Diagnosing Difficulties During Pump Operation
Usually the pump operator can diagnose pumping trouble and make the required correction on the fire scene or training ground. But there are times when the problem requires a trained mechanic to correct the cause, particularly when the problem is mechanical rather than environmental. By environmental, we refer to conditions relating to water supply.
Such conditions are: hydrant residual pressure too low to permit use of more lines and still maintain 10 psi at the pump inlet; lift at draft too high for the desired volume; water temperature from the source too high (above 90°F); one or both suction strainers clogged; suction hose too small for the desired volume at the lift and number of lengths of suction hose. All these conditions are usually recognized by the pump operator and corrected to permit continuing operation.
Other problems may not he so easily solved on the fireground and may require
checking by the mechanic to correct. Most of these are due to lack of preventive maintenance, although some do occur without prior warning. One such is loose fining in a length of suction hose.
Engine speedup: One problem which has been experienced with some dieselpowered pumpers is a gradual increase in engine speed and pump pressure above the preset engine speed at the desired pressure. This condition usually occurs when pumping with small lines, booster lines or 1%-inch lines, or a single 2Viinch line. The run from the station to the fire scene is not long enough to bring the cooling system up to proper operating temperature. As the engine continues to operate, it warms up, increasing in efficiency. The throttle is set at the initial speed to provide a fixed amount of fuel injected into each cylinder. Thus, with the increase in efficiency as the engine warms, the speed will increase without a change in throttle setting. If the initial setting was for example, at 1800 rpm, the engine speed may increase to 2300 rpm with a corresponding increase in pump discharge pressure.
The engine governor, usually of the variable-speed type, does not control this condition. The governor does control the idle speed and limits the maximum speed. For road operation there is no problem. It is a speed control between idle and maximum that is needed. While there is some difference of opinion on how to provide this control of intermediate speeds, the constant speed control governor has provided the type of control needed on pumpers. A number of applications made to date have proven highly successful.
The constant speed governor, technically known as the “limiting speed mechanical fuel modulating governor,” controls engine idle speed, acts to maintain a constant preset speed by varying the throttle setting and injection of fuel to match the load, and controls the maximum engine speed.
If you have such a problem maintaining preset engine speed and pump pressure, refer the problem to the manufacturer.
Mechanical difficulty: Operating problems that get a quick call to the mechanic to solve are usually one or more of the following:
1. No increase in pump discharge pressure over the hydrant residual pressure when the throttle opening is increased,
a. Road transmission gear shift in neutral.
b. Pump drive not engaged.
c. Relief valve in bypass setting.
2. Pump discharge pressure decreases as throttle is opened.
a. Road transmission gear shift in reverse drive.
3. Unable to get over 70 percent of rated capacity discharge from the pump.
a. Transfer valve in PRESSURE setting, change to CAPACITY setting.
b. Water main too small to deliver desired flow while holding residual pump inlet pressure at 10 psi minimum.
c. Relief valve improperly set, pressure setting too low.
d. Pump not properly primed, all air not exhausted. This applies only to operation at draft.
4. Operator unable to prime the pump for draft operation. Do not assume any of tire following are OK. Check each item.
a. Make sure all drain valves are closed. Also, water tank to pump valve, radiator fill valve, bleeder drain valves, cooling bypass valve, governor control valve or relief valve.
b. Disconnect the suction hose and replace the blind cap on pump suction inlet. Operate the primer to create a vacuum in the pump. If a vacuum is readily created (in less than 30 seconds), shut down the primer and observe the vacuum gage. The decrease in vacuum must not be more than 10 inches in 10 minutes. If the loss in vacuum is faster than this rate, the pump packings are most probably worn sufficiently to require adjustment or repacking.
c. If the above test is satisfactory, the cause of the trouble is outside the pump. Look for the following after connecting the suction hose:
(1) Check the gaskets at each coupling in the suction line. Dirt or scoring of a gasket will permit air to enter, making a prime impossible, or the pump will be unable to hold a prime.
(2) Make sure the suction strainer is submerged at least 18 inches below the surface of the water. If this is not possible, a float type of strainer should be used.
(3) Loose lining in a length of suction hose may be collapsing, making it impossible to get water even with a high vacuum in the pump.
5. Engine speed too high, surge in tachometer and discharge pressure gage readings, not steady.
a. Road transmission shift lever not in proper gear ratio. May be in second or low.
b. Pump transfer valve in the PRESSURE setting when it should be in the CAPACITY setting for the volume being pumped (over 50 percent of rated capacity). Several makes of centrifugal pumps deliver the 200-psi rating with the pump operating in the capacity setting. While this rating is at 70 percent of the rated pump capacity, the general ride that can apply to all makes of pumps is to use capacity discharge for all volumes above 50 percent of the rated capacity.
c. Pump is “running away from water.” The supply is less than the desired quantity. This may be due to one or more of the following: suction strainer clogged, lift too high, too many lengths of suction hose, suction hose too small, mains too small to provide the required flow, partial collapse of lining in suction hose or an air leak in the suction line, couplings not tight or strainer not submerged sufficiently.
d. Pump only partially primed. Some operators discontinue the priming operation before the air in the pump is exhausted. Without a full prime, it is impossible to get full rated capacity discharge and discharge will be erratic. Take a few seconds longer to be sure the pump is fully primed.
e. When the lift is too high, cavitation will usually be present, as indicated by the sound emanating from the pump. It will sound like many small stones in the pump. This is the sound of collapsing air bubbles in the impeller. This will damage the impeller if not corrected by reducing the volume of water discharged or lowering the lift.
f. Pump may be worn enough to require an overhaul. Clearance rings, worn excessively, bypass too much water from discharge back to suction. Sand is present in all water systems and is not limited to draft operation. These particles of sand are mostly silica, which is harder than bronze or cast iron, so the rate of pump wear is proportional to the amount of sand carried in the water supply. The pump can be tested by operating at capacity and comparing the engine speed during this test with the rpm shown on the manufacturer’s plate. Under the same conditions of temperature, height of lift, and suction size and length, the engine speed readings are a good indicator of pump condition.