What is an API mechanical seal flush plan?

An API flush plan (API 682, Annex G) is a standardized piping arrangement that conditions the fluid at a mechanical seal: cooling it, cleaning it, supplying a buffer or barrier fluid, or applying a quench. Each plan carries a number (Plan 11, 23, 52, 53A and so on) so engineers, vendors and operators describe the same arrangement consistently on a P&ID.

What is the difference between API Plan 11, Plan 23 and Plan 32?

Plan 11 recirculates fluid from pump discharge through a flow-control orifice into the seal chamber, the default for clean general service. Plan 23 uses a pumping ring to circulate seal-chamber fluid through a cooler and back, the most efficient cooling plan for hot service. Plan 32 injects a clean flush from an external source into the seal chamber, used for dirty, abrasive or polymerizing service.

When should I use API Plan 53 instead of Plan 52?

Both are dual-seal plans. Plan 52 supplies an unpressurized buffer fluid kept below process pressure (Arrangement 2) for containment and leakage monitoring, while Plan 53 (53A/53B/53C) supplies a barrier fluid pressurized above process pressure (Arrangement 3) so no process fluid escapes to atmosphere. Use Plan 53 for hazardous, toxic or no-leak-to-atmosphere service; use Plan 52 where a small amount of contained, monitored leakage is acceptable.

Mechanical Seal Flush Plans Guide | API 682 Piping Plans

1. Why Seals Need a Flush Plan

A mechanical seal closes the gap where the rotating shaft passes through the stationary pump casing. Two lapped faces — one rotating with the shaft, one held stationary — run against each other with a microscopically thin film of fluid between them. That film is what keeps the faces from touching, but it also means the seal environment has to be deliberately managed. Left to its own devices, the fluid trapped at the seal faces will overheat, flash, deposit solids, or crystallize, and the seal fails.

A seal flush plan (also called a seal piping plan) is the small piping circuit that conditions the fluid at the seal so the faces survive. Depending on the service it does one or more of the following:

Removes heat generated by the rubbing faces (and conducted in from a hot process), keeping the film below its vaporization point.
Keeps the fluid clean at the faces, diverting abrasive solids that would grind the lapped surfaces.
Prevents flashing or crystallizing by holding the seal-chamber pressure and temperature in a safe window.
Contains leakage with a buffer or barrier fluid, so process fluid does not reach atmosphere.

API 682 — Pumps: Shaft Sealing Systems for Centrifugal and Rotary Pumps — standardizes both the seals and the piping plans. Its Annex G gives a numbered schematic for every plan, so “Plan 11” or “Plan 53A” means the same thing to the designer, the seal vendor, and the operator. API 610 in turn requires the seal system on a centrifugal pump to comply with API 682.

The plan is chosen after the seal arrangement, because the arrangement decides which family of plans even applies. So the right starting point is the arrangement.

2. Seal Arrangements 1, 2 & 3

API 682 sorts sealing configurations into three arrangements, distinguished by how many seals there are and whether an externally supplied fluid is present and pressurized.

Arrangement	Configuration	What it does
Arrangement 1	Single mechanical seal	One seal sees the process fluid directly. A flush plan conditions that process fluid at the faces. A small amount of process fluid leaks to atmosphere by design.
Arrangement 2	Two seals; outer runs on an unpressurized buffer (below process pressure)	The inner (primary) seal does the sealing; the outer (containment) seal runs on a buffer fluid that is below process pressure. Provides containment and a point to monitor for primary-seal failure.
Arrangement 3	Two seals; barrier fluid pressurized above process pressure	A barrier fluid is held above process pressure between the two seals, so any leakage is barrier fluid — no process fluid escapes to atmosphere. The choice for hazardous, toxic and no-emission service.

The buffer-vs-barrier distinction is the heart of dual seals. A buffer fluid (Arrangement 2) sits below process pressure and merely contains and monitors. A barrier fluid (Arrangement 3) sits above process pressure and positively prevents process fluid from reaching atmosphere. Pressurized = barrier = zero emission.

How the arrangement steers the plan

Arrangement 1 → single-seal flush plans: 01, 02, 11, 13, 14, 21, 23, 31, 32, 41 (plus quench 62 and detection 65/66).
Arrangement 2 → unpressurized buffer plans: 52, 55 (liquid), 72 (buffer gas), with 75/76 handling containment leakage.
Arrangement 3 → pressurized barrier plans: 53A, 53B, 53C, 54 (liquid), 74 (barrier gas).

3. Single-Seal Flush Plans (Arrangement 1)

Single-seal plans take process fluid and condition it before it reaches the seal chamber. They range from a bare recirculation line to a recirculation through a separator and cooler.

Plan	Function (API 682 Annex G)	Typical use
01	Integral / internal recirculation from a high-pressure pump region to the seal chamber	Built-in recirculation; no external piping
02	Dead-ended seal chamber, no circulation	Low pressure / low temperature, benign service
11	Recirculation from discharge through a flow-control orifice to the seal chamber — the default plan	Clean general service
13	Recirculation from the seal chamber through an orifice back to suction	Standard for vertical pumps; high-head service
14	Combination of 11 + 13 (discharge→seal and seal→suction)	Vertical pumps needing positive flow-through
21	Recirculation from discharge through an orifice and a cooler into the seal chamber	Hot service
23	Recirculation by a pumping ring through a cooler and back to the seal chamber — most efficient cooling	Hot water, boiler feed water
31	Recirculation from discharge through a cyclone separator; clean fluid to the seal chamber	Dirty / abrasive service
32	Clean flush injected from an external source into the seal chamber	Dirty, abrasive or polymerizing service
41	Discharge → cyclone separator → cooler → seal chamber	Dirty and hot service

Reading the family: the bare number 11 is just an orifice; add a cooler and you get 21, or the more efficient pumping-ring-plus-cooler 23 for hot duty; add a cyclone separator and you get 31 for dirty duty, or 41 if it is also hot; when the process itself is unusable at the faces, bring in clean fluid from outside with 32.

4. Dual-Seal Buffer & Barrier Plans (Arrangements 2 & 3)

When a single seal cannot contain the fluid — because it is hazardous, toxic, flashing, or simply must not leak to atmosphere — a second seal is added and a buffer or barrier fluid is supplied to the space between the two seals.

Unpressurized buffer (Arrangement 2)

Plan	Function (API 682 Annex G)
52	Unpressurized external buffer-fluid reservoir for the outer seal of an Arrangement 2 dual seal (buffer below process pressure; vented to vapor recovery). Thermosiphon or pumping-ring circulation.
55	Unpressurized external buffer-fluid system with forced circulation (Arrangement 2) — used when natural circulation is inadequate.

Pressurized barrier (Arrangement 3)

Plan	Function (API 682 Annex G)
53A	Pressurized external barrier reservoir, gas-pressurized (typically nitrogen blanket), supplying barrier fluid (no process leakage to atmosphere). The most common Arrangement 3 plan.
53B	External barrier system pressurized by a bladder accumulator — keeps pressurizing gas out of the barrier liquid; suits higher pressures.
53C	External barrier system pressurized by a piston accumulator that tracks process pressure — maintains a set differential as process pressure swings.
54	Pressurized external barrier system circulated by a pump or external source — for high heat loads or large/multiple seals.

Dry-gas dual seals

Plan	Function (API 682 Annex G)
72	Externally supplied buffer gas (unpressurized) for an Arrangement 2 containment / dry gas seal. Almost always paired with a Plan 75 or 76 leakage-handling plan.
74	Externally supplied barrier gas (pressurized) for an Arrangement 3 dry gas seal — the standard zero-emission gas-seal plan.

Buffer is not barrier. An Arrangement 2 buffer plan (52 / 72) contains and monitors but, because the buffer is below process pressure, a primary-seal failure can still pass process fluid outward. If the requirement is truly zero process emission, an Arrangement 3 pressurized barrier (53A/B/C, 54, or 74) is the conservative choice.

5. Quench & Containment Plans

A final group of plans acts on the atmospheric side of the seal, or collects and routes whatever leakage does occur.

Plan	Function (API 682 Annex G)
51	External reservoir providing a dead-ended quench blanket (often nitrogen) to the gland (Arrangement 1).
62	External quench stream (steam, water or nitrogen) applied to the atmospheric side of the seal faces — washes away crystals/coke, prevents icing on flashing fluids.
65A / 65B	Atmospheric leakage collection & detection for condensing leakage — 65A normal drain, 65B with an orifice.
66A / 66B	Throttle bushing (66A) or orifice (66B) that limits leakage and allows seal-failure detection.
71	Tapped and plugged connections provided for a future dry barrier gas (Arrangement 2).
75	Containment-seal leakage collection for condensing / mixed-phase leakage (Arrangement 2).
76	Containment-seal vent for noncondensing leakage to vapor recovery or flare (Arrangement 2).

75 vs 76: both handle the leakage that gets past the inner seal of an Arrangement 2 dual seal. If that leakage condenses (or is mixed-phase), collect it with Plan 75; if it stays a gas, vent it to vapor recovery or flare with Plan 76. They are routinely paired with the Plan 72 buffer-gas supply.

6. Selecting a Plan

Plan selection works top-down: settle the arrangement, then read the service conditions onto the plan family.

STEP 1 — ARRANGEMENT Single seal, leakage to atmosphere acceptable? → Arrangement 1 Need containment + leakage monitoring? → Arrangement 2 (unpressurized buffer) Need zero process emission (hazardous/toxic)? → Arrangement 3 (pressurized barrier) STEP 2 — ARRANGEMENT 1 SERVICE Clean + ambient → Plan 11 (vertical pump → 13 / 14) Clean + hot → Plan 23 (or 21) Dirty / abrasive → Plan 31 (cyclone) or 32 (external clean flush) Dirty + hot → Plan 41 Polymerizing / fouling → Plan 32 (+ Plan 62 quench) STEP 2 — ARRANGEMENT 2 (buffer) Liquid buffer → Plan 52 (forced circ → 55) Dry gas buffer → Plan 72 + condensing leakage → add Plan 75 + noncondensing leakage → add Plan 76 STEP 2 — ARRANGEMENT 3 (barrier) Liquid barrier → Plan 53A (gas) / 53B (bladder) / 53C (piston) / 54 (circulated) Dry gas barrier → Plan 74

This is engineering guidance, not an API mandate. The plan numbers and their functions above are from API 682 Annex G, but the mapping from service to a recommended plan reflects common application practice. The seal chamber pressure and temperature, fluid vapor margin, solids loading, and site standards all bear on the final choice — always confirm the flush plan with the seal vendor and per API 682.

Common mistakes to avoid

❌ Treating an Arrangement 2 buffer plan as if it were an emission barrier (it is not pressurized above process).
❌ Specifying a bare Plan 11 on a hot service where the film flashes — add cooling (21/23).
❌ Recirculating dirty process fluid straight to the faces — use a cyclone (31) or external clean flush (32).
❌ Forgetting that a Plan 72 buffer-gas seal needs a 75/76 leakage-handling plan with it.
❌ Picking a plan number before settling the seal arrangement.

Key standards & references

API 682 (4th ed., 2014), Annex G — Pumps: Shaft Sealing Systems for Centrifugal and Rotary Pumps (normative source for the piping-plan numbers and functions).
API 610 (12th ed.) — requires the seal system to comply with API 682.

Mechanical Seal Flush Plans: An API 682 Guide to Flush, Buffer & Barrier Piping

Cool / Clean / Contain

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API 682 Annex G

Contents

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