PRIMING MIDSHIP FIRE PUMPS: QUESTIONS AND ANSWERS

BY DOMINIC COLLETTI

What would happen if a pump priming system malfunctioned or failed during a drafting operation at a working structure fire? The resulting delay in delivering water or no water delivery at all would likely lead to catastrophic structure damage and may compromise firefighter safety-not a pleasant picture. However, a few conditions considered “minor,” such as loose pump shaft packing, a bad suction hose connection, and lack of lubricant in the primer oil reservoir, when combined, can also 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. Although the discussion in this article focuses on priming midship-mounted fire pumps, the principles apply to all centrifugal pumps-including front-mounted, rear-mounted, auxiliary engine, and power take-off (PTO) driven pumps.

Question: Why is a priming system needed on a centrifugal fire pump?


A centrifugal, midship-mounted fire pump on a fire apparatus must be primed before it will operate. When a pump is “primed,” that means that the pump is fully flooded with water and that all the air inside the pump casing (preferably) has been 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 is a problem when drafting from a static source where the water supply level is lower than the pump itself, such as when operating from a portable tank or pond. An operating air-filled midship pump will not produce enough of a vacuum to provide lift (the difference in elevation between the pump inlet and the top of the static water source), which is needed for the pump to flood itself when the pump is above the supply source. Under most drafting conditions, an air-filled pump will not initiate a prime and thus requires 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 (1) a priming valve; (2) an electrically operated, positive displacement vacuum pump powered by the truck’s electrical system; and (3) a primer lubricating oil reservoir. Various manufacturers recommend different types of lubricants. Typically, the vacuum pump and oil reservoir are located inside the apparatus pump house. The design of the vacuum-priming pump is usually a rotary-vane or rotary-gear pump type.

The latest technology available today is a new priming pump that completely eliminates the need for lubricant (i.e., it is oil-less). The rotary-vane vacuum pump 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 happens simply because pressures are 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 pump casing. Once the pump is flooded (primed) and is 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 pump from inside the cab, how long should I wait before priming a dry pump?

As a rule of thumb, midship pumps should be primed immediately after being engaged. This prevents the serious damage that can occur from running the pump dry (drained of water) for an extended time. Check with your fire pump manufacturer for specific recommendations.

Damaging a pump by running it dry for an extended time can happen after positioning the engine at a pond for a drafting operation and engaging the pump. Caution! Always chock the rear wheels immediately after setting the parking brake! If problems ensue, it may take several minutes to remove the pump suction cap and connect and tighten lengths of hard-suction hose from the supply source. In a situation such as this, it is better to assemble the suction connections first, prime the pump second, and engage the midship pump third.


If a pump is damaged by running it dry, the apparatus must be taken out of service for a potentially costly repair.

Question: After activating a vacuum priming system, how do I know when the pump is primed 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 (i.e., 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 prime. To verify pump prime, listen-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 check the pressure readings on the master discharge gauge to verify pump prime. 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 time to get a pump primed, there are problems with the primer itself or there are air leaks into the pump. This will be discussed later.

Do not run the primer continuously for more than 45 seconds. The electric primer motor amperage draw may cause the primer to become too hot if it is run for an extended time without a cool-down period, resulting in damage. Check with the primer manufacturer for its recommended maximum activation time and cool-down period.

Question: Every time a pump is placed in service, must the priming system be activated?

In many instances, activating the electric vacuum primer is not required at all if the pump is wet (i.e., water filled).

If the pump is dry (i.e., drained) and you are working from a booster tank or a 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 onboard 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 the tank-to-pump valve is opened.

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, start operations with a dry pump and proceed to open the tank-to-pump valve; the slightly pressurized incoming water from the booster tank will fill the pump casing and prime the pump.

The slight head pressure that the column of water in the booster tank exerts forces water into the pump without requiring manual activation of the electric vacuum pump priming system. A variant of this procedure will work when the fire pump is connected to a hydrant fed by a municipal water supply, except that the hydrant has much more pressure to do the job.

Caution! When using large-diameter (LDH) supply hose, always open a bleeder valve on the hydrant side of the pump inlet valve/supply hose connection before opening the hydrant so trapped air can escape from the supply hose!

Question: You sometimes can encounter problems when setting up a pump drafting operation, such as the inability to achieve a pump prime. What causes a no-prime condition?

A no-prime condition will always occur where the pump lift is too high. For example, if the maximum capability of your priming system is 24 inches of mercury (Hg), the maximum water lift capability (at sea level) is 27.2 feet.

In addition to remembering maximum static lift limitations, keep in mind that when operating a pump at lifts of more than 10 feet and up to maximum vacuum primer lift capacity, maximum fire pump gpm capacity may be significantly reduced. To illustrate, your vehicle-mounted NFPA 1901, Standard for Automotive Fire Apparatus, 1999-compliant pump is rated at 10 feet lift. Pumps of 2,000-gpm capacity and higher are rated at a six-foot lift. Therefore, using lifts greater than 10 feet but less than the maximum primer lift inherently reduces the pump’s maximum gpm rated capacity while increasing risk of cavitation and vacuum leaks because of the higher vacuum required.

When drafting, another factor in reduced pump capacity is suction hose friction loss. As extra lengths are added to the suction hose assembly for deep lifts or for hard-to-get-at water sources, friction loss increases, also reducing pump performance.

“Watch them mountains.” Because of lower atmospheric pressure at higher elevations, pump lift capability and gpm performance naturally decrease as geographic elevation increases.

If excessive lift is not the cause of a no-prime problem, check the integrity of the priming system for the origin of a recurring no-prime condition. There can be a variety of possible causes for this condition, and the maintenance shop is a good place to start looking for possible causes.

First, perform a cursory check for the following:

  1. Primer reservoir oil level. If the primer is using an excessive amount of oil, check to see if the vent hole in the oil line (which prevents a siphon) is blocked.
  2. Correct priming valve operation.
  3. Sound electric primer motor power and electrical connections. Now, check which gauge wire is connected to the primer motor/solenoid. How long is the power cable? Is the wire gauge sufficient for its total length in feet vs. its amp-carrying capacity? Check with the primer manufacturer for wire-gauge/amp draw/total length requirements. Does the primer motor have a good ground connection? A primer needs a good electrical supply and a properly sized electrical connection to work correctly and keep on working overtime.

    In addition to the above items, one cause of a no-prime condition is a damaged electric vacuum primer that does not turn or, if turning, is worn and not able to pull the required amount of vacuum.

    VACUUM PRIMER TEST

    To test for a worn or damaged primer in the maintenance shop, isolate the primer by disconnecting the vacuum primer/fire pump tubing connection at the vacuum priming pump. Connect a manometer or master vacuum gauge to the primer. Activate the primer and see how deep a vacuum it will pull.


    NFPA 1901 contains guidelines to be used during a vacuum primer test: “The maximum vacuum attained shall be at least 22 in. Hg.”

    See how the vacuum reading compares with the NFPA minimum requirements and the primer’s original vacuum specification from the manufacturer. Repair or replace the priming pump if required.

    If a no-prime condition still persists when drafting from a static water source, air leaks into the fire pump are the likely culprits. Some common reasons for these air leaks include the following:

    • The pump operator left the pump discharge, suction, or drain valves open;
    • The fire pump suction caps are loose or have bad gaskets;
    • The pump’s discharge and suction valves leak, even when fully closed;
    • The pump’s external relief valves may be damaged and leak;
    • The pump casing, flanges, and discharge or suction manifold piping have pinholes, the pump shaft seal leaks, the pump shaft packing needs adjustment, or a gasket is defective; or
    • The hard-sleeve suction hose has pinholes, bad gaskets at connections, leaking couplings, or loose connections.

    PUMP, MANIFOLD, AND SUCTION HOSE ASSEMBLY VACUUM TEST

    Once the above causes of air leaks are checked and any obvious deficiencies are corrected, run a vacuum test on the entire pump and suction assembly.

    To do this

    1. Fully charge the truck batteries;
    2. Drain the pump and close the tank-to-pump valve;
    3. Install the suction hose you normally use on the intake of the pump;
    4. Cap the end of the suction hose;
    5. Remove all caps from the pump’s discharge valves (keep the discharge valves closed);
    6. Open all pump suction valves, and cap every pump intake opening;
    7. Check and clean out the test gauge connector line;
    8. Connect a manometer to the test gauge connector line;
    9. Run the primer until the vacuum reading is 22 inches/Hg or greater, as stated in the primer manufacturer’s original specifications; and
    10. Watch the manometer over the next five minutes to see if or how far the reading drops.

      NFPA 1901 contains the following guidelines for a vacuum test:

      “The vacuum shall not drop more than 10 in. Hg. in 5 minutes. The primer shall not be used after the 5-minute test period has begun.”

      If the vacuum drops faster than 10 inches/Hg in five minutes, look for and correct any additional air leaks.

      Note: 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 the NFPA. In reality, this 10 inches/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 by just 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 half a minute. If there is a vacuum leak, you can feel the suction on your hand. Caution: 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 a 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 supply source for a priming time test. NFPA 1901 recommends the following:

      “The time required to prime the pump if the rated capacity is 1250 gpm or less shall not exceed 30 seconds. If the rated capacity is 1500 gpm or more, the time to prime the pump shall not exceed 45 seconds. An additional 15 seconds shall be permitted to meet these requirements when the pump system includes an auxiliary 4 in. or larger intake pipe ellipse.”

      If you have a 1,500- or 2,000-gpm midship pump that has a large-diameter suction connection located at the front or rear of the apparatus, expect extended priming times.

      Suction and discharge piping manifolds found on industrial foam pumpers also contain large volumes of air. For example, Hale manufactures midship- and rear-mounted pumps used for industrial applications that are rated at 3,000 gpm. These pumps have eight-inch suction and six-inch discharge pump body connections. Apparatus builders normally install two vacuum priming pumps within the priming system to reduce the time required to evacuate the large volume of air found inside the eight- and six-inch pump suction and discharge manifolds. (These pumps, when equipped with these manifolds, will normally move more than 5,000 gpm when connected to a suitable positive water supply.)

      Other possible causes of extended priming times (beside large piping manifolds) include a damaged primer pump electric motor (which runs the priming pump underspeed), a damaged priming pump (caused by a stuck rotary vane in the priming pump rotor), or a priming pump that has 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 from 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 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 reprime the pump.

      Another reason for loss of prime is a whirlpool developing by the suction strainer. This allows air to enter the suction hose, causing a temporary loss of prime. Debris can also wrap itself around and block flow into the suction strainer, resulting in lost prime.

      Question: Every so often when you set up a drafting operation, you 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 that allows air to enter the suction hose and pump.

      Question: What steps must you take to maintain your priming system?

      Recommendations for each priming system will vary by manufacturer. Contact your priming system manufacturer and ask for testing and maintenance information. Make priming system checks an integral part of your scheduled fire pump preventative maintenance and testing program. Failing to maintain the priming system is foolish. Think about it-your engine may cost between $150,000 and $300,000. If your priming system fails because of neglected maintenance, your engine and your investment in it could be useless at the next fire at which drafting is required. A few suburban departments have been lulled into a false sense of security because their fire districts have lots of hydrants and, therefore, they don’t need to use the primer much. Don’t let it happen to you.

      DON’T OVERLOOK THE PRIMING SYSTEM

      While a priming system is an important fire pump accessory, it is often thought of secondarily when planning apparatus specifications and is 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 are up to par to prevent a fireground catastrophe.

      PRIMING SYSTEM DIAGNOSIS

      Symptom: Priming pump electric motor doesn’t turn.

      Possible cause:

      • Dead battery.
      • Main power feed wire has faulty connections.
      • Faulty electric primer motor ground.
      • Solenoid switch that activates primer is faulty.
      • Wiring to control solenoid has faulty connection or is disconnected.
      • Defective solenoid.
      • Defective motor.
      • Defective switch on priming valve.

      Symptom: Vacuum priming pump is slow to pull a prime, or priming maximum vacuum is lower than normal (vacuum holds).

      Possible cause:

      • Low battery charge.
      • Battery size insufficient.
      • Cable between battery and primer insufficient size.
      • Dirty electrical connectors on cable.
      • Faulty ground.
      • Extremely large quantity of suction hose or piping.
      • Piping between prime and pump is too small or too long.
      • Priming valve not opened fully (operator error).
      • Lift too high.
      • Primer vanes worn.
      • Loose or broken V-belt (on belt-driven pumps).

      Symptom: Priming pump pulls some prime but does not hold vacuum.

      Possible cause:

      • Air leaks somewhere in system.
      • Faulty suction hose gasket.
      • Pump or discharge drain left open.
      • Gauge or draining tubing line disconnected.
      • Leaking or stuck priming valve.
      • Rusted piping.
      • Leaking suction hose.
      • Faulty mechanical seal.
      • Leaking packing.
      • Faulty pump flange gasket.
      • Cracked or broken casting.

      Symptom: Pulls full prime that holds, but pump loses water when starting to pump or pump cavitates prematurely or pump cavitates at high pressures.

      Possible cause:

      • Pump impeller damaged or severely worn.
      • Suction hose submerged in inadequate water supply.
      • Air leak during water flow (see preceding “Possible cause” list).
      • Clogged inlet strainer on pump suction hose.
      • Pump impeller prerotation baffle missing.
      • Air pocket in piping or air trap in suction hose.
      • Air accumulation in piping.
      • Turbulence in piping.
      • Vertical distance to water source too high or too many lengths of suction hose.
      • Failing (collapsing) suction hose.

      References

      1. Davis, Larry. Rural Firefighting Operations-Book Two, International Society of Fire Service Instructors, 1986, Chapter 8, p. 159
      2. Handwerk, Gary. Priming System Diagnosis, Hale Products, Inc., 1999, an unpublished work.

      DOMINIC COLLETTI is the director of North American OEM accounts at Hale Products, Inc. A firefighter with the Humane Fire Company in Royersford, Pennsylvania, he is a member of the National Fire Protection Association committees for NFPA 1500, Standard on Fire Department Occupational Safety and Health Program-1997, and NFPA 18, Standard on Water Additives for Fire Control and Flammable Vapor Mitigation. He is the author of the book Class A Foam-Best Practice for Structure Firefighters (Lyons Publishing, 1998, distributed by Oklahoma State University/IFSTA).

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