# Understanding Domestic Water Booster Systems and Discharge Pressure – Part 2

Norm Hall
/
December 21, 2015

In last week’s Monday Morning Minutes post, we defined the information needed to determine if a pressure booster is required. We also discussed what pressure capacities are needed. Next, let’s turn our attention to discharge pressure.

## Example of a Discharge Pressure System

Let’s look at an example of a system requiring a pressure booster system. For the system side:

• Minimum fixture pressure required: 35 PSIG
• Maximum fixture pressure required: 85 PSIG
• Your design fixture pressure required: 40 PSIG
• Elevation from the booster location to the highest fixture: 96 feet
• Maximum friction loss per 100 feet: 3 PSIG
• Horizontal distance on discharge, including fittings (TEH): 300 feet
• Pressure drop of equipment on discharge: 0 PSIG

The friction loss is found by adding 300 feet of horizontal pipe, plus 96 feet of vertical pipe, for a total of 396 feet. Using a friction loss of 3 PSIG/100 ft of pipe equals 11.88 PSIG. The elevation of 96 feet converted to pounds equals 41.56 PSIG. Let’s add 5 PSIG for safety and unknowns during construction.

The total discharge pressure (MINIMUM) at zero GPM is (40 + 0 + 41.56) 81.56 PSIG. There is no friction loss if there is no flow rate. The total discharge pressure we DESIGN for is (40 + 11.88 +41.56 + 5) 98.44 PSIG. This is above the 85 PSIG maximum. We will have to use pressure reducing valves or multiple pumping systems or take some of the lower floors directly from the cold water main and bypass the booster system if there is enough pressure. We’ll explore these topics in another Monday Morning Minutes post.

## Pressure Booster—Suction Considerations

Now let’s turn our attention to the suction side of the pressure booster. Many people just guess at incoming water pressure, but that can lead to unhappy surprises later on. You can get the information you need by calling the municipal water board or fire marshal and asking for the minimum and maximum expected pressures. They will often have test pressures at a nearby intersection. Let’s look at our example system again.

For the suction side:

• Minimum or residual city water pressure: 45 PSIG
• Maximum or static city water pressure: 67 PSIG
• Backflow preventer on main at full flow: 12 PSIG
• Backflow preventer on main at zero flow: 8 PSIG
• Water meter on main at full flow: 2 PSIG
• Water meter on main at zero flow: 0 PSIG
• Maximum friction loss per 100 feet: 3 PSIG
• Horizontal distance on discharge including fittings (TEH): 35 feet
• Elevation of the booster pump above the main (in this example, the booster system is BELOW where the water comes in): minus (-6) feet

The horizontal and vertical distance is a total of 41 feet of pipe at 3 PSIG/100 feet of pipe, which equals 1.23 PSIG; the elevation to the booster pump is negative 6 feet, which equals (-2.6 PSIG).
My MINIMUM suction pressure for design is (45-12-2-1.23+2.6) for a total of 32.37 PSIG. Our MAXIMUM suction pressure is (67-8-0-0+2.6) for a total of 61.6 PSIG.

The table below shows the design conditions for selecting the booster system and the minimum conditions at times. If the minimum required pressure was negative, we would be able to shut the booster system down at times and save even more energy.

In our next post, we will explore the number of pumps to select for the design flow rate and look at an example selection.

Catch up on the entire series: