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Plan 72

Plan 72 uses an external low pressure buffer gas, usually nitrogen, which is regulated by a control panel before it is injected into the outer seal cavity of a dual unpressurized seal arrangement. It is almost always used in conjunction with either a Plan 75 or Plan 76 to lead inboard seal leakage to a collection system.

The control panel should contain a pressure control valve to limit buffer gas pressure in order to prevent reverse pressure on the inboard seal and/or limit pressure applied to the secondary containment seal. This is followed by either an orifice or needle valve to control the gas flow rate. The control panel should also have a coalescing filter to prevent solids and/or liquids within the buffer gas from contaminating the secondary containment seal.

A very important feature of this plan is that the gas purge is introduced close to the seal faces, whereas the vent and drain are located away from the seal faces. In API 682/ISO 21049 a bushing is required to physically separate the buffer inlet and the vent/drain. Plan 72/75 is used for primary seal leakage that is condensing (returning to liquid form). Plan 72/76 is used for non-condensing (vapor) leakage. This helps to minimize process fluid from affecting the containment seal faces and aids in diluting leakage to the atmosphere.

It is recommended that a Plan 72 not be used in a dead-ended containment chamber. The gas pressure regulator should control the pressure upstream of the flow control system to slightly less than the Plan 75 or 76 alarm setting to ensure buffer flow over the complete system operational range.

The minimum gas flow rate for dry containment seals should be in the 3-scfh to 6-scfh range, which will provide adequate product leakage dilution at normal inboard leak rates. Gas flow rates at this level can only be controlled by a needle valve. If dilution is still required at Plan 75 or 76 alarm points and/or a flow control orifice is specified by the purchaser, buffer gas flow rates are likely to be in excess of 20-scfh.

Seal Flush Plan 72

Advantages

• Protects the outer containment seal.
• Reduces fugitive emissions.
• Prevents icing in cryogenic services.
• Introduction of nitrogen keeps the outer seal chamber cooler. This is an advantage where the process fluid has a low vapor pressure margin.
• Temporary loss of nitrogen should not affect the performance of the containment seal.
• Lower maintenance and operating costs than a Plan 52 system.

 Disadvantages

• Temporary loss of nitrogen can result in temporary increase in fugitive emissions.
• Contacting containment seals run better with the moisture from the process vapor leakage. A dry nitrogen purge reduces the moisture in the containment seal chamber and can decrease the operating life of the containment seal.

Plan 74

Plan 74 is a pressurized plan for dual gas seals that utilizes an inert gas, typically nitrogen, as the barrier fluid. As with all pressurized dual seal arrangements, the barrier fluid is at a pressure greater than the process pressure being sealed.

Dual gas seals differ from other pressurized dual seal arrangements in that they do not require circulation of a fluid between the seals since the seal generated heat is minimal. The flow of inert gas is the result of leakage past the outboard seal faces and, to a lesser extent, the inboard seal faces due to the low differential between the barrier pressure and process pressure.

As with the Plan 53A system, the inert gas normally is supplied by a plant wide distribution system. In some very special cases the nitrogen source can be a bank of nitrogen bottles. However, this can be an expensive, unreliable system, and the maintenance is high to ensure that the bottles have sufficient pressurized gas at all times.

Plan 74 includes a control panel to regulate the pressure going to the dual gas seals and API 682 includes some details of the panel. The panel also acts to remove moisture and filter the inert gas. The panel should contain a low pressure alarm along with flow meters. Flow alarms warn against problems with the gas supply and are optional.