be checked within the package. A better solution is a variable speed booster pump. By tracking pressure, flow or electrical current, a variable speed booster pump can deliver constant pressure at any flow rate. This provides a more predictable system pressure and saves electricity at the same time.
Regardless of pump type, the lower zones in a high rise will need PRVs.
In most cases, for economical reasons, direct acting PRVs are used. A more consistent pressure can be maintained by using two valves piped in parallel (figure 1). The smaller valve may be sized to handle 1/3 of the flow rate at an acceptable falloff pressure. The larger valve is then sized for 2/3 of the flow rate at the same falloff pressure. If the smaller valve is set for 75 psi and the larger valve is set for 70 psi, then under low flow the larger valve will be closed and the smaller, more accurate valve will regulate the pressure. A relief valve is required downstream of the PRVs and will require an indirect waste receptor, which is often overlooked in the design of these stations. In many cases, the lowest of all zones may not require a boost in pressure. If so, a separate branch in the main, prior to the booster pump, could serve several lower floors, saving installation and utility costs.
The maximum number of floors that can be served depends on the materials used. The booster pump, valves, piping and appurtenances must all be capable of handling the maximum pressure at the base of the riser. Understanding pressure ratings can get quite involved. Bronze, threaded, class 150 valves are limited to 200 psi at 150 F, while the more expensive class 200 valves are limited to 400 psi. Iron, class 125 valves up to 12" in size are also limited to 200 psi at 150 F, while the more expensive class 250 valves are limited to 500 psi. The correct valves must be specified in the booster pump package and in the piping system, at least for the lower floors.
At higher floors, the pressure falls; good practice is to reduce the class of valves when a safe working pressure has been reached. Pressure gauges and other small devices are often overlooked, along with, surprisingly, the piping. The maximum safe working pressure of 6" hard drawn copper tube at 150 F is 376 psi, and the maximum gauge working pressure of the solder joint (assuming 95-5 tin-antimony solder) is 375 psi, but the rated internal working pressure of the fitting is only 213 psi. As such, serving more than 40 floors can be difficult at best.
One solution, to add a few more floors, is to use stainless steel pipe. The typical joint working pressure of schedule 10S can be 300 psi and schedule 40S can be 600 psi depending on the couplings used. When serving even taller high rise buildings, a secondary pumping station must be used (figure 2). In this scenario, a lower pump serves the bottom half of the building and also feeds the suction side of the higher pump, which in turn serves the top half of the building.
Particular attention must be given to the simultaneous control of these pump sets; a buffer tank may be necessary to maintain a constant suction pressure at the higher tank Alternatively, two lower pumps can be provided, one for the lower fixtures and one to feed the higher pump. This separation of the upper and lower building systems will allow for more independent control over pressures and can be useful for maintenance.
In most high rises, the water is pumped up to the PRV stations that are located at the top of the zones. The downstream risers and branch piping then downfeed to the fixtures. This decision, however, can be affected by the type of fixtures and the location of the hot water heater. In an upfeed system, the pressure loss due to friction and the pressure loss due to elevation are additive; the worst case is the top of the system where the pressure is lowest. In a downfeed system, at least for smaller pipe sizes, the friction pressure loss will be somewhat offset by the pressure gain from downfeeding. Also, since the friction loss is greatest at the bottom of the system where the pressure is greatest, smaller branch pipes can be utilized. The result is a more consistent static and dynamic pressure, providing a better experience for the end user.
An economic analysis often reveals that the cost of the express riser, the upfeed pipe that has no connections, is less than the savings from the smaller branch piping. It is strongly recommended that the hot and cold water in any building feed in the same direction. Otherwise, the cold water friction losses may be at a minimum where the hot water friction losses are at a maximum. Even with pressure balancing shower valves, a differential pressure of 50% could have disastrous results. If the water heater is on the roof, a downfeed system makes good sense.
The design of hot water systems is outside of the scope of this article. It would be unfair, however, not to address hot water circulation from several zones of different pressures. Engineers often return this hot water to the central water heater. Doing so can create a system that is very difficult to balance. Even when each zone is protected by a check valve, the pressure from the higher zone will often prevent the lower zones from circulating at all. A better approach is to circulate within each zone (figure 3). A fractional horsepower pump and a small electric tank type heater work well. Five gallons and three to nine kilowatts will handle six floors of almost any footprint since the water is only reheating from 110 F to 120 F. Since the pressure is already reduced, the circulating pump and reheat tank can be placed on any floor. Don't forget about the main hot water riser. It must still be circulated back to the central system to ensure that this large column of water does not get cold overnight.
One final issue to consider in both hot and cold water distribution is the ability to purge air from the system. There are automatic air vents that leak and fail, and there are manual air vents that are soon forgotten, but the best way to purge air from the system is simply to provide horizontal distribution on the floor below the highest floor in each zone. This allows the air to collect in each riser and float to the top, where it is purged every time that a fixture on the top floor is used. This is rarely noticed by the end user unless the fixture is seldom used. Once again, why would anyone want to live in the penthouse?
Peter Kraut is a licensed mechanical engineer in 20 states. He founded South Coast Engineering Group, near Los Angeles, California, in 2001. He designs plumbing and HVAC systems for commercial projects including high rises, hospitals and even amusement parks. He can be reached at (818) 224-2700 or via e-mail at firstname.lastname@example.org.
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