Pump Seal Leaks: How to Tell an Acceptable Weep From a Failing Mechanical Seal or Packing

• Gland packing must drip. Roughly 10 to 60 drops per minute is the commonly cited normal range. A packed pump with no drip and a hot gland is a warning sign.
• A mechanical seal should look dry. One manufacturer's guide puts acceptable leakage at about 10 drops per hour or less, so a visible, steady drip means the seal has failed.
• Most mechanical seal failures start elsewhere: dry running, abrasive water, vibration, misalignment, cavitation, sudden temperature changes or incompatible water treatment.
• The floor under a pump keeps a record. White crust, rust trails, corroded anchors and a bucket under the casing all show a leak that has been running for a long time.

A pump keeps water in along its shaft with one of two devices: gland packing or a mechanical seal. Both sit where the shaft leaves the casing, in an area called the stuffing box or seal chamber.

The problem they solve is simple. A shaft that turns thousands of times a minute cannot be bolted tight to a stationary casing, so something has to seal a moving surface against pressure.

Packing seals by squeezing soft rings against the shaft and letting a little water through to cool them. A mechanical seal moves the sealing point to two polished faces that run against each other on a thin film of fluid.

In HVAC (heating, ventilation and air conditioning) plants, both are common. Older chilled water, condenser water and heating water pumps are often packed; most newer end suction and in line pumps use mechanical seals. Knowing which one each pump has is the first step in reading any leak.

Gland packing is a set of braided rings squeezed around the shaft by a gland follower. It must drip, because the leakage cools and lubricates the packing. It cannot be a zero leak device.

The gland follower is pulled in by two gland nuts. Tightening them squeezes the rings harder against the shaft or shaft sleeve, which reduces the drip.

The rings rub the turning shaft all the time. The water seeping past carries away the friction heat and keeps the packing soft enough to seal.

What over tightening does

Over tightening packing to stop the drip builds friction heat fast. The packing glazes or chars and scores the shaft sleeve, sometimes within hours (Fabrico).

Once the sleeve is scored, no amount of adjustment will seal it properly again. That is why a hot stuffing box with no drip is a warning sign, not a success.

Heat shows the problem before the sleeve is ruined. Thermal imaging makes an over tightened gland stand out against its sibling pumps at a glance.

When packing leaks inward instead

On pumps under suction lift, a leaking packing gland can draw air in instead of letting water out (Cornell Pump). The leak becomes invisible, but the air feeds cavitation and air binding, covered in Rotating Equipment Part 3.

Air drawn through a gland makes the high frequency turbulence that airborne ultrasound picks up even when nothing drips. A packed pump on suction lift that runs rough with a dry gland is worth that check.

Normal packing drip is commonly given as roughly 10 to 60 drops per minute, depending on shaft size, speed and service. Larger, faster shafts need more leakage to stay cool.

Field guidance often targets 40 to 60 drops per minute at start up, while new packing beds in. The gland nuts are then adjusted in small, even steps, for example a quarter turn at a time.

What the drip adds up to

A properly adjusted packed pump at about 10 drops per minute loses roughly 0.2 gallons (about 0.75 L) a day. At 60 drops per minute, six times the rate, that becomes about 1.2 gallons (about 4.5 L) a day.

That water is not a fault, but it is not free. It has to drain somewhere, and on a closed loop it is replaced by makeup water, a point covered below.

Packing versus seal leakage, side by side

The difference between the two technologies is large. A packed pump dripping normally leaks many times more than a mechanical seal at its acceptable limit (Swagelok Northern California).

That gap is why the same puddle means opposite things. Under a packed pump it is expected; under a sealed pump it is a failure.

You adjust gland packing in small, even steps while the pump runs, then wait and watch both the drip and the gland temperature. The goal is a steady drip and a gland that is warm, not hot.

1. 1. Confirm the pump is packed, not fitted with a mechanical seal, and that guards stay in place while it runs.
2. 2. Note the current drip rate and how warm the gland feels or reads.
3. 3. Tighten both gland nuts by the same small amount, such as a quarter turn, so the follower stays square.
4. 4. Wait several minutes for the packing to settle before judging the result.
5. 5. Repeat only if the drip is still well above the target range, and stop if the gland starts running hot.

Uneven tightening cocks the follower, crushing the packing on one side and leaving a gap on the other. Small steps and patience prevent most packing damage.

Adjustment should be recorded with the date and who did it. A gland that needs tightening every few weeks has worn packing or a scored sleeve, and repacking is due.

A mechanical seal uses two flat, polished faces, one rotating with the shaft and one stationary, pressed together by spring force. A microscopic fluid film between the faces lubricates them, so the seal leaks only a barely measurable vapor or film.

The faces are lapped extremely flat. The spring keeps them in contact as they wear, and secondary seals, usually elastomer O-rings or bellows, seal the faces to the shaft and the housing.

That film is the whole design. Too little and the faces overheat; too much and the seal leaks. A healthy seal lives in between, and to the eye it looks dry.

Some pumps also use a seal flush or cooling line that feeds clean or cooler liquid to the seal chamber. A blocked flush line, or a cold flush line on a hot pump, is a seal failure in waiting.

One manufacturer's guide, from CIRCOR, puts acceptable mechanical seal leakage at about 10 drops per hour or less per seal (CIRCOR). Anything you can see dripping steadily is far beyond that.

The refinery seal standard is stricter still. American Petroleum Institute (API) 682 sets less than about 0.2 US fluid ounces (about 6 mL) per hour per pair of seal faces. HVAC pumps are not built to API 682, but it shows how little a seal is meant to pass.

Visible, continuous dripping or spraying from a mechanical seal means it has failed. There is no adjustment to make, and the leak will only grow.

A brief weep after start up can settle as the faces bed in. A drip that is still there after the pump has run steadily is a replacement, not a wait and see.

Mechanical seals in HVAC pumps fail mostly because something disturbs the fluid film between the faces. The seal is usually the victim, not the cause.

• Running dry, even for a few seconds: the faces have no film and overheat.
• Abrasive particles in the water, such as scale, sand or corrosion debris, scoring the faces.
• Shaft vibration and bearing play opening the faces, covered in Rotating Equipment Part 1.
• Misalignment between pump and motor, covered in Rotating Equipment Part 2.
• Cavitation vapor at the faces, covered in Rotating Equipment Part 3.
• Thermal shock, such as cold water hitting hot faces.
• Chemical attack on elastomers from incompatible water treatment.

This is why a replaced seal so often fails again. If the cause was a worn bearing or a misaligned coupling, a new seal inherits the same problem.

Recording a cause with every seal replacement is the cheapest reliability improvement a plant can make. A failed seal kept for inspection often shows the cause plainly: scored faces, heat checking or a swollen O-ring.

Dry running in practice

Dry running happens more often than people expect. A pump started before the system is filled can do it in seconds. So can a standby pump run while valved off, or air trapped at the seal after maintenance.

The evidence under a pump tells you that a leak has been running for weeks or months, even when the pump looks dry today. Water leaves minerals and corrosion behind every time it runs and evaporates.

• Mineral crust and white scale on the casing below the seal.
• Rust trails running down the casing or baseplate.
• Corroded baseplates and anchor bolts.
• A stained or eroded housekeeping pad.
• A drip tray or bucket someone placed under the pump.

The bucket is the most telling of all. It means someone saw the leak, decided it could wait, and contained it instead of fixing it.

On a packed pump, some staining near the drain is expected. Below a mechanical seal, crust means a long running leak. Routine visual inspection records these marks round after round, so a new stain stands out against the last photo.

Reading the full history of a room is the subject of Part 4 in this series, Reading the Scars.

A seal leak costs water, corrosion and treatment in both building types. In a data center it can also cost cooling capacity and create an electrical hazard.

In an office building

Water on the floor, corroded baseplates and anchors, and lost system water are the usual costs. Lost water on a closed loop is replaced by fresh, untreated makeup water, which brings in oxygen and minerals and dilutes the treatment.

That makes a seal leak a corrosion problem for the whole loop, not just the pump. Pump checks belong in routine HVAC maintenance in office buildings.

In a data center

A leaking seal loses chilled water or coolant volume and puts water on a floor full of electrical equipment, a slip and electrical hazard. Makeup water dilutes treatment chemicals in loops that serve critical cooling.

Catastrophic seal failure can drain a loop faster than makeup can keep up. A seal that is weeping today is the warning before that happens. Broader leak response is covered in water leak detection and damage prevention.

Converting packed pumps to mechanical seals usually makes sense where water loss, floor wetting or constant adjustment are real costs. Packing still has a place where the water is dirty or the shaft condition is poor.

The case for conversion is the drip itself. A mechanical seal removes routine leakage and the labor of adjusting glands, and it keeps the pump room drier.

The case against is sensitivity. Mechanical seals tolerate dry running, abrasive water and vibration far less well than packing. Converting a pump with those problems trades a drip for repeated failures.

A sound approach is to fix the pump's alignment, bearings and water quality first, then convert. The pump manufacturer can confirm whether a seal kit fits the existing seal chamber.

Several sources put water near a seal without the seal being at fault. Where the water appears, and what it looks like, separates them.

• Condensation on a cold chilled water pump: it appears on the whole cold surface, not one point. Condensation is covered in the Moisture and Air sub-series.
• A leak from above: a casing gasket, drain plug or flange above the pump can run down onto the seal area. Joint leaks are covered in Part 2.
• A grease purge: an over greased bearing purges grease near the seal. It is oily rather than watery, covered in Rotating Equipment Part 1.

Tracing the wet path upward usually settles it. Water that starts at the shaft is the seal; water that starts above it is something else.

A walk past can prove that a seal is dripping, that packing has been over tightened and that a leak has run for a long time. It cannot prove the condition of the seal faces or why they failed.

• Directly observable: an active drip or spray at the seal, a packed pump with no drip and a hot gland, mineral crust, a drip bucket and puddles.
• Strong indicator: a seal area running hotter than the same seal on a sibling pump, and a rising makeup water meter reading.
• Suspected: seal face condition and the root cause. Those need disassembly or a seal failure analysis.

That is the limit of any route based check. It finds the failing seal and the over tightened gland; disassembly finds the cause.

Thermal imaging compares each seal area with its sibling pump. The makeup water meter shows whether the loop is losing water. This is why no single sensor is enough: the drip, the heat and the meter each tell part of the story.

Trending those readings over time is the core of predictive maintenance, and a walkthrough record keeps the history.

• Packed pumps: a steady drip present, and the gland not hot.
• Mechanical seals: dry, with no spray, crust or staining below.
• Mineral crust, rust trails or staining on the casing and baseplate.
• Drip trays, buckets or rags placed under the pump.
• Seal flush lines intact and not blocked.
• Seal area temperature compared with a sibling pump.

Coming Soon

1. 1. Which of your pumps are packed, and which have mechanical seals?
2. 2. How is packing adjusted, and by whom?
3. 3. When was each seal last replaced, and was a cause recorded?
4. 4. Is there a bucket or tray under any pump right now?
5. 5. Does your makeup water meter reading trend with seal problems?

Is a pump supposed to leak?

A packed pump is supposed to drip, because the leakage cools and lubricates the packing. A pump with a mechanical seal should look dry, and a visible, steady drip means the seal has failed.

How many drops per minute should gland packing leak?

Normal packing drip is commonly given as roughly 10 to 60 drops per minute, depending on shaft size, speed and service. Field guidance often targets 40 to 60 drops per minute at start up, then adjusts the gland nuts in small, even steps.

How much leakage is acceptable from a mechanical seal?

One manufacturer's guide, from CIRCOR, puts acceptable leakage at about 10 drops per hour or less per seal. API 682, the refinery seal standard, sets less than about 0.2 US fluid ounces per hour per pair of faces. Continuous dripping or spraying means failure.

Why is my pump packing hot?

Hot packing usually means it has been tightened too far and is not getting enough leakage to cool it. Friction heat can glaze or char the packing and score the shaft sleeve within hours. Back the gland off in small, even steps until a steady drip returns.

What causes mechanical seals to fail?

Common causes are running dry, abrasive particles in the water, shaft vibration and bearing play, misalignment, cavitation, thermal shock and chemical attack on elastomers. The seal is often the victim of a problem elsewhere in the pump.

Can running a pump dry damage the seal?

Yes. Running dry, even for a few seconds, leaves the seal faces with no fluid film, so they overheat. Dry running often happens when a pump starts before the system is filled or with trapped air at the seal.

How do you tell a seal leak from condensation?

Condensation forms over the whole cold surface of a chilled water pump, not at one point. A seal leak starts at the shaft where it leaves the casing. Tracing the wet path back to its highest point usually settles it.

Should I convert packed pumps to mechanical seals?

Often, yes, where water loss, wet floors or constant adjustment are real costs. Mechanical seals tolerate dry running, abrasive water and vibration poorly, so fix alignment, bearings and water quality first.

What does white crust under a pump mean?

White crust is minerals left behind as leaked water evaporated. Under a mechanical seal it means the seal has been leaking for a long time. Under a packed pump some staining near the drain is normal, but heavy crust on the baseplate means the drip is not draining away.

• Water Leak Detection and Water Damage Prevention for Commercial Buildings
• Visual Inspection in Data Centers: What a Camera Catches That No Sensor Can
• Bearing Failure, Stage by Stage: Why Ultrasound Hears It Before Vibration Feels It and Heat Shows It
• Pump Cavitation in Chilled Water and Condenser Water Systems: The Gravel Sound and What the Suction Gauge Already Knows

• Fabrico: gland packing leakage ranges and adjustment
• CIRCOR: the ABCs of mechanical seal leaks
• Swagelok Northern California: packing leakage versus API 682
• Cornell Pump: common pump leak locations and suction air leaks