Mechanical Seal: How we select dual seal API 52 and API 53 (A/B/C)?

Centrifugal and rotary pumps require a sealing system to provide sealing integrity of the pump package and minimize leakage or preventing leakage of pump fluid to the atmosphere or environment. These sealing systems range from traditional mechanical packing to complex mechanical seal systems as required by the pump operating fluid and condition.

Selection of mechanical seal depends on correctly specifying liquid conditions e.g., pump operating condition, pumping fluid contamination/solid content, corrosiveness, toxicity, flammability, and hazardous classification. 

Single Seal is normally used in the following application:

- General application for clean fluid. 

- Non-toxic/non-hazardous/non-flammable fluid.

- Leakage to environment is allowed.

- For hot pump fluid (higher than 80 deg C) using API Plan 23.

When it come to the option that single seal is not acceptable due to pump fluid leakage is not allowed to environment then we shall go to the dual seal arrangement. In this case we are discussing about the option for API Plan 52 and API Plan 53 (A/B/C).

To select between API Plan 52 and API Plan 53 (A/B/C), then it starts from how they are working on sealing the pump. Both of these API Plan is used for hazardous liquid (toxic, flammable) but there are some specific different that we should go to one of them during the pump mechanical seal selection.

 

Fig-1 Dual Non-Pressurized Seal

Dual Non-Pressurized (API Plan 52)

The principle of API Plan 52 is pump fluid leakage will be always to the outer side seal as the pump fluid pressure is always greater than buffer fluid circulated in Plan 52. Gas that accumulated on the reservoir need to be vented to safe area (or flare). So, Seal operates on process fluid on inner seal and buffer fluid on outer seal. Inner seal handle full pressure and outer seal runs at atmospheric or low pressure.

With above principle of operation, this API Plan system will be suitable in clean, non-polymerizing, non-abrasive pump fluid.  There is also an advantage that no contamination to pump product.

Fig-2 Dual Pressurized Seal

Dual Pressurized (API Plan 53A, 53B, 53C)

The principle of API Plan 53 is pump fluid leakage will be always to the inner side seal as the pump fluid pressure is always smaller than barrier fluid circulated in Plan 53. So, Seal operates on barrier fluid on both inner and outer seals. Barrier fluid pressure must be higher than stuffing box pressure and therefore barrier fluid leakage into the pump stuffing box and process.

With above principle of operation, this API Plan system will be suitable in dirty, abrasive or polymerizing pump fluid. The important thing that barrier fluid shall be compatible with pump product.

So, what different between API Plan 53A, 53B and 53C? They are different from how they are getting pressurized barrier fluid. 

API Plan 53A: Use an external reservoir to provide barrier fluid for pressurized the system. Reservoir pressure is produced by gas usually nitrogen.

API Plan 53B: Use accumulator to isolate the pressurizing gas from barrier fluid. Heat exchanger is included in the circulation loop to cool barrier fluid

API Plan 53C: Use piston accumulator to provided pressure to the system. It uses a reference line from the seal chamber to provide a constant pressure differential over the chamber pressure. A water or air-cooled heat exchanger provides for barrier fluid cooling.

Another consideration

API Plan 52: It need venting to safe area or flare depend on the safety requirement on the project (Some cases in non-oil and gas Plant, flare is not available). With API Plan 52, buffer fluid is contaminated and the usual practice is to change the buffer fluid every 4-6 months, depending on the severity of the buffer fluid contamination. 

Plan 53A: It require continuous supply pressure (N2) from outer source. It will give more complicated in the piping system, especially almost all pump use dual seal and pump installation is indoor. If supply pressure fails, it will impact to all pump failure  

Plan 53B: It does not require external source to provide pressure. It is also independent each other.  

Plan 53C: It will be necessary if there is condition that fluctuation pressure is considered in seal chamber and need to be controlled.