Challenging plumbing systems demand a pressure boost
Booster pumps elevate water pressure and flow to meet the water requirements of larger facilities.
Keith W. Seier, PE, CPD, Environmental Systems Design Inc., Chicago
From office buildings to retail centers to hospitals and commercial structures, packaged booster pump systems help large facilities meet the unique pressure and flow demands required of their plumbing fixtures, specialized plumbing equipment, irrigation systems, and HVAC equipment. By increasing the water pressure, the booster pump moves the required amount of water from the building’s water source to the plumbing distribution system.
Each building type has varying flow demands and pressure requirements. When the water pressure from the local water source falls short of meeting the required flow pressures at the fixtures, a booster pump is required by code. Flow pressure consists of the supply pressure from the municipal water supply, the static height of the most remote fixture, the minimum residual pressure at the most remote fixture (up to 45 psi for blowout-type water closets with flushometer valve per the International Building Code), and friction losses through the booster pump, distribution system, and any pressure-regulating valve (PRV). PRVs are included on high-rise buildings where multiple pressure zones are required. Pressure losses through the PRVs must be accounted for when sizing the booster system.
Flow demand is the amount of water, measured in gallons per minute (gpm), required to meet the specific fixture and equipment demand at any given time. The most popular method to determine flow demand is to use water supply fixture units and Hunter’s Curve. Hunter’s Curve is a probability curve that converts fixture units to the equivalent volumetric flow rate in gpm. Peak flow demands are experienced at different times of the day based on building type. For example, condominium buildings might experience peak demands between 6 a.m. and 9 a.m. when residents are getting ready for their day, while a sports stadium would experience peak demands at half-time for football or between innings at a baseball game.
Simple booster pumps
Back in the 1900s, a building’s plumbing system commonly lacked efficiency and sanitation. Booster pump arrangements typically included constant speed pumps that fed a water tank on the roof of the building. The water tanks were elevated above the roof to create enough static pressure to operate fixtures and equipment on the top floors. While most water tanks have been decommissioned due to bird nesting and unsanitary conditions, they still grace the top of some high-rise buildings in large metropolitan cities.
In the 1950s, PRVs were introduced to help stabilize the pressure at the discharge side of the booster pump. Hydropneumatic tanks became prevalent as well. The hydropneumatic tank moved rooftop water tanks from outside the building to inside the penthouse or mechanical rooms. The pumping systems of yesterday met the demands for pressure and flow but lacked efficiency. The booster pump would turn on “full throttle” to meet the design pressure and the PRVs would knock it down to meet the actual building demand pressure, wasting energy and money. Variable speed pumps and controllers became available in the late 1960s and early 1970s, but they were very cost prohibitive and were typically as much as two-and-a-half times the size than those of constant speed, lacking the technology for precision control.
The most prevalent booster pumps of the past consisted of split case, vertical turbine, and end suction pumps. Split case pumps were installed vertically or horizontally and could be found in single or multistage arrangements. Split case pumps were typically installed in high-flow, low-head applications. Due to the high costs of the cast iron body and bronze impeller, split case pumps were, and are still today, used mostly in municipal projects.
Vertical turbine pumps, typically used for high-pressure applications with high shut-off heads, can also be installed in the vertical or horizontal position with an air-cooled or water-cooled motor. To meet high-head demands, vertical turbine pumps can be multistage, meaning multiple centrifugal pumps can be joined in series by combining multiple impellers into a common shaft. These pumps are primarily used in high-rise applications such as offices, condominium buildings, or hospitals.
End suction pumps, once again, can be mounted horizontally or vertically and are typically used for low-to medium-head applications. End suction pumps are easy to maintain and are very reliable for typical commercial applications including mid-size office buildings, low-rise condominium buildings, or multi-use facilities.
Yesterday’s pump controls were simplistic, without much more than level or pressure switches for on/off operation. A typical arrangement included: overload cut-off, remote alarm centers reporting a low-flow or low-level condition or a non-operational pump, and amp current sensor relays to stage the pumps. Per the IBC, a low pressure cutoff was and still is required on all booster systems to prevent negative pressure on the suction side of the pump when a positive pressure of 10 psi or less occurs on that side. Before the days of building automation system connections, alarms consisting of a light or horn would alert the building engineer on-site to any issues requiring attention.
The pumps in this era were installed with a smaller lead pump that would run constantly while one or two larger pumps ran as needed for peak demands. The size and number of pumps was dependent on the size and height of the facility, municipal water pressure, and the water pressure requirements at the most remote fixture or piece of equipment. Plumbing systems in the past also had larger capacities prior to the Energy Policy Act of 1992. Toilets were 3.5 gal/flush compared to the 1.6 gal/flush today, while lavatories functioned at 2.2 gpm instead of the 0.5 gpm used today.
Intelligent, efficient booster pumps
In the last decade, booster pump technology has improved dramatically, with the biggest technology advancement coming from variable frequency drives (VFDs). VFDs allow for varying flow and water pressures to accommodate only the momentary demand, dictated by the building. Like all technologies over time, the economies of scale have reduced the cost of VFDs to be comparable to the cost of the traditional constant speed system.
Advantages of VFDs over constant speed drives include: eliminating energy losses of PRVs and the high-head conditions as flow decreases, extending motor life without constantly being on, reduced cost of operation over time, no in-rush current, extended life of the motor bearings and pump seals, minimizing “water hammer” in the system, and new pump efficiency rates as high as 80%.
Today’s split case, vertical turbine, and end suction pumps have remained primarily the same, but variable speed pumps are much more common than the older constant speed version. The controls have also remained relatively the same, but now include control sequencing, setpoint control, monitoring, and the automatic alteration of pumps. These new controls allow the booster pump to deliver greater energy and operational efficiencies, which provide an increase in energy savings.
The pressure of the future
Tomorrow’s plumbing systems will be challenged to meet increasing demand and pressure requirements within an aging municipal infrastructure. Green initiatives like the U.S. Green Building Council’s LEED Rating System and Abu Dhabi’s Estidama Pearl Rating System have made low-flow fixtures popular in the U.S. and throughout the world. However, these low-flow fixtures are requiring higher amounts of pressure. In fact, the newest demand pressures are as high as 50 psi. The low-flow fixtures substitute water with high pressure to create a similar experience while using a fraction of water. This may eventually lead to a demand for more pressure zones within the building as typical pressure zones increase from 30 to 50 psi while the maximum pressure remains at 80 psi. It will also lead to larger booster pumps as pressure requirements at the top of the building continue to increase.
Aging municipal infrastructure is also a major concern, especially in large metropolitan cities. In some areas, the city water mains cannot hold the current water pressure demands. In order to reduce the stress on the infrastructure, including the avoidance of breaks in the water mains, the city water pressure is lowered. Cities like Chicago still have some wooden water mains within their system. While water departments throughout the U.S. continue to update their infrastructure, there are not enough dollars allocated to fix the entire domestic water supply system. Inevitably, this will result in the need for booster pumps in more applications.
Tips for sizing a booster pump
- Take seasons into account. For example, water pressure is usually higher in the winter months and lower in the summer months due to irrigation and cooling tower loads.
- Calculate the total plumbing fixture load using Hunter’s Curve or another approved method. Include the cooling tower load and other miscellaneous mechanical loads in the calculations. The engineer needs to use adequate judgment and should consult the owner on whether the booster should be sized to handle 100% of the capacity or be reduced since the probability of all fixtures operating at the same time is extremely low.
- Find out the static and residual water pressures from the local municipality. Be sure to include losses from the water meter and backflow preventer when sizing the booster pump.
- Size hydropneumatic tanks to be large enough to handle low-flow conditions. Remember that low-flow conditions for a hospital are much larger than for an office building, where most of the tenants leave at night.
- Specify steep curves (diagonal vs. flat curve), which allow the pump to find the optimal flow and pressure condition.
- Make sure there is enough—but not too much—pressure at the top of the building, especially for the cooling tower. The engineer must also account for any losses through the backflow preventer.
- Balance the correct number of pumps for desired redundancy and the building type.
- Choose the appropriate electrical feed(s) to the pump. Critical facilities may require dual electrical feeds or emergency power to the booster pump so that it can remain operational during an electrical shortage or power outage.
- Power and redundancy requirements will need to be researched at the building with the facility engineer to ensure compatibility prior to the upgrade or replacement.
- Coordinate the timing of the pump replacement with the owner to minimize tenant disruption.
- Maximize the efficiency of the pump system by providing a hydropneumatic tank sized to meet the low-flow conditions of the building.
Understanding booster pump retrofit
To retrofit the booster pump in an existing building, the engineer must complete an initial assessment of the current system. The first step is to do a fixture count on the facility to find the flow requirements for the building. When existing floor plans are not available or if the tenant spaces are difficult to access, placing a flow meter on the discharge side of the existing pump system is another way to measure the flow requirements for the building. When using a flow meter, take readings for a two-week period to achieve more consistent results. Take into account the season and occupancy of the building and adjust your flow calculations accordingly to size the booster pump correctly.
The second step is to check the pressure requirements for the building. Typically, the head pressure matches the existing pump system unless the building is experiencing pressure problems on the top floors.
The third step is to check for a hydropneumatic tank in the penthouse or on the pump skid. If the existing pump system does not have a hydropneumatic tank, it is recommended to provide one in the new installation to manage the building’s low-flow conditions. Hydropneumatic tanks come in all shapes and sizes, but 120- and 240-gal sizes are the most popular. The hydropneumatic tank acts as a buffer, slowly releasing water to the system in low-flow condition. This prevents the booster pump from short-cycling and helps extend the life of the booster pump.
We should expect booster pumps to become more common, such as in residential buildings, and to increase in size due to increased pressure requirements. Additionally, we should also expect critical and institutional facilities to require commissioning on all booster pump installations. These types of facilities (hospitals, data centers, etc.) rely on the domestic water supply for their operations and cannot have a booster pump failure. The commissioning agent makes sure the booster pump is running as designed and that controls are operating per the specifications. On the controls horizon, expect fancier hardware and more compatibility, but similar system functionality down the road.
Seier is vice president and a group leader for the Plumbing and Fire Protection Engineers Group at ESD and has more than 10 years of experience supervising plumbing/fire protection engineers, developing design concepts and specifications, policies, standards, and procedures for the department. Seier is also a board member of the Chicago Chapter of American Society of Plumbing Engineers.