Net Pump Pressure = Outlet Pressure - Residual Pressure
What if you are pumping at draft and the gauge on the pump panel reads 150 psi (1,035 kPa)? Is that the correct reading? Not really, for you must take into account the pressure that was lost just to get the water to the pump inlet - the amount of energy that the engine and fire pump used to get the water up to the fire pump.
Using the table in Fig. 1 on page 34, look up a 1,250-gpm (4,732-lpm) pump with a 6-inch (150-millimetre) suction hose and a lift from the water level to the centre of the pump impeller eye of 10 feet (3 metres). From the chart you can see that a 1,250-gpm (4,732-lpm) pump with one six-inch (150-millmetre) inlet will have a friction and entrance loss (FEL) of 5.2 feet (1.6 metres). Now add the FEL to the distance from the water to the pump eye (this is called the static lift), in this case 10 feet. Now 5.2 feet (1.6 metres) plus 10 feet (3 metres) equals 15.2 feet (4.6 metres). This 15.2 feet (4.6 metres) is called the dynamic lift.
FEL + Static Lift = Dynamic Lift
Substituting: 5.2 feet (1.6 metres) + 10 feet (3 metres) = 15.2 feet (4.6 metres).
Remember that one psi (6.89 kPa) equals 2.035 inches (5.17 centimetres) of mercury (mercury can be used to measure either a pressure or a vacuum). The 15.2 feet (4.6 metres) of dynamic lift will be divided by the 2.035 inches (5.17 centimetres) of mercury (Hg) to give us a vacuum reading of 7.47 psi (51.5 kPa) (vacuum). If the pump panel outlet pressure gauge reads 150 psi (1,035 kPa) you must add the 7.47 psi (51.5 kPa) to the pump outlet to get a true reading of 157.47 psi (1,085.7 kPa). In other words, when you are conducting the 150 psi (1,035 kPa) test you do not need to go to 150 psi (1,035 kPa) if you are at draft. If you could get the proper water flow at the proper engine r.p.m. you could pass this test at a pump panel gauge reading of only 142.5 psi (982.5 kPa) in this case. This may seem like a lot of work for such a small difference in pressure. However, if you combine this with a low air pressure day, a day with a high relative humidity, high air temperature, low-power winter diesel fuel and an engine that needs a tune-up, your pump could fail the test and be mistakenly deemed deficient and taken out of service. Thousands of dollars and many hours of work could be expended to repair a pump that is still serviceable just because you did not do the math.
On the older, mechanically controlled diesel engines there was no compensation for changes in altitude, barometric pressure or air temperature. These engines ran very smoky when travelling over a mountain pass and also when they were pumping. On the newer, electronically controlled, engines there are sensors that inform the computer of the incoming air temperature, the barometric pressure, the temperature of the diesel fuel and the temperature of the water in the engine block. All these factors will affect the way the engine runs but they will not compensate for large changes in altitude. In other words, an engine that has 300 horsepower at sea level will have less at 5,000 feet (1,524 metres) above sea level; it just will not smoke (pollute) as much at this height as the older engines. If the apparatus must perform at locations above 2,000 feet (609.6 metres) the fire department must inform the manufacturer during the bidding process.
- Service tests
A service test should be performed any time the vehicle has undergone a repair to a system that may affect the ability of the apparatus to pump, when the operator suspects a problem or, at least, annually.
What could trigger a service test? The replacement of a wheel bearing, a steering adjustment or a change of engine oil or batteries are not generally causes for a service test. The replacement of an engine water pump, a tune-up of the valves or a replacement of the fuel injector would likely require a service test. The new NFPA 1911 Standard for the Inspection, Maintenance, Testing, and Retirement of In-Service Automotive Fire Apparatus, chapter 18, 2007 edition, clearly lays out the requirements for this test.
- Test conditions
The conditions for the tests are wide ranging. Air temperature may be between 0 F and 101 F (-18 C to 43 C), water temperature can range from 35 F to 90 F (2 C to 32 C) and the barometric pressure should be at least 29 inches of Hg (98.2 kPa) (this can be corrected to sea level). All engine-driven accessories must be connected and operational during the tests. You cannot cheat and disconnect the alternator. The alternator must be providing the total connected load during these tests. One of the greatest problems that exists with fire-truck design is heat (generation/dissipation); therefore, you cannot remove any panels, grills or gratings that are not meant to be opened during pumping to alleviate this problem, nor can you lift the hood if the engine overheats during the tests. During these tests, the engine, transmission and any other parts cannot exhibit undue heating, loss of power or any other defect. With the large variation of air and water temperature allowed in the standard, a truck could very easily pass the tests at an air temperature of 32 F (0 C) and a water temperature of only 40 F (4.4 C) but these readings may not represent the conditions that the truck might experience on a normal summer day in your location. If the truck is manufactured in a northern location with a cool climate and sent to the interior of British Columbia, it is your responsibility to inform the manufacturer of the conditions that must be in place during the acceptance tests. If you are dealing with a reputable manufacturer, it may be able to install an engine fan with more blades or a radiator with more cores, for example, to address the special climactic conditions you specify for the tests. Be up front with the manufacturer during the bidding process so it has time to make alterations to the design and ensure the apparatus will pass the pre-service tests under the conditions that you specify.
The electronic tachometer operated from the engine or transmission can be used to measure engine r.p.m. if it has been proven to be accurate, otherwise you will need to connect a mechanical drive tachometer off the port provided for this. The mechanical tachometer installed on the pump panel or in the cab is not considered accurate enough. Accuracy must be ±50 r.p.m. at the actual speed being measured. Make sure you use a hand tachometer of know accuracy.
- Annual pumping test
The first test will take at least 40 minutes. It will be broken down into the following: a continuous 20-minute test with the pump operating at its rated capacity (flow) at a net pump pressure of 150 psi (1,035 kPa); a 10- minute test that requires the pump to operate at 70 per cent of rated capacity (flow) at a higher pressure of 200 psi (1,380 kPa); a third test of 10 minutes and at 50 per cent of rated capacity (flow) at a pressure of 250 psi (1,725 kPa).
- Overload test
If the pump is of 750 gpm (2,850 lpm) or larger, an overload test must be performed immediately after the 150-psi (1,035 kPa) test. The pump is run for 10 minutes at a pressure of 165 psi (1,138 kPa). During all of the above tests the discharge pressure, intake pressure and, most importantly, engine speed (r.p.m.) must be recorded.
If the test conditions are equal to those at the time of delivery of the apparatus and the speed of the engine increases by more than 10 per cent of the original engine speed, the reason for the decrease in performance should be determined and the deficiency corrected. Where test conditions are significantly different from the original test conditions at the time of delivery, results should be compared with previous years' tests. The test conditions should be maintained as consistently as possible from year to year.
It has been my experience that if a pump marginally passes the 200 psi test and passes the 250 psi test but fails the 150 psi test it is because of a water intake problem, i.e., intake hose collapses or too small of an intake hose. If the pump passes the 150 psi test, marginally passes the 200 psi test and fails the 250 psi test it is because of a worn impeller or impeller wear rings. If the overload test fails but all others pass I normally look for an engine problem.
- Pressure control system test
This test will be done if the fire pump has a very simple relief valve or a very complex electronic engine governor. The pump will be operated from draft at three different pressures in this order: 150 psi (1035 kPa) test first; then the 90 psi (620 kPa) test; and lastly the 250 psi (1,725 kPa) test. All the above pressures are ±5 per cent. The purpose of this test is to ensure that, as discharge valves are closed, the system's water pressure does not rise (spike) by more than 30 psi (207 kPa). Discharge valves cannot be closed faster than three seconds or slower than 10 seconds. If the water pressure were to spike as discharge valves are closed it may be very difficult for firefighters to maintain their grip on the hose lines. A loose charged hose line is very dangerous. Therefore, this test has very important safety consequences.
- Solving Pumping Problems
If any operational problems arise during any of the tests described above the most efficient way to identify the cause is to adopt a systematic and logical approach based on elimination. An experienced pump operator will often be able to recognize the symptoms of the common faults and pinpoint the cause, but if the anticipated difficulty is not the actual problem, the time spent is wasted.
The logical process, when diagnosing an operational problem, is to follow the power flow from engine to pump impeller and water flow from source to nozzle.
- Drive, Supply, Output
To remember the sequence, think of the key words or letters: drive, supply, output: D.S.O.
D - Drive: Is the pump running at the correct speed?
S - Supply: Is water able to enter the pump, air-free, and in sufficient volume?
O - Output: Is anything hindering the required pressure and volume from getting out of the pump?
The term "drive" refers to all things involved in the mechanics of turning the pump. In order to check the drive system, answer these questions.
If the pumper is a front mount:
· Is the main transmission in neutral?
· Is the pump transmission in pump gear?
· Is the pump actually turning?
If the pumper is a mid-ship mount:
· Is the main transmission in the correct gear?
· Is the pump transmission in pump gear?
· Is the transfer valve in the correct stage (if multi-stage)?
· Is the pump turning at the right speed? (Check the speedometer.)
· If the pump is a power take off, or PTO, drive:
· Is the main transmission in neutral?
· Is the PTO fully engaged?
· Is the pump shaft turning?
The term "supply" refers to all equipment and conditions that are involved with the delivery of water into the pump. In order to check the supply, answer these questions.
If the supply is at draft:
· Is the correct suction hose being used?
· Is the strainer located at the correct depth?
· Are there any air leaks in the suction line?
· Are there any air leaks in the truck?
· Is there any blockage of any of the strainers?
· Is the intake valve fully open (if so equipped)?
· Are there any air traps in the suction hose?
· How high is the lift?
· Did the primer operate correctly?
· Are any other valves opened?
· How long is the suction line?
· What is the interior condition of the suction hose?
If the supply is a hydrant:
· Is the suction hose large enough for the required flow?
· Is the hydrant fully open?
· Is there a residual pressure?
· Are the intake valves in use fully open?
· Are there any air or water leaks?
· Is the tank valve open?
· Are any intake strainers plugged?
· Are any hose lines kinked?
The term "output" refers to all of the equipment that is involved with the delivery of water, at satisfactory volume and pressure, to the nozzles. In order to check output, answer these questions.
· Is the governor operating correctly?
· Is the relief valve operating correctly?
· Are the appropriate discharge valves open?
· Is the engine able to reach the required r.p.m.?
· Is the flow pressure required within the pump's capacity?
· Are valves open that should not be?
· Is there any unusual leakage to the ground?
· Are the gauges operating properly?
· Are any hoses kinked?
· Are you attempting to move too much water through that size of valve or hose?
- Finding the unknown
Every time we diagnose a problem in anything we do, we begin a process of elimination. Anything that we know is not part of the problem, we dismiss until we have found the problem. When a problem's cause is unknown, begin by assembling all of the information that is known. When you have completed this process, the unknown will become obvious.
an individual piece of information can answer a number of questions,
saving a lot of steps. A good example is the speedometer of a midship
pumper. If the speedometer is reading a speed in the range from nine
m.p.h. (15 km/h) to 19 m.p.h. (30 km/h) at engine idle, you know that
all of the correct steps involved in putting the pump transmission into
pump gear have been followed. This eliminates the need to check all the
procedures involved in putting the pump into gear individually.
It is important to follow a logical sequence in all troubleshooting, so use the Drive-Supply-Output sequence in the process of working from the known to find the unknown.
Here is an example of how the process can work on a simple operational problem. A midship pumper is connected to a hydrant with soft suction, but no water is getting to the nozzle.
The first step is to check the drive system.
Drive - The speedometer is reading 12.5 m.p.h. (20 km/h) at engine idle.
We can assume that all is well with the pump drive unless, of course, there is a mechanical failure in the pump transmission or the pump.
The second step is to check the supply system.
Supply - The pump intake pressure gauge is reading 65 psi (450 kPa). This pressure reading tells us that water is in fact getting into the pump (if the gauge is working).
Now check the output system.
Output - The pump master pressure gauge reads 145 psi (1,000 kPa). This indicates that the pump is producing pressure within the pump discharge manifold.
From the information we have at this point, we know that the problem is somewhere between the pump manifold and the nozzle. The most likely fault in this case would be a closed discharge valve.