Fire pump primers may not be glamorous, but priming systems are critical to pump drafting performance. Pulling a prime quickly is key to fast water delivery, a basic fireground necessity. Last month, we answered several questions about pump priming. This month, we will continue our look at priming...
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NFPA 1901 contains the following guidelines for a vacuum test: "The vacuum shall not drop more than 10 in. Hg in five minutes. The primer shall not be used after the five-minute test period has begun." If the vacuum drops faster than 10 in. Hg in five minutes, look for and correct any additional air leaks. Even though you may meet the NFPA minimum leakage requirement, a small air leak in the wrong spot will reduce pump performance. Preferably, your performance should exceed the minimum set by NFPA. In reality, this 10 in. Hg. in five minutes was allotted for pump shaft packing adjustment and not for plumbing or valve leaks! If a mechanical seal is installed on your pump, the vacuum drop over a five-minute period should be insignificant.
During the test, major air leaks normally cause a hissing sound and can be located just by listening for them. A leak in a discharge valve can be located by placing the palm of your hand over the discharge outlet for about 30 seconds. If there is a vacuum leak, suction can be felt on your hand. (Do not open the valve while your hand is on it - serious injury can occur!) If you believe you have several small air leaks in the suction hose assembly after the vacuum test, you can use water to pressurize the suction hose and fittings and check for leaks, but be careful not to exceed the maximum pressure recommended by the suction hose manufacturer. Vacuum testing the suction hose alone is also good idea because some pinholes in the hose may self-seal under pressure.
Priming Time Test
Now that you have repaired all the air leaks and the vacuum test proves satisfactory, it's time to drop the suction hose in a static water source for a priming time test. NFPA 1901 recommends: "The time required to prime the pump shall not exceed 30 seconds if the rated capability is 1,250 gpm or less. The time to prime the pump shall not exceed 45 seconds if the rated capacity is 1,500 gpm or more. An additional 15 seconds shall be permitted to meet these requirements when the pump system includes an auxiliary four-inch or larger intake pipe."
If you own a 1,500- or 2,000-gpm midship pump that has a large-diameter suction connection at the front or rear of the apparatus, expect lengthened priming times. Suction and discharge piping manifolds found on industrial foam pumpers also contain large volumes of air.
Other reasons for extended priming times (beside large piping manifolds) are a damaged primer pump electric motor (which runs the priming pump under-speed) or a damaged priming pump (caused by a stuck rotary-vane in the priming pump rotor, or a priming pump that ingested a large amount of solid particulate). Check these out and repair or replace the components.
Question: During some of our drafting operations, we prime the pump, start to move water and then immediately lose prime. Why does this occur?
Many times, it is because of an air trap in the suction hose. If you have quite a few lengths of hard-suction hose off the pump into a static water supply, and the elevation of the hose does not follow a downward slope from the pump to the water source, an air trap occurs. Even though the suction hose has this air trap, it is still possible to initially prime the pump. However, the air pocket stays trapped inside the suction hose. As soon as the fire pump discharges a significant volume of water, flow in the suction hose increases and sweeps the air pocket up into the pump, resulting in a temporary loss of prime. Try to avoid suction hose air traps - if they are unavoidable, immediately re-prime the pump.
Another reason for loss of prime is a whirlpool developing by the suction strainer. This lets air enter the suction hose, causing a temporary loss of prime. Debris can also block flow into the suction strainer resulting in lost prime.
Question: Every so often, when we set up a drafting operation, we prime the pump, start discharging water and find pump pulsation (air in the discharge water). What causes this?
Constant pump pulsation or a ragged discharge stream indicates that air is being entrained in the water flow at some point in the suction side of the pump. This can happen after a pump achieves a prime and moves water, even though there are air leaks in the suction assembly. The result is lowered pump efficiency and reduced maximum pump capacity. This problem magnifies itself on lifts that are higher than normal and at high altitudes. Find and fix the air leaks in the suction hose fittings or assembly. A ragged discharge stream can also occur because of a whirlpool at the suction hose strainer allowing air to enter the suction hose and pump.
Question: What steps must we take to maintain our priming system?