To find why a newly replaced seal on a vertical multistage pump leaks, work in order: confirm the liquid is coming from the seal, read when the leak appears, then check the three causes that belong to this pump type — air left at the top of the pump, the seal and coupling setting, and the suction and operating conditions.

Slips common to any seal installation, such as dirty faces or a pinched O-ring, are covered in why a mechanical seal leaks after installation. How much leakage a running seal may show is a separate question, answered in acceptable mechanical seal leakage.

Confirm Where the Leak Starts

Dry the pump head and watch where liquid first appears, because a wet pump is not proof of a seal leak.

Where the liquid first shows points to the leak path:

  • At the shaft, under the coupling: the seal faces or the cartridge’s O-ring on the shaft.
  • Around the outside of the cartridge, where it screws into the pump head: the seal’s own O-ring on its seat.
  • At the joint between the pump head and the outer sleeve: a sleeve O-ring, and the seal is not involved.

Then stop the shaft and keep the pump under pressure. A leak that continues with the shaft stopped points to an O-ring or its seat, or to faces that are out of flat. A leak that appears only with the shaft turning points to the faces.

A new O-ring will still leak on a damaged seat. On a Grundfos CR, the seal’s O-ring seats directly on the casting of the motor stool, or of the pump head on CRI and CRN models. Grundfos ties pitting on the motor stool casting to leaks and has rusted seating areas cleaned before the O-rings go back.

Read When the Leak Appears

The moment the leak first shows tells you which check comes first:

  • While filling, before the shaft turns: the leak is at an O-ring or its seat, or at faces that are out of flat, as in the previous section.
  • At first start-up: usually the faces wearing in; see the rule below.
  • After hours or days of steady running: points to air collecting at the top of the pump or to the suction and operating conditions.
  • After a change of duty: a change in operating conditions starts a new wear-in, so a short rise that falls again is expected.
  • More at standstill than while running, on a seal that has already run: running has left the faces out of flat.

A start-up leak on its own is not a fault. Grundfos expects a run-in period on every new CR shaft seal.

As a working rule, a new seal whose start-up leakage is more than 100 times the rate expected for that seal is very likely damaged or wrongly installed, and it should come out so you can read the faces. A new seal whose start-up leakage is 10 to 100 times the expected rate needs its trend measured. If a new seal’s start-up leakage holds constant for 48 hours, pull the seal; if it is falling, keep running until it settles and review the settled rate with the seal maker.

Air Left at the Top of the Pump

On a vertical multistage pump the shaft seal sits at the top of the pump, in the motor stool or pump head, so air that is not vented out collects around the seal faces and the new seal runs dry from the first turn.

The face damage dry running leaves is described in signs of a dry-running mechanical seal.

On a Grundfos CR, the priming plug and the vent valve are both in the pump head. Grundfos has the pump filled through the priming plug with the outlet valve closed, the motor started and its rotation checked, and the pump then vented through the vent valve while the outlet valve is opened in steps. The vent valve is closed only when a steady stream of liquid runs out of it.

One vent at start-up is not always enough. If the leak starts only after the pump has run for a while, look for air entering on the suction side, then vent again.

Cartridge Seal Setting and Coupling Height

On a Grundfos CR the cartridge seal is locked to the shaft by set screws and the shaft’s height is set at the coupling, so set screws tightened at the wrong step, or a height set without the maker’s spacer, leave the rotating face in the wrong place and the new seal leaks.

Grundfos does not use one setting method across its CR line, and two neighbouring ranges set the seal in opposite order:

  • CR 1s, 1, 3 and 5: the seal’s set screws stay loose while the shaft is lifted to its full travel, lowered half that distance and the coupling bolts are torqued; the set screws are tightened only after that.
  • CR 10, 15 and 20: the set screws are tightened first, with the shaft pushed all the way down; the shaft is then lifted on a spacer tool placed between the seal collar and the seal’s hex nut, and the coupling bolts are torqued with the spacer in place.

Use the procedure for the exact model and seal arrangement on the nameplate, on any maker’s vertical multistage pump. Before the guards go back on, turn the shaft by hand: Grundfos requires it to spin freely, and a shaft that binds means taking the setting apart and starting again.

If the seal itself is in doubt, we build most brands’ mechanical seals, cartridge seals for the Grundfos CR among them, from the old seal’s part number, its drawing, or the old seal sent as a sample.

Suction and Operating Conditions

A new seal that was fitted, set and vented correctly can still leak when the pump runs short of suction pressure, runs against a closed outlet, or starts and stops often, because each of these strips or keeps disturbing the liquid film between the faces.

  • Low suction pressure: dissolved air and vapour come out of the liquid at low inlet pressure, and on a vertical pump that gas ends up at the seal; cavitation at the first stage adds vibration on top.
  • Running against a closed outlet or below minimum flow: the liquid held in the pump heats up and can flash to vapour at the seal. Grundfos tells CR users not to run the pump against a closed outlet valve.
  • Frequent starts and duty swings: a seal on a pump that cycles all day may never finish wearing in.

These are system causes, and the next new seal will leak the same way until the suction or the duty is corrected.