ASK ERNIE: Mechanical Seals, Pump Packing & More

Apparatus Maintenance Q & A

CarolinaFireJournal -

10/14/2011 -

What is the difference in mechanical seals and pump packing? Pros and cons of each?

Prior to mechanical seals, every pump had a mechanical seal of some style. Early centrifugal and positive displacement pumps had a rope style packing with a grease fitting of one of may types to keep the packing lubed and resilient. Today, we have a pump packing of newer and more resilient composites that require no lubrication.


Darley Pump Company uses both the old ‘Plastalic’ injection packing that assumed the role of the common rope packing with a much easier means of adjustment. As the packing became washed out and ‘worn,’ it would no longer stop leaking or would leak at a much too high rate. This situation led to ‘over-adjusting’ the packing in an attempt to achieve a drip free condition while on the apparatus floor. The over-tightening led to high heat and scoring of the shaft. Once the shaft was substantially damaged by scoring, no amount of new packing would cure the problem of leakage. The packing style pump had the ability of being ‘re-packed’ by removing the old packing and installing new. It was not fun, but could restore the packing to its original leak rate while pumping if the shaft had not been scored and could be accomplished without major disassembly of the pump.

Mechanical seals in firefighting pumps are likened to the mechanical seal in the modern day centrifugal water pump that circulates coolant through the motor. Coolant water pumps can go without changing for the life of the automobile if the coolant is maintained in proper condition. The mechanical seal in the firefighting pump can also last the life of the pump if clean water is flowed through the pump and the seal is not allowed to EVER overheat due to loss of water. However, every time the water tank goes dry with no water available to the seal surfaces, those surfaces — ceramic and hard carbon — become extremely hot. When cool/cold water does make contact with these overheated seal surfaces, the result is usually a cracked component of the seal surfaces. The result: an external leak and a negative pressure leak during drafting situations.

Replacement of the mechanical seals is usually a very labor intensive task. Some pumps utilize a single mechanical seal, some use double shaft-end seals. The preferred seal today is the mechanical seal due to its total leak free ability and long service life. Packing style pumps are still available, but are fewer each year.

What is the correct drip rate for pump packing Waterous and Hale?

Old Uncle Ernie is not falling for this one. Get out your manuals and check the drip rate for the pump you are asking about.  A mechanical seal pump has a zero drip rate allowed. A packing style pump has a drip rate as designed into a particular pump by the engineers to keep the pump shaft and packing cool while pumping water. A drip rate is one that occurs only under hydrant situations where a single stuffing box is utilized — like Hale — and on a double stuffing box pump — like Waterous. If the pump is a single stage, the packings only leak while at hydrant. If the Waterous style pump with dual stuffing boxes on a two stage pump is used, the pressure/series mode will leak only at the compound side of the pump.

How often should I perform a drift test on my aerial hydraulics?

The aerial drift test is an annual test performed at 60 degrees elevation and full extension. The drift test is for a period of one hour and may not drift more than the manufacturer’s specified amount.  Refer to NFPA 1911; Standard for the Inspection, Maintenance, Testing, and Retirement of In-Service Automotive Fire Apparatus, 2007 Edition. Section;; and others, depending on the type of aerial apparatus. Please note that all aerials must operate with no Class 3 fluid leaks and should be evaluated for immediate hose replacement or component repair if it has a Class 2 leak. 

What interlocks does my truck have if it is after 2010 and what interlocks does it have if it is older?

Today’s apparatus have many more interlocks than in the past. Electronic motors and multiplex’s electrical systems allow for more interlocks for the safety of the operator. On a pumper, things like: if you have not achieved parking brake set confirmation, pump engagement confirmation; idle confirmation, the pump panel throttle will not activate. In an older mechanical motor, if an error was made — usually through the stress of the moment — and the transmission was left in “D” gear and then a move was made to shift the lever from “Road Mode” to “Pump Mode,” no movement of the pump gearing would occur because of the torque on the slider gear in the Road Mode. The result, when the operator got to the pump panel and began the unrestricted throttle up on the mechanical engine, the torque in first gear converter would finally overcome the parking brake and the pumper would leave the scene. 

Many occurrances of this type happened in years gone by. Thank goodness for these interlocks today. If you are operating an aerial device, interlocks abound. If the stabilizers are not deployed, the aerial may not be elevated. If it does, the interlocks are in failure, or someone has placed the aerial interlocks in ‘override,’ which is a no-no. If the aerial can only be operated in override mode, the aerial is in an ‘Out-of-Service’ condition and must be repaired before further use. You will note that I did not say ‘outriggers.’ Most heavy duty modern aerials have both outriggers and downriggers, the combination of which are called stabilizers  Regardless of the style of stabilization components are utilized, the aeril utilizes ‘Stabilizers’ for that function.

What is the allowable variance in my relief valve when I check it before it needs to be repaired.

Wow, trying to catch old Ern in a tangle? To which relief valve do we refer? A system pressure relief valve in the aerial hydraulic system? Power steering stop pilot relief valve? End-of-stroke cylinder hydraulic relief valve? Water pump relief valve? Let’s take a shot at the firefighting water pump relief valve. Once set, it must function with no pressure rise greater than 30 PSI above the setting selected in any of the following situations. It must be annually checked at the time of the annual in-service pump testing and must be tested at 100 percent capacity at 150 PSI, again at 90 PSI, and finally at 50 percent rated capacity at 250 PSI.  Routinely, the relief valve shall be tested during daily, weekly, and/or monthly inspections, depending on the type of department schedule is dictated. Relief valves must be checked at a minimum of monthly.

When putting my pump in gear why should I stop in the center for air actuated pump shift?

Let’s make it clear. Not all manufacturers stipulate a stop in the middle. Not all actuators have a middle position. Consider the Waterous electrically switched pump mode — Road and Pump are your only two positions.  For those that do stipulate a stop in the middle it is for cause. With full air pressure of 120 PSI holding the gear shift in the Road Mode, and then immediately applying 120 PSI to the Pump Mode side, there is a softening of the shift movement while the air from the Road Mode exits the switch. This has led to some ‘butt tooth’ situations where the pump would not attain a completion of shift from Road to Pump modes. To ovecome that issue, when the lever is placed in the middle position for a couple of seconds, you will hear all air from the Road Mode exit the switch, leaving zero pressure holding the shift cylinder in Road Mode. When you then move the selector to the Pump Mode, the applied 120 PSI or system pressure will provide a resounding and rapid movement to the Pump Mode position and overcome a potential butt tooth situation. If the pump still refuses to drop the butt tooth issue, and not shift,  the approved method is to shift the main transmission from “N” to “R” and quickly back to “N”.  The pump will most assuredly achieve pump mode.

What is urea and why do I have to add it to my truck?

If you are adding urea, you have a diesel engine in you vehicle and it is manufactured with a serial number indicating the motor was manufactured to meet 2010 emissions standards.  Urea is essentially a synthetic pig urine (urea) and is injected into the exhaust stream after the DPF (Diesel Particulate Filter) and flows into an additional ‘calalytic’ unit where it purifies the exhaust stream to meet EPA standards. Urea can be purchased at any truck stop, and at fueling islands, some with dispensing nozzles, some using gallon jugs. All diesel manufacturers today use urea with the exception of Navistar, MAN, and the new Caterpillar vocational truck. The process is used in vehicles as small as 3500 (class 3) diesel pickups.

What is the NFPA recomendation on servicing my apparatus annually or semi-annual?

Once again, let’s get the book out. The list is far too extensive to cover here. NFPA 1911, 2007 Edition covers those items that must be covered along the firefighting vehicle line. But the NFPA list is small in comparison to the manufacturer’s list of service items. The manufacturer of the chassis, body components, aerial and the like are set forth by the manufacturer, not NFPA.  NFPA will stipulate XYZ must be maintained at least annually, or more often as stipulated by the manufacturer’s recommendations. A manufacturer’s recommendation is one that carries warranty ramifications if not done to the manufacturer’s minimum standards.

To submit your questions to Ask Ernie, e-mail [email protected]
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