Depending on the nature of the potential liquid in the seal gas streams, different methods of liquid removal and control may need to be employed. Liquids can be carried into the seal gas system directly, or they can condense out of the gas stream in various parts of the seal gas system, including at the primary seal faces. Careful thermodynamic evaluation of each seal gas stream (normal and back-up) under existing and estimated future conditions is key to identifying problematic conditions and designing the appropriate facilities. External seal gas supplies can also contain lubricating oil if reciprocating compressors are part of the supply system. For retrofits, confirm process conditions by analyzing actual gas samples, with special attention to capturing and identifying liquids. On new installations, pessimistic expectations for a clean and dry gas are recommended. It is important to design the entire seal gas supply system to achieve the desired goal of delivering clean and dry seal gas to the seals. Design strategies include:
• Use of stainless materials for supply system lines and components downstream of the filters.
• Use of, and proper location of, coalescers: Depending on how the gas behaves as it is let down, the location of the coalescer could be upstream or downstream of the supply control valve. The coalescer should be properly selected and sized for the normal and extreme conditions (note that over sizing can allow gas to cool excessively or reduce coalescer efficiency). The device should include a sight glass, a differential pressure indicator and valving to allow element changes on-line without disrupting flow. Services that are normally dry may use a bypass rather than a duplex arrangement, but the bypass should be routed to avoid low points.
• Use of heaters and tracing downstream of the liquid removal device: The intent is to provide adequate margin (some users target 35-50 degrees F) from saturated conditions. Note that conventional steam tracing may not provide the needed reliability, necessitating either temperature monitoring or alternative tracing/heating methods.
• Prevention of liquid accumulation in lines/vessels: Piping should be routed without low points and also sloped to avoid liquid accumulation. Idle lines (bypass loops, duplex filter legs) should also be arranged to prevent liquid accumulation, or should include low point bleeders where this is not practical. In addition, liquid removal vessel low points should be piped to the compressor suction for continuous blow down operation. The blow down line should include an orifice to avoid excessive recycling of gas back to suction.
• Promoting controlled condensation: In cases where it is not practical to heat the seal gas to achieve adequate superheat, a cooler upstream of the liquid removal device can be installed to knock out additional liquid. A lesser amount of gas heating downstream of liquid removal can then provide the desired superheating.