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?
As a rule of thumb, midship pumps should be primed immediately after being engaged (run). This is to prevent serious damage that can occur from running the pump dry (drained of water) for an extended period. (Check with your fire pump manufacturer for pump-brand specific recommendations.)
Damaging a pump by running it dry for an extended period can happen after positioning an engine for a drafting operation at a pond and engaging the pump. If problems ensue, it may take minutes to remove the pump suction cap and connect and tighten lengths of hard suction hose from the supply source. In a situation like this, you are better off assembling your suction connections first, priming the pump second and engaging the midship pump third.
Damaging the fire pump by running it dry for an extended period means that the apparatus must be taken out of service for a potential costly repair.
Question: After activating a vacuum priming system, how do I know when the fire pump has achieved a "prime," so I can turn off the vacuum primer?
Assuming a good suction hose connection from the static water supply source to the pump inlet and a "tight" pump (no air leaks) after primer activation, you should see a small amount of primer lubricant being discharged onto the ground from the vacuum primer outlet. This changes to a steady lubricant/water mixture after the pump achieves a prime. As an audible verification of pump prime, you may be able to hear a noticeable difference in the sound of the primer as soon as it starts discharging the lubricant/water mixture. Always verify a fire pump prime by checking the pressure readings on the master-discharge gauge. If the pressure readings vary with corresponding increases in pump rpm speed, the pump is indeed primed.
If you have to operate a vacuum primer for an extended period to get a pump primed, you have problems either with the primer itself, or with air leaks into the pump (we'll talk more about these later).
Do not continuously run the primer for more than 45 seconds because the electric primer motor amperage draw may cause it to become too hot if run for an extended time without a cool-down period, causing damage. (Check with the primer manufacturer for their recommended maximum activation time and cool-down period.)
Question: Every time a fire pump is placed in service, is priming system activation required?
In many instances activating the electric vacuum primer is not required at all if the pump is "wet" (water filled). If the pump is "dry" (drained) and you are working from a booster tank or pressurized hydrant water supply, you can purge the air out of the pump without activating the primer by using incoming water pressure. For example, most midship pump equipped engines carry an on-board water supply in a booster tank. Booster tanks generally hold from 500 to 1,000 gallons of water. Since the water level in a full booster tank is at a higher level (elevation) than the pump, it naturally exerts force called head pressure on the pump, after opening the tank-to-pump valve.
The slight pressure of incoming booster tank water is enough to expel air inside the pump if the air is bled off by momentarily cracking open a discharge valve or the booster-tank fill valve. Therefore, if we start operation with a "dry" pump (the pump was previously drained) and proceed to open the tank-to-pump valve, the slightly pressurized incoming water from the booster tank fills the pump casing and "primes" the pump.
The slight head pressure exerted by the column of water in the booster tank forces water into the pump without the need to manually activate the electric vacuum pump priming system. This same procedure will work when the fire pump is connected to a hydrant fed by a city water supply - only the hydrant has much more pressure to do the job.
Caution: When using large diameter hose (LDH), always open a bleeder valve on the hydrant side of the fire pump inlet valve/supply hose connection prior to opening the hydrant, so trapped air can escape from the supply hose!
Question: We sometimes encounter problems when setting up a fire pump drafting operation, such as not being able to achieve a pump prime. What causes lack of prime?
One situation where a no prime will always occur is where the pump lift (the elevation between the pump inlet and the top of the static water source) is too high. For example, if the maximum capability of your priming system is 24 inches Hg., the maximum water lift capability (at sea level) is 27.2 feet.
Besides remembering maximum static lift limitations, keep in mind that when operating a fire pump at lifts over 10 feet and up to as many feet high as maximum vacuum primer lift capacity — maximum fire pump gpm capacity may be significantly reduced. To explain, your vehicle- mounted National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, 2003 Edition, compliant pump is rated at a 10-foot lift. Pumps 2000 gpm and over are rated at a six-foot lift. Therefore, using lifts greater than 10 feet, but less than the maximum primer lift, inherently reduce the fire pump's maximum gpm rated capacity while increasing risk of cavitation and vacuum leaks due to the higher vacuum required. When drafting, another factor in reduced pump capacity is suction hose friction loss. As you add extra lengths to the suction hose assembly for deep lifts or for hard to get at water sources, friction loss increases, which also reduces pump performance.
"Watch them mountains." Due to lower atmospheric pressure, pump lift capability and gpm performance naturally reduce as geographic elevation increases.
While a priming system is an important fire pump accessory, it is often thought of secondarily when planning apparatus specifications, and not given adequate attention during pump testing and maintenance. Good priming system design, installation and maintenance are critical for effective drafting operations. Make sure the integrity of your priming system, fire pump and pump operation training is up to par to prevent a fireground catastrophe. Next time, in "Fire Pump Priming Basics - Part 2," we will review troubleshooting common priming system problems.
DOMINIC COLLETTI is the author of The Compressed Air Foam Systems Handbook and Class A Foam - Best Practice for Structure Firefighters. He also is co-author of Foam Firefighting Operations 1 and The Rural Firefighting Handbook. Colletti is a former assistant fire chief and serves on the technical committee of NFPA 1500 Fire Department Occupation Safety and Health Program. He is the Global Foam Systems Product Manager for Hale Products and can be reached at [email protected].
