Pipe it, unless the seal was built to run without a flush. A cartridge seal leaves the factory with its flush port plugged for shipping whatever plan it was designed for, so the plug tells you nothing about how the seal should run.

A plugged port is legitimate only when the fluid and the seal chamber remove the face heat and vent vapour on their own: a non-volatile liquid at low pressure and low speed, or a chamber shaped to circulate and vent itself. In every other case the port takes a line, because the faces need the cooling and lubricating liquid a flush supplies.

That line comes from the pump discharge on a horizontal pump, goes back to suction on a vertical one, or comes from a clean outside source when the pumped fluid is too dirty, too hot or too poor a lubricant. Which of these the seal was built for is written on its own drawing.

When the Flush Port Can Stay Plugged

The flush port can stay plugged when the seal runs in a non-volatile liquid at low pressure and low speed, or in a seal chamber designed to cool and vent the seal without a line.

“Clean” is not one of the conditions. A clean aqueous chemical qualifies for a plugged port only when it is well away from boiling and the pump is a low-pressure, low-speed duty.

These are the designs that let a cartridge seal run dead-ended:

  • Taper-bore chamber (API Plan 03): suits seals whose faces generate little heat, and only with a steep taper, 30 to 45 degrees.
  • Taper bore with axial stop ribs: the ribs improve self-venting and draining enough that liquids close to their boiling point can run with no flush.
  • Jacketed chamber on hot service (Plan 02): most hot refinery bottoms and gas-oil pumps above 300 °C run bellows seals dead-ended, with a steam or cooling jacket on the chamber in place of a flush.
  • Seals made for a conical chamber: a cartridge seal built specifically for a conical seal chamber can ship with every port plugged and run with no flush.

An old-style narrow stuffing box gives good seal life only on clean, ambient water with a flush off the pump discharge.

A cylindrical large-bore chamber should not run without a flush when the liquid carries solids, and should not be used where the seal needs to self-vent and drain. On a vertical pump the port cannot stay plugged.

A port that stays plugged in service needs a metal plug or a pressure-rated fitting. A plastic shipping plug cannot hold pressure and has to come out before start-up.

When the Flush Port Must Be Piped

The flush port must be piped whenever the conditions above do not hold: a volatile or near-boiling liquid, high pressure or speed, solids in the pumped fluid, a fluid that lubricates the faces poorly, a narrow stuffing box, or a vertical shaft.

Piping From the Discharge

Pipe the flush port from the pump discharge (API Plan 11) when the pumped liquid is clean, lubricates the faces well and the pump is horizontal.

On a horizontal shaft, put the flush connection at top dead center so the chamber vents itself. On a multistage pump the discharge pressure can push far too much flow through the line, so the flush is taken from the crossover or an intermediate stage.

A pump with an integral passage from discharge to the seal chamber (Plan 01) already recirculates without external lines, so the gland’s flush port stays plugged. Plan 01 is for clean liquid only, typically on pumps with less than 125 ft of total discharge head, and is not recommended on vertical pumps.

Piping to Suction on a Vertical Pump

Pipe the flush port back to pump suction (API Plan 13) on a vertical pump whose seal chamber sees discharge pressure, because without that line air collects at the seal.

Venting once before start-up is not enough, because the liquid keeps carrying entrained air into the chamber. Put the outlet connection as high as possible, above the seal faces; on a vertical seal with a separate flush inlet, the inlet sits below it.

The line only flows if seal chamber pressure is higher than suction pressure, so check both with a gauge. On a vertical sump pump the chamber sits close to atmospheric pressure and Plan 13 will not flow, so the chamber is filled from an external source before start-up and given its own vent.

Plan 13 cools the faces less well than Plan 11. Where a vertical pump needs both cooling flow and venting, Plan 14 pipes both lines at once.

Piping a Clean External Flush

Pipe the flush port to a clean external source (API Plan 32) when the pumped fluid cannot be made to cool and lubricate the seal: abrasive solids, a liquid at or near its boiling point, a product that polymerises or solidifies, or one that lubricates the faces poorly. Plan 32 is almost always installed with a close-clearance throat bushing between chamber and pump.

The flush ends up in the product, so it dilutes it and has to be compatible with it; in water-based services it is usually plant service water or city water. The supply also has to run whenever the pump runs.

Set the external flush by flow, not by a fixed pressure above the chamber: the pressure only has to push the required flow through the bushing.

What Happens When a Port That Needs a Flush Is Left Plugged

A seal that needs a flush and runs with its port plugged loses the liquid film at the faces: the heat stays in the chamber, air or vapour collects at the faces, and the seal runs partly or fully dry. Wet seals run without enough liquid typically squeal, then overheat and score their faces, leak more and fail early.

On a vertical shaft the damage comes from trapped air. In one published field case on a vertical split-case cooling water pump with no vent connection, both rotary faces came out with a phonographic finish and chipped edges, the pattern of dry running.

How to Tell Which Plan the Seal Was Built For

The flush plan is written on the seal arrangement drawing; if the drawing is missing, the seal maker can give it from the seal’s model or part number.

On a seal built to the 4th edition of API 682 the plan is also the last field of the seal code. A code such as 31B-LIN-075-53A ends in 53A, meaning the seal was built for Plan 53A. A cartridge seal for an ANSI pump may carry no such code, so the drawing is the place to look.

Some makers leave the decision to the user. In that case the plan comes from the conditions in the sections above.