KEYSTONEIndustrial Services
Interactive trainer

Mechanical seals & packing

A leaking pump seal is usually a bad environment, not a bad seal. A mechanical seal runs on a thin film between two lapped faces and lives on its flush — lose it and it runs dry and heat-checks in seconds; a clogged flush only under-cools it. Packing is meant to weep a little: over-tighten it and it burns dry and scores the sleeve; under-tighten and it floods. Six pumps, four reads — leak, flush, faces, pressure — including the two look-alike pairs you can only split by the flush flow and the face temperature.

By Francisco Padilla Interactive Technician level

Before you start — job brief

A pump is leaking at the shaft and you've been sent to diagnose it. The trap is assuming a leaking seal is a bad seal — most of the time it is a bad environment. A mechanical seal runs on a micro-thin film of liquid between two lapped faces and lives or dies on its flush; compression packing is a different animal and is meant to weep a little. Read the leak, the flush, the faces and the box before you condemn anything.

  • A mechanical seal needs its flush — lose it and it runs dry, overheats and heat-checks in seconds
  • A clogged flush merely under-cools the seal: it runs hot and worn, not cracked and blistered
  • Packing is meant to weep a few drops — over-tighten it and it burns dry and scores the sleeve; under-tighten and it floods
  • Read four things: the leak rate, the flush/cooling, the face condition & temperature, and the seal-chamber pressure

A leaking seal is usually a bad environment, not a bad seal

The most expensive mistake at a leaking pump is condemning the seal. Most of the time the seal did nothing wrong — its environment did. A mechanical seal is two flat, lapped faces, one turning with the shaft and one held still in the gland, squeezed together by springs and running on a film of liquid a fraction of a thousandth of an inch thick. That film both lubricates and cools the faces. Keep it fed with clean, cool liquid and the faces glide for years; starve it, boil it, or corrode it and the seal fails in ways that look like a seal defect but are really a flush, a box, or a materials problem. This trainer walks six pumps: read the leak, the flush, the faces and the box pressure on each, and let the readings — not the leak alone — name the fault.

The mechanical seal lives on its flush

The single most important thing you provide a mechanical seal is its flush — a controlled stream of clean, cool liquid piped to the seal chamber, usually to an API plan such as a recirculation line off the pump discharge through an orifice (Plan 11), or from an external source (Plan 32). The flush carries away the heat the faces generate, keeps the box flooded so the film can't flash to vapor, and washes solids away from the faces. Lose the flush completely — a plugged line, a valved-off plan, a lost suction that empties the box — and the film boils off in seconds. The faces run dry, the temperature spikes, and the carbon heat-checks, cracks and blisters. At the same time the starved box loses pressure and the remaining liquid flashes to vapor, which finishes off whatever film was left. That is a dry run, and it is catastrophic and fast.

Field note — the two heat-and-leak faults that look the same

A hot, weeping seal has two very different causes, and only three readings tell them apart. If the flush has stopped entirely — zero flow — the faces are cracked and blistered, and the box pressure has dropped and is flashing to vapor, the seal ran dry: it is destroyed and must be replaced, and you have to restore the flush before it happens again. If instead the flush is merely restricted — some flow, just not enough — the faces are hot and worn but still intact, and the box pressure is normal, the seal is only under-cooled by a clogging orifice or strainer. Caught there it usually survives a good cleaning of the flush path. Zero flush, cracked faces and a flashing box is a dry run; reduced flush, intact faces and a normal box is a clogged flush. Same hot weep, different seal, different fix — always read the flush flow and the face condition before you decide.

Packing is supposed to weep — a little

Compression packing is the older way to seal a shaft: rings of braided material stuffed into the box and squeezed by a gland follower until they grip the shaft. Here is the part that trips people up — packing is designed to leak. It must weep a slow, steady drip, a few drops a minute, because that leakage is what lubricates and cools the packing against the turning shaft. A packed pump that runs bone dry is not sealed well; it is overheating. That single fact splits the two packing faults. Someone sees a drip, cranks the gland down to stop it, and now the packing is bone dry and running hot — with no cooling flow the friction heat builds until the packing chars and grooves the shaft sleeve. That is over-tightened, and the cure is to back the gland off until the weep returns. The opposite fault looks nothing like it: an under-tightened or worn gland streams and sprays, wasting product, but runs cool because all that flow carries the heat away. There you take the gland up a little at a time until the stream settles to a controlled drip. Dry and hot means loosen; streaming and cool means tighten — the temperature and the leak rate together tell you which, and getting it backwards ruins either the sleeve or the day.

Chemical attack — the seal the fluid ate

Not every seal failure is thermal. Change the process fluid — or the cleaning chemicals, or the temperature — and the seal's materials can suddenly be wrong for the duty. An elastomer that was fine on the old product swells, softens or hardens on the new one, and the leak path opens up right past a distorted O-ring. The hard faces themselves can be etched and pitted by an aggressive fluid until they can no longer hold a film. The giveaway is that the cooling side is innocent: the flush is flowing, the box pressure is normal, and the faces aren't especially hot — yet the seal leaks, the O-ring is visibly swollen, and the faces are corroded rather than heat-checked. The fix isn't a better-built seal of the same materials; it is the right materials, chosen against a chemical-compatibility chart for the actual fluid, temperature and chemistry the pump now sees.

The discipline

Four reads name every fault in this trainer. The leak rate tells you the seal is unhappy but not why — dry, weeping, streaming or leaking past an O-ring each mean something different. The flush / cooling flow separates a lost flush (dry run) from a restricted one (under-cooled) — zero versus reduced. The face condition and temperature confirm it: cracked and very hot is a dry run, hot but intact is under-cooling, hot and dry on packing is over-tightening, cool but flooding is under-tightening, and etched-but-cool is chemical attack. The box pressure catches the flashing box of a starved seal. Read all four, compare each to what a healthy seal should show, and the pump tells you whether you have a bad seal or — far more often — a bad environment around a seal that was fine.