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Having an adequate water supply is critical for any fire sprinkler or standpipe system. When the water supply doesn’t have enough pressure, a fire pump can be an excellent solution to the problem, as long as the pump is sized correctly.
It is important to note that if the water supply does not have sufficient flow, a fire pump by itself is not going to be a solution to the problem. There is no magical device inside a fire pump combining hydrogen and oxygen molecules to create water. Fire pumps cannot create additional flow. All a fire pump can do is take the flow that is provided by the water supply and increase the pressure available for fire protection.
This column will focus on selecting an appropriately sized fire pump for any given situation.
There are three general considerations when selecting a fire pump. First, the pump has to be large enough for the necessary flow to move through the pump. Second, the pump needs to provide the pressure required to allow the fire protection system to function correctly. Third, the pump cannot create more pressure than the components can handle. If the pump creates too much pressure, there is a risk of blowing apart the system and creating a catastrophic failure.
Each of these three general considerations will be discussed in detail in this column. It assumes the reader understands the concepts of suction pressure, discharge pressure and net pressure. In addition, this column assumes the user can calculate friction loss in pipe, including the use of equivalent lengths of fittings and valves.
Flow Ratings of Fire Pumps
Stationary fire pumps (designed and installed in accordance with NFPA 20) are all given a rated flow by the manufacturer of the pump. Ratings range from 25 gallons/minute to 5,000 gpm, but the most commonly used ratings are 250 gpm, 500 gpm, 750 gpm and 1,000 gpm. The flow rating of a stationary fire pump is not a direct limitation on its use. Fire pumps are allowed to be used beyond their rated flow. NFPA 20 allows fire pumps to be used up to 150 percent of their rated flows.
So, a pump rated at 750 gpm can be used with fire protection systems having flow demands up to 1,125 gpm, and a pump rated at 1,000 gpm can be used with fire protection systems having flow demands up to 1,500 gpm (as long as they meet the pressure demands discussed later).
For anyone reading this column who also is a firefighter, you may be thinking this is not what you learned regarding the ratings of pumps in firetrucks — and you would be correct. NFPA 1901 takes a different position with respect to the flow rating for pumps on a firetruck. Firetruck pumps are not supposed to be used past their rated flows. A pump on a firetruck rated at 1,000 gpm is not supposed to be used to supply any flow greater than 1,000 gpm.
This difference in philosophies sometimes causes confusion between AHJs and fire protection system designers. When the AHJ is a firefighter familiar with NFPA 1901 sees a stationary fire pump rated at 750 gpm being used to satisfy a standpipe system demand of 1,000 gpm, he gets concerned and rejects the plans, thinking this is a problem.
But, in reality, it is perfectly acceptable for a 750 gpm pump to supply a 1,000 gpm standpipe demand (assuming the pressure conditions discussed later are met) because 1,000 gpm is only 133 percent of the rated flow of the pump; it is permitted by NFPA 20 to be used up to 150 percent of its rated flow.
Performance Curves of Fire Pumps
Fire pumps are rated by the manufacturer to produce a certain amount of net pressure (typically between 40 and 250 psi) at their rated flow. It is important to note that the rated net pressure usually only occurs at the rated flow of the pump. At flows less than the rated flow, the net pressure is typically higher than the rated net pressure. At flows greater than the rated flow, the net pressure generally is lower than the rated net pressure.
To accurately describe the performance of its fire pump, the manufacturer will produce a performance curve for each model fire pump it makes. The performance curve is plotted as the flow (typically the “x” axis or the horizontal axis of a graph) against the net pressure produced by the pump on the “y” axis or vertical axis of the graph.
Using the performance curve, a designer can tell exactly how much net pressure is produced by the pump at any of the flow conditions between churn (no flow) and maximum flow (150 percent of rated flow).
There are two pieces of information from NFPA 20 that everyone seems to be able to quote (although many people misquote this information) regarding the performance curves of fire pumps. Many people seem to think all fire pumps produce 140 percent of their rated pressure at churn (no flow) and 65 percent of their rated pressure at maximum flow. This is incorrect and can be an extreme problem if these values are assumed when selecting a fire pump.
The actual language from NFPA 20 is that the pump is not allowed to exceed 140 percent of its rated pressure at churn and is not allowed to go below 65 percent of its rated pressure at maximum flow. These maximum and minimum values are not intended for designers to use when selecting a pump. Instead, these values are intended for pump manufacturers to let them know how to make an acceptable pump.
These values are extremely dangerous to use when selecting a fire pump and need to be ignored. The proper way to choose a fire pump is to use the actual performance curve for a specific model of fire pump when working to decide whether any particular fire pump will be acceptable. Since all models of fire pumps perform differently, even those with the same flow and pressure ratings, the only way to properly select a fire pump is to work with the specific performance curve for the model of fire pump you want to consider.
Selecting Fire Pumps
The following is a simple six-step procedure for selecting a fire pump:
1. Perform the hydraulic calculations of the fire protection system from the most remote portion of the system to the place where the discharge flange of the pump will be located.
2. Calculate the suction pressure at churn (no flow) and the flow demand of the fire protection system by taking the residual pressure at the water supply and subtracting the friction loss between the water supply and the pump. Also, take into account the elevation change between the water supply and the pump.
Note that for this calculation, the suction pressure at churn should be calculated using the highest consistent pressure from the water supply. The suction pressure at the system flow demand should be calculated using the lowest reasonable residual pressure from the water supply, taking into account daily and seasonal fluctuations.
3. Select a model of a fire pump to consider where the flow demand of the fire protection system is less than or equal to 150 percent of the rated flow of the pump.
4. Using the manufacturer’s performance curve for the model of fire pump you want to use, determine the net pressure from the pump at churn (no flow) and the demand flow of the fire protection system.
5. Add the suction pressure at the system flow demand (from step 2) to the net pressure from the pump (step 4). If this discharge pressure is greater than or equal to the system demand pressure from step 1, then go on in the procedure. If the pressure is less than the demand pressure, go back to step 3 and select a new pump.
6. Add the suction pressure at churn (from step 2) to the net pressure at churn (step 4). If this discharge pressure is less than or equal to the rated pressure for the components downstream of the fire pump, then this pump is acceptable. If the discharge pressure is greater than the rating of the fire protection system component(s), then go back to step 3 and select a different pump.
A fire protection system has a demand of 900 gpm at 60 psi at a location where a fire pump’s discharge flange will be installed. The water supply has a static pressure (no flow) of 50 psi (the consistent highest pressure considering daily and seasonal fluctuations). The water supply has a residual force of 25 psi at 900 gpm (the lowest reasonable pressure taking into account daily and seasonal fluctuations).
The pump is 30 feet higher than the fire hydrant, where the water supply pressures were determined. Will a fire pump rated at 750 gpm at 67 psi that produces a net pressure of 87 psi at churn and 56 psi at 900 gpm be acceptable to use? To answer the question, follow the six steps:
1. The fire protection system demand has been calculated to be 900 gpm at 60 psi at the discharge flange of the pump.
2. PS at churn = 50 – (30 x 0.433) = 37 psi
PS at 900 gpm = 25 – (30 x 0.433) – (friction loss from water supply to pump)
If we calculate the friction loss from the water supply to the pump as 5 psi, this becomes:
PS at 900 gpm = 25 – (30 x 0.433) – (5) = 7 psi
3. The pump under consideration is rated at 750 gpm at 67 psi.
4. Net pressures of 87 psi at churn and 56 psi at 900 gpm.
5. PD at 900 gpm = 7 + 56 = 63 psi. Since this is greater than 60 psi, this is acceptable.
6. PD at churn = 37 + 87 = 124 psi. Since this is below 175 psi, which is the pressure that all fire protection system components are listed for, this is acceptable.
Earlier in the column, the statement was made that you need to use the manufacturer’s performance curve for the model pump under consideration to accurately determine whether the pump will work. But there are times, particularly in the early planning stages of a job, where you might need to estimate a size for a pump without selecting any particular model to consider.
In these cases, conservative estimates can be used. Assume a rating for the pump of flow and pressure. Then for the net pressure (step 4), assume two lines — one horizontal line between churn and the rating of the pump (assumes the pump stays at 100 percent, but no more, of the pump’s rated pressure at churn and every flow between churn and rated flow).
The other line would be between the rated pressure of the pump and 65 percent of the rated pressure at maximum flow. For step 6, you need to assume that the pump goes to 140 percent of rated pressure at churn. Using these simplifications, a conservative estimate can be achieved.
Using this basic six-step procedure, fire pumps can be appropriately selected for any fire protection system.