Choose pump packing based on pump speed and media
In my work with customers, it is not uncommon to come across instances where there are repeated failures of pump packing. This situation can be frustrating and expensive for the customer, both in terms of downtime and the additional costs incurred to try and remedy the problem.
The conventional way to choose a particular compression packing for an application is to use the STAMPS process: Size, Temperature, Application, Media, Pressure, and Speed. The STAMPS process defines the critical criteria that should be taken into account when selecting the correct packing for a specific application. Of the five STAMPS criteria, Speed and Media are two critical factors can help you to choose a packing which will work well with the least chance of failure in a given application.
Reciprocating pump speed can be, and usually is, the main cause for packing failure. If compression packing is left to run without water or lubrication for any period of time, it will burn out and fail. In this case, the heat generated within the stuffing box at elevated pump speeds causes the compression packing to quickly burn out and fail.
To avoid this type of failure, two things must be done. First, it is critical that the packing be lubricated. Secondly, you must know the maximum speed recommended by the pump manufacturer. These two variables guide the selection of the appropriate packing. For example, for a high speed condensate pump, which operates at 3,600RPM, there are limited types of packing that will work in this application – typically pure exfoliated graphite packing, designed to perform up to a maximum speed of 4,000 fpm. In this type of application, the graphite will also self-lubricate the stuffing box at elevated speeds, making this the packing of choice for applications involving elevated pump speeds.
Difficult media also have special needs and requirements that must be taken into account to successfully resolve packing issues. For example, abrasive and corrosive applications, such as pumps used in mine slurry, or pulp stock packing processes having a high pH requirement, can also be very difficult to resolve. In these cases, a combination of packing which uses carbon and graphite packing together will usually provide the best results
For pumps that operate at slower speeds (i.e., below 500 RPM), such as low pressure feeders and the like, choosing an optimal packing may be less easy. Teflon packing usually does work at these slower speeds, but is typically too soft and weak to work in applications involving heavy, aggressive slurries. Graphite packing is also soft, but this weakness can be overcome by adding carbon corners to improve the performance of the packing in heavier applications. The addition of gasket spacers in the bottom end of the stuffing box can prevent the extrusion of aggressive slurries or pulp stock through the pump impeller shaft (see diagram at right).
In summary, when faced with chronic, unresolved compression packing issues on pumps, first look at the pump speed and the media being pumped to ensure that the correct packing is being used. In many cases, choosing an alternative packing that is better optimized for the pump speed and media being used will resolve many issues.
Bill Searle welcomes comments or questions on any aspect of this blog post. Contact Bill directly at crg@canadarubbergroup.com.
CRGI stocks a full range of compression packing. Contact our sales department to find out how we can help with all your compression packing needs.