API Plan 51 is used on a single mechanical seal to keep a still blanket of liquid, fed from an unpressurized external reservoir, on the atmospheric side of the seal faces, so that ice or crystals cannot form there and damage the seal. Nothing flows through it: the reservoir and the gland’s quench connection form a dead end, and the liquid only moves when it is topped up or changed.
It is mostly found on vertical pumps, and on services where a flowing steam or nitrogen quench is unavailable or more than the job needs.
How a Plan 51 Dead-End Quench Works
A Plan 51 system pipes liquid from a reservoir into the quench connection on the seal gland and leaves it there. On a single seal, the quench port opens into the cavity between the seal faces and the outboard bushing of the gland, the same cavity that collects the leakage passing between the faces. The liquid therefore sits outboard of the faces as a quench; it does not enter the seal chamber and is not a flush.
The reservoir works at atmospheric pressure. That separates Plan 51 from Plan 53A, which also uses a reservoir but pressurizes it to feed the barrier fluid of a dual seal.
What Holds the Quench Liquid at the Gland
An auxiliary sealing device at the outboard end of the gland keeps the dead-ended liquid in the cavity. Most often this is a throttle bushing, a close-clearance ring between the gland and the shaft sleeve that restricts how fast liquid escapes to atmosphere.
How well the blanket holds depends on which device is fitted:
- Fixed bushing: a solid ring pressed into the gland, which restricts the least.
- Floating bushing: a ring free to move radially with the shaft, which lets it run at a closer clearance than a fixed one.
- Segmented bushing: a floating ring split into segments held by a garter spring, which runs closest of the three.
- Lip seal: used specifically to retain quench liquids, where the liquid lubricates the lip or a small amount of leakage to atmosphere is acceptable.
What Plan 51 Is Used For
Plan 51 is applied to two atmospheric-side problems: ice forming at the seal on cold service, and product leakage crystallizing as it meets air.
Preventing Icing on the Atmospheric Side
Plan 51 prevents icing by filling the atmospheric-side cavity with a liquid that does not freeze at the process temperature, keeping out the humid air that would otherwise condense on the chilled gland and freeze at the seal.
On cold hydrocarbon pumps, a dry nitrogen quench on Plan 62 is often the first choice. Plan 51 is used where there is no nitrogen supply at the pump.
Preventing Crystallization at the Seal
A Plan 51 blanket prevents crystallization by catching the leakage of products that crystallize as they dry or cool on the atmospheric side of the faces, holding it in a liquid that keeps it dissolved or wet so it never dries into crystals against the faces. The quench liquid is picked to suit the product it has to take up.
Quenching the atmospheric side is one of three ways to stop crystallization, alongside temperature control of the product and a double seal with buffer or barrier fluid. Plan 51 is the one that keeps the single mechanical seal already in the pump.
Plan 51 or Plan 62: When a Dead-End Quench Is Enough
A static Plan 51 blanket is enough when the task is to keep air and moisture away from the seal and the leakage it collects stays small enough to dilute between liquid changes. The service needs a flowing Plan 62 quench when deposits form faster than a still liquid can hold them and have to be carried away.
Hot hydrocarbon service, where leakage congeals and carbonizes on the atmospheric side of the faces, is the typical case for Plan 62.
Plan 62 needs a steam, nitrogen or clean water supply piped to the pump. Plan 51 needs no utility at all, which is its advantage wherever a static blanket can do the job.
Why Plan 51 Is Used on Vertical Pumps
Plan 51 is typically used on vertical pumps because there the atmospheric side of the seal faces up: the quench liquid lies on top of the faces, and the outboard bushing sits above the liquid.
On a horizontal pump the bushing clearance runs all the way around the shaft, and its lower part sits below the liquid level in the cavity, so the auxiliary device must be much tighter, such as a lip seal, and the reservoir level needs closer watching.
Check the seal maker’s gland design before applying Plan 51 to a horizontal pump.
Where Plan 51 Is the Wrong Choice
Plan 51 does not suit dirty, abrasive or polymerizing products, because a quench outboard of the faces does nothing to keep particles or reacting product out of the faces of a single seal. These products are usually sealed with a pressurized dual seal on Plan 53A, 53B or 53C.
It is also the wrong plan when product leakage must be contained. Plan 51 collects leakage in an unpressurized reservoir at the pump, so a toxic, hazardous or flashing product needs one of the dual-seal API plans instead: Plan 52 for an unpressurized buffer, or Plan 53A, 53B or 53C for a pressurized barrier.