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Booster pumps Continued from page 32


life. Calculating the pressure requirement for tall build- ings is a function of several variables. The booster system pressure is the sum of the height of the building, the resid- ual pressure at the top of the system for operation of plumbing fixtures and the friction losses in the piping. From this total, one can subtract the available municipal water pressure. As an example, using a 40-story building with a 12 foot


floor-to-floor profile, the static height of the building is 480 feet or 205 psi. For flush valve fixtures, a residual pressure of 20 psi is acceptable. Friction losses could be anywhere from 10 to 25 psi, considering backflow pre- venters and piping. Using the higher figure gives a required pressure of the booster system of 250 psi. If the municipal water system has an available pressure of 100 psi at maximum demand, the booster system would be sized for 150 psi. I’ll add that the pressure rating of piping and fixtures


must be taken into account. As a result, tall buildings can be divided into pressure zones to limit the maximum pres- sure in any one area of the piping system. In order to size the pumps, we also have to size the flow


rate through the booster system. This is generally done using fixture counts or fixture units and converting this to


and flow rate, you can make an intelligent decision about the pumps required and how best to control them. There are several ways to convert a constant flow rate to vari- able demand. This could be accomplished with equal sized pumps in a duplex or triplex configuration, although, if the pumps were of equal size and there was a wide variation in flow (as in an apartment building), it would be difficult to keep the pumps from short cycling. Systems using on-off control typically use a smaller


(jockey) pump in combination with a larger pump(s). The jockey pump is controlled to maintain the pressure at low flows. The larger pumps (in combination with the jockey pumps) are controlled to maintain pressure at high flow. Widely regarded as the best solution, especially now,


considering the much more competitive price of variable frequency drives (VFDs) for pumps is to operate pumps in combination with VFDs. With variable speed/variable frequency drives, duplex and triplex systems can use pumps of equal size and best efficiencies. In essence, the pumps are sized for maximum efficiency rather than maximum flow. Municipal water systems are also converting from


open atmospheric water towers to variable speed systems for the same reasons, abandoning water towers for new uses.


Other considerations The other key element in a contemporary pressure


booster system is the control function. The control system consists of a remote-mounted pressure sensor or trans- ducer, a controller and a controlled device or pump. The pressure transducer converts a mechanical signal to an analog signal. The signal is sent back to the digital pump controller, which compares this input to the system pres- sure setpoint. This differential is converted to a control signal and sent to the pump’s variable speed drive to alter pump speed. Digital controllers can now use a time function in the


Affordable advanced technology provides optimal control and energy efficiency. Pictured: Taco CI triplex booster set, model number TB-CI-VS.


flow rate or gpm, with formulas that are available in plumbing codes or the ASHRAE Guide. The conversion of fixture units to flow rates is accom-


plished by use of the Hunter Curves, formulas that convert fixture units directly to gpm. The curves are based on use patterns observed in actual buildings and take into account the demand factor for domestic water systems, since not all the fixtures will be calling for water at the same time. What we want to avoid, of course, is oversizing the


pumps, because that leads to unnecessary expense. Oversizing pumps could also shorten equipment lifespan if the pumps short cycle frequently.


Constant or variable? Once you’ve calculated the maximum needed pressure


Page 34/Plumbing Engineer


control algorithm to prevent hunting of the VSD, a con- dition in which the drive ramps up and down trying to reach the control pressure setpoint. This is generally a result of using just a P or proportional control algorithm. With digital controllers, this can take the form of an inte- gral function in a PI (proportional plus integral) con- troller or an additional derivative function in a PID (pro- portional plus integral plus derivative) controller for more stable operation. Another benefit of variable speed control is energy


consumption. Pumps can now be selected based on their best efficiencies. This is a big improvement when com- pared with pump selection based on maximum system flow rates. Next time you face a need for domestic water booster


pumping, consider a variable speed booster pump system to efficiently deliver water to the top story of a building. Affordable advanced technology provides optimal con- trol and energy efficiency. n


Greg Cunniff, P.E., is application engineering manag- er for Cranston, R.I.-based Taco Inc.


June 2011


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