Single seal flush plans, API 682 Plans 01 to 41, condition the pumped fluid around one seal, while dual seal flush plans, Plans 52 to 55, supply and monitor a separate buffer or barrier liquid held between two seals. Which family a pump needs depends on whether it needs one seal or two.
A flush plan gives the seal faces fluid at the right temperature, pressure and cleanliness, and API 682 (4th edition, 2014, reaffirmed 2022) defines both families. The dual-seal family adds emissions control and a leak alarm before process fluid reaches atmosphere, in exchange for a liquid system someone has to fill, pressurize and watch.
What Is a Single Seal Flush Plan
A single seal flush plan is an API 682 piping plan from the 0 to 4 series that conditions the fluid in the seal chamber of an Arrangement 1 seal, one set of faces running in the pumped fluid.
API 682 defines 13 plans in the 0 to 4 series. They differ in what they do to the fluid at the faces:
- Plans 01, 02 and 03: no external flush piping.
- Plans 11 to 14: recirculate pumped fluid between the pump and the seal chamber. Plan 11, from discharge through an orifice into the seal chamber, is the common choice.
- Plans 21, 22 and 23: recirculate the fluid through a cooler.
- Plans 31 and 41: clean the fluid in a cyclone separator, with a cooler added in Plan 41.
- Plan 32: injects clean fluid from an outside source in place of the pumped fluid.
What Is a Dual Seal Flush Plan
A dual seal flush plan is a 5-series plan that supplies, circulates and monitors a separate liquid held between the two seals of a dual seal.
API 682 splits dual seals into two arrangements by the pressure of the liquid between the seals:
- Buffer, Arrangement 2: the liquid is held below seal chamber pressure. The inner seal seals the process and leaks into the buffer, and the outer seal takes over if the inner seal fails.
- Barrier, Arrangement 3: the liquid is held above seal chamber pressure, so both seals run on the barrier liquid.
The dual-seal plans, by mode:
- Plan 52: buffer liquid from an external reservoir, circulated by a pumping ring in the seal.
- Plan 55: buffer liquid circulated by an external system.
- Plan 53A: barrier liquid from an external reservoir pressurized by gas, usually nitrogen.
- Plans 53B and 53C: barrier liquid in a closed loop with a make-up system, pressurized by a bladder accumulator in 53B and a piston accumulator in 53C. Reservoir versus closed loop is the main difference between Plan 53A and 53B.
- Plan 54: barrier liquid pressurized and circulated by an external system.
The Difference Between Single and Dual Seal Flush Plans
Single and dual seal flush plans differ in process emissions, leak detection and the support hardware they need, and every one of those differences comes from the seal arrangement each family serves.
Process Emissions
A single-seal plan lets a very small amount of process fluid weep to atmosphere in normal running. A dual-seal plan cuts that with a buffer, and to essentially none with a barrier as long as one of the two seals holds.
With a buffer plan, emissions depend on where the reservoir vents: they are very low when it goes to vapor recovery or a closed vent system.
Leak Detection
A single-seal plan shows a leak only after process fluid is outside the seal; a dual-seal plan shows it as a change in reservoir level or pressure while the second seal still holds.
On a single seal, leaks are found by inspection at the gland, or by a Plan 65 add-on from the 6-series that collects gland leakage and alarms on high level.
On a dual seal, the reservoir readings show the leak:
- Buffer plans: an inner-seal leak raises the reservoir level or pressure and trips an alarm. A falling level points to the outer seal.
- Barrier plans: both seals use up barrier liquid slowly in normal running, so a fast loss of level or a drop in pressure marks a failed seal.
Support Hardware and Upkeep
A single-seal plan is piping on the pump itself, while a dual-seal plan is a separate liquid system that has to be filled, pressurized, cooled and monitored.
The only utilities a single-seal plan needs are cooling water for a cooler and, for Plan 32, a clean flush supply.
A dual-seal plan adds:
- A reservoir or accumulator for each seal: a seal pot on Plans 52 and 53A, a bladder or piston accumulator on 53B and 53C. Under the 4th edition its working volume must last at least 28 days without topping up.
- A pressure source on barrier plans: nitrogen or an accumulator holding the barrier pressure at least 1.4 bar (20 psi) above seal chamber pressure.
- Cooling: a cooling-water coil in the reservoir or a cooler in the loop.
- Instruments: level and pressure transmitters with local indicators, the 4th edition’s default for monitoring a plan.
- Make-up: a way to replace liquid lost through normal seal leakage, from a hand pump to an automatic fill system.
How to Choose the Right One
Pick a single or double seal for the pumped fluid and the plan family comes with it: a single-seal plan where process leakage to atmosphere is not critical, a dual-seal plan where that leakage has to be captured or stopped.
- Single-seal plan: benign fluids whose leakage to atmosphere is not critical or is easily controlled. It is the lowest-cost option.
- Buffer plan, 52 or 55: leakage has to be captured and disposed of, and small amounts reaching atmosphere are acceptable.
- Barrier plan, 53A, 53B, 53C or 54: no process leakage to atmosphere is allowed. It also suits dirty, abrasive or polymerizing fluids and pumps that may run with no liquid in the seal chamber. The product must tolerate a little barrier liquid leaking into it.
Under the federal leak standard for pumps in US light liquid VOC service, a single-seal pump is monitored monthly with an instrument, while a pump with a dual seal and a qualifying barrier fluid system is exempt from that monthly monitoring.
Among the rule’s conditions, the liquid between the seals must either stay above stuffing-box pressure at all times or degas in a reservoir routed to a process, a fuel gas system or a closed vent system with a control device, and a sensor must detect failure of the seal system. A Plan 52 vented that way can therefore qualify as well as a Plan 53.
A dual seal can also carry a single-seal plan for its inner seal, as in Plan 11/52. On a buffer arrangement the inner seal usually has its own flush plan, chosen the same way as for a single seal.
With a Plan 53 the default is no separate flush. A severe chemical service can still add one: in hydrofluoric acid, which is both highly hazardous and corrosive, Plan 32 can be paired with Plan 53.
