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Similar to Plan 53B, there is no gas pressurizing the barrier fluid so there is no chance of gas entrainment. Also, like Plan 53B flow is generated by a pumping ring through a heat exchanger. The heat exchanger can be water cooled, air cooled or can be finned tubing if the heat load is small enough. This system should be used with caution, as the reference line to the accumulator is subject to the process fluid. The process fluid may be corrosive, abrasive, or a slurry that could potentially clog the pressure reference line threatening the tracking ability of the system.

Seal Flush Plan 53C

  Advantages & Disadvantages (vs. other Plan 53 systems)

• The advantages and disadvantages are the same as the Plan 53B system. Additionally, the disadvantage of this system is that pressure spikes or pressure drops in the process pressure will vary the pressure on the outer seal that may create a temporary leakage condition. Also, tracking pressures can always be subject to delays that can cause a temporary loss of positive pressure differential across the inboard seal.

Plan 54

Plan 54 utilizes an external source to provide a clean pressurized barrier fluid to a dual pressurized seal. Strictly speaking, there is no "Plan 54 System" specified by API. That is, the details of the external lubrication system are not included by simply specifying Plan 54.

The external lubrication system for Plan 54 can be as simple as a basic reservoir, pump/motor, heat exchanger and relief valves to a complex system per API Standard 614 system. Plan 54 can even be pressurized from a process stream (the so-called "Process Plan 54").

The more complex systems can be supplied with redundant systems for uninterrupted service, accumulators to maintain pressure in the case of a power outage, and any number of alarms to detect operational problems. The complexity of the external lubrication system should be in line with the severity of the service or importance of the equipment operation it is supplying barrier fluid to.

The flush rate for a Plan 54 system must take into account not only energy from the mechanical seal (heat soak, seal generated heat, and turbulence), but also the heat added to the barrier fluid from the pump supplying the barrier fluid. On low pressure/flow systems this is minimal, but can become significant on larger systems operating at high pressures and flows.

The system reservoir should be sized for a retention time of 5 minutes, so if the flow rate is 4-gpm the reservoir size should be a minimum of 20-gal. The flow rate is usually controlled by the size of the pump on the system. In applications where one system is supplying barrier fluid to multiple seal chambers, flow can be controlled with simple manually adjustable needle or globe valves to control valves utilizing a variety of mechanical or pneumatic systems.

Like other pressurized systems, the barrier pressure should be above the maximum pressure that the inboard seal will be subject to. This differential can range from a minimum of 25-psi to large differentials to account for possible upset conditions.

Seal Flush Plan 54

Advantages

• The barrier fluid is typically one that has good to exceptional lubricating properties, that when applied properly can result in extended MTBPM for the seal.
• When properly instrumented, the system can safeguard the seal against process pump upset conditions as well as power outages.
• The mechanical seal is exposed to a neutral fluid, with the exception of parts of the inboard seal, so that corrosion and other chemical related problems are eliminated.
• Positively eliminates leakage of harmful and fugitive emissions to the atmosphere.
• Can provide pressurized flow to multiple seal installations with one system to reduce costs.
• Is not constrained by nitrogen ingress into the barrier fluid as in a Plan 53A.

Disadvantages

• Systems can be costly compared to other flush plans, depending upon the number and type of redundant and safeguard systems utilized.
• The system is dependent upon a separate pumping system (pump and motor) that can cause seal failure if power to the "system" is lost.
• Damage to the inboard seal can result in contamination of the process from barrier fluid leakage.
• If used on multiple seal installations, the failure of one can have an effect on all of the other installations unless proper precautions are taken to isolate the failed seal.
• Dependent upon a reliable electrical supply.

  

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