
Testing Pumps on Fireboats
Most fire departments periodically exercise the pumps on their marine units. The pumps are started, monitor control valves opened, and the streams noted. This usually is done to signify some event, such as the arrival or departure of a foreign or large naval vessel, or to celebrate a local or national holiday. It’s an impressive exercise, but it may not indicate the conditions of the pumping units.
NFPA (National Fire Protection Association) 1911, Standard on Acceptance and Sendee Tests of Fire Department Pumping Apparatus, discusses the acceptance and service tests for mobile apparatus. Service tests are recommended “at least once per year.” The purpose of these tests is “to assure the user that the pump is capable of the required performance.”
The NFPA is developing a standard on marine firefighting vessels that will address acceptance and service tests. The fire boat pumps I have seen appear to be more like those installed in buildings covered by NFPA 20, Standard for the Installation of Centrifugal Fire Pumps. Those pumps were generally horizontal, splitcase, and single or multistage types with most fixed fire pumps driven at a constant speed. Marine pumps, like mobile firefighting units, however, are driven at variable speeds.
TESTING PROCEDURE
The procedure for testing fire pumps on a marine unit are similar to procedures used in testing a building’s pump. Fixed fire pump test results are compared with the pump manufacturer’s certified discharge curves. Test pressures and flows are adjusted, using the pump affinity laws, by correcting pump test speed (rpm) to the pump rated speed.1 For boats operating in salt water, the flow results are corrected to fresh water flow for the comparison.2
The manufacturer’s curve for a particular pump can usually be obtained by submitting the information from the data plate on the pump to the manufacturer. These data would at least include pump model and serial number(s), pump rated speed, impeller diameter(s), rated capacity (gpm), and head (feet or psi).
The manufacturer’s curves for a particular model and capacity of pump generally will include curves for a number of examples with different impeller diameters. The curves also may be plotted for pump speeds other than the rated speed of the installed unit. Those curves usually are based on fresh water.
The new curve can be approximately plotted by correction for impeller diameter.’ speed, and salt water (if appropriate) for evaluating the test results of the installed units.
Each pump should be tested separately. The test should begin by flowing from the smallest nozzle, adding flowing nozzles until the rated capacity is noted. At each step, the pump speed should be maintained at its rated rpm. For each point in the test, the following data should be recorded:
- pump speed (rpm),
- suction pressure (psi or “Hg (inches of mercury)],
- discharge pressure (psi), and
- nozzle diameter (inches) and pitot pressure (psi) at each nozzle being flowed.
During the test, the pump and engine should be observed for overheating, leaks, excessive noise, or vibrations.
Immediately after the test, results should be plotted on the curve and compared, after
correction of flow (gpm) and pressure (psi), with rated speed (rpm). The pumps operate under a lift; therefore, suction and discharge pressures are added to provide net pressures to be plotted for each flow point. Suction gauges are a compound type with a range from 30 “Hg to at least 100 psi. (Multiply “Hg by 0.491 to get psi.) A comparison of the test curves with the manufacturer’s curve can indicate a deterioration of performance due to worn components, plugged impeller suction pipe or sea chest inlet, dragging bearings, etc. Inability to reach rated capacity and head at rated speed might indicate engine performance loss.
Endnotes
1. To correct pump test results to rated pump speed:
Capacity (gpm): = Qiwhere:
Q, = capacity at test speed
0; = capacity at rated speed
N, = test speed in rpm
N, = rated speed in rpm. and
Head: H, = X H,. where:
H, = head in feet at test speed
H = head in feet at rated speed
Feet of head X 0.433 = psi
Inches of mercury (“Hg) X 0.491 = psi
Psi X 2.31 = feet of head
2. Convert Qf (gpm fresh water) to (gpm sea water): Q X 0.988 = Q
3. To correct a performance curve for a pump with a different impeller diameter:
Head (ft ): H, = X H,. and:
Flow (gpm): Q = (X Q,. where:
Q, = flow from manufacturer’s curve for impeller diameter D, (inches)
D, = actual diameter of impeller tested at pressure point H,
Q, = flow (gpm) for impeller D, at pressure point H,
To calculate flow from a tapered smooth-bore nozzle:
Qf = 29.83 cdvp (fresh water), and
Q = 29.47 cd VP (sea water), where
c = discharge coefficient of nozzle,
generally c = 0.97 or less
d = nozzle outlet diameter (inches)
p = pitot pressure at nozzle outlet (psi)