Second in a Series
You have probably heard the old saying “If you don’t have time to do it right, then you’ll need to make time to do it over.” Understanding basic compression packing installation steps is key to getting the job done right the first time.
The goal of this article is to provide an overview of the critical steps necessary to ensure outstanding packing performance.
The first step is to remember the “5 Ps of Packing Installation”:
- proper selection
- preparation for installation
- product form
- patience (and a little pampering)
1. Proper Selection
Several braid configurations, packing fibers and combinations of packing fiber materials are available. Choosing the correct one can be intimidating and confusing. If an equipment manufacturer has not provided guidance or a specification, how do you decide what to use? One acronym commonly used as a guideline when making a packing selection is STAMPS.
As cited in the Compression Packing Technical Manual published by the Fluid Sealing Association (FSA), the acronym STAMPS can be used to remember key considerations for selecting a packing material and construction.
- Size: What are the stuffing box dimensions?
- Temperature: What is the temperature of the medium?
- Application: What type of equipment is it?
- Media: What medium is being sealed?
- Pressure: What is the internal pressure being sealed?
- Shaft speed: What is the surface speed of the shaft?
These attributes relate to the chemical compatibility of the packing material with the sealed media, its functional shaft speed capability, operational temperature and pressure parameters, and equipment service (rotating, reciprocating or valves).
The compression packing manual provides a breakdown of the performance properties of different classes of materials and fits into the STAMPS selection process.
Once the correct packing material has been selected, make sure the pump and accessories are properly prepared for installation of the packing.
The stuffing box must be clean, and the worn-out packing must be completely removed. Leftover packing debris can create uneven seating, which can cause leakage or plug the flush port line and lead to overheating problems.
Check the stuffing box’s condition, and inspect the shaft or sleeve wear for corrosion, nicks, burrs or scoring. Ensure that the gland follower and fasteners are in good condition. Check other components for cracks or signs of wear that can limit packing operational life.
A heavily corroded stuffing box may prevent the packing from sealing because of deviations on the stuffing box’s sealing wall. Corrosion can cause the outside diameter of the stuffing box to become slightly larger, resulting in a poor fit of the new rings. This can lead to the packing rings spinning within the stuffing box and a dramatic increase in leak rate.
A damaged shaft or sleeve will not allow the packing to seal properly. Even the highest-performing packing materials cannot function properly on a misaligned or damaged pump shaft and will experience a shortened operational life.
3. Packing Form
The next step is to determine if the packing rings were cut on-site from bulk spools or if they were preformed by the packing supplier and offered as individual ring sets customized for a specific pump or piece of equipment.
When using rings that are cut on-site, ensure that the packing is properly sized. The packing rings must reflect the shaft’s true size.
Avoid measuring a used packing ring as a reference to cut a new ring. Doing so can result in an improper cut length because the used ring may have been stretched during removal or it may have shrunk from loss of lubricant during use. Rings should be cut around a mandrel or a calibrated measuring tool equal to the shaft’s diameter. In some cases, the unworn area of a shaft sleeve may be an ideal measurement tool.
Rings that are cut too short will leave gaps in the seal along the stuffing box wall and pump shaft, causing excessive and uncontrollable leakage. Rings that are cut too long will not fit into the stuffing box.
Use the square “Butt” cut or a diagonal “Skived” cut for the ends (see Figure 1). During installation, stagger the cut ends of each so they are not all lined in a row (45-, 90- and 180-degree offsets). Offsetting the joints will eliminate a possible leak path that could form if the joint ends are all in a straight line.
Always follow recommendations that the packing or equipment manufacturer offers in the instructions. In recent years, preformed ring sets have grown in popularity for their more precise and uniform sizing, resulting in improved sealing and time savings compared with cutting on-site.
Proper placement, or positioning, is another critical step in packing installation. The first ring sets the stage for the alignment and positioning of subsequent rings. If the first ring is not seated and properly compressed, the remaining rings will be negatively affected for the following reasons: