What is a Pump’s Packing

No, it’s it has nothing to do with guns or firearms. Packing is a type of material used within a seal box in a pump to keep fluid away from the pump’s bearings and or electrics. Originally, packing was formed from materials such as leather, rope or flax. These materials have long since been replaced by specialized materials often impregnated with graphite or other lubricants to improve performance, but the principle is still the same.

The shaft or shaft sleeve is measured to determine its circumference, and a length of packing material is cut to match that circumference. The ends are typically cut at 45 degrees, and when wrapped around the shaft or shaft sleeve, they fully overlap at the point of union.

As the illustration below shows, it is generally installed with multiple rings, each forming an independent ring but compressed as a unit by a single gland.

The packing in the seal box or packing box as it is commonly referred to, forms an effective seal when the compression or packing gland is tightened. This gland effectively squeezes the packing material around the shaft to form a tight seal when the pump is not running. However this tight seal creates friction when the shaft begins to rotate during normal pump operation. If a form of lubrication and or cooling were not employed, the friction would  heat the packing material causing it to burn or carbonize in very short order. This would result in seal failure and liquid leaking into sensitive components.

The most common way to avoid friction and dissipate heat in clear liquid applications is to allow a small amount of product to leak by the seal during operation. This provides a level of lubrication but more importantly it also carries away any heat created by friction.

Flush water is used to preclude solids in slurry applications, or in chemical applications it is used to provide a liquid that can be acceptable to vent to the environment. It must be 100 % solids free. Any particles, no matter how few or how small will embed themselves in the packing material and form a grinder to destroy the shaft or shaft sleeve.

The rule of thumb is flush water pressure be set to pump shut off pressure, plus 10% or plus 20 psi, which ever is greater.

As flush water is always introduced into the seal box at a pressure that is higher than the pumps discharge pressure it will naturally migrate in both directions along the shaft axis. The water heading toward the impeller side serves to flush solids away from the packing material as well as providing lubrication and cooling . The water heading toward the bearing side of the box is solely for lubrication/cooling of the packing on that side of the injection port. A metal ring, often made of brass , acts a spacer between the multiple rings of packings. This lantern ring as it is called, has grooves and holes in it to provide liquid around the complete circumference of the shaft.

If you are interested in the causes of packing gland failure and how to replace and adjust the compression gland, I will cover that under a separate lesson soon.

Cheers

RJ