Demand for water in multi-story buildings such as hotels, multifamily, office and other institutional applications — require pressure-boosting equipment to raise incoming municipal water pressure to serve upper floors. Booster systems or packages contain one or more pumps and related accessories and controls.
Until the early 1990s, pressure regulator valves were typically used to control booster system pressure. Many times these pump systems would operate at top speed and “bleed off” excess pressure to reach the desired output. The more energy-efficient option is to design a booster system that ramps up to meet the specific demand.
Today’s sophisticated booster systems integrate multiple multi-stage pumps and variable frequency drive-controlled motors, along with software that adjusts pump speed and the number of pumps in operation to meet frequently changing system demand. These systems are designed to deliver the minimal pump output necessary to achieve optimal performance — all without direct human intervention.
Booster system sizing
In sizing a pressure booster system, we must first determine an application’s flow and head requirements.
For commercial building applications, the flow will be determined by the total number of fixtures or fixture units (Fu) being served (sinks, WCs, urinals, hose bibbs, showers, drinking fountains, cooling towers, irrigation, etc.).
The fixture unit computation is based on the average use of each fixture and its corresponding gpm. Since it’s unlikely that all fixtures will be used simultaneously, methods to calculate a “reasonable” maximum flow have been established. Such fixture unit values may be found in sources such as the ASPE Design Handbook (International Plumbing Code & Uniform Plumbing Code versions) and Engineered Plumbing Design II by Alfred Steele. Within these references, tables provide the fixture unit values for both private installations (residential or multi-family settings) and institutional installations (public spaces with multiple users such as a restaurant) for each type of fixture.
Once the fixture unit values are determined, multiply each fixture type by its corresponding rate to calculate the total fixture unit. A fixture vs. flow curve graph or chart can then be used to figure the overall flow rate (gpm) for the application.
Calculating flow rate (gpm): To better illustrate these calculations, let’s look at a simple example of a 56-unit apartment building that contains the following features:
The first step in calculating a flow rate is to find the sum of each type of fixture and multiple by the number of fixture units assigned to that type of fixture, referencing either the ASPE Design Handbook or Engineered Plumbing Design II:
Project Total: 1,092 fixture units
The second step to determine an application’s flow rate is to locate the estimated fixture units on a flow chart or graph. Using the 1,092 total fixture units in the above example, we find a flow rate of approximately 220 gpm.
For a conservative approach to calculating total fixture units and resulting flow (gpm), cross reference multiple sources for fixture unit values and use the highest count of fixture units when calculating flow (gpm). Variations in fixture units can occur between sources or how different terms are interpreted. For example, some sources calculate the total, simultaneous flow generated from a “bathroom group;” rather than individual flows from a toilet, urinal, lavatory sink or shower.
Once the required flow rate is calculated, we need to determine the head pressure required by a specific application.
Four main elements are needed to determine head:
Using the example above of the 56-unit, 18-floor apartment building, we can calculate the project’s required head as follows. Let me first add the following details: 1) the pump is to be installed in the building’s basement, so there are 18 floors located above the pump, and 2) these are 12-foot floors.
Calculating static head HS: We multiply the number of floors by the height of each floor:
(18 floors x 12 feet per floor = 216 feet)
Then, using the following conversion: 1psi = 2.31 feet of head, we convert this figure to psi by dividing the total distance by 2.31:
(216 feet/2.31 = 93.5 psi)
HS = 94 psi
Calculating friction head HF: Next, we need to calculate friction head HF. To do so we first need to determine the friction loss, or the resistance to flow caused by friction as the water moves along the walls of the pipe, as well as the resistance caused by its own turbulence. Added together, these losses are referred to as friction loss and may significantly reduce system pressure.
Distance, pipe diameter and gpm all affect friction loss and standard friction figures vary between 4 feet to 10 feet per 100 feet of pipe.
For this example, pipe friction loss per 100 feet of pipe is 6 feet, which means we need to overcome 6 feet of head loss for every 100 feet of vertical pipe that we have in the system. Next we need to determine the longest horizontal run (LH) of pipe from the vertical riser. In this example, the longest horizontal run (LH) is 100 feet.
Now we add the HS (static head/lift) value of vertical pipe to the LH (longest pipe run) of horizontal pipe. In our example, this would be expressed:
(216 feet + 100 feet = 316 feet)
Next, because pipe runs are not straight, elbows and other fittings must be taken into account when determining friction loss. Pipe valves and fittings generate a lot of turbulence that can add up to a significant amount of friction. Therefore, as a rule of thumb, we assign a 5 percent loss for fittings by multiplying the longest pipe run value by 0.05:
(316 feet x 0.05 = 16 feet)
In general, friction losses increase as the flow rate increases or as the pipe size decreases (if the flow rate doubles for a given pipe size, friction losses quadruple).
The next value we must find is the total equivalent length of pipe. We can find this value by adding our two previous measurements: the fitting friction loss value (16 feet) and the longest pipe run value (316 feet):
(316 feet + 16 feet = 332 feet)
In order to calculate the total amount of friction loss in our example, we need to multiple the total equivalent length of pipe value and the amount of friction loss that occurs every 100 feet. For this example, the friction loss every 100 feet would be 6 feet. After multiplying these two values, divide the answer by the longest horizontal run (LH), which was previously noted as 100 feet. This value represents the total amount of friction loss:
(332 feet x 6 feet)/100 feet) = 20 feet
Once the total amount of friction loss is calculated, we must convert the value from feet to the correct psi measurement using the following conversion: 1 psi = 2.31 feet of head. To convert the total amount of friction loss value, divide by 2.31:
(20 feet/2.31 = 8.7 psi).
HF = 9 psi
Calculating residual pressure HR: In our example, the pressure needed at the top floor is 30 psi. In real world commercial applications, typical residual pressure (HR) rates range between 20 to 40 psi.
HR= 30 psi
In order to find the required discharge pressure for our example building, we just add the HS, HF, and HR values:
(94 psi + 9 psi + 30 psi = 133 psi).
Just as the residual pressure (HR) is provided for us, the minimum inlet pressure (HI) is also provided for our example. The minimum inlet pressure (HI), or the city supply main pressure, is 30 psi:
HI= 30 psi
Calculating total pressure boost or total dynamic head
In order to calculate the pressure boost requirement for our example, we need to subtract our previous two values, the required discharge pressure and the minimum inlet pressure (HI). In other words, we know the 133 psi (discharge pressure) requirements, as well as the existing incoming (inlet pressure) HI. We simply subtract the two values (133 psi - 30 psi = 103 psi). This is the amount of pressure the booster pump system will need to deliver in order to provide adequate pressure to the building.
Next, we convert the 103-psi boost requirement value into feet by multiplying by 2.31. This conversion changes our pressure boost requirement to the appropriate measure of total dynamic head (TDH), expressed in feet, which is featured on pump selection graphs. This conversion would be expressed as:
(103 psi x 2.31 = 237.9 feet)
After successfully completing each equation and conversion, we would use a pump selection graph to select the booster pump that would meet the project’s requirements.
For our example, we know that the apartment building requires a flow (Q) of 240 gpm and head of 239 feet.
Other selection considerations
But before we search for a booster system that can deliver a design flow 240 gpm at 239 feet head, there are additional factors that influence pump selection.
For optimal pump consistency and efficiency, you should create a flow profile for your building, tracking pump demands and the high usage point throughout your pump’s daily cycle. Typical applications require high rates of flow only 4-6 hours/day. In other words, the apartment building would only need to achieve 240 gpm at 239 feet head for roughly 17-25 percent of each day with the majority of operation at lower volumes.
This daily pump cycle, complete with usage spikes provides additional insight into your building’s pump demands. Usually, high flow rates occur during morning hours, the noon lunch hour, and evening hours when meals are prepared, clothes are washed and showers are taken.
As a result, actual pressure demand is typically less than 20 percent of the design flow, or the flow you calculated by fixture unit, around 70-80 percent of the time. During your pump and booster selection process, you must account for this fact: actual flow will almost never reach calculated flow or design flow. Software is available to help you create a flow and usage profile tailored to your building.
When selecting a booster pump system, you need to pick a system that can achieve the head requirements that your application demands, even though it may only need to reach this design flow for a fraction of each day. Rather than selecting one large pump, consider employing multiple pumps with smaller horsepower. Pumps that are too big can waste energy (kWh/yr), present higher costs, produce excessive cycling resulting in inconsistent pressure.
According to Grundfos, power consumption accounts for 85 percent of all costs incurred during the life cycle of a pump. The initial pump purchase price and the cost of regular maintenance account for the remainder. Therefore, even the smallest improvement in energy efficiency can translate to sizeable savings. In addition, system designers can squeeze an additional 10 to 35 percent in energy savings by transitioning from pumps that use pressure-reducing valves to those that leverage variable-speed drives, depending on the system parameters.
Reece Robinson is the senior product training specialist at Grundfos Pumps Corp. He actively welcomes reader comments and can be reached at rrobinson @grundfos.com.