A double mechanical seal fails at one of three points: the inner seal, the outer seal, or the barrier or buffer system between them. You find which one from what the reservoir’s level and pressure did and where the fluid went, then correct the failure at that point, which can be the support system rather than the faces.

Each seal in the pair fails the way a single seal does, so the face damage you find at teardown is read by material type in the same way. The most common causes of any seal failure, such as poor lubrication, installation errors, misalignment and contamination, are covered in the reasons mechanical seals fail, and elastomer damage in the causes of O-ring failure.

Telling Which Seal Failed

The barrier or buffer reservoir tells you before teardown: its level, its pressure and where the fluid went point to the inner seal, the outer seal or the barrier system.

In a Pressurized Double Seal

In a pressurized double seal, barrier fluid lost with nothing visible outside points to the inner seal, and barrier fluid at the gland or on the atmospheric side points to the outer seal.

  • Level falling, barrier fluid at the gland: the outer seal is leaking to atmosphere.
  • Level falling, no external leak: the inner seal is losing barrier fluid into the product. Barrier fluid found in the product confirms it.
  • Pressure falling with the level holding (Plan 53A): the gas supply or the regulator has failed, not a seal.
  • Pressure falling (Plan 53B): barrier fluid is leaving through one of the seals. A Plan 53B accumulator’s pressure also drops with ambient temperature, so check the temperature before reading a drop as a leak.
  • Level rising, or process fluid in the barrier: process fluid has crossed the inner faces, which happens only when seal-chamber pressure rises above barrier pressure. This pressure reversal starts in the barrier system or on the process side, and the inner faces may still be sound.

In an Unpressurized Double Seal

In an unpressurized double seal on a Plan 52 buffer system, a rising buffer level or pressure means the inner seal is leaking, and a falling level means the outer seal is.

  • Level rising: the inner seal is leaking process fluid that stays liquid in the reservoir.
  • Pressure rising: the inner seal is leaking process fluid that flashes to vapor and collects above the liquid. On volatile services, a pressure switch sees this failure and a high-level switch does not.
  • Level falling: the outer seal is losing buffer fluid to atmosphere.
  • Process fluid in the buffer: normal, because some inner-seal leakage always ends up there. The trend in level and pressure is what shows the inner seal failing.

With a dry containment seal as the outer seal (Plans 72, 75 and 76), there is no buffer liquid. Inner-seal leakage is read as pressure for vapor, or as level in a leakage collection reservoir for liquid.

A Plan 76 can fail without an alarm. A 2013 AIChE paper describes set screws that slipped on the shaft: the uncooled containment seal failed first, so when the inner seal followed, the high-pressure alarm never tripped.

Barrier and Buffer System Failures

The barrier or buffer system fails in four ways: it loses pressure, it overheats, it stops circulating, or its fluid degrades.

Loss of barrier pressure. A pressurized system must keep barrier pressure above seal-chamber pressure at all times. A common cause of losing it is taking pressure off the system while the pump runs, often to top up the reservoir.

Plan 53C loses its pressure margin in its own ways. Fine abrasives can damage its piston seal, and a process fluid that solidifies as it cools can plug its reference line.

Pressure reversal can also come from the process side. In a published case, isolating a water-injection pump’s suction after shutdown over-pressurized the seal chamber far above the Plan 54 operating pressure, large amounts of water got into the barrier oil tank, and the faces broke.

Overheating and over-pressure. In gas-charged systems (Plan 53A and 53B), heat from the sun, the surroundings and the seal itself expands the gas and drives barrier pressure far above its target, and the over-pressure can end in seal failure.

A Plan 53B pressure alarm can also be wrong in both directions because of temperature. On a very hot day the accumulator can run out of liquid without tripping the low-pressure alarm. On a cold night the alarm can trip with liquid still in it, and the refill that follows over-pressurizes the system when the day warms up.

Circulation loss. Circulation stops when the pumping ring can no longer move fluid around the loop, and inadequate venting is the usual cause. A barrier fluid that is too viscous at start-up or stand-by temperature slows the flow, and one that freezes stops it. Long runs, extra fittings and a restrictive cooler add pressure drop that the pumping ring may not overcome.

Fluid degradation. Barrier fluid that runs hot degrades faster, and heat-transfer fluids used in very hot services can decompose into coke. The inhibitors in an inhibited glycol such as antifreeze can come out of solution and damage the faces. A Plan 52 buffer fluid also takes process leakage by design, so it degrades from contamination as well as heat and needs changing more often than a barrier fluid.

A viscous barrier oil can blister carbon faces, particularly at ISO Grade 32 or heavier.

Inspecting a Failed Double Seal

Inspect a failed double seal as two seals and a fluid: record what the reservoir did, sample the barrier or buffer fluid before it is drained, and examine the inner and outer seals separately.

  • Reservoir history: the level, pressure and alarm trend up to the failure, and on a Plan 53B the ambient temperature when the alarm tripped.
  • Barrier or buffer fluid: color, viscosity, consistency, pH and solids against fresh fluid, plus any process fluid, darkening or coke in it.
  • Each seal: faces and elastomers kept apart and labeled inner and outer, then read as you would a single seal. Blistered carbon points back to barrier viscosity.
  • Circulation path: vents, lines, cooler and pumping ring checked for blockage, coke or trapped gas.

Sound faces beside a contaminated or overheated fluid mean the correction belongs in the support system.

Corrective Actions

Correct the failure point the diagnosis found, and after any seal upset or failure, drain and replace the barrier or buffer fluid before restart.

  • Inner seal failed in an unpressurized seal: plan an orderly shutdown for seal replacement as soon as possible.
  • Pressure reversal from the barrier side: keep barrier pressure up while the pump runs, refills included, and set it against the maximum seal-chamber pressure. API 682 recommends a barrier pressure at least 1.4 bar (20 psi) above seal-chamber pressure; the seal pot pressure calculation works it through per plan. Give each seal its own reservoir, or fit a robust pressure alarm, where several share one.
  • Pressure reversal from the process side: make the suction isolation valve and the dump valve act together so a stopped pump cannot over-pressurize the seal chamber, and add a suction pressure transmitter with a high-pressure alarm.
  • Plan 53C on dirty or solidifying process fluid: change to a plan that does not take its pressure from the process, such as 53A, 53B or 54.
  • Heat-driven over-pressure (Plan 53A, 53B): shade the tank or accumulator, use the largest tank that fits, and add a relief valve or a pressure regulator that can vent. Skip the regulator where process vapor mixes with the barrier fluid. Where barrier pressure on the inner seal runs high, fit a seal designed for it.
  • Plan 53B false alarms: insulate or temperature-control the accumulator, and set the alarm strategy for the site’s range of ambient temperatures.
  • Circulation loss: vent the loop fully, keep piping short with few bends and fittings, slope the lines up from the gland to the reservoir, and check that the fluid circulates at start-up temperature.
  • Overheated or degraded fluid: change hot-running fluid more often, use uninhibited glycol instead of antifreeze, and consider synthetic barrier fluids, which perform well at a higher cost.
  • Blistered carbon faces: change to a barrier oil lighter than ISO Grade 32, or to a hard-vs-hard face pair.
  • Plan 76 silent failure: replace and torque the set screws correctly so the seal cannot slip on the shaft.
  • Repeat failures: track barrier fluid consumption against the seal maker’s estimated leak rate, so rising consumption flags the next failure before it happens.