Mechanical Seals Explained: API 682 Types, Arrangements and Piping Plans

Quick Answer

A mechanical seal is the sealing device between a rotating pump shaft and its stationary casing, replacing packing with two flat faces held in near-contact by springs and hydraulic load. API 682 (4th Edition) classifies seals into three types by flexible-element design — Type A (rotating flexible element with elastomer bellows, the default for temperatures from -40°C to 260°C), Type B (stationary flexible element, elastomer bellows) and Type C (metal bellows, for -40°C to 400°C) — and three arrangements by number of seals: Arrangement 1 (single seal), Arrangement 2 (dual seals with an unpressurised buffer fluid between them), and Arrangement 3 (dual seals with a pressurised barrier fluid held 1.4-2 bar above process pressure, giving effectively zero process emission). The sealing faces run on a fluid film typically 0.25 to 2.5 microns thick; lose that film and a seal that should last five years fails in minutes. Flush, quench and barrier-fluid routing is specified using API piping plans — Plan 11, 13, 21, 23, 32, 52, 53A/B/C, 54 and 62 cover the great majority of refinery and chemical pump applications.

A fertiliser plant in the Gulf replaced the same mechanical seal on the same ammonia carbamate pump eleven times in fourteen months. Each replacement cost about 2,800 USD in parts, plus four hours of a two-man crew, plus whatever the production loss was that shift. Nobody counted the production loss, which is usually how these things survive for fourteen months.

The seal was not the problem. The seal was an Arrangement 1 single seal with a Plan 11 flush — recirculation from the pump discharge, through an orifice, back into the seal chamber. On paper, textbook. In practice the carbamate solution was crystallising in the orifice, the flush starved, the faces ran dry, and the carbon face shattered. Eleven times.

They changed to Arrangement 2 with a Plan 52 buffer system and a Plan 62 steam quench on the atmospheric side. The seal has now been running thirty-one months.

That is the whole lesson of mechanical seals in one story: the seal itself is rarely what fails. What fails is the environment the seal was asked to live in. API 682 exists because the industry got tired of learning this one plant at a time.

What a Mechanical Seal Actually Is

Before packing, before seals, pumps leaked. Then they leaked less. A mechanical seal is the device that took shaft sealing from "manage the drip" to "expect zero visible leakage."

Every mechanical seal has four functional elements, and understanding them makes every later decision legible:

1. The primary sealing faces. Two flat rings, one rotating with the shaft, one stationary in the housing, pressed together face-to-face. This is where sealing happens. One face is usually a soft material (carbon-graphite), the other hard (silicon carbide, tungsten carbide, ceramic). Lapped flat to within one or two light bands — about 0.3 microns.

2. The secondary seals. O-rings, wedges, or bellows that seal the faces to the shaft and to the housing. These are static or semi-static seals. They are also the single most common cause of chemical-compatibility failures, because the elastomer is the weakest chemical link in the assembly.

3. The loading mechanism. Springs, a bellows, or both, that push the faces together and keep them together as the faces wear and as the shaft moves axially. Single-spring designs tolerate solids better; multi-spring designs load more evenly.

4. The drive mechanism. Set screws, keys, or drive bands that transmit torque from the shaft to the rotating face.

The fluid film is the entire mechanism

Here is the point everyone misses: the faces are not supposed to touch. They are supposed to be separated by a film of liquid 0.25 to 2.5 microns thick — thinner than a red blood cell, thinner than a fingerprint ridge. That film carries the load, lubricates, and carries away the heat.

The film forms because the sealing faces are not perfectly parallel under load — they coning slightly, hydrodynamic and hydrostatic effects pull fluid in, and an equilibrium establishes itself. Face flatness, face pressure, fluid viscosity and temperature all govern it.

Leakage through a properly functioning mechanical seal is typically 0.5 to 5 cubic centimetres per hour, which evaporates before you see it. That is "zero leakage" in industrial language.

When the film breaks down — because the fluid flashed to vapour, because the flush stopped, because solids wedged in, because the faces distorted from heat — the faces touch. Dry contact at 3,000 rpm on a 60 mm seal face means a rubbing velocity around 9 m/s. Carbon against silicon carbide at 9 m/s with no lubricant generates heat faster than it can be conducted away. Faces reach several hundred degrees in seconds. Carbon blisters and cracks; the elastomer cooks; the seal fails.

Every piping plan in API 682 exists to protect that film.

Balanced vs Unbalanced Seals

The hydraulic closing force on the sealing faces comes from the process pressure acting on the seal's cross-sectional geometry.

Unbalanced seals let full process pressure act across the whole face area. Simple, cheap, robust. But face loading rises linearly with pressure, and above roughly 14-20 bar the film cannot survive the load. Unbalanced seals are used in low-pressure water, general utility, and clean service.

Balanced seals use a stepped shaft or sleeve so that process pressure acts on a reduced area. The balance ratio — typically 0.65 to 0.80 — sets how much of the pressure reaches the faces. This keeps face loading in a workable band up to 35-50 bar and beyond, and it is mandatory for light hydrocarbons, where excess face heat would flash the fluid film to vapour.

API 682 seals are balanced by default. If a vendor offers you an unbalanced seal for hydrocarbon service, that is a specification error, not a cost saving.

API 682 Seal Types: A, B and C

API 682 (ISO 21049) is titled Pumps — Shaft Sealing Systems for Centrifugal and Rotary Pumps. It was written to stop every refinery specifying its own seal from scratch. It defines standardised, pre-engineered cartridge seals that are interchangeable between vendors.

All API 682 seals are cartridge seals: the rotating element, stationary element, gland plate and sleeve ship as one pre-assembled, pre-set unit. You bolt it on, pull the setting clips, and it is installed. No field face-setting, no measuring, no "the fitter compressed the springs by eye." This alone eliminated a large category of installation failures.

Type A is the default. Pusher design: the dynamic O-ring slides axially along the shaft sleeve as the faces wear. Multi-spring, rotating flexible element. Covers the overwhelming majority of refinery, chemical and water pump duties.

The Type A weakness is hang-up: if the fluid leaves a deposit on the sleeve where the O-ring slides — coke, salt, polymer, crystallised product — the O-ring sticks, the springs cannot push the face forward as it wears, a gap opens, and the seal leaks. In clean service, irrelevant. In fouling service, this is the failure mode.

Type B replaces the dynamic O-ring with a metal bellows. No sliding element, so no hang-up. The bellows flexes to accommodate wear and axial movement. Used where fouling or crystallisation would hang up a pusher seal, but temperature is still within elastomer range for the static seals.

Type C is the high-temperature seal: stationary metal bellows, metal-to-metal secondary sealing (graphite gaskets rather than elastomers), rated to 400°C. This is the hot-oil seal — vacuum tower bottoms, heavy residue circuits, hot asphalt. Expensive, and it should be specified only when temperature genuinely requires it.

API 682 Arrangements 1, 2 and 3

The arrangement decides what happens when the inner seal leaks — and every seal eventually leaks.

Arrangement 1 — Single Seal

One set of sealing faces. Process on one side, atmosphere on the other. When the seal fails, process goes to atmosphere.

Fine for water, for non-hazardous fluids, for anything where a leak is a housekeeping issue rather than a safety or environmental event. Cheapest and simplest. Easiest to maintain. Still the right answer for the majority of general-service pumps in a plant.

Arrangement 2 — Dual Seals, Unpressurised Buffer

Two sets of sealing faces in series, with a buffer fluid between them at a pressure below process pressure — usually near atmospheric, vented to a flare or a vapour recovery system.

The inner seal does the real work and sees nearly full process pressure. The inner seal's small normal leakage goes into the buffer cavity and is swept away to flare, not to atmosphere. The outer seal sees only buffer-fluid pressure, so it lives an easy life and acts as a containment seal: if the inner seal fails suddenly, the outer seal holds process pressure long enough for an orderly shutdown instead of a release.

Arrangement 2 is the workhorse of modern hydrocarbon service. Combined with a Plan 52 buffer system, it meets most emissions regulations while keeping maintenance manageable.

Arrangement 3 — Dual Seals, Pressurised Barrier

Two sets of faces, with a barrier fluid between them held at a pressure above process pressure — API 682 requires the barrier to be maintained at least 1.4 bar (20 psi) above the maximum seal chamber pressure, and 2 bar is common practice.

Because the barrier pressure is higher, leakage flows inward, from the barrier into the process. No process fluid reaches the atmosphere at all. Emission from the process side is effectively zero.

This is what you use when the fluid must not escape under any circumstances: hydrofluoric acid, isocyanates, acrylonitrile, benzene above regulated thresholds, chlorinated solvents. It is also what you use when the process fluid is a hopeless lubricant — the barrier fluid lubricates both seals instead, so you can seal slurries, dry gases and crystallising solutions reliably.

The cost is a barrier system that must never lose pressure. If barrier pressure falls below process pressure, you have an Arrangement 3 seal running as a badly-configured Arrangement 2, and you will find out quickly. Arrangement 3 systems need pressure monitoring with an alarm, not just a sight glass.

Face Materials and Pairings

Face material selection governs wear life, chemical resistance and thermal behaviour.

Carbon-graphite — self-lubricating, forgiving of brief dry contact, conforms to minor face distortion, inexpensive. Resin-impregnated grades are standard; antimony-impregnated grades are used for higher pressures and in applications where the resin would be attacked. Carbon is the "soft" face in most pairings. It is attacked by strong oxidisers and by hot concentrated acids.

Silicon carbide (SiC) — very hard, excellent thermal conductivity, outstanding chemical resistance including strong acids and caustics, low thermal expansion. Reaction-bonded SiC contains free silicon and is attacked by strong caustic above about pH 11; sintered (direct-sintered) SiC has no free silicon and handles caustic. SiC is the default hard face in modern seals.

Tungsten carbide (WC) — tougher than SiC, better in abrasive service with impact or vibration, harder to break. Nickel-bound grades resist corrosion better than cobalt-bound. Thermal conductivity is good but below SiC.

Ceramic (alumina) — cheap, chemically inert to many fluids, but low thermal conductivity and brittle. Used in water, general utility, and some chemical duties. Not for thermal-shock applications.

The trade-off: hard-vs-hard pairings last far longer in abrasive duty but will not survive dry running. Carbon-vs-hard pairings tolerate upsets but wear faster in abrasives. If you specify SiC vs SiC, you have committed to a flush system that never fails.

Elastomer selection is not a detail

The secondary seals are usually the chemical limit of the assembly:

• FKM (Viton) — general hydrocarbon default, to about 200°C. Attacked by amines, ketones, esters, hot water and steam.
• EPDM — water, steam, caustic, amines, most inorganic acids. Not for hydrocarbons.
• FFKM (Kalrez, Chemraz) — near-universal chemical resistance, to 300°C+. Expensive but often cheaper than one unplanned shutdown.
• NBR — oils, water, lower temperature. Cheap general service.
• Flexible graphite — high temperature, used in Type C seals where elastomers cannot survive.

A Viton O-ring in amine service will swell, soften and extrude. The seal will be blamed. The elastomer selection was the failure.

API Piping Plans — The Part That Decides Reliability

A piping plan is the external circuit that controls the fluid environment at the seal. Specifying the right seal with the wrong plan is the most common expensive mistake in pump sealing.

Flush plans — conditioning the process-side environment

Plan 02 — No flush. Dead-ended seal chamber, no circulation, usually with a cooling jacket. Simple, no external hardware. Only for clean, cool, low-duty service. Heat removal is poor.

Plan 11 — Recirculation from pump discharge through an orifice into the seal chamber. The most common plan in the world. Provides positive flow across the faces, raises seal chamber pressure above vapour pressure, flushes heat away. The orifice is the weakness: small, and it plugs in fouling or crystallising service — this was the Gulf fertiliser plant's failure.

Plan 13 — Recirculation from the seal chamber back to pump suction. Standard for vertical pumps, where Plan 11 would trap vapour at the top of the seal chamber. Also used in high-suction-pressure service.

Plan 14 — Combination of 11 and 13: flow in from discharge, out to suction. Used in vertical pumps where venting and positive flow are both needed.

Plan 21 — Plan 11 with a heat exchanger (cooler) in the recirculation line. For hot service where the fluid must be cooled before it reaches the faces. Continuous flow of hot fluid through the cooler means high cooling duty.

Plan 23 — A closed loop: a pumping ring in the seal chamber circulates seal-chamber fluid through a cooler and back. Only the small volume in the loop is cooled, not a continuous stream of hot process fluid. For hot water and boiler feed service Plan 23 is dramatically more effective and cheaper to run than Plan 21 — far lower cooling duty for better face temperature control. Still specified as Plan 21 far too often out of habit.

Plan 31 — Plan 11 with a cyclone separator to remove solids before the flush reaches the seal. Works when the solids are substantially denser than the liquid; does not work for low-density or near-neutral-density solids.

Plan 32 — Clean flush injected from an external source: utility water, clean product, condensate. The most reliable solution for dirty and slurry service, because the seal never sees the dirty fluid. The cost is dilution of the process and consumption of flush fluid. Flow, pressure and reliability of the external source all need to be engineered — "there's a water line nearby" is not a Plan 32 design.

Plan 41 — Plan 21 plus a cyclone separator. Hot and dirty.

Buffer and barrier plans — for Arrangements 2 and 3

Plan 52 — Unpressurised buffer system for Arrangement 2. An external reservoir of buffer fluid, circulated by a pumping ring, vented to flare or vapour recovery. Level and pressure instruments detect inner-seal leakage: rising level or rising pressure means the inner seal is degrading. This is the diagnostic value of Arrangement 2 — the system tells you the seal is dying before it dies.

Plan 53A — Pressurised barrier system for Arrangement 3, pressurised by nitrogen or plant air directly on the barrier fluid. Simple. Limited to about 15-20 bar, and gas slowly dissolves into the barrier fluid, which degrades its lubricating properties over time.

Plan 53B — Barrier pressurised by a bladder accumulator. The gas never contacts the barrier fluid, so no gas entrainment. Suits higher pressures. Accumulator pre-charge must be maintained and checked.

Plan 53C — Barrier pressurised by a piston accumulator referenced to process pressure, so barrier pressure automatically tracks process pressure and stays above it. The right answer for high pressures and for processes with large pressure swings. Most complex and most expensive of the 53 family.

Plan 54 — Barrier fluid supplied from an external pressurised circulation system, often shared across several pumps. Used in large installations with many Arrangement 3 seals. Reliability of the central system becomes critical to every pump on it.

Quench plans — the atmospheric side

Plan 62 — External quench on the atmospheric side of the seal: steam, water, or nitrogen. Steam quench prevents coking in hot oil service and prevents crystal formation where product would solidify on contact with air. Water quench cools and washes away salts. Nitrogen quench excludes oxygen and moisture. Cheap and remarkably effective — the Gulf fertiliser pump needed a Plan 62 as much as it needed Arrangement 2.

Plan 65A / 65B — Leakage detection: the atmospheric-side drain is piped to a collection point with a level switch, so leakage is measured rather than guessed at. 65A uses an orifice in the drain line; 65B uses a standpipe. Low cost, high value for condition monitoring.

Plan 66A / 66B — Throttle bushing arrangements that restrict leakage flow for added containment on single seals.

Plan 72 — Buffer gas (nitrogen) supply to a containment seal in Arrangement 2 dry-running configuration, sweeping leakage to vapour recovery.

Plan 75 — Collection of condensing leakage from a containment seal, routed to a drain or vapour recovery system.

Plan 76 — Collection of non-condensing leakage from a containment seal, routed to a vapour recovery or flare.

A Practical Selection Sequence

1. Define the fluid completely — not "crude oil" but composition, specific gravity, viscosity at seal chamber temperature, vapour pressure at seal chamber temperature, solids content and particle size, pH, and whether it crystallises, polymerises or cokes.
2. Decide the arrangement from consequence of leakage — toxic, carcinogenic or regulated? Arrangement 3. Hydrocarbon or VOC? Arrangement 2. Water or benign? Arrangement 1.
3. Check temperature to pick the type — up to 260°C, Type A unless fouling demands Type B; above 260°C, Type C.
4. Check for hang-up risk — if the fluid deposits solids on the sleeve, a pusher seal will hang up. Go to Type B or to Arrangement 3 with clean barrier fluid.
5. Verify the margin over vapour pressure — seal chamber pressure must exceed the fluid's vapour pressure at seal chamber temperature. API 682 asks for a margin; typically at least 3.5 bar or a specified ratio for light hydrocarbons. Insufficient margin means the film flashes and the seal fails, whatever else you specified.
6. Pick face materials — abrasives push toward hard-vs-hard with a reliable flush; upset-prone service pushes toward carbon-vs-hard.
7. Pick the elastomer against the actual full fluid composition, including trace amines, chlorides, and cleaning chemicals used during turnaround.
8. Select the piping plan to fix the specific environmental problem — heat (21 or 23), solids (31 or 32), vapour (11 or 13 with margin), air-side reaction (62).
9. Check shaft and sleeve material against the fluid — a 316 sleeve in chloride service with stagnant seal chamber flow will pit.
10. Specify instrumentation — Plan 52 and 53 systems need level and pressure alarms wired to the control room, not sight glasses someone reads during rounds.

Why Seals Actually Fail

In failure analyses across refineries and chemical plants, the dominant causes repeat:

Dry running. The pump ran at shut-off, lost suction, or was started with a closed suction valve. The faces had nothing to lubricate them. Seconds, not minutes.

Flush failure. Plugged orifice, failed cooler, lost external flush supply, pumping ring not working. The seal was fine; its support system was not.

Flashing. Seal chamber pressure too close to vapour pressure, or face heat raising local temperature past the boiling point. The film becomes vapour, the faces touch, the faces fail. This is why vapour pressure margin is a specification item and not a nicety.

Off-design operation. A pump running far from its best efficiency point has high radial loads, shaft deflection and internal recirculation. The seal sees misalignment and vibration it was not designed for. Seal life is a symptom of hydraulic selection.

Vibration and misalignment. Coupling misalignment, bearing wear, worn baseplate grouting. The faces separate and reclose repeatedly; the elastomer fretts; the springs fatigue.

Solids. Abrasives lap the faces, or wedge between them, or hang up the pusher O-ring.

Wrong elastomer. Swelling, extrusion, hardening or cracking of secondary seals in a fluid they were never suitable for.

Installation error. Cartridge seals have removed most of this, but shaft runout out of tolerance, gland bolts tightened unevenly, setting clips left in place, or a sleeve set screw not torqued will still kill a seal in the first hour.

Notice how few of these are about the seal. A reliability programme that only changes seal designs will keep buying seals.

Common Specification Mistakes

1. Specifying only "mechanical seal" on the pump datasheet. You will receive the vendor's cheapest compliant offer. Specify type, arrangement, face materials, elastomer and piping plan explicitly.

   Prevention: Use an API 682 seal code and state the piping plan, e.g. "Type A, Arrangement 2, carbon vs sintered SiC, FFKM, Plan 11 + 52 + 62."

2. Specifying Arrangement 1 for a fluid that cannot be released. Someone decides a single seal is adequate because the pump is small, and a toxic service ends up with one set of faces between process and atmosphere.

   Prevention: Decide arrangement from consequence of release, never from pump size or cost.

3. No vapour pressure margin check for light hydrocarbons. The most frequent cause of short seal life in LPG, propane, butane and light naphtha pumps.

   Prevention: Calculate seal chamber pressure and compare to vapour pressure at seal chamber temperature — not at process line temperature. Require the vendor to show the margin.

4. Plan 11 in crystallising or fouling service. The orifice plugs, the flush stops, the seal dies. Repeatedly, with everyone blaming the seal.

   Prevention: For fouling, crystallising, polymerising or dirty fluids, use Plan 32 with clean external flush, or Arrangement 3 with a clean barrier fluid.

5. Plan 21 where Plan 23 belongs. Cooling a continuous stream of hot process fluid when a closed loop would cool a small circulating volume instead. Larger cooler, higher utility cost, worse temperature control.

   Prevention: For hot water, boiler feed and hot clean service, specify Plan 23 with a pumping ring.

6. Elastomer selected against the main constituent only. The main fluid is compatible; the trace amine, the chloride, or the turnaround cleaning solvent is not.

   Prevention: Give the seal vendor the full composition including trace constituents, and state which chemicals will be used for cleaning and flushing.

7. Arrangement 3 with no barrier pressure monitoring. Barrier pressure falls, nobody knows, and the zero-emission seal is no longer zero-emission.

   Prevention: Specify pressure transmitter with low-pressure alarm to the DCS on every Plan 53 and 54 system.

8. SiC vs SiC faces specified for abrasion resistance in a pump that cavitates or runs dry occasionally. Hard-vs-hard has no tolerance for dry contact.

   Prevention: If the operating regime includes occasional dry-run or cavitation risk, use carbon against a hard face and accept faster abrasive wear, or fix the operating regime.

9. Reusing the old seal's specification on a repeat order after the process changed. Throughput was debottlenecked, the fluid got hotter or dirtier, and the seal spec stayed from 2009.

   Prevention: Re-verify fluid conditions at every seal repurchase, not just at pump purchase.

10. No spare seal in stores for critical pumps. A 2,800 USD seal with a ten-week lead time stops a 40,000 USD-per-day unit.

    Prevention: Hold cartridge spares for all A-criticality pumps; standardise seal types across the plant to reduce the number of spares required.

Supply from Kasko Makine

Kasko Demir Çelik Makine supplies mechanical seals and sealing systems alongside the pumps, piping and fittings that surround them:

Mechanical seals

• API 682 cartridge seals, Types A, B and C
• Arrangements 1, 2 and 3, including dual pressurised configurations
• Component seals for general service and OEM replacement
• Face material options: carbon-graphite, reaction-bonded and sintered silicon carbide, nickel-bound tungsten carbide, alumina ceramic
• Elastomer options: NBR, EPDM, FKM, FFKM, flexible graphite for Type C
• Split seals for large shafts and difficult-access equipment
• Replacement cartridges interchangeable with major OEM envelopes

Seal support systems

• Plan 52 buffer reservoirs with level and pressure instrumentation
• Plan 53A, 53B and 53C barrier systems, including bladder and piston accumulators
• Plan 23 cooling loops with pumping rings and seal coolers
• Plan 32 clean flush skids with filtration
• Plan 62 steam, water and nitrogen quench arrangements
• Plan 65 leakage detection assemblies
• Seal pots, coolers, cyclone separators, orifice assemblies, instrumentation

Associated supply

• API 610 centrifugal pumps and general-service pumps
• Pump spares: bearings, sleeves, wear rings, couplings
• Gaskets, stud bolts and flanges for the pump and auxiliary piping
• Tubing, fittings and valves for seal support piping

Engineering support

Send a pump datasheet and full process conditions and we will review the seal selection against API 682 — type, arrangement, materials, piping plan and vapour pressure margin — before quoting. If you have a repeat failure, send the failure history and photographs of the failed faces; the wear pattern usually identifies the cause, and the right fix is often a piping plan change rather than a different seal.

Certification

Material certificates to EN 10204 3.1 for metallic components, elastomer compound certification, dimensional and face-flatness inspection reports, and API 682 compliance documentation where specified.

Logistics

Standard cartridge seals in common sizes and materials ship from stock in 1-2 weeks. Engineered seals and seal support systems typically 6-10 weeks. Shipping from Istanbul by air for urgent spares — 2-4 days to Gulf and North African destinations — or by sea freight for systems and bulk orders.

Send your pump datasheet and process conditions and we will return a seal selection review and quotation within three working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Rotating Equipment Series

• Industrial Pumps: Complete Guide
• Centrifugal Pumps: Types, Curves and Selection
• API 610 Centrifugal Pumps for Refinery Service

Frequently Asked Questions

Q: What is the difference between API 682 Arrangement 2 and Arrangement 3?
A: Arrangement 2 uses two seals with an unpressurised buffer fluid between them, held below process pressure and usually vented to flare, so inner-seal leakage is contained rather than released. Arrangement 3 uses two seals with a barrier fluid pressurised at least 1.4 bar above process pressure, so leakage flows inward into the process and no process fluid reaches atmosphere. Arrangement 3 gives zero emission and is used for toxic fluids and poor lubricants.

Q: What are the API 682 seal types A, B and C?
A: Type A is a pusher seal with a rotating multi-spring flexible element and an elastomer dynamic O-ring, rated -40°C to 260°C, and is the default choice. Type B replaces the sliding O-ring with a rotating metal bellows to avoid hang-up in fouling service, same temperature range. Type C uses a stationary metal bellows with metal and graphite secondary seals and is rated to 400°C for hot oil and high-temperature duty.

Q: What is an API seal piping plan?
A: An API piping plan is a standardised external circuit that controls the fluid environment at the mechanical seal. Flush plans such as 11, 13, 21, 23, 31 and 32 condition the process side by providing circulation, cooling or clean flush. Plans 52, 53A/B/C and 54 supply buffer or barrier fluid to dual seals. Plan 62 provides steam, water or nitrogen quench on the atmospheric side.

Q: Why do mechanical seals fail so quickly?
A: Most premature failures are environmental rather than design faults. The sealing faces run on a fluid film 0.25 to 2.5 microns thick, and anything that removes that film causes failure within minutes: dry running, a plugged flush orifice, insufficient pressure margin over vapour pressure causing the film to flash, or solids wedging between the faces. Correcting the piping plan usually fixes repeat failures better than changing the seal.

Q: What face materials are used in mechanical seals?
A: Common faces are carbon-graphite as the soft face paired with a hard face of silicon carbide, tungsten carbide or alumina ceramic. Carbon against silicon carbide is the general default because it tolerates brief dry contact and conducts heat well. Silicon carbide against silicon carbide lasts much longer in abrasive service but will fail quickly if the seal ever runs dry.

Q: Do I need a cartridge seal or a component seal?
A: Cartridge seals arrive pre-assembled and pre-set, so installation requires no field face setting and a major category of installation error disappears. API 682 requires cartridge construction. Component seals are cheaper and still used in general-service and OEM replacement applications where skilled fitters and good procedures are available, but for critical or hazardous service cartridge seals are the correct choice.

Q: What is the minimum pressure margin over vapour pressure for a mechanical seal?
A: The seal chamber pressure must stay above the fluid's vapour pressure at seal chamber temperature so the lubricating film remains liquid. For light hydrocarbons a margin in the region of 3.5 bar is commonly specified, and the vendor should demonstrate the calculated margin using seal chamber conditions rather than process line conditions. Insufficient margin is a leading cause of short seal life in LPG and light naphtha pumps.