Depending on the service and seal arrangement, seal gas can either be supplied from the compressor gas stream or from an external source. As previously described, double seals will inherently require externally supplied inert seal gas. Tandem seals or single seals can use either compressor discharge gas or an external supply of gas. Examples of the latter include nitrogen, hydrogen, fuel gas and other by-product gases. Determining factors include the availability and cost of a suitable and reliable external gas supply and the characteristics of the gas from each source (cleanliness, liquid/moisture content, toxicity, thermodynamic properties, etc.).
Note In order for dry gas seals to operate reliably, it is essential that a constant and sufficient supply of seal gas be delivered in a clean and dry condition. Although this appears straightforward, gas seal failures are often a result of not meeting this requirement.
Some of the unanticipated conditions that may be encountered include:
• Loss of externally supplied seal gas
• Insufficient seal gas while compressor is at idle speed or stopped
• Excessive supply system pressure drop in low pressure (close to 1 atmosphere) services
• Reverse pressurization if suction pressure and seal gas fall below vent pressure (possible during startup in low pressure services)
• Reverse pressurization if vent pressure rises above seal gas pressure (possible during flare system excursions in low pressure services)
• Reverse pressurization on loss of seal gas supplied to double seals
• Saturated seal gas due to changes in the process
• Liquid formation in the seal gas due pressure letdown cooling
• Oil mist contamination from external seal gas source compression system
• Filter element failures due to pressure pulsations from an external seal gas source supply system