Pump Packing vs. Mechanical Seal
The large nut with the bolt going through it is a Darley packing nut and tube that pushes the injection pellet into the “stuffing box.”

By Andy Biron

In this article, we are going to discuss what packing and a mechanical seal are. But first we need to know what the purpose of these “seals” is in our pumping systems. To put it very simply, they keep the oil and water separated, thus allowing the oil to lubricate bearings, chains, and gears in the drive systems for the impeller. Contaminated oil, commonly from water, causes premature failure of these parts.

Pump Packing

First, we will start with packing. We need to describe what packing material is and its function in the pump assembly and the difference by manufacturer of the application.

Packing in general is designed to allow leakage between the sleeve, shaft, and packing to lubricate and dissipate heat emanating from the rotating impeller shaft. In a fire apparatus application, there is a sleeve is the area around the impeller shaft called the “stuffing box” and is surrounded by “packing” material.

Speaking of packing material, let’s identify the three major brands of class A pump manufacturers and what they use.

This is Darley “injection” packing.

This is Darley “injection” packing.

  • Darley uses a pellet-style packing. This material is “injected” into the stuffing box
  • Waterous uses a “ring” that surrounds the rotating shaft to the impeller in a front and rear stuffing box.
  • Hale uses a “ring” plus two lantern (plastic) rings that surround the rotating shaft to the impeller.
    • The lantern rings are designed to allow for more cooling with water flowing through the stuffing box.

Note that the number of rings for Waterous and Hale depends on the specific pump model.

This is Hale packing. It’s showing the order they are installed on the shaft. The white plastic units are the “lantern” rings. The three packing rings before the lantern rings are where the adjustment gland sets the tension.

This is Hale packing. It’s showing the order they are installed on the shaft. The white plastic units are the “lantern” rings. The three packing rings before the lantern rings are where the adjustment gland sets the tension.

This is Waterous packing. This shows that no lantern ring is used.

This is Waterous packing. This shows that no lantern ring is used.

Now let’s talk drip rate. Drip rate is the amount of water that is allowed to pass through the packing material. Packing needs to be cooled, and this happens from the water in the main body of the pump being forced into the stuffing box.

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Packing needs to allow water through it as a manufacturer-specific rate. On average, a drip rate of one drop per second is acceptable at 80 to 150 psi total pump pressure (refer to specific manufacturer manuals).

Now let’s discuss when to adjust and how. Packing drip rate needs to be seen from the packing area. Looking at it after it hits the spinning apparatus drive shaft will give a false drip rate, leading to improper adjustment.

While we are talking adjustments, what is over and what is under? Over adjusted is too tight. This adjustment does not allow for water to pass between the packing material and cool the area of the rotating shaft. Depending on the manufacturer there may be a specific coating for the packing to ride on the impeller shaft. With the excessive heat, this coating will be damaged and “eat” packing and not seal this area, allowing excessive water to flow by.

Under adjustment allows extreme amounts of water to flush out the packing material and the water to potentially change pump performance and/or damage the components below the packing area.

Either condition over an extended period will cause damage and problems to the pump and drive transmission.

Packing life will be determined by several factors. First is the mineral content in the water. Hard water after years of flowing through the packing will make the rope like material become extremely hard, not allowing it to crush and create a seal. Heat is the second big factor in this material’s life span. When the impeller experiences a major temperature spike, commonly caused when not flowing water and the impeller is rotating, the material will breakdown and exit the stuffing box.

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Mechanical Seal

Now we are one to mechanical seal. By definition, a mechanical seal is simply a method of containing the fluid within a vessel (pump, mixer, etc.) where a rotating shaft passes through a stationary housing or, occasionally, where the housing rotates around the shaft.

This pic shows a Waterous mechanical seal, the portion that is pressed into the stuffing box area.

This pic shows a Waterous mechanical seal, the portion that is pressed into the stuffing box area.

There is a static portion fitted into a housing (stuffing box) with a static seal. This may be an O-ring or ceramic material, a shroud, or it may be clamped in position. Millimeters of water will pass by both these faces to “cool” the mechanical seal. At no time should a mechanical seal have a “drip rate”.

The rotary portion of the seal is sealed onto the shaft by similar means. This seal can also be regarded as static as it rotates with the shaft at the same speed. As the relative velocity between the rotary part of the shaft and the seal is zero, it can be regarded as static.

This shows the other half of the same seal (photo 5) that is on the rotating shaft of the impeller. Note the spring on the back of this portion.

This shows the other half of the same seal (Waterous mechanical seal) that is on the rotating shaft of the impeller. Note the spring on the back of this portion.

Most often, the rotating portion of the seal is accompanied by a spring that provides tension to keep the two faces mated together. This tension is to accommodate any shaft movements and maintain the millimeters of cooling water between the two surfaces.

The mechanical, if treated correctly, will typically last the life of the impeller. The most common cause of mechanical seal failure is heat. Just like with packing when that impeller is rotating, and water is not flowing in and out of the pump, heat is generated. When the operator introduces cold water rapidly, one or both faces of the seal will fracture and the seal will fail.

This pic show both halves of the seal mated together when installed in a pump assembly.

This shows both halves of the seal mated together when installed in a pump assembly.


ANDY “SIPPY” BIRON retired from the City of Manchester (NH) Fire Department in 2023 after 22 years of line firefighting service. For the past nine years, he has been an EVT for W.D. Perkins Fire Pump Specialist based in Bedford, New Hampshire.

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Topics

In this article, we are going to discuss what packing and a mechanical seal are. But first we need to know what the purpose of these “seals” is in our pumping systems. To put it very simply, they keep the oil and water separated.

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