Mechanical seal weepage is the small amount of process fluid that crosses the faces of a seal in normal operation; you detect it by tracing the moisture to the gland drain with the shaft turning, and you measure it by collecting the liquid over a timed interval or, when it leaves as vapor, by probing the seal with a vapor analyzer.

No seal is leak-free, and a seal has failed only when its leakage becomes excessive. How much mechanical seal leakage is acceptable is a separate question with its own limits.

Where Weepage Shows Up

Weepage shows up at the bottom of the seal gland. On most single seals face leakage goes to a drain port machined at the 6 o’clock position, where it runs out by gravity.

On a horizontal pump the bearing bracket usually sits under the gland, so that drain port is often left unpiped and the leakage falls into the bracket. On rounds it appears as a drip from the seal or as a pool on the baseplate under the pump. Some pumps, large between-bearings pumps especially, collect seal leakage in the bracket and drain it from there.

Weepage can also be present without a visible drop. Fluid that flashes across the faces leaves the seal as vapor, and water-like weepage can evaporate before anyone sees it. A pool or stain under a seal that is dry at inspection shows leakage that happened earlier.

Confirming the Seal Is the Source

Confirm the seal is the source by wiping the gland, the flush and quench connections and the casing joint dry with the pump running, then watching which surface wets first: weepage comes back at the drain side of the gland below the shaft, while a weeping fitting or gasket wets from its own joint. A standby seal that has run before should also be checked at rest: its faces may have worn to a non-flat pattern that leaks more at rest than in operation.

A static pressure check, with the shaft stopped and the seal chamber still pressurized, finds the seal’s other leakage paths: a degraded O-ring or, rarely, a seal ring porous enough to leak through its body.

On a quenched seal, the quench leaves by the same drain port as face leakage, so a wet drain does not show weepage until the quench is shut off or its flow is known. A steam quench also condenses on the equipment and can look like leakage.

Measuring Liquid Weepage

Measure liquid weepage by catching the drain-port leakage in a graduated container over a timed interval with the pump in steady operation, and record the result as a volume per hour.

Plan the interval for very small volumes: a seal in general service leaks so little that its rate is measured in drops per hour or mL per hour.

When the leakage is fast enough to count, drops convert to volume at about 20 drops per mL:

Leakage (mL/hr) ≈ 3 × drops per minute

The drop conversion is approximate; a collected volume is the better figure.

Record in mL/hr, or in g/hr by mass; leakage limits and calculated leakage rates use the same units. A fluid that crystallizes at the gland cannot be collected as a liquid; weigh what builds up and record grams per day.

Let the pump run steady for several minutes before you start collecting, both after start-up and after any change in operating conditions.

Record each reading with the speed, pressure and temperature it was taken at, and keep the first readings on a new seal as the baseline that later readings are compared against.

Measuring Vapor and Gas Leakage

Vapor leakage cannot be collected, so it is surveyed with a portable organic vapor analyzer, and the result is the vapor concentration at the seal in ppm, classified against a leak definition. Under EPA Method 21, which sets the survey procedure, that reading locates and classifies a leak and is not a measure of the leakage rate.

Hold the probe inlet within 1 cm of the shaft-seal interface and move it around the full circumference, or around every accessible part of it when the housing blocks a full traverse. Where the reading rises, find the point of maximum reading and hold the probe there for about twice the analyzer’s response time. Sample the other joints on the pump housing as well.

If the reading is high, wash the equipment down and retest before you treat it as a leak.

The ppm reading that counts as a leak comes from the rule that covers the unit. In EPA’s 2007 leak detection and repair guide, most NSPS rules defined a leak at 10,000 ppm and many NESHAP rules at 500 or 1,000 ppm. State and local rules can set lower figures, so use the rule or permit that applies to your unit.

Soap solution is allowed as a screen only on parts that do not move and are not leaking liquid, such as static gland joints and fittings. At the rotating shaft-seal interface the survey has to be made with the analyzer. Ultrasonic detectors and fluorescent dye can help locate a leak, but neither gives the ppm reading a leak definition is written in.

Monitoring Weepage Online

Online monitoring puts a level or pressure transmitter on the seal’s piping plan, so a rising level in the vessel the leakage drains to, or a pressure rise in a cavity it passes through, raises an alarm without anyone at the pump.

Leakage normally rises for several minutes after start-up, so set a time delay of that length on the leakage alarm to keep it from raising false alarms.

Single Seals

On a single seal, leakage is watched either by the level it builds in the drain line (Plans 65A and 65B) or by the pressure it builds in the gland (Plans 66A and 66B), and only Plan 65B gives an actual leakage rate.

  • Plan 65A: the drain line runs to a detection vessel that empties through an orifice, and a level transmitter alarms when leakage arrives faster than the orifice passes it. At weepage-level flow the leakage crosses the orifice without raising the level, so Plan 65A detects growing leakage but never weepage itself.
  • Plan 65B: a normally closed block valve replaces the orifice, so the leakage accumulates in the vessel. How fast the level rises, or how often the vessel has to be drained, gives the seal’s leakage rate.
  • Plan 66A: a highly restrictive bushing between the faces and the drain cavity lets pressure build in the cavity between the faces and that bushing as leakage increases, and a pressure transmitter reads it. It works on liquid and vapor leakage.
  • Plan 66B: an orifice plug in the gland of a conventional single seal does the same job with a pressure transmitter in the drain cavity. It is less sensitive than Plan 66A.

Dual Seals

On a dual seal, leakage past the inner seal is read from the buffer or barrier system: a level transmitter for liquid leakage, and a pressure transmitter for vapor leakage or barrier loss.

In an unpressurized dual seal with a liquid buffer (Plan 52), the level and pressure transmitters sit on the seal pot. Some process fluid always crosses the inner seal into the buffer, so process fluid in the buffer is normal and a change in level or pressure is the signal.

With a containment seal, the level and pressure transmitters sit on a collection reservoir (Plan 75), or a pressure transmitter sits on the vent line (Plan 76).

In a pressurized dual seal (Plans 53A, 53B and 53C), barrier fluid leaks out across the seals, so its loss is the leakage measurement: a falling level or a decaying pressure in the barrier system, depending on the plan.