A high-pressure mechanical seal is a pump seal designed to hold a seal-chamber pressure beyond what a standard seal is built for, with its balance, faces and secondary seals reworked so the faces do not overload, distort or leak as pressure rises. A high-pressure design limits how much of the sealed pressure reaches the faces and keeps the rings stiff enough to stay flat under it.

These seals run on pipeline pumps, water injection pumps and boiler feed pumps, wherever the seal chamber sits well above the pressures a catalogue seal is qualified for.

What Pressure Counts as High

No industry standard sets one pressure at which a mechanical seal becomes a high-pressure seal. The closest thing to a threshold is API 682: its seal categories are qualified up to a maximum pressure of about 20 bar for Category 1 and 40 bar for Categories 2 and 3.

API 682 calls any seal outside its scope, by design or by operating window, an “Engineered Seal”.

Supplier definitions of “high pressure” vary widely, so the figure to ask for is the seal-chamber pressure the design was tested to.

How a High-Pressure Seal Is Built

A high-pressure seal is built to keep face load, face shape and the secondary seals under control as chamber pressure climbs. On a standard seal, rising pressure raises the force closing the faces, and more face load means more heat at the faces and more distortion of the rings.

Hydraulic Balance

Hydraulic balance keeps face loading in check by shrinking the area that seal-chamber pressure pushes on, so that only part of each added bar of pressure reaches the faces. The balance ratio is that hydraulically loaded closing area divided by the face contact area: above 1 the seal is unbalanced, below 1 it is balanced.

Balanced mechanical seals are typically designed for a balance ratio of 0.6 to 0.9.

Balance has a lower limit. A 1978 symposium paper on seal balance traced some premature failures to face instability when the balance ratio was set too low, so the ratio is chosen for the duty, and a lower one is not automatically safer.

Face Materials

High-pressure seals run either carbon against silicon carbide, with the carbon ring shaped and sized to stay flat, or two hard faces such as silicon carbide against silicon carbide or tungsten carbide when carbon would deflect too much. Carbon against silicon carbide is still API 682’s default face pairing in all three seal categories, up to the standard’s 40 bar ceiling.

On larger, higher-pressure seals a stiffer face replaces carbon. Solid tungsten carbide has the highest modulus of the common face materials and resists pressure deflection best.

Face Distortion and Stiffness

A high-pressure seal keeps its faces flat by making the rings stiff and shaping them so pressure cannot bend them out of their lapped flatness.

Beyond a stiffer face material, the ring’s cross-section is shaped so that the hydraulic forces balance out about the centroid of the face. A more constant section also behaves more predictably under both high and changing pressure.

The O-ring behind the face is placed so its tolerances no longer shift that force balance, leaving face twist to depend only on the groove dimensions.

Fluctuating pressure is harder to seal than steady pressure, so a design for variable duty is checked across its whole pressure range.

Preventing O-Ring Extrusion

A high-pressure seal keeps its O-rings from extruding with harder, higher-durometer compounds and, where pressure demands it, a PEEK backup ring fitted behind the O-ring to raise its pressure capability. U-cups and V-rings are the other secondary-seal shape used at higher pressure, mainly at low temperature.

The pressure an O-ring is rated to hold before extruding is set by the size of the clearance gap on its low-pressure side and by its Shore A hardness.

For extrusion, the premium compound is not the most resistant. In O-ring tests funded by the US offshore regulator BSEE, 90 Shore A HNBR and FKM O-rings held more pressure before tearing at 150 to 175 °C than 90 Shore A FFKM and FEPM O-rings. Those were oilfield tests on plain O-rings outside a mechanical seal; in a seal, FFKM is chosen when the fluid’s chemistry calls for it.

Drive, Springs and Bushings

In a high-pressure seal the springs do less of the closing and the drive does more of the holding. Hydraulic pressure supplies most of the force that closes the faces, so the springs are there for the moments when pressure is low: start-up, shutdown and upsets.

The drive has to hold the seal against the larger axial load that pressure generates. API 682 sizes the holding capacity of a cartridge seal’s set-screw drive collar against the load the seal produces at the maximum pressure of its category.

On API 682 Category 2 and 3 seals, the bushing outboard of the faces is a floating carbon throttle bushing, which runs closer to the shaft than a fixed bushing and limits leakage if the faces fail.

Seal Types Used at High Pressure

High pressure is mostly served by balanced O-ring pusher seals, single or in a dual arrangement, while metal bellows seals are the usual choice where temperature is the limit. Metal bellows handle more pressure than elastomer bellows.

A single seal covers high-pressure duty when the process fluid may leak at a seal’s normal rate. When it may not, a dual seal runs in one of two modes:

  • Dual pressurized (API 682 Arrangement 3): A barrier fluid is held above seal-chamber pressure. Process fluid cannot reach atmosphere and the inner faces run on barrier fluid, at the cost of some barrier fluid leaking into the process.
  • Dual unpressurized (API 682 Arrangement 2): A buffer fluid is held below seal-chamber pressure. It is simpler, keeps the process uncontaminated, and adds a backup seal over a single seal.

For dual pressurized seals, back-to-back and face-to-face orientations are the usual choice.

A pressurized dual seal on high-pressure duty needs a seal support system that holds the barrier above an already high seal-chamber pressure, typically an accumulator plan such as Plan 53B (bladder) or Plan 53C (piston), or an external barrier system under Plan 54.