Packing Tips

By Rick Kushinski, Service Manager

One of the most overlooked preventive maintenance tasks for a NASH vacuum pump is replacing the stuffing box packing annually. Regular replacement helps keep the packing pliable, maintain an effective seal, and reduce the risk of shaft, bearing, and pump damage.

Old packing causes problems

As packing ages, it becomes hard and brittle and can no longer seal properly between the shaft and the stuffing box bore. This poor seal allows air to enter the Nash Vacuum pump through the stuffing box, especially when the pump operates at higher vacuum levels. Air ingress reduces the pump’s ability to maintain the required vacuum, lowers performance, and can increase operating time and energy use. At lower vacuum levels, worn packing may instead allow excessive seal water to leak along the shaft. If severe, this water can enter the bearing housing and cause premature bearing failure.
When these leaks occur, the packing gland is often tightened too much in an attempt to restore the seal. Excessive compression increases friction against the shaft, causing the shaft coating to overheat and potentially crack. Once this damage occurs, the problem becomes more serious and the pump may need to be removed for repair.

Changing the packing annually makes removal easier because the material remains pliable. Before installing new packing, shut down and isolate the pump, relieve all pressure, and remove the gland. Extract every old packing ring from the stuffing box; do not replace only the first two or three accessible rings. Use a packing hook or suitable removal tool, taking care not to scratch the shaft or damage the stuffing box. Inspect the removed rings for hardening, brittleness, burning, uneven wear, or signs of water leakage. Clean the stuffing box thoroughly, then inspect the shaft and bore for grooves, corrosion, or scoring. Any remaining debris or damaged surfaces can prevent the new packing from sealing correctly and may cause the same problems to return. Install new packing only after the stuffing box and shaft are clean and in suitable condition.

Another Tip

While this task is being performed, make sure that the drip tray between the stuffing box and the bearing housing is not full of debris and that the drain is clear. A clogged drain will allow water tobuild up in the drip tray and possibly enter the bearing housing and contribute to a premature bearing failure.

When repacking a pump stuffing box, first shut down and isolate the pump, relieve all pressure, and remove the gland. Extract every old packing ring with a packing hook or suitable tool, taking care not to scratch the shaft or stuffing box. Clean the box and inspect the shaft and bore for debris, grooves, corrosion, or scoring before installing new packing. Cut and fit each ring individually, staggering the slits so they do not align. Seat each ring fully with the gland before adding the next one. Staggered joints help control excessive leakage during start-up, but they should not block water from reaching the outer rings. Install the gland without forcing it into the box, then start the pump with the packing loose and adjust it gradually while monitoring leakage and temperature.

The packing drip is decided by the two outer rings. Adjusting the gland in only these rings will compress the packing and make it tighter on the shaft. The last ring opening or slit should be at the bottom of the shaft, and the gland should be started or registered in the stuffing box. You do not want it to drop on the shaft.

To start the gland in the stuffing box, cut ¼ inch from the new ring and flatten it a bit. This will allow it to go into the stuffing box more easily. Do not force the gland into the packing box, as it will put too much pressure on the shaft and packing. Most of the packing is graphite impregnated Teflon and can wear into a shaft and groove it if you are not careful.

Install the packing rings one at a time in the stuffing box, seating each ring fully with the gland before adding the next. Stagger each ring joint so the openings do not line up, and keep the final ring opening at the bottom of the shaft. After installing the first three rings, use the gland to confirm they are seated evenly and correctly. Add the fourth ring, install the gland without forcing it, and start the pump with the packing loose. Adjust the gland gradually while monitoring leakage and packing temperature. If space allows, install the final ring only after confirming that the packing is seated properly. Maintain a slow drip during break-in; no leakage combined with excessive heat may indicate that the packing is too tight.

Packing that runs excessively hot or begins to smoke is a warning that it may be overtightened or not receiving enough cooling leakage. A gland tightened too much increases friction between the packing and shaft, which can overheat or crack the shaft coating, groove the shaft, and damage the packing. Excessive heat can also transfer to the bearings and contribute to premature bearing failure. During start-up, maintain a slow, controlled drip and monitor packing and bearing temperatures. If there is no drip and the temperature continues to rise instead of peaking and returning to normal, stop the pump, allow it to cool, and loosen the gland slightly. Adjust the gland gradually until leakage and temperature remain within the recommended operating range.

When starting a new pump, or a pump with new packing, the following tips that will help extend the life and performance of your pump packing:

• Ensure that the gland is loose, but not riding on the shaft
• Let it leak for at least 30 minutes while the equipment is broken in
• An optimal leak is approximately one drip per second, to achieve this adjust one end of the pump at a time, monitoring the pump and its operating temperatures
• Adjustments should be made by turning gland nuts one flat at a time, on both sides of the shaft, and letting it run for at least 20 minutes
• Some pumps will not drip at all, in such cases, it is crucial to monitor pump temperature. If the pump is running at 54°C; or below, the packing is fine
• If the shaft is grooved from overtightening, it will be difficult to adjust the pump packing. To avoid overtightening, allow the seal to break in, then tighten as noted above