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The nitrogen source in a typical plant has pressure on the order of 100-psig. If the dual gas seal is sealing product pressures in excess of 75-psig, then the typical plant nitrogen gas source alone may be inadequate. In these applications a pressure amplifier (piston pump) can be utilized to boost system pressure.

The high pressure gas from the booster should be fed into a receiver of suitable capacity, as piston pumps are not designed for continuous operation. For this reason it is advisable to oversize the piston pump to minimize its operation and prolong maintenance cycles. It is not recommended to just hook the inert gas line directly to the seal cartridge. This is very unreliable, prevents regulation of pressure, and can allow for contamination of the seal faces that results in seal performance problems.

The schematic for Plan 74 shows two connections, a gas barrier inlet and outlet. The outlet is normally plugged as flow beyond makeup for seal leakage is typically not a requirement for these seals.

Seal Flush Plan 74

Advantages

• Lower system cost than liquid dual pressurized systems, especially for between bearing pumps.
• Lower maintenance requirements and associated cost compared to dual liquid systems that utilize a pressurized reservoir (Plan 53).
• Leakage from the inboard seal into the process is an inert gas and is easily separated from the process downstream.
• Barrier fluid leakage to atmosphere is an inert gas. Drainage and cleanup is not an issue as with dual liquid systems.

Disadvantages

• Entrained gas from seal leakage can cause pump problems, especially on closed loop systems.
• Gas entrainment problems can cause pump performance problems on some installations with both low suction head and low flow conditions.
• Entrained gas under certain conditions can influence net positive suction head testing at the pump OEM level.
• Not recommended for services where dehydration of the pumpage causes solids buildup.

Plan 75

Plan 75 is designed for use with a dual unpressurized seal utilizing a dry running containment seal, where primary seal leakage is collected into a reservoir. It is intended to be used when the process sealed by the primary seal will condense to a liquid at lower temperatures or is always in a liquid form.

In this arrangement, the drain is located at the bottom of the outer seal gland and is routed to the reservoir. Liquid leakage is collected in the reservoir and the gaseous portion is further routed through an orifice to a flare or vapor recovery system.

The reservoir does contain a pressure gauge and a high pressure switch to indicate a buildup in pressure in the reservoir from excessive primary seal gaseous leakage or a primary seal failure of some magnitude. Some users prefer to isolate the secondary containment device with valves to the reservoir in the event of a primary seal failure. A level switch to warn of excessive liquid leakage is optional on the reservoir.

The secondary containment seal can be subject to clogging in this arrangement. Some sort of baffle or close clearance bushing between the seal and gland should be used to isolate the containment seal from the leakage of the primary seal per API 682/ISO 21049. As noted earlier, Plan 75 can be used in conjunction with a gas purge from Plan 72. Typically, contacting secondary containment seals are used with this plan.

Seal Flush Plan 75

  

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