# API Seal Flush Plans: How to Read an API 682 Piping Plan and Choose the Right One
A mechanical seal does not fail in isolation. More often than not, it fails because the environment around the faces — temperature, cleanliness, pressure, lubrication — was wrong, and the piping plan is what controls that environment. The plan number is not paperwork for the data sheet. It is the seal’s life-support system. Specify the wrong one and the best seal on the market will still be leaking before the pump reaches operating temperature.
This page maps the full set of API 682 piping plans, what each one actually does, and when to use it. Each plan links to a detailed guide where one exists.
How to read a plan number
API 682 plans are grouped by the side of the seal they serve and the job they do:
- Plans 01–41 condition the process side of a single seal — recirculating, cooling, cleaning, or injecting flush across the inboard faces.
- Plans 51, 62, 65, 66 serve the atmospheric side — quench, drain, leakage detection, and leakage limiting.
- Plans 52–76 support dual seals — supplying the buffer (unpressurized) or barrier (pressurized) fluid between two seals.
The number itself is a label, not a ranking. Plan 53B is not “better” than Plan 11 — they solve different problems. Start from what the seal needs, then find the plan that delivers it.
Choosing a plan: start with what the seal needs, not the plan number
Before looking at any plan number, answer four questions about the fluid at the faces:
- Is it clean? Abrasives or solids → you need separation or external injection (Plan 31, 32, 41), not a straight recirculation.
- Is it cool enough? Faces generate heat; hot or marginal-margin fluids need cooling (Plan 21, 23) or the film flashes and the faces dry-run.
- Will it stay liquid at the faces? Fluids near their vapor point need pressure margin or a cooler — vaporization at the interface is one of the most common seal killers.
- What happens if it leaks? Hazardous, toxic, or fugitive-emission-controlled fluids push you from a single seal to a dual seal with a buffer or barrier (Plan 52, 53A/B/C, 54).
Only after those answers does the plan number follow. A flush plan that ignores the actual failure risk is just decoration on the data sheet.
Single-seal plans (01–41): conditioning the process side
These plans manage the fluid film across a single seal’s faces — by recirculating, cooling, separating solids, or injecting an external flush.
| Plan | What it does | Typical use | Detail |
|---|---|---|---|
| 01 | Internal recirculation from discharge to the seal chamber through an internal passage — no external piping | Clean, non-fouling, ambient-to-warm service | — |
| 02 | No flush — dead-ended seal chamber, often with a cooling jacket | Clean, cool services where chamber design alone suffices | — |
| 11 | Flush from pump discharge through an orifice into the seal chamber | The default single-seal plan for clean fluids | Plan 11 guide |
| 12 | Plan 11 with a strainer ahead of the orifice | Clean fluid carrying occasional debris that would plug the orifice | — |
| 13 | Recirculation from the seal chamber back to suction through an orifice | Vertical pumps, where Plan 11 cannot vent | Plan 13 guide |
| 14 | Plan 11 + Plan 13 combined — flush in from discharge, bleed out to suction | Vertical pumps needing both venting and positive flow | Plan 14 guide |
| 21 | Plan 11 with a cooler in the line — cooled flush to the faces | Hot service where the flush must be cooled before reaching the faces | Plan 21 guide · 11 vs 21 |
| 22 | Plan 21 with a strainer ahead of the cooler | Hot service where the flush also carries debris | — |
| 23 | Seal-chamber fluid circulated through a cooler by a pumping ring (closed loop) | The efficient hot-service plan — cools only the small seal-chamber volume. Boiler feed, hot water | Plan 23 guide |
| 31 | Discharge fluid through a cyclone separator — clean fraction to the seal, solids to suction | Dirty or abrasive services | Plan 31 guide |
| 32 | Clean fluid injected from an external source into the seal chamber | Dirty, abrasive, or flashing service where the process fluid is unsuitable as flush | Plan 32 guide · 11 vs 32 |
| 41 | Plan 31 + a cooler — cyclone separation plus cooling | Service that is both dirty and hot | — |
Plans 01-31 all recirculate the pump’s own fluid, while Plan 32 brings in an outside source — a distinction with its own failure modes, covered in internally vs externally flushed seals and what an external flush is.
Atmospheric-side plans (51, 61, 62, 65, 66): quench, drain, and leakage control
These plans act on the outboard (atmospheric) side of a single seal — keeping the faces clean, excluding air, or catching and measuring leakage.
| Plan | What it does | Typical use | Detail |
|---|---|---|---|
| 51 | Dead-ended external reservoir supplying quench fluid to the atmospheric side (no circulation) | Fluids that crystallize, coke, or freeze on the outboard side | — |
| 61 | Tapped and plugged atmospheric-side connections — a provision only, with no quench hardware installed | The default when no atmospheric-side support is needed yet; ports stay ready for a future Plan 62 or 65 | — |
| 62 | External quench — steam, nitrogen, or water — on the atmospheric side | Polymerizing, oxidizing, or coking service; excludes air and washes the faces | Plan 62 guide · Flush vs quench |
| 65A / 65B | Leakage collection with level/flow monitoring and alarm | Detecting single-seal failure early | Plan 65 guide |
| 66A / 66B | Leakage limiting through an orifice or restriction | Capping emissions when a single seal fails | — |
Dual-seal plans (52–76): buffer and barrier support
When a single seal’s leakage is unacceptable, two seals are run in series with a fluid between them. The distinction that governs everything here is buffer vs barrier: a buffer fluid sits at a pressure below the seal-chamber pressure (unpressurized, containment role); a barrier fluid sits above it (pressurized, zero-process-emission role). See buffer and barrier fluids for the full distinction.
| Plan | Buffer / Barrier | What it does | Detail |
|---|---|---|---|
| 52 | Buffer (unpressurized) | External reservoir, circulated by a pumping ring, vented to vapor recovery; buffer pressure below seal-chamber pressure | Plan 52 guide · 52 vs 53 |
| 53A | Barrier (pressurized) | External reservoir pressurized directly by gas (N₂); moderate pressure; the most common pressurized plan | Plan 53A guide · 53A vs 53B vs 53C |
| 53B | Barrier (pressurized) | Pressurized via a bladder accumulator — gas isolated from the barrier fluid; higher pressure, no gas absorption | Plan 53B guide |
| 53C | Barrier (pressurized) | Pressurized via a piston accumulator referenced to process pressure — tracks process pressure at a fixed differential | Plan 53C guide |
| 54 | Barrier (pressurized) | External pressurized barrier from a separate circulation unit/console; clean, high-duty, precisely controlled | Plan 54 guide · 53A vs 54 |
| 55 | Buffer (unpressurized) | External unpressurized buffer circulated by an external system (the console-style equivalent for buffer duty) | — |
| 72 | Buffer gas (unpressurized) | Unpressurized buffer gas to a containment (dry/gas) seal; used with Plan 75/76 | — |
| 74 | Barrier gas (pressurized) | Pressurized barrier gas (N₂) for dual gas-barrier, non-contacting seals | — |
| 75 | — | Collects condensing leakage from the containment seal into a collection vessel | — |
| 76 | — | Vents non-condensing leakage from the containment seal to flare or vapor recovery | — |
The reservoir on a Plan 52/53 system is the seal pot; how it is sized and pressurized is covered under API 682 seal pot requirements by plan type.
Common plan-selection mistakes
- Defaulting to Plan 11 on hot service. Plan 11 recirculates the hot process fluid straight back to the faces. If the fluid is already near its margin, you have just removed your only cooling. This is where Plan 21 or 23 earns its cost.
- Specifying Plan 32 without checking flush volume. External injection dilutes or contaminates the process and consumes utility fluid continuously. It solves a dirty-service problem but creates an operating-cost one — confirm the plant can supply the flow before committing.
- Confusing buffer and barrier. A Plan 52 buffer does not stop process leakage to atmosphere on its own — it contains and dilutes it. If zero emission is the requirement, you need a pressurized barrier (Plan 53/54), not a buffer.
- Treating the plan as set-and-forget. A flush plan has a pressure and flow window. When it drifts, the seal starts dying before anyone sees a leak — which is why checking flush-plan pressure belongs on the maintenance round.
Matching a plan to an application comes down to working the service conditions in order — how to select the right flush plan walks that sequence, and the single-seal vs dual-seal flushing plans comparison settles the branching question first. Matching the seal hardware to the plan you have chosen is where a seal supplier earns its keep — a seal built for Plan 23 with the wrong pumping ring will never circulate the way the plan assumes.
Frequently asked questions
Are seal plans a must for a mechanical seal?
Not always. Seal plans are important, but whether one is required depends on the application, fluid properties, and operating conditions. A clean, cool, well-behaved fluid in a generously sized seal chamber may run reliably on Plan 02 (no flush) or a simple Plan 01/11. The plan exists to fix a specific problem — heat, solids, vaporization, or emissions — so if the fluid presents none of those, an elaborate plan adds cost without benefit.
What is the API code in a mechanical seal?
The API code refers to the American Petroleum Institute’s standardized system (API 682 / ISO 21049) for classifying mechanical seals and their support systems by configuration, category, type, and arrangement. The piping-plan numbers on this page are part of that system — they let a buyer and supplier specify the seal’s support environment unambiguously, without describing the piping from scratch each time.
What is the most common flush plan?
For clean single-seal services, Plan 11 (recirculation from discharge through an orifice) is the workhorse. For hot service, Plan 23 is the most efficient choice. For dual seals where emissions matter, Plan 53A is the most widely specified pressurized arrangement.
What is the difference between a buffer and a barrier fluid?
A buffer fluid (Plan 52) runs at a pressure below the seal-chamber pressure and contains or dilutes leakage. A barrier fluid (Plan 53A/B/C, 54) runs above seal-chamber pressure, so any leakage is clean barrier fluid into the process, giving effectively zero process emission to atmosphere.
How do I know if my flush plan is working?
Every plan has a flow and pressure window. Monitor the seal pot level (dual seals), the cooler outlet temperature (Plan 21/23), and the flush pressure at the seal chamber. Drift outside the window is the earliest warning that the seal is being starved or overheated — long before a visible leak appears.