To troubleshoot a leaking or failed seal on a chiller pump, find where it leaks and when, trace that to a cause in the loop or in the way the pump is run, and correct the cause before a new seal goes in. The steps are the same for chilled-water and condenser-water pumps; only the water differs.
What to Record Before You Pull the Seal
Record how the pump was running and what the seal looks like before anything is cleaned, because the deposits and wear marks that identify the cause are lost once the parts are wiped.
Before the pump is shut down, note:
- Loop and fluid: chilled or condenser water, the glycol concentration if any, and when the water was last tested.
- Operating point: flow, suction and discharge pressure, VFD speed, and the positions of the discharge and balancing valves.
- Leak timing: whether it leaks running, stopped or only at start-up, and whether the pump is the lead, standby or a seasonal unit.
- Mechanical condition: vibration, bearing noise or heat, and any sign that the piping pulls on the pump.
- Water sample: a jar from the casing drain shows solids and colour, and gives you a pH reading.
As the seal comes out, photograph the faces, the O-rings and the atmospheric side before anything is wiped, and keep the parts. Read the face damage from the width and position of the wear track, cracks, tacky residue, white mineral crust or black sludge.
Where the Seal Is Leaking
Tell a face leak from a gasket, O-ring or sleeve leak by where the water comes out and whether it keeps coming with the shaft stopped. Suspect the faces first: the Fluid Sealing Association puts most seal leakage at the interface between the two faces.
Each of the seal’s sealing points leaks at a different spot:
- Where the shaft leaves the seal, between the rotating and stationary parts: the faces, or the rotating unit’s O-ring or bellows on the shaft.
- Around the outside of the gland or the stationary seat: the gland gasket, or the seat’s O-ring or cup.
- Along the shaft from under a sleeve: the sleeve O-ring of a cartridge seal.
Then stop the pump with the loop still under pressure. A leak that continues with the shaft stopped points to an O-ring, a gasket, or faces too damaged to close.
On a chilled-water pump, wipe the area dry first and make sure the water is not condensation running off the cold casing and piping.
Some leakage is normal; how much leakage is acceptable depends on the seal and the duty.
When the Seal Leaks
A leak that grows while the pump runs points to the shaft moving the faces apart or to worn faces, and a leak that continues with the pump stopped points to an O-ring, a gasket or faces that no longer meet flat.
- Only while running: a running leak that has grown points to vibration, shaft movement or misalignment forcing the faces apart, or to worn and damaged faces.
- With the pump stopped and the loop pressurized: look at the O-rings and gaskets, or at faces that no longer meet flat. A standby seal that has run before can wear into a non-flat pattern that leaks more at rest than when running.
- At start-up, then settling: a new seal leaks more until it wears in. A new seal that keeps leaking past wear-in points to how it was fitted or set.
- At the first start after an idle spell: faces that stuck together, or a seal chamber full of air, damage the seal in its first minutes of running.
- With load: a leak that appears when valves close at low load or the drive slows points to the operating point.
Common Causes of Chiller Pump Seal Failure
Most chiller pump seals fail because of conditions in the loop or in the way the pump is run and fitted; a defect in the seal itself is the rarer case.
Vendors variously name dirt or installation as the most common cause, and the Fluid Sealing Association’s seal technical committee, in a compilation of several seal makers’ failure investigations, names pumps run outside their design range. None gives percentages.
Dirt, Magnetite and Weld Slag in a Closed Loop
Solids in a closed chilled-water loop get between the faces and grind them. They are weld slag, pipe scale and sand left from construction or a pipework change, and magnetite, the black iron oxide that steel piping sheds as it corrodes.
On the seal they show as grooves and a scored, widened track on the softer face, usually the carbon, and as black sludge packed around the springs and moving parts.
Seals that fail soon after a new installation or a piping change point to construction debris. A slow, repeating failure on an older steel loop points to magnetite.
Scale and Debris in Condenser Water
An open condenser-water loop adds wind-borne dirt and debris, biological growth and minerals concentrated by evaporation, none of which a closed loop has to the same degree.
At the seal, the minerals come out as scale. A white or tan crust on the outboard side of the seal, with a leak that grew gradually, points to the condenser water.
Glycol and Water Chemistry
Glycol and water chemistry reach the seal through pH: degraded glycol turns acidic, and an elastomer outside its pH range swells, softens or hardens and stops sealing.
Bell & Gossett, for example, limits the Buna version of its Series 1522 standard seal to pH 7–9 and the EPT version to pH 7–11. Degraded glycol pulling the pH down, or treatment chemicals pushing it up, can take the loop outside the range of the elastomer fitted.
On the removed seal, O-rings or bellows that are swollen, soft or sticky point to chemical attack. Elastomer that is hard and flattened into its groove points instead to compression set or heat.
Air in the System and Dry Running
Air in the loop collects at the highest point of the pump, and on a vertical in-line pump that is the seal chamber, where it leaves the faces running dry while the pump still moves water.
Venting once at start-up does not solve it while an air-bound loop, with low fill pressure or a failed air separator, keeps supplying the air.
A seal that ran dry shows a glazed or heat-discoloured track, carbon dust around the seal and, on ceramic faces, fine radial cracks or crazing from overheating. If cold make-up water then reaches a face that has run hot and dry, the sudden cooling can crack it through (thermal shock).
Running Near Shutoff or Off the Curve
Running near shutoff or far from the best efficiency point damages the seal through the shaft: at low flow the radial load on the impeller rises and deflects the shaft, and at high flow the pump can cavitate, with vibration reaching the faces either way.
In a chiller plant, flow drops near shutoff when two-way valves close at low load or when a pump runs against a closed isolation valve. The seal shows a wide or uneven wear track and a leak that comes and goes with load.
Misalignment, Bearing Wear and Pipe Strain
Misalignment, worn bearings and pipe strain all move the shaft relative to the seal, and they show up as a wider or off-centre wear track and a leak that rises with vibration.
Look for a wear track wider than the narrower face and for polished or fretted bands on the sleeve under the O-ring.
Installation Errors
A chiller pump seal fitted with one of a few installation mistakes fails early:
- Damaged or dirty faces: a face chipped, scratched or handled with dirty fingers during fitting never seals.
- Wrong setting: a rotating unit fixed at the wrong position on the shaft gives the springs too much or too little load.
- Petroleum lubricant on EPDM: oil or grease swells EPDM elastomers; use water or a lubricant the seal maker approves.
- Cocked seat: a stationary seat pressed unevenly into its bore runs out of square with the shaft.
- Dry start: starting before the pump and seal chamber are vented runs the new faces dry.
A cracked stationary face whose crack starts away from the drive-pin slot, with clean residue on the face, points to impact during installation.
Standby and Seasonal Pumps
Seals on standby and seasonal pumps fail at restart because the faces stick together while the pump stands, and the torque of the first turns cracks a face or tears the elastomer.
A stationary face cracked from the drive-pin slot, with tacky residue on the face, points to faces that stuck.
Condenser-water pumps that stand through winter can also freeze: water left in the casing and flush lines jams the seal’s sliding parts.
Fixing the System Before Fitting a New Seal
Before the replacement goes in, correct whichever loop or operating fault the evidence points to.
- Solids in a closed loop: flush the loop, clean the strainers, and fit a dirt and magnetite separator or a side-stream filter. Flush new or modified pipework before the first start.
- Condenser water: bring the tower’s water treatment and blowdown back under control so the minerals stay in solution, and filter a side stream to remove debris.
- Chemistry: test pH, glycol concentration and inhibitor level, and match the seal’s elastomer to the loop’s measured pH.
- Air: restore fill pressure, service the air separator and vents, and on vertical pumps vent the seal chamber continuously from its highest port, with the flush entering below it.
- Operating point: check the duty point against the pump curve, set a minimum drive speed or a bypass that keeps flow above the pump’s minimum, and never run a pump against closed valves.
- Mechanics: realign the coupling, replace worn bearings, and support the suction and discharge piping independently near the pump so it does not load the casing.
- Standby and seasonal pumps: alternate duty and standby pumps every two or three months, an interval that depends on how the bearings are lubricated, and drain idle pumps that could freeze. Some Grundfos manuals, for example, call for a few drops of silicone oil on the shaft before a pump is drained for a long idle period, so the seal faces do not seize.
Fit a replacement of the same design unless the evidence shows the seal itself was wrong for the duty, such as an elastomer outside the loop’s pH or faces too soft for solids that cannot be removed. We build HVAC pump seals from the old seal’s drawing, its part number, or the seal itself.