Leakage from the primary seal in a tandem seal arrangement must generally be vented to a safe location such as a flare system. The venting system should allow for safe disposition of leakage under normal and emergency situations, and also have capability for leakage monitoring and excessive leakage warning for each primary seal. In low pressure services, any check valves used in the vent system must have sufficiently low opening pressures to avoid creating excessive back pressure that could result in reverse pressurization of the primary seal faces. The following devices and locations are suggested for leakage monitoring:
• Flow indication (and recording for critical service) at each primary seal vent. Orifice-type flow meters are recommended. Sizing should be for normal leakage rates, up to 3-4 times the normal design rate.
• A pressure switch located between the seal and an orifice in the vent line. Sizing of the orifice and setting of the pressure switch should trigger a high pressure alarm at about 4 times the design leakage rate.
• A pressure switch with a setting to trigger a high, high pressure alarm at a rate or pressure that would allow for a safe manual or automatic shutdown (see the Shutdown Protection section).
Design of the above systems should also take into consideration keeping the intermediate seal cavity pressure low enough to allow continued inert buffering of an intermediate labyrinth, if so equipped. Likewise, the normal flow of the buffer gas out the primary seal vent should be considered in sizing of the above devices.
Note Seal gas supply and venting system panel design has been very variable and a potential source of reliability, operability and maintainability problems. It is important to specify requirements, review designs, and where possible, test the auxiliary panels to better assure satisfactory performance.