Prevent braided shaft seal problems

Sleuthing compression packing failures: Forensic analysis can point to root causes and corrective action.

By Chris Boss, Garlock Sealing Technologies

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Compression packing materials are used to seal pumps and valves in a wide range of industrial applications. As such, packing often is exposed to aggressive media and operating conditions that can cause failure. Troubleshooting and analyzing failures requires some detective work, gathering clues from different sources and putting them together to solve the crime, so to speak.

Effective troubleshooting calls for bringing to the investigation some degree of knowledge of various compression packing materials, the equipment in which they are installed and the system in which the equipment is installed. Investigating packing failure root causes logically begins with those closest to the “victim.” Seal installers, maintenance personnel, operators, process engineers and others can shed light on potential failure causes.

These “witnesses” to the crime, to complete the detective metaphor, can answer critical questions, including how failure is defined — excessive leakage, overheating, high flush water consumption, excessive friction load or blowout. It’s also important to determine if the application is a source of chronic failures or if the failure was unexpected. Were there any changes in the seal material, equipment or process, as well as system upsets or cleaning cycles preceding the failure? The installation procedure also is a vital piece of information, because improperly installed packing will invariably fail prematurely.

STAMPS

The first, and perhaps most important, step in troubleshooting compression packing failures is knowing the limitations of different types and configurations of packing. STAMPS, a simple acronym for size, temperature, application, media, pressure and speed, can serve as a useful guide to ensure selecting the right packing type for a particular application.

Size: Is the packing’s cross-section correct? Are the rings cut or formed to the correct length? Most pumps and valves conform to API/ANSI standards; otherwise, they should be field-measured.

Temperature: Check the system temperature against the manufacturer’s temperature rating for the seal. Knowing this limits the number of viable material choices.

The importance of matching seals to specific application variables and operating conditions can’t be overstated because many of the more common failure modes are directly or indirectly related to initial misspecification.

– Chris Boss, Garlock Sealing Technologies

Application: Some packings are designed specifically for rotary equipment, while others are intended for valves or static seals. Ensure that the packing is suitable for the equipment in which it’s being used.

Media: Check compatibility charts or contact the manufacturer to ensure that the packing is compatible with fluid being sealed. Slurries might require abrasion-resistant materials. Media that is toxic, explosive or subject to maximum allowable leakage requires packing capable of sealing at very low emission levels.

Pressure: System pressure shouldn’t exceed the manufacturer’s ratings for the product. Most systems operate at fairly consistent pressure, but it’s important to take into account severe spikes or surges that might occur.

Speed: Surface speed at the seal-shaft interface of rotating equipment determines how much frictional heat will be generated. High speeds call for materials that can withstand and effectively dissipate this heat.

In addition, try to observe the equipment in operation. Smoke, scent of burning fibers, vibration, grinding noises and pressure fluctuations are obvious clues to a problem. Also, inspect the equipment; most packings can accommodate equipment in less-than-perfect-condition, but there are limits. Check valve stems and pump shafts or sleeves for scratches, pitting and general surface roughness, all of which can damage the sealing surface.

Figure 1. When compression packing extrudes, abrasive particles can enter the stuffing box and the seal might fail.
Figure 1. When compression packing extrudes, abrasive particles can enter the stuffing box and the seal might fail.

Excessive clearances in a stuffing box can lead to extrusion of the seal material, ingress of particulates and, in severe cases, seal blowout (Figure 1).

In rotating equipment, bearings in poor condition can result in shaft runout, “wallowing out” a seal’s inside diameter. Shaft-stuffing box offset resulting from misalignment can cause one side of a packing set to become more heavily compressed than the other. Similar asymmetrical loading can occur in large, horizontally oriented valves, where the packing set bears the stem’s weight.

Parts can be misplaced during repair. Equipment can be reassembled with missing seat rings, bushings, lantern rings, O-rings and other essential parts. Also consider how the equipment interacts with other system equipment and control devices. For example, a downstream valve might produce pressure spikes in an upstream pump seal when it closes while the pump is still operating.

Forensic analysis

Expired packing can tell you a great deal about the failure cause. If possible, remove the entire packing set and arrange the pieces in the same sequence for reconstruction and examination. If the entire set can’t be retrieved, pieces of the remaining portions might indicate the failure mode, which, once detected, can point to a course of corrective action.

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