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These seals and their piping plans offer a simpler alternative to wet containment systems.
Figure 3. Typical Plan 76 system
Plan 76 is intended for use when the pumped fluid sealed by the process seal will not condense at lower temperatures or pressures. This sealing support system connects to the mechanical seal containment seal vent (CSV) connection in the upper part of the containment seal cavity between the process seal and close clearance bushing (see Figure 3). This location allows the vapor leakage to accumulate and be directed away from the mechanical seal with minimal impact to the operation of the dry running containment seal.
The leakage vapor is routed through an orifice to a flare or vapor recovery system. A pressure transmitter monitors the pressure in the containment seal cavity. In the event of a process seal failure, the orifice plate causes a back pressure that triggers a high-pressure alarm. The piping continuously slopes upward, and provisions are incorporated so that any condensate that forms can be captured and drained away.
Lower emissions and enhanced safety can be achieved by diluting the vapor leakage in the containment seal cavity with a buffer gas, typically nitrogen. The buffer gas is introduced into the containment seal cavity through the mechanical seal’s gas buffer inlet (GBI) connection between the containment seal faces and the close clearance bushing, purging the area immediately around the containment seal faces with the buffer gas.
A control panel filters the buffer gas and regulates the pressure and flow before delivering it to the containment seal (see Figure 4). The system includes a pressure transmitter upstream of the flow control device to indicate the buffer gas supply pressure and to trigger an alarm if the buffer gas supply fails. The system also includes a flow transmitter to monitor the consumption of the buffer gas and to trigger an alarm in the event that the containment seal fails.
Combining Piping Plans
Many facilities use a combination of the piping plans described above.
- Condensing leakage only: Plan 75 or Plan 72/75
- Non-condensing leakage only: Plan 72 or Plan 76 or Plan 72/76
- Mixture of condensing and non-condensing leakage: Plan 75/76 or Plan 72/75/76
In addition, these piping plans can be used in combination with process flush piping plans, such as Plans 11, 13 and 21.
Figure 4. Typical Plan 72 system
Instrument Settings and Alarms
To ensure that alarms function correctly, the alarm guidelines should be set carefully to ensure that false alarms are not triggered. Similarly, alarm points must be sufficiently sensitive to activate in the event of a failure.
When using a pressure control valve (Plan 72) in conjunction with Plan 75 or 76, the pressure should be regulated to a level above the flare or vapor recovery system pressure but less than the Plan 75 or 76 alarm pressures and less than the pumped fluid pressure in the seal chamber. A low output pressure is desirable.
When Plan 72 is used in conjunction with Plan 75 or 76, the low-pressure alarm level should be set at a pressure above the flare or vapor recovery system pressure and below the pressure control valve outlet pressure.
A high-pressure alarm that is set close to the control valve outlet pressure is desirable to ensure the early detection of a supply gas failure. The high-flow alarm point will vary with the shaft diameter, mechanical seal type and manufacturer. Consult the mechanical seal supplier for advice on selecting the high-flow alarm setpoint.
Plan 75 & Plan 76
The high-pressure alarm should be set at a level above the flare or vapor recovery system pressure and below the pumped fluid pressure in the seal chamber. A low-pressure alarm set close to the flare or vapor recovery system pressure is desirable for the early detection of a process seal failure.
Compared with traditional wet containment seals that use Plan 52, dry containment seals offer the same benefits with sealing support systems that are less complex, have lower initial and operational costs, and generate significantly less waste.