Are you fed up with pump packing salesmen trying to sell you the latest and greatest packing? Selecting the right pump packing for your application requires some careful thought before falling victim to sales hype. To guide you in choosing the optimal packing for your application, we have compiled the following questions to consider asking and answering before you make your next pump packing purchase.
Question #1: What are we pumping?
What you are pumping has a direct relationship to the raw material you select when you make the choice of packing for each service. For example, pumping abrasive products such as slurries requires a different packing than for pumping water. It is also important to consider whether the state of the media being pumped can change when it comes into contact with either air or a contrasting temperature. For example, with cold water it is relatively easy to control leakage, but hot water can be a problem because its higher temperature requires a pump packing that is a good conductor of heat.
Question #2: What is the pressure?
When considering pressure, we need to be clear about what pressure we are referring to. For example, are we referring to suction, discharge, differential, or stuffing box pressure (SBP)? SBP is the critical pressure to consider and, depending upon the pump design, there are formulae for calculating the SBP when only suction and differential pressures are known.
Question #3: Are we using flush water?
Related to Question #2, if flush water is being used in the pump, the flush water pressure should be no more than 10 – 15 psi over the maximum stuffing box pressure. If higher flush water pressures are used, more gland adjustments will be required to control leakage, removing lubricant from the packing and shortening its life considerably.
Question #4: What is the pump speed?
What we really want to know is not the shaft rpm but rather the shaft surface speed, expressed as either feet per minute (fpm) or meters per second (m/s). This is important because you will want to choose a packing that can support the surface speed with becoming overheated or, worse yet, scorched. Formulae for calculating the shaft surface speed are as follows:
Shaft surface speed (fpm) = (Dia. inches X RPM) / 4
Shaft surface speed (m/s) = (Dia. millimeters X RPM) / 18750
Question #5: What is the media pH?
The pH factor is a numerical measure of the intensity of an acid or caustic. On the pH scale, 0 is the maximum concentration of hydronium ions as is found in concentrated acids, while 14 is the maximum concentration of hydroxyl ions as is found in concentrated alkalis or caustics. Knowing the pH factor is important when choosing a packing – in applications involving strong acids or caustics, one will always want to select a packing that can withstand the corrosive effects of these media.
Question #6: What is the pump start-up temperature?
It is important to note that the start-up torque in a pump can be 5 times the running torque for the break-in period. Consequently, the temperature limit of a pump packing may be exceeded within the first 10 to 15 minutes after pump start-up. This factor should be considered in the selection of a pump packing.
Question #7: What packing size do we need?
Packing comes in 1/16 inch increments so it is important to select the right size for your equipment. To determine the correct packing size you require, first measure the shaft size and then measure the bore of the stuffing box. Subtract the shaft size from the bore of the stuffing box, and then divide by 2 to obtain the correct packing size.
Question #8: What is the condition of the equipment?
Older equipment in a well-maintained condition may require a different packing than the same equipment which has been poorly maintained. In the latter case, the presence of excessive wear and defects, such as scoring on the shaft, may require the selection of a packing that is more “forgiving.”
Question #9: What are the service conditions?
Continuous service is by far the best service for packing or any sealing device. When packing runs continuously, the gland adjustments are less frequent which allows longer life for the packing. In contrast, intermittent service can be extremely hard on packing. The more frequent starting and stopping associated with intermittent service can be bad if the process fluid changes its state, in turn damaging the packing and requiring frequent gland adjustments.
Asking and answering these questions can help guide the right pump packing choice. At Canada Rubber Group Inc. (CRGI), we are always available to help customers choose from the many fine Teadit pump packings that we stock and supply.
For further information on any of these packings, or for assistance with your application, please contact our sales department directly at firstname.lastname@example.org.