Seal Gas Control Methods
The flow of seal gas to a primary seal is controlled by either a flow control method or a differential pressure control method. The main objective of both types of control methods is to positively sweep the seal gas across the process gas labyrinth to prevent a reverse flow of process gas into the dry gas seal.
A flow-control system controls the supply of seal gas to the seals by regulating the seal gas flow through an orifice upstream of each seal. It includes a valve with a remote flow controller that compares the seal gas flow to each seal and adjusts the flow control valve to maintain a minimum 16 fps (4.9 m/s) flow velocity across the inner process labyrinth seal based on “high select reference pressure” for each seal, measured downstream of the flow orifices.
A differential pressure control system controls the supply of seal gas to the seals by regulating the seal gas pressure to a fixed value (typically 10 psig [70 kPa]) above the sealed (reference) pressure. It includes a differential-pressure control valve with a remote controller. A bypass line with manual block valves is provided around the control valve.
A dry gas seal control system must have an adequate range and controllability to maintain a minimum 16 fps (4.9 m/s) gas velocity across the inner process labyrinth as well as at the labyrinth clearances ranging from the minimum to the maximum and up to two times the maximum design clearance. This is critical to avoid seal face contamination and to maintain the required seal face temperature below the safe upper operating limit. A proper balance of the seal gas consumption and seal gas flow velocity is essential, regardless of the type of seal gas control system used. A high seal gas flow requires more energy and makes the control system inefficient. It also requires relatively larger system components.
A flow control system minimizes the seal gas consumption and can maintain the minimum acceptable seal gas velocity. Except in the double-seal arrangements and in seals with very low sealed pressure (below 100 psig [690 kPa]), a flow control system is usually the ideal choice for all seal types and applications. Also it does not require measurement of the reference pressure that is so critical in a differential pressure control system.
Seal gas conditioning
Seal gas entering the primary seal area must be clean and dry (99.98% free of entrained liquid particles 3 microns and larger) and should be filtered to at least 10 micron solid particles. In addition, at least 36°R (20°K) dew point margin (superheat) is essential throughout the dry gas seal system. To ascertain this margin, a phase map computer simulation of the dry gas seal system from the primary seal gas supply point to the primary vent must be carried out to evaluate any potential for seal gas condensation. The temperature of the seal gas must be measured at the point of seal gas entry to the seal, not at the source of seal gas supply. Figure 9 shows some phase map curves.
To achieve the seal gas quality mentioned above, it often becomes necessary to integrate the seal gas treatment system(s) with the overall dry gas control system. Seal gas conditioning hardware consists of the units that provide clean and dry seal gas. Coolers, wet gas prefilters and if necessary, a seal gas heater, are used to provide dry seal gas. Wet gas demisters and dual filters clean up the seal gas. A unit that boosts the supply gas pressure and creates a sufficient positive differential pressure becomes necessary in many compressor applications in order to avoid seal contamination during transient conditions such as startup, shutdown, process sequence, slow roll, recycle and settle-out. The differential between the seal gas supply pressure and the sealed pressure should be at least 50 psi (3.4 bar) to avoid primary seal contamination.
Figures 10 and 11 show the cross-sectional view of a seal gas heater and a coalescing prefilter. The schematic of a seal gas booster is presented in Figure 12.
Nitrogen, as the source of separation gas, is not available in many remote compressor locations. In such cases, a nitrogen generator can be installed at the job site to provide pure nitrogen (between 95 to 99.5% purity) from compressed air. Most dry gas seal manufacturers offer a nitrogen generator either as a standalone unit or as a unit integrated with the dry gas seal control panel.
Dry gas seal vent system
The primary vent from a dry gas seal system is routed to the plant’s flare when the sealed gas is a hydrocarbon or a hydrocarbon gas mixture. The normal flare pressure is generally close to the atmospheric pressure and the maximum pressure can be as high as 50 psig. If the compressor suction pressure is below the maximum primary seal vent pressure, the seal chambers must be equalized to some intermediate pressure above the compressor suction pressure.
Bypass relief valves provided on the primary seal vent lines should be installed as close to the dry gas seals as possible. The dry gas seal system piping and vents should be sized to prevent over-pressurization of the bearing housings in the event of a seal failure.
The secondary vent from a dry gas seal is routed to the atmosphere unless the hydrocarbon-plus-air mixture is above the upper explosive limit. It is mentioned in the previous discussion that presence of a rich mixture in the secondary vent cavity requires it to be routed to the plant’s flare for obvious safety reasons.