Chapter 4: Fire Service Pumps
Introduction
1) Three Basic Types of Fire Pumps
a. Piston Type Fire Pump
b. Rotary Type Fire Pump
c. Centrifugal Type Fire Pump
2) Pump Mounting on Apparatus
a. Mid-ship (middle) – 2 ways
i. Between road transmission and rear axle in line with drive shaft (most common)
ii. Ahead of clutch and transmission with flywheel and power take off. This type allows for direct engine power to pump transmission connection. It allows for driving and pumping simultaneously.
b. Front Mounting
i. From front of engine crankshaft connected to pump transmission. This type also allows for driving and pumping simultaneously.
3) Pump Ratings
a. Standard pumper capacity ratings start from 500 gpm and increase in 250 gpm increments (NFPA 19 Specification) up to 2000 gpm.
i. 500 gpm, 750 gpm, 1000 gpm, 1250 gpm, 1500 gpm, 1750 gpm, 2000 gpm
ii. Pumpers must have one 2 ½” gated outlet per 250 gpm rated capacity.
b. Fire pumps are designed to perform as follows:
100% of rated capacity at 150 psi net pump pressure
70% of rated capacity at 200 psi net pump pressure
50% of rated capacity at 250 psi net pump pressure
Net pump pressure is found by subtracting suction or inlet pressure from the discharge pressure.
Work Problem: A fire pump is discharging 200 psi through a 2 ½”
hoseline and is receiving 50 psi from a fire
hydrant. What is the net pump pressure?
Net Pump Pressure = Discharge Pressure – Intake Pressure
Net Pump Pressure = 200 psi – 50 psi
Net Pump Pressure = 150 psi
Work Problem: A pumper has a capacity rating of 1000 gpm.
Using net pump pressure find the efficiency of the
pump:
At 150 psi net pump pressure _______gpm
At 200 psi net pump pressure _______gpm
At 250 psi net pump pressure _______gpm
The answers are 1000 gpm (1000 gpm X 100%), 700 gpm
(1000 gpm X 70%) and 500 gpm (1000 gpm X 50%).
note: Fire pumps are most efficient at 150 psi or less.
4) Cavitation
a. Causes of cavitation
i. Lift too high for volume and pressure discharged
ii. Suction hose too small for volume and pressure discharged
iii. Suction strainer or hose clogged
iv. Partial collapse of hose lining
v. Temperature of water too high
b. Signs of cavitation
i. Pump vibrations with loud pinging noises. This is caused by air bubbles that form in the pump that collapse violently when they enter the impeller.
ii. “Pump running away” This happens when the pump is pumping air or steam instead of water. The pump speed will increase with no increase in discharge pressure or volume. The pump operator will hear the engine revving and running away. If this should occur, shut down pumping operations immediately.
Centrifugal Fire Pumps
1) General Information
a. Water enters the centrifugal pump through the eye and is delivered to the impeller through the vanes. The impeller increases the speed of the water and discharges it through the volute. (see figure 4.1)
figure 4.1 – Centrifugal Fire Pump
c. Centrifugal pumps may be connected in stages to each other. Centrifugal pumps can have 1 – 4 stages. Each impeller and volute is one stage in a multi-stage centrifugal pump.
2) Capabilities and Limitations
a. Volume (gpm) varies directly as the pump speed. The faster the pump speed, the greater the volume.
b. Pressure (psi) varies as the square of the pump speed. If you double the pump speed, the pressure increases 4 times (22 = 4)
c. Centrifugal pumps are not self-priming. A separate rotary vane priming pump unit is used to prime centrifugal pumps.
3) Single-Stage Centrifugal Fire Pumps
a. Basic Designs
i. Single suction impeller
1. Limited up to 750 gpm pumpers
ii. Double suction impeller
1. 1000 – 1500 gpm pumpers
iii. Double Volute design
1. Limited to single stage pumps
2. High efficiency at rated capacity
b. In general, single stage pumps have a high efficiency rating (about 70%), which is generally slightly higher than multi-stage pumps.
4) Two-Stage Centrifugal Fire Pumps
a. Basic design
i. Two impellers with separate volute chambers for each impeller
b. Limited use in fire service
i. Pumps may be front mounted for smaller trucks
1. Provides vehicle movement while pumping
2. Able to provide high-pressure for operations (300-400 psi).
5) Parallel – Series Two-Stage Centrifugal Pumps
a. Basic design
i. Two impellers with separate volutes
ii. Addition of transfer or changeover valve for parallel or series pumping operations
iii. The efficiency of parallel – series two-stage pumps is around 65-70%. This is slightly less than a single stage pump.
b. Parallel (Volume) Operations (see figure 4.2)
i. When a pump is placed in the parallel position, water enters both stages of the pump simultaneously from the suction side. This means that the pump will be able to deliver twice the volume at half of the pressure.
ii. An example of this is: A 1000 gpm rated capacity pumper operating at 150 psi net pump pressure. In the parallel position, each impeller will deliver 500 gpm at 150 psi. Note that the pump will double the volume of water, but at half the speed.
figure 4.2 – Parallel – Series pump in parallel position
c. Series (Pressure) Operations (see figure 4.3)
i. When a pump is placed in the series position, water enters one stage of the pump from the suction side. The water is then delivered to the second stage via the first stage of the pump. This means that the pump will be able to deliver half the volume at twice the pressure.
ii. An example of this is: A 1000 gpm rated capacity pumper operating at 150 psi net pump pressure. In the series position, the first impeller will deliver 500 gpm at 150 psi to the second stage. The second impeller will discharge the 500 gpm, but it will double the pressure. In the series position, this pump will deliver 500 gpm at 300 psi. Note that the pump will double the pressure, but at half the volume.
figure 4.3 – Parallel – Series pump in series position
d. Transfer Valve
i. A transfer valve is the device that is used to change a pump from series to parallel or vice versa. The pump operator must decide which position would best meet the needs of a given situation.
ii. Transfer valves may be either powered or manual
iii. Transfer valves may be disk type or cylinder type
iv. Transfer valves are normally left in the series position for normal day-to-day operations.
v. As a general rule, the transfer valve should be kept in the series position when pumping up to 70% of the pump’s capacity. The transfer valve should be switched to the parallel position when evolutions or circumstances require a pump to deliver more than 70% of it’s rated capacity.
vi. When switching the transfer valve from one position to another, the pump pressure should be lowered to below 60 psi. This is especially crucial when switching from parallel to series because the pressure will immediately double. This sudden increase in pressure could damage the pump, hoses, or seriously injure the firefighters on the hoselines.
6) Piston Pumps
a. General Remarks
i. Piston pumps were the first pumps developed for firefighting.
ii. Piston pumps are preferred by fire departments that get their water supply mostly by drafting operations.
iii. Piston pumps have a high efficiency (around 75-80%)
b. Basic Design
i. Use of piston to displace water
1. Single action (water gets moved on piston’s up stroke)
2. Double action (water gets moved on up stroke and down stroke)
ii. Piston pumps are positive displacement pumps. This means that they are self-priming
iii. Different gear ratios between engine and pump are needed to obtain higher pressure.
7) Rotary Fire Pumps
a. General Remarks
i. Rotary fire pumps were first used by the fire service around the 1600’s. In the 1900’s they were adapted to work with the steam engines.
ii. Rotary pumps are preferred where all pumping operations involve drafting, high lift conditions, or
long suction hoses.