Sealing — API 682 piping plans — ANSI/ASME B73.1
An API piping plan, or seal flush plan, is the standard numbered way of describing how liquid or gas reaches a mechanical seal, circulates through it, or is taken away from it. The numbering comes from API 682 and is used industry-wide, including on ANSI/ASME B73.1 chemical process pumps that are not API 610 machines. For the pump itself the plan decides two things: which seal chamber is fitted, and which tappings are machined into the casing and the chamber cover. Both are settled at order.
Why the plan is a pump decision, not only a seal decision
The seal maker selects faces, elastomers and the arrangement. The plan, though, lands on the pump. A Plan 23 needs a chamber that accepts a pumping ring and a close-clearance throat bushing. A Plan 32 needs a flush connection sized for the external supply. A Plan 62 needs a quench and drain gland. None of these can be added from a catalogue after the casting is machined, which is why the plan is asked for with the duty and the liquid rather than after the order.
The YSM ranges carry the chamber options the plans are built on. The G196 programme is supplied with standard-bore, large-bore and taper-bore seal chambers and a packed stuffing box; the D Mark III programme carries the CBS, CBL, FMS and FML seal housings that cover packed, mechanical-seal and flush-plan arrangements. Casing drain and gauge tappings and seal chamber flush tappings are machined to the arrangement stated on the order.
Process-side plans
These plans take, condition and return the pumped liquid itself. They are the plans that matter most on a single-seal chemical process pump.
| Plan | What it does | Typical service | What the pump has to provide |
|---|---|---|---|
| Plan 01 | Internal recirculation from discharge to the seal chamber through a passage cast in the casing. | Clean, cool liquid where external flush piping is not wanted. | A casing with that internal passage cast and machined. |
| Plan 02 | No circulation. The seal chamber is dead-ended, often with a jacketed chamber or cover for cooling. | Clean, cool, low-duty services. A large-bore or taper-bore chamber does most of the work. | Large-bore or taper-bore chamber; jacket connections if cooling is required. |
| Plan 11 | Pump discharge, through a flow-control orifice, into the seal chamber. | The default single-seal plan for clean, non-flashing liquid. | A tapped flush connection into the chamber and a discharge-side tapping. |
| Plan 13 | Seal chamber, through an orifice, back to pump suction. | Vertical pumps, which vent themselves this way, and services that run better with a lower chamber pressure. | A tapped chamber connection and a suction-side connection. |
| Plan 14 | Plan 11 and Plan 13 together: flush in from discharge, recirculation out to suction. | Where both positive flush flow and continuous venting are wanted. | Both sets of connections. |
| Plan 21 | Discharge, through an orifice and a cooler, into the seal chamber. | Hot liquid where the flush has to be cooled before it reaches the faces. | Flush connections, plus room and support for the cooler. |
| Plan 23 | A closed loop: chamber, pumping ring, cooler, back to the chamber, with a close-clearance throat bushing holding the process out of the loop. | Hot liquid. Only the small chamber volume is cooled, so it is the efficient answer where Plan 21 was once used. | A chamber that accepts a pumping ring and a close-clearance throat bushing, with inlet and outlet connections. |
| Plan 31 | Discharge, through a cyclone separator: the clean stream goes to the chamber, the heavy stream to suction. | Liquid carrying a modest solids load that a cyclone can actually separate. | Flush connections and a suction return. |
| Plan 32 | Clean liquid from an external source, into the seal chamber. | Slurries, crystallising and polymerising services, and anything the pumped liquid itself cannot flush. | A flush connection sized for the external supply, and a throat bushing so the flush stays in the chamber. |
| Plan 41 | Discharge, through a cyclone separator and then a cooler, into the chamber. | Hot liquid that also carries solids. | As Plan 31, plus the cooler. |
Dual-seal and atmospheric-side plans
These plans work outboard of the inner seal: between two seals, or on the atmospheric side of a single seal.
| Plan | What it does | Typical service | What the pump has to provide |
|---|---|---|---|
| Plan 51 | Dead-ended external reservoir supplying a quench fluid to the atmospheric side. | Where a static blanket on the outboard face is enough. | A quench and drain gland. |
| Plan 52 | Unpressurised buffer fluid circulated between two seals by a pumping ring; the reservoir is vented, usually to a vapour recovery or flare header. | Dual unpressurised arrangements on hazardous or volatile liquid. Product leaking into the buffer shows as a level or pressure rise. | A chamber and gland that take a dual cartridge seal, with buffer inlet and outlet connections. |
| Plan 53A | Barrier fluid from a reservoir pressurised by an external gas supply, above seal chamber pressure, circulated by a pumping ring. | Dual pressurised arrangements where no product may reach atmosphere. Barrier fluid leaks into the product, so it has to be compatible with it. | As Plan 52, with barrier connections. |
| Plan 53B | Pressurised barrier held in a bladder accumulator, so the barrier fluid never contacts the pressurising gas. | Where gas absorption into the barrier fluid is unacceptable. Pressure decays as barrier fluid is consumed. | As Plan 53A. |
| Plan 53C | Pressurised barrier using a piston accumulator referenced to seal chamber pressure. | Where barrier pressure has to track a process pressure that moves. | As Plan 53A. |
| Plan 54 | Barrier fluid supplied by an external pressurised circulating system. | Several seals on one system, or a heat load beyond what a reservoir can carry. | As Plan 53A. |
| Plan 62 | External quench — steam, water or nitrogen — on the atmospheric side of the seal. | Crystallising, oxidising or freezing services, where the outboard face has to be kept clean or warm. | A quench and drain gland with the quench connection. |
| Plan 65A / 65B | Leakage collection and detection on the atmospheric drain. | Where leakage has to be measured rather than only contained. | A drain connection. |
| Plan 72 / 75 / 76 | Buffer gas sweep to a containment seal, and the condensing and non-condensing containment-seal vents that go with it. | Containment-seal arrangements. Mostly API 610 practice rather than B73.1 chemical service. | A containment-seal gland. |
The plans used most often, in detail
API Plan 11
Plan 11 takes liquid from the pump discharge, drops it across a flow-control orifice and feeds it into the seal chamber. The flow leaves through the throat bushing towards suction, so the chamber is swept, the faces are cooled by the flow, and chamber pressure is held above suction pressure. That last point is why Plan 11 suits liquids that would otherwise flash at the faces: raising the pressure raises the margin to vapour pressure. It is the plan most single-seal ANSI pumps leave the works with, and the one most often specified by default. The orifice size is the whole design: too small and the chamber does not turn over, too large and the pump recirculates useful flow. On a horizontal pump the chamber has to be vented on start-up, which is the usual argument for Plan 13 or Plan 14 instead.
API Plan 13
Plan 13 runs the other way: out of the seal chamber, through an orifice, back to pump suction. Because the flow leaves at the top of the chamber it is self-venting, which is why it is the standard plan on vertical pumps where a Plan 11 would leave a vapour pocket at the faces. Chamber pressure sits closer to suction pressure than it does with Plan 11, which helps a seal that cannot take discharge pressure and hurts one that needs pressure margin against flashing. It is the same trade-off read from the other end.
API Plan 21
Plan 21 is Plan 11 with a cooler in the flush line: discharge, orifice, cooler, seal chamber. It works, and it is expensive to run, because it cools a continuous stream of hot process liquid and then returns it to the pump. On new work it has largely been replaced by Plan 23, which cools far less liquid for the same result. Plan 21 still appears when a Plan 23 cannot be fitted — no room for a pumping ring, or a chamber that will not take a close-clearance throat bushing.
API Plan 23
Plan 23 closes the loop. A pumping ring in the seal chamber circulates the chamber fluid through a cooler and back, while a close-clearance throat bushing keeps the hot process liquid out of that loop. Only the small chamber volume is cooled instead of a continuous discharge stream, so the heat load on the cooler falls sharply and the seal sees a stable, cool environment. It is the preferred hot-service plan in current practice. The cost is on the pump side: the chamber has to accept the pumping ring and the throat bushing, and the loop has to be filled and vented properly or the ring circulates vapour.
API Plan 32
Plan 32 abandons the pumped liquid as a flush and brings clean liquid in from outside. It is the answer for slurries, for crystallising and polymerising services, and for anything where the process liquid would coat or abrade the faces. Two conditions decide whether it is usable: the external supply has to stay above seal chamber pressure at all times, including during upsets, and the dilution of the product by the flush has to be acceptable to the process. A throat bushing is normally fitted so the flush stays in the chamber rather than running into the pump.
API Plan 52
Plan 52 is the unpressurised dual-seal plan. Buffer fluid sits between the inner and outer seals at close to atmospheric pressure, circulated by a pumping ring, with the reservoir vented to a vapour recovery or flare header. The inner seal does the sealing; the outer seal contains whatever passes it. Because the buffer is at a lower pressure than the process, leakage runs into the buffer, and a rising level or pressure in the reservoir is the leak detector. It suits hazardous or volatile liquids where a small, contained and measurable leak is acceptable but an emission is not.
API Plan 53A, 53B and 53C
The 53 family is the pressurised dual-seal answer. Barrier fluid is held above seal chamber pressure, so any leakage runs inwards — barrier into product, never product to atmosphere. That is also the constraint: the barrier fluid has to be compatible with the process, because some of it ends up in it. The variants differ only in how the pressure is made and held. 53A pressurises the reservoir with an external gas supply, simple but the gas dissolves into the barrier fluid. 53B uses a bladder accumulator so gas and barrier never touch, at the cost of a pressure that decays as fluid is lost. 53C uses a piston accumulator referenced to seal chamber pressure, so barrier pressure tracks a process pressure that moves.
API Plan 62
Plan 62 sprays a quench — steam, water or nitrogen — onto the atmospheric side of the seal, behind the faces. It does not flush the seal chamber and it is not a substitute for a process-side plan. What it does is stop the outboard face building a deposit: it washes away a crystallising salt, keeps air off a liquid that oxidises, or keeps a cold service above its icing point. It needs a quench and drain gland, and the drain has to be piped away rather than left to run down the baseplate.
Choosing a plan by service
Plan selection belongs to the process owner and the seal maker, on the complete liquid and operating data. The table below is the shape of that discussion, not a substitute for it.
| Service condition | Plans usually considered | Why |
|---|---|---|
| Clean, cool, non-flashing liquid | Plan 11, or Plan 02 with a large-bore chamber on light duty | There is nothing to separate, cool or dilute. The flush only has to turn the chamber over. |
| Liquid close to its vapour pressure at the faces | Plan 11, or Plan 23 if it is also hot | Chamber pressure has to be held above vapour pressure. Plan 13 moves it the wrong way. |
| Hot liquid | Plan 23; Plan 21 where a pumping ring or throat bushing will not fit | Cooling the chamber volume is cheaper than cooling a discharge stream. |
| Vertical pump | Plan 13, or Plan 14 where flush flow is also wanted | The chamber has to vent upwards or vapour collects at the faces. |
| Separable solids | Plan 31, or Plan 41 if the liquid is also hot | A cyclone removes what it can; what it cannot remove still reaches the seal. |
| Slurry, crystallising or polymerising service | Plan 32, often with Plan 62 on the atmospheric side | The pumped liquid cannot be used as a flush. Dilution has to be acceptable to the process. |
| Hazardous or volatile liquid, emission limited | Plan 52 for unpressurised dual, Plan 53A/B/C for pressurised dual | The choice is whether any product may reach the buffer at all. |
| Service that freezes, oxidises or salts at the outboard face | Plan 62 alongside the process-side plan | A quench treats the atmospheric side; it does not replace the chamber flush. |
What to send with the enquiry
To quote a pump against a sealing plan, the review needs the liquid and the arrangement together:
- Liquid, concentration, temperature, vapour pressure and solids — the data that decides whether the pumped liquid can be used as a flush at all.
- Duty point, suction pressure and the seal chamber pressure that follows from it.
- The plan number if the plant has already set one, and the seal arrangement — single, dual unpressurised or dual pressurised.
- For an external flush or a barrier or buffer fluid: what it is, at what pressure, and whether it may enter the product.
- For a changeover, the installed pump nameplate and drawing. The nameplate and drawing guide lists the fields that matter, and the enquiry checklist covers the rest.
API seal flush plan FAQ
What is an API seal flush plan?
It is a standard, numbered description of how liquid or gas is supplied to, circulated through, or removed from a mechanical seal. The numbering comes from API 682 and is drawn as a schematic, so that one number carries the whole arrangement between the pump maker, the seal maker and the plant. A plan number on its own is not a specification: the orifice size, the cooler duty, the barrier fluid and the chamber it all sits in still have to be engineered for the service.
What is API Plan 11?
Plan 11 recirculates liquid from the pump discharge, through a flow-control orifice, into the seal chamber, and lets it leave through the throat bushing towards suction. It sweeps and cools the chamber and holds chamber pressure above suction pressure, which is what gives the faces their margin against flashing. It is the most common single-seal plan on clean, non-flashing liquid.
What is the difference between API Plan 11 and Plan 13?
They run in opposite directions. Plan 11 feeds the seal chamber from discharge through an orifice; Plan 13 recirculates from the seal chamber back to suction. Plan 11 raises chamber pressure, which helps against flashing but leaves a horizontal chamber to be vented on start-up. Plan 13 vents itself, which is why it is standard on vertical pumps, and keeps chamber pressure nearer suction pressure. Plan 14 is both together.
What is API Plan 14?
Plan 14 is Plan 11 and Plan 13 combined: flush enters the seal chamber from discharge through an orifice and a second line recirculates from the chamber back to suction. It gives positive flush flow and continuous venting at the same time, at the cost of two sets of connections and two orifices to size.
Do ANSI B73.1 pumps use API piping plans?
Yes. The plans are numbered in API 682, which is written around API 610 machines, but the numbering is used across the industry and chemical process pumps built to ANSI/ASME B73.1 are specified with the same plan numbers. What differs is the hardware available: a B73.1 pump is specified with its seal chamber type and its tappings, not with the full instrument and reservoir scope an API 610 package would carry.
Which plan suits a hot chemical service?
Plan 23 in most current work. It circulates only the seal chamber volume through a cooler by a pumping ring, behind a close-clearance throat bushing, so the cooler carries a fraction of the heat load a Plan 21 would. Plan 21 is still used where the chamber will not take a pumping ring or a close-clearance bushing.
What does the pump have to provide for a Plan 32?
A flush connection into the seal chamber sized for the external supply, and normally a throat bushing so the clean flush stays in the chamber instead of running into the pump. The supply pressure, the flush rate and whether the process tolerates dilution are decided with the plant, not by the pump.
Can the flush plan be changed after the pump is installed?
Sometimes, and it depends on what the change asks for. Moving between Plan 11, 13 and 14 is mostly external piping and orifices. Moving to Plan 23 or Plan 32 usually means a different seal chamber, a throat bushing, or tappings that are not there, which is chamber and cover work rather than pipework. It is cheaper to settle the plan before the pump is built, which is why it is asked for at quotation.
Related reading
- Mechanical seals for process pumps — arrangements, chambers and what the seal sees.
- Choosing seal face materials — carbon, ceramic, silicon carbide and tungsten carbide against the liquid.
- Mechanical seal failure root cause checklist — including what a failed seal says about the support system.
- Types of mechanical seal — pusher, bellows, cartridge, single and dual.
- Replacement ANSI pump parts — seal chambers, covers and glands as spare parts.