A mechanical seal goes in without leaking when the shaft runs true and smooth, the faces meet clean and dry, the stationary seat sits square, the rotating assembly is locked at the working length on the seal drawing, and the pump is full of liquid before the shaft turns. These steps are for a component seal, the kind supplied as a separate seat and rotating head.

Have the seal’s drawing or instruction sheet on the bench before you start, along with a micrometer, a dial indicator, lint-free cloth, a quick-drying solvent, a lubricant compatible with the seal’s elastomers, and a torque wrench.

Step 1: Isolate and Drain the Pump

Before the seal area is opened, the pump is isolated by its valves, the driver is locked out, the casing is drained and all pressure is released. Expect liquid to stay trapped in the seal chamber after draining, and check the safety data sheet for the pumped fluid before you break the gland joint.

As you remove the old seal, note which way each part faced and mark where the rotating head sat on the shaft.

Step 2: Check the New Seal Against the Old One

Check that the new seal is the one on the order and that it fits this pump, by comparing it with the old seal and with the pump’s dimensions. Lay the two seals side by side and compare size, face materials, elastomers and spring type. If the old seal has lost its markings, identify it before going further.

Then measure the shaft or sleeve diameter, seal chamber bore and depth, gland pilot, stud size, bolt pattern and the distance to the first obstruction, and check each against the seal assembly drawing.

Inspect the faces for chips and cracks. A seal that has been dropped or knocked in transit does not go in, even when nothing is visible: a lapped face can be cracked or pulled out of flat without showing it.

Step 3: Inspect the Shaft and Seal Chamber

The shaft or sleeve must be smooth and true where the seal sits, and the seal chamber clean.

Shaft and Sleeve Condition

Replace the sleeve, or repair the shaft, if there is any groove, fretting, pitting or scoring along the path of the seal’s O-ring or bellows that you can feel with a fingernail. Check the shaft or sleeve finish under the seal against the finish on the seal drawing.

Measure the diameter with a micrometer against the tolerance on the seal drawing. Remove burrs and break the sharp corners on every step, thread and keyway the elastomers will cross. Renew the sleeve’s own O-ring or gasket with the seal.

Shaft Runout

Acceptable runout at the seal is the limit on that seal’s own instruction sheet. Measure it with a dial indicator on the shaft where the seal will sit, turning the shaft by hand; the full setup is in measuring shaft runout before seal installation.

A reading over the limit means a bent shaft or worn bearings to fix first.

Cleaning the Seal Chamber and Gland

Clean the seal chamber along its full length and scrape every trace of old gasket off the chamber face and the gland, using a solvent compatible with the process fluid. Deburr as you go.

Step 4: Clean the Faces and Lubricate the Elastomers

Lubricant goes on the elastomers and on the shaft where they slide; the seal faces stay dry unless the seal drawing says otherwise.

Keeping the Seal Faces Clean

Take each face out of its packaging only when its part is fitted, wipe the lapped surface with a lint-free cloth and rubbing alcohol or another quick-drying solvent just before the two faces meet, and do not touch it again.

Which Lubricant to Use and Where

Use a lubricant compatible with both the elastomer and the pumped fluid, preferably water-soluble so the pumped liquid washes it away once the pump runs (oil-filled seal chambers excepted). Apply it sparingly to the shaft or sleeve along the elastomer’s path and to the static O-rings.

Which lubricant is safe depends on the elastomer:

  • Nitrile, FKM and chloroprene: light mineral oil sparingly, or silicone grease checked against the pumped fluid.
  • EPDM: no mineral oil, petroleum jelly or other hydrocarbon; glycerine, or soap and water.
  • Rubber bellows of any compound: no grease of any kind, silicone included. Use soft hand soap and water or glycerine, not washing-up liquid or liquid soaps.

The dynamic O-ring of a pusher seal can be lubricated lightly or left dry so nothing can reach the faces. If the seal’s own sheet specifies one, do what it says.

Step 5: Press the Stationary Seat In Square

Press the stationary seat into the gland or housing evenly and squarely, by hand or with an arbor press, with the cardboard disc supplied with the seal over the lapped face so nothing hard touches it. Lubricate the seat’s O-ring or rubber cup first, line up any anti-rotation slot with its pin, and never tap the seat in with a metal tool. Check the seat from behind the bore that it sits evenly all round.

If the seat sits in the gland, slide the gland onto the shaft afterwards and park it clear of the seal position, keeping the seat from knocking against the shaft.

Which Way the Seat Faces

The seat goes in with its lapped, shiny face out toward the rotating face and its back into the bore. Where the seat comes with its own instruction sheet, follow that sheet.

Step 6: Slide the Rotating Assembly Onto the Shaft

Slide the rotating assembly onto the shaft with a steady push on its back, the drive collar or the tail of a rubber bellows, with the primary ring’s lapped face toward the seat. Never hammer it or push on the face, and do not let the primary ring drop out of its holder or its bore catch on the shaft.

A rubber bellows has to slide, not roll along the shaft, and it has to reach its working length promptly. Left too long, the bellows neck grips the shaft permanently.

Getting O-Rings Past Keyways and Threads

Cover any keyway, thread or step the O-ring must cross with thin tape or a plastic sheet wrapped round the shaft, slide the lubricated O-ring over it, then remove the cover. Chamfer the lead-on end of the shaft or sleeve.

Matching the Spring to Shaft Rotation

On a seal that drives through a single conical spring, the spring’s coil must turn the same way as the shaft, looking at the sliding face of the rotating part; multiple-spring, wave-spring and rolled-bellows seals run in either direction.

On a single-spring seal the hand is a separate part, a right-hand or left-hand spring with its own part number. Check the spring’s part number against the pump’s rotation before fitting.

On a pump with a seal at each end of the shaft, the rotation seen from each seal is opposite, so the two seals take opposite-hand springs.

Step 7: Set the Working Length

On a component seal, the working length is the setting dimension on the seal’s own drawing or size table, set by marking the shaft from the seal chamber face and locking the drive collar at the mark. No general compression figure replaces it.

With the shaft in the position it runs in, bearings and thrust bearing fitted, mark the shaft or sleeve in line with the seal chamber face. Measure the drawing’s dimension from that mark to where the back of the seal’s collar or retainer sits, mark again, and lock the set screws there once the rotating assembly is in place.

A mark left from the old seal is only a check; the location has to be measured again even when the replacement is the same seal and seat.

On some small pumps the impeller sets the length, compressing the spring when it is fitted. A cartridge seal arrives with its working length preset, so this step becomes locking and centering the cartridge.

Step 8: Fit the Gland and Tighten the Bolts

Fit the gland squarely over the studs with a new gasket or O-ring in place, run the nuts down by hand, then tighten them evenly in small steps, alternating between opposite bolts, to the torque in the pump manual. Recheck that the gasket is still seated before the final pull.

Step 9: Turn the Shaft by Hand

Turning the shaft by hand checks that it rotates freely, with no binding and no metal-to-metal rubbing between the seal, the gland and the shaft, before the driver is coupled. If it binds or scrapes, the rotating assembly is touching the gland or the gland is off centre; take it apart before start-up. Check the coupling alignment before you reconnect the driver.

Step 10: Vent, Prime and Start the Pump

Open the suction valve and fill the pump, vent all air and vapour from the casing and the seal chamber, turn on any flush or cooling the seal needs, and only then start the motor. Keep the suction valve fully open while the pump runs so the seal always has liquid.

A squeal from the seal area after start-up means the seal is running dry; stop and vent again. If the seal runs hot, or the pump vibrates or cavitates, shut down and remove the cause before restarting.

How Much Leakage Is Normal at Start-Up

A new seal can leak a little more while its faces wear in, but on an ordinary process pump normal leakage is measured in drops per hour and often evaporates before you see it. A steady drip of drops per minute is not bed-in on such a pump.

A Texas A&M Pump Users Symposium paper puts leakage in drops per hour as the norm for seals running at no more than 3,600 rpm and 22 bar (319 psi), between −40 and 180 °C, on shafts up to 100 mm; seals in high-duty services such as boiler feed pumps normally leak 1 to 2 drops per minute.

Pull the seal if its start-up leakage is more than about 100 times what the service should produce: it is very likely damaged or wrongly fitted. When start-up leakage is 10 to 100 times the expected rate, track the trend; falling leakage can run until it settles, but leakage that stays constant for 48 hours means the seal comes out for inspection.

Leakage beyond bed-in points to an installation fault: troubleshooting a seal that leaks after installation works through the causes, and acceptable mechanical seal leakage gives the limits by service.