Under Pressure?
Give a boost to bigger buildings
By Greg Cunniff, P.E., Taco Inc. P
ity the life of commercial booster pumps, among the least understood, least appreciated and yet hardest working members of the mechanical team on any
building site. Here in America, the demand for comfort and instant gratification rule. This burden is no more apparent, or demanding, than the universal expectation that, at any height within a building or at any distance from the source of water entry, sufficient water pressure be available for any need. The advantage with new buildings is that, with the involvement of a professional design team, a building owner can assume that a smartly designed, fully function- al structure will meet or exceed the expectations of its occupants. But what of a remodeled or expanded facility? An older structure in need of renovation? Or a building plagued with inconsistent pressure from the water supply? Will the domestic water system get the attention it’s due? And will sufficient pressure be available on the top floor for the luxury suite’s multi-jet shower? Welcome to the questions, the life, intricacies and joys
of the modern, pumped world. Supplying large buildings with ample domestic water pressure can be a challenge, though some basic considerations and tenets apply. It’s generally understood that, in tall buildings, the pressure provided by a municipal water system isn’t capable of meeting adequate domestic water system needs beyond a certain height. As an example, municipal water systems typically provide maximum pressures in the range of 100 to 150 psi. This may be sufficient for buildings of 20 to 30 stories. Above this height, a building will require a domes- tic water pressure booster system.
Out with the old
Given today’s market conditions, with reduced new construction, we’ll begin with a look at building renova- tion and some of the variables that may play into provid- ing adequate pressure for a large domestic water system. Consider this: At what point in the life of a building do facility managers decide to replace a domestic water sys- tem or at least the pumps that maintain pressure within it? The answer to that question depends on many factors, including the age and condition of system components, operational performance and efficiency and the cost and long-term advantages of system replacement. One answer may be to conduct a rigorous energy audit
of the property. It’s likely that not one, but several, issues, may play into answering the question of whether or not it’s a good time to get serious about maintaining or replac- ing key components. I’ve visited many buildings where
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high maintenance and energy costs, deteriorating domes- tic water pressure, mechanically fatigued pumps and the limitations of older, proprietary controls leave owners with no easy options. In instances like these, it’s not uncommon for a new pump system to be specified. Of course, one of the key challenges to such “major
surgery,” especially when the building is occupied, is maintaining service to the building’s tenants, even when replacement or retrofit work is being done. If it’s been determined that a booster system should be installed, the next issue to address may be how to supply a substantial- ly varying flow rate with relatively constant flow rate pumps. To accomplish this, manufacturers use several types of pressure booster systems. Open tank storage systems use an open, atmospheric
tank located at the top of the building. These systems are the easiest solution in terms of pump control. Pump oper- ation is controlled by a simple level control in the tank. The tank volume and pump flow rate are typically sized to prevent short cycling of the pump. Since the tank is open to the atmosphere, it’s subject to corrosion. Also, the tank requires additional floor space. As a result, these systems have gradually lost popularity. Closed tank hydropneumatic systems include a closed, pressurized storage tank. The advantage to a “captive air” pressurized storage tank, when compared to an open, atmospheric storage tank, is in the size of the tank. The captive air tank is much smaller than the open tank. Pump control, typically with a pressure switch to activate and stop the pump, is relatively simple. Pressure difference between pump on and off is generally in the range of 20 to 30 psi. Here, too, tank volume and pump flow rate are typ- ically sized to prevent short cycling of the pump. During periods of low demand, the pump may be off for
long periods of time. For this reason, the system is better suited for applications where demand reaches design flow for a relatively short time and is low for a high percentage of time. Even with a captive air tank, however, corrosion can be an issue. Tankless systems are better suited for larger buildings
where the size of a storage tank is prohibitive and the long term maintenance of a storage tank is a consideration. Also, closer control of water pressure is possible with tan- kless systems employing staged or variable volume pumps. The design objective of a booster system is to provide a
stable pressure to the fixtures at the very top of the build- ing and to avoid short cycling of the pumps and shortened
Continued on page 34 June 2011
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