A seal support system is the piping, vessels and instruments outside a mechanical seal that keep the fluid at the seal faces in a condition the seal can run in, and that collect and monitor whatever leaks past it. API 682 standardizes these systems as numbered piping plans, so “Plan 11” or “Plan 53B” on a seal data sheet names the support system.

Which system a pump needs follows from two things: the seal arrangement, which sets the family of system, and the condition of the pumped fluid, which sets the plan within that family.

What a Seal Support System Does

A seal support system keeps the seal faces supplied with a fluid film they can survive on, and handles the leakage that gets past them.

It does four jobs:

  • Cooling: carries away the heat the faces generate, by circulating fluid through the seal chamber and, where needed, through a seal cooler.
  • Pressure: keeps the liquid at the faces far enough above its vapor pressure that it stays liquid, and in a dual seal sets the pressure of the fluid between the two seals.
  • Cleanliness: sweeps solids away from the faces, or replaces a dirty process fluid at the faces with a clean one.
  • Leakage control: contains what leaks past the seal, routes it somewhere safe and measures it, so a failing seal shows up before it fails outright.

Components of a Seal Support System

A seal support system is built from piping plus a short list of accessories, meaning any hardware the seal or its piping plan needs to work:

  • Orifice: a restriction in a recirculation line that sets the flow and pressure reaching the seal faces.
  • Seal cooler: a heat exchanger that cools the flush or the buffer or barrier fluid.
  • Cyclone separator or strainer: removes solids from a flush taken off the pump before it reaches the faces.
  • Reservoir: the seal pot that holds buffer or barrier fluid for a dual seal, with level and pressure instruments and often a cooling coil.
  • Accumulator: a bladder or piston vessel that pressurizes barrier fluid in place of a nitrogen blanket over a reservoir.
  • External supply hardware: the supply line, filtration and flow control that bring a clean outside fluid to the seal, as in an external flush system.
  • Instruments: pressure, level, temperature and flow measurement.

The piping between these parts is part of the system too: tubing from the seal gland should rise all the way to the reservoir, with a vent at the high point and a drain at the low point.

Types of Seal Support Systems

Seal support systems are classified by API 682, 4th edition, which defines the standard piping plans in its normative Annex G and groups them by the first digit of the plan number:

  • Plans starting with 0 to 4: flush plans for single seals, acting on the process side of the faces.
  • Plans starting with 5: buffer and barrier liquid plans for dual seals.
  • Plans starting with 6: quench and leakage plans on the atmospheric side of the seal.
  • Plans starting with 7: gas seal and containment seal plans, including the plans that collect leakage from containment seals.

Flush Systems for the Process Side

A flush system feeds liquid into the seal chamber of a single seal so the faces run at the right temperature, pressure and cleanliness.

Most flushes reuse the pumped liquid. Plan 11 feeds it from the pump discharge into the seal chamber, and its variants add a cooler in the line (Plan 21) or a cyclone separator that strips out solids (Plan 31).

When the pumped liquid is too dirty, too hot or too close to vaporizing to protect the faces, an external flush (Plan 32) injects a clean, compatible liquid from outside and displaces the process fluid at the faces.

Buffer and Barrier Systems for Dual Seals

Buffer and barrier systems fill the space between the two seals of a dual seal with a liquid that lubricates and cools the seals and separates the pumped fluid from the atmosphere.

The two differ in pressure. A buffer fluid is held below seal chamber pressure (API 682 Arrangement 2, typically Plan 52), so the inner seal leaks process fluid into the buffer. A barrier fluid is held above seal chamber pressure (Arrangement 3, Plan 53 or 54), so the inner seal leaks barrier fluid into the process and no process fluid reaches the atmosphere. Which way the inner seal leaks is the main difference between Plan 52 and Plan 53.

A buffer system is usually a Plan 52 reservoir, circulated by a pumping ring inside the seal and vented to a flare or vapor recovery system. A barrier system pressurizes the fluid with nitrogen over a reservoir or in a bladder accumulator, or with a piston driven by seal chamber pressure (the Plan 53A, 53B and 53C variants), or with a separate unit that pressurizes and circulates it (Plan 54).

Quench and Leakage Collection Systems

Quench and leakage collection systems work on the atmospheric side of the seal, where leakage leaves the faces.

A quench applies a fluid such as steam or nitrogen to the atmospheric side to stop leaking product from crystallizing or coking there; in API 682 this is Plan 62.

Leakage collection plans pipe what leaks past a single seal to a vessel where it is measured: Plan 65A detects a leakage rate and Plan 65B detects accumulated leakage.

Gas Seal Support Systems

A gas seal support system supplies clean, pressure-regulated gas, usually plant nitrogen, to a gas-lubricated seal in place of a liquid buffer or barrier.

Plan 74 supplies barrier gas to a dual pressurized gas seal (Arrangement 3), held above seal chamber pressure, and Plan 72 supplies buffer gas to a dry-running containment seal. Compressor dry gas seals use their own support systems under API 692 and ISO 6368, and the pump plans above do not apply to them.

How to Choose a Seal Support System

Two choices settle it: the seal arrangement, picked by how much leakage the plant can accept, fixes the family of support system, and the pumped fluid’s condition fixes the plan within it.

The three arrangements and the support each one needs:

  • Single seal (Arrangement 1): for fluids where some leakage to atmosphere is acceptable or easy to control; usually a simple flush such as Plan 11 that nobody has to watch or top up.
  • Unpressurized dual seal (Arrangement 2): captures inner-seal leakage for disposal; needs a buffer reservoir (a containment seal may use buffer gas instead), possibly cooling water, and a connection to a flare or recovery system.
  • Pressurized dual seal (Arrangement 3): for fluids that must not leak to atmosphere at all; needs a barrier system with a pressure source, most often nitrogen, and someone to keep the barrier level up, which makes it the most operator-intensive choice.

Within the family, a hot fluid needs a cooler in the loop. Solids are handled by a cyclone separator only if the particle size is steady; otherwise a clean external flush isolates the faces, Plan 32 on a single seal and Plan 54 on a dual seal. Dirty, abrasive or polymerizing services that a single seal or a Plan 52 cannot handle are the usual case for a pressurized Plan 53.

Under API 682, the purchaser or the seal vendor is responsible for making sure the seal and its auxiliaries suit the service. The standard’s Annex A sets out a recommended procedure that takes the application data through to seal category, type, arrangement and piping plan.

On a smaller, lower-duty ASME B73 chemical pump, check the seal chamber first: a tapered bore with flow modifiers circulates the liquid by itself (Plan 03 in the 4th edition) and can remove the need for a piped Plan 11.

Slurry and mining pumps and pulp and paper pumps commonly run on a plant-water flush. A closed-loop seal water system with a pressurized pot cuts water use, but it costs more to install and needs regular maintenance.