Consider the consequences of a priming system malfunction or failure during a drafting procedure at a working structure fire. A delay in delivering water, or no water delivery at all, is likely to lead to catastrophic structure damage and may compromise firefighter safety. Not a pleasant picture...
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Consider the consequences of a priming system malfunction or failure during a drafting procedure at a working structure fire. A delay in delivering water, or no water delivery at all, is likely to lead to catastrophic structure damage and may compromise firefighter safety. Not a pleasant picture. And a few conditions considered "minor" - like loose pump shaft packing, a bad suction hose connection and lack of lubricant in the primer oil reservoir - when combined, can cause priming system failure.
Fire pump accessories might 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. This article gives priming systems the much-needed attention they deserve. We will review how they work, the theory behind priming centrifugal pumps and procedures used to prime a fire pump. Our discussion will focus on priming midship-mounted fire pumps. However, the principles apply to all centrifugal pumps - including front mount, rear mount, auxiliary engine and Power Take Off driven.
Question: Why is a priming system needed on a centrifugal fire pump?
A centrifugal, midship-mounted fire pump on a fire department engine must be primed before it will operate. Primed means fully water flooded and all the air inside the pump casing preferably removed.
An operating midship pump filled with water is self-sustaining in that it creates quite a high vacuum at the eye of the impeller. This provides a continuous intake of water when drafting from a static supply source. However, when water is drained and the pump is filled with air, the centrifugal pump impeller is a very poor vacuum pump. This presents a problem when trying to draft from a static source where the physical level of the water supply is lower than the pump itself, such as when operating from a portable tank or pond. Operating an air-filled midship pump fails to produce enough vacuum required to provide lift, which is needed for the fire pump to flood itself when the pump is above the supply source. Under most drafting conditions, operating an air-filled pump will not initiate a prime and it therefore requires the aid of an external priming system.
Question: What are the main hardware components found in a priming system?
A priming system's main components normally consist of:
- A priming valve
- An electrically operated positive displacement vacuum pump powered by the truck's electrical system
- A primer lubricating oil reservoir (various manufacturers recommend different types of lubricants)
The vacuum pump and oil reservoir are typically located inside the apparatus pump house. The design of the vacuum-priming pump is usually a rotary-vane or rotary-gear pump type.
One technology that has taken hold in the last 10 years is a priming pump that totally eliminates the need for lubricant - it is "oil-less" - a rotary-vane vacuum pump that runs dry with no lubrication required.
Question: How does a priming system work?
An electric vacuum priming pump is a positive displacement air pump. Activating the vacuum primer removes air from inside the suction and discharge casings of the fire pump. Removing air lowers the pressure inside the pump casing below atmospheric pressure. It is atmospheric pressure - 14.7 pounds per square inch absolute (psia) at sea level - pushing on the static supply source that ultimately provides the force to move water into the hard sleeve suction hose and up to the eye of the impeller. This activity happens simply because of pressures trying to reach equilibrium - the higher pressure exerted by the atmosphere on the static water supply moves water toward the low pressure area inside the fire pump casing. Once the fire pump is flooded (primed) and then discharging water, it creates its own vacuum at the impeller eye to carry on this low-pressure area for continuous operation.
Question: After arriving at a fire scene and engaging the fire pump from inside the cab, how long can I wait before priming a dry fire pump?