What is an API Seal Plan?

An API Seal Plan (also called a piping plan or flush plan) is a standardized support system that circulates, cools, filters, or pressurizes fluid around a mechanical seal to keep it operating reliably These plans and associated instrumentation are defined in Annex G of API Standard 682, the American Petroleum Institute’s standard for mechanical seals and seal support systems used on centrifugal and rotary pumps in oil, gas, and chemical industries.

API 682 4th edition defines 32 piping plans in total though in practice, a small group of plans covers the vast majority of real-world pump installations.

• What is API 682?
• Why Seal Plans Matter
• How Seal Plans Are Classified
• Common API Seal Plans Explained
• Quick Reference
• Conclusion

API 682 is an internationally recognized standard developed by the American Petroleum Institute for mechanical seals used in centrifugal pumps and rotating equipment.

The standard provides recommendations for:

• Mechanical seal design
• Seal arrangements
• Seal support systems
• Piping plans
• Testing requirements
• Installation practices

Today, API 682 is widely followed across industries that require high reliability and safety.

Two identical seals installed on the same pump can have very different service lives depending on the flush plan used, because the plan controls face temperature, the presence of solids or vapor at the faces, and the pressure margin above the fluid’s vapor pressure. Get the plan wrong, and even a correctly selected seal can fail from dry running, coking, or face damage.

A well-chosen seal plan helps to:

• Remove heat generated at the seal faces
• Keep the seal chamber pressure above the fluid’s vapor pressure (preventing flashing)
• Prevent solids, polymers, or contaminants from reaching the seal faces
• Provide backup lubrication if the primary seal fails (dual seal arrangements)
• Support leak detection and emissions compliance

API 682 applies primarily to balanced, cartridge-type mechanical seals, and each seal is classified into one of three Types, which are further broken into Arrangements and Configurations. The seal plan you select depends heavily on which Arrangement is in use:

• Arrangement 1 (single seal): One seal per chamber. Common plans: 11, 13, 21, 23, 31, 32.
• Arrangement 2 (dual unpressurized / tandem seal): Two seals with an unpressurized buffer fluid between them. Common plan: 52.
• Arrangement 3 (dual pressurized seal): Two seals with a pressurized barrier fluid between them, at a pressure higher than the process. Common plans: 53A, 53B, 53C, 54.

Plan 11 — The Default Flush Plan

Plan 11 is the default flush plan for most single mechanical seals. It uses fluid from the pump discharge, routed through an orifice, into the seal chamber through the flush port, which increases lubricity at the seal faces and reduces heat in the chamber. It’s simple, low-cost, and requires no external equipment — which is why it’s the starting point for the majority of clean-service pump applications.

Plan 21 — Flush With Cooling

When Plan 11 alone cannot hold the seal chamber temperature within limits, Plan 21 adds a cooler: discharge flow is routed through an orifice and a heat exchanger before returning to the chamber. It’s used on moderately hot services where basic recirculation isn’t enough.

Plan 23 — Closed-Loop Circulation

Plan 23 uses a closed loop instead of continuous discharge flow — fluid already inside the seal chamber is circulated through an external cooler with only a small makeup flow, giving tighter temperature control. This makes Plan 23 more thermally efficient than Plan 21, and it’s the preferred choice for hot services such as boiler feedwater pumps. However, Plan 23 is generally not recommended when process temperature is below about 60°C (140°F), since the small pumping ring depends on thermal circulation that needs sufficient heat to work efficiently.

Plan 32 — External Flush Injection

Plan 32 injects a clean, compatible fluid from an external source directly into the seal chamber at a controlled flow and pressure. It’s the standard solution when the process fluid itself is unsuitable for lubricating the seal faces — for example, when it’s abrasive, polymerizing, or too viscous.  Because the external fluid dilutes the process stream, compatibility with the process must always be verified before selecting this plan.

Plan 52 — Unpressurized Buffer (Tandem Seals)

Plan 52 supports dual unpressurized (tandem) seal arrangements. A buffer reservoir, held at a pressure below the seal chamber pressure, circulates fluid between the two seals using thermosiphon effect or a pumping ring.  This arrangement gives the outer seal a controlled environment and provides early warning if the inner (primary) seal starts to leak, without introducing barrier fluid into the process.

Plan 53A / 53B / 53C — Pressurized Barrier Systems

These three variants support dual pressurized seal arrangements, where the barrier fluid pressure is kept above the process pressure so any leakage moves into the process rather than out to atmosphere:

•  Plan 53A uses a reservoir with a nitrogen gas cap in direct contact with the barrier fluid — simple and widely used, though gas can gradually dissolve into the fluid as pressure rises.
• Plan 53B uses a bladder accumulator pre-charged with nitrogen, so the gas never contacts the barrier fluid directly. It reaches higher pressures with less gas absorption, but the set pressure doesn’t automatically track process pressure changes.
• Plan 53C uses a piston accumulator referenced to the seal chamber, so barrier pressure automatically tracks process pressure — making it the preferred option for services with large pressure swings, though it’s limited to clean fluids.

These Plan 53 systems are common on high-hazard hydrocarbon and LPG services.

Plan 62 — External Quench

Plan 62 is an external quench, typically steam or water, injected at low pressure on the atmospheric side of a single seal. It doesn’t lubricate the seal faces — its job is to prevent the product from solidifying or coking there. It’s common on hot, waxy services, and is frequently paired with Plan 11.

Plan 74 — Dry Gas Barrier

Plans like 72 and 74 use a gas rather than a liquid barrier, avoiding the cost, contamination risk, and disposal burden of a liquid barrier fluid while generating less drag — which is why they’re favored on refinery and gas plant pumps. For dry dual-seal systems, Plan 74 is the typical choice, compared with Plan 53A (or sometimes 53B/54) for wet systems.

Selecting an API seal plan isn’t a one-size-fits-all decision — it depends on your specific fluid, pressure, temperature, and reliability requirements. At Micro Seals, our engineering team helps plant operators across India select and implement the correct API 682 seal plan for their pump fleet, from simple Plan 11 flush systems to fully pressurized dual-seal barrier systems.

Need help selecting the right seal plan for your application? Get in touch with our seal engineering team for a technical recommendation based on your process conditions.