A mechanical seal needs a liquid film between its faces, but that film normally comes from the pumped liquid itself, and a separate seal water supply is needed only when the pumped liquid cannot give the faces a clean, stable film.

A few seal types, gas-lubricated and dry-running seals, need no liquid at all.

What Seal Water Does at the Seal

Seal water gives the seal faces a better liquid to run on than the one being pumped: clean water fed into the seal chamber from an outside supply. It does four things there:

  • Cooling: it carries away the heat the rubbing faces generate, and heat from the shaft.
  • Lubrication: it forms the film between the faces where the pumped liquid would form a poor one.
  • Purging: it flushes solids and debris out of the seal chamber before they reach the faces.
  • Isolation: fed above seal chamber pressure, usually with a close-clearance bushing at the chamber throat, it keeps the pumped liquid off the faces, and only a small, steady amount of the water crosses the faces into the process.

In piping plan terms, seal water on a single seal is an external flush, API Plan 32.

When the Pumped Liquid Is Enough

A mechanical seal can run on the pumped liquid alone when that liquid reaches the faces clean, stays liquid there, and is moved past them fast enough to take their heat away. A clean, cool water service, such as an HVAC circulating pump, usually meets all three with no outside line.

The heat is handled by circulating the pump’s own liquid. An API Plan 11 line takes liquid from the pump discharge into the seal chamber and over the faces. On smaller, lower-duty pumps such as ASME B73 chemical pumps, a tapered seal chamber with flow modifiers often circulates the pumped liquid by itself, with no line at all.

The liquid also has to stay liquid at the faces. For the petroleum, petrochemical and chemical duty pumps it covers, API 682 requires the seal chamber pressure to be at least 3.5 bar (50 psi) above the liquid’s vapor pressure, or at least 1.3 times that vapor pressure where the 3.5 bar margin cannot be met.

When a Seal Needs Seal Water

A single seal needs seal water when the pumped liquid cannot be turned into a clean, stable film at the faces, which in practice means it carries abrasive solids or fibres, or is a poor lubricant that pressure and cooling cannot fix.

Slurries and fibrous stock such as paper pulp are the usual cases. Solids do not always require an outside supply, though, because some seal and seal chamber designs for slurry service keep abrasives off the faces without one.

Hot water on its own does not call for seal water. In one Fluid Sealing Association example, 180 °F water in a single-stage overhung pump is sealed with only a Plan 11 line and a throat bushing. Boiler feed water at 100 to 200 °C is sealed either by injecting cool water from outside (Plan 32) or by cooling the pump’s own water through a heat exchanger (Plan 21 or Plan 23).

Toxic or hazardous liquids point to a double seal. A double seal runs on a buffer or barrier fluid held between its two face pairs, which can be water in a closed loop but is a separate system from once-through seal water.

A seal that needs seal water and loses it runs dry: the faces overheat and the seal fails.

Flush From the Pump vs Seal Water From Outside

A flush taken from the pump is enough whenever the pumped liquid, cleaned or cooled if needed, makes a good face lubricant; the water has to come from outside only when it does not.

Before going to outside water, the pump’s own flush can sometimes be cleaned instead, with a strainer or a cyclone separator on the line from the discharge removing solids before they reach the seal.

A water line connected to the atmospheric side of the gland is a quench, which does not feed the faces and is not seal water.

Seals That Run Without Any Liquid

Gas-lubricated seals and dry-running contact seals run with no liquid at the faces, and both are chosen for duties where a liquid film cannot be relied on.

A gas-lubricated pump seal is usually a pressurized dual seal whose faces ride on a film of nitrogen, steam, clean air or another inert gas without touching. It is used where the pumped liquid is volatile, non-lubricating, heat-sensitive or hazardous, and a liquid barrier fluid would contaminate the process.

Dry-running contact seals rub with no lubrication at all; dry mixer seals and vacuum seals are typical. Dry-running containment seals work the same way as the outer seal of a dual arrangement, behind a liquid-lubricated inner seal that still has its own flush.

Seal Water Lines Left Over From Packing

A mechanical seal fitted in place of packing needs the old seal water line only if the seal itself needs outside water, and that depends on the pumped liquid and the stuffing box, not on what the packing used.

A stuffing box designed for packing removes a seal’s heat poorly, so a seal there may need a flush that the same seal in a purpose-built seal chamber would not. A box with an enlarged bore and an enlarged throat lets a single seal run with no flush at all.

Where the pumped liquid is clean, a flush from the pump’s own discharge usually serves the seal as well as the outside water does, without diluting the product. On a pump under suction lift, the seal chamber can sit below atmospheric pressure, and the seal is normally given a flush from the discharge to keep the chamber full of liquid.

Before removing the line, check two things: whether the pumped liquid carries solids or lubricates poorly, and whether the stuffing box has been opened up for a seal. If the liquid is clean and the box has been reworked, or the seal already has a flush from the pump, the outside line can usually be removed.