Fuel Gas Conditioning Skids are needed now more than ever…
With the advancement of technology and tighter emission control requirements, the need for an efficient and economical fuel gas conditioning system that delivers clean, dry and superheated gas is crucial. Fuel gas conditioning is a critical optimization process for the removal of damaging impurities from gas streams to increase the life of downstream equipment. It is designed to protect natural gas fueled engines and turbines from liquid slugs, solids, and liquid aerosols. These contaminants can cause corrosion, plug nozzles, and even erode combustion hardware damaging equipment. The absence of proper gas conditioning equipment can lead to costly unplanned outages or shut downs for repairs and equipment replacement.
Fuel gas conditioning may consist of several key components including a knock out drum, fuel gas scrubber, pressure regulating station, and filter/separator coalescer. Depending on the application or strictness of the gas quality, these components can be combined to meet the job specific requirements.
Knock Out Drum
The initial stage for gas separation is a knock out drum. They are used primarily for high slug handling capacity by means of gravity separation for bulk separation of gas and liquid. By knocking out the heavy slugs, KO drums prevent overloading of downstream final filter elements. A fuel gas scrubber is another tool utilized to remove gas entrained water droplets through gravity separation. If large quantities of gas entrained liquids are expected, a scrubber downstream of the knock out drum may be required. The efficiency of a gas/liquid separator is highly dependent on the liquid droplet size distribution and liquid load rate. This will dictate the type of vessel internals to be utilized to get optimal performance. The internals of the knock out drum is minimal with half pipes and/or deflectors. The internals of a fuel gas scrubber can vary from a mist pad, vane unit, or even higher efficiency cyclones. Each drum and scrubber will have a minimum instrument package that includes level switches, a level gauge, and pressure relief valve. The liquid collection sump of the vessel may be furnished with a manual or drain system that automatically removes liquids from the vessel based on the monitored sump level.
The gas must be operating at a high pressure to effectively and efficiently move through the pipeline. As a result, it is often necessary to reduce the pressure before it gets to its final destination. When the pressure of the gas is reduced, its corresponding temperature is also reduced. Depending on how much the pressure is reduced, water can condense or drop out and introduce water in the pipeline and cause damage to downstream equipment. One way to mitigate this is to heat the gas before the pressure is reduced to keep the gas temperature above the dewpoint or hydrate formation threshold. This may be accomplished with the use of a pressure reduction station and dew point heater. The reduction station will consist of spool piping assemblies with a series of pressure regulators and isolation valves that set the pressure to a desired set point. The pressure regulators are typically pilot operated which provide a high degree of reliability, speed of response, and noise attenuation. A dew point or fuel gas heater will preheat natural gas above the hydrocarbon and water dewpoints with superheat. This will provide the margin to compensate for the temperature loss due to the pressure reduction. The type of heater to be used may be dictated by the site utilities available. A heat exchanger (shell and tube) can take advantage of available steam or feedwater while as an electric type heater will require power being made available to the site.
The final stage of a fuel gas system will include a filter/separator/coalescer. This is an additional step to after-cleaning of already demisted gas when maximum liquid removal efficiency is required. A standard filter separator may be utilized for wet gas filtration removing solid particles from the gas. A coalescing filter is utilized for the removal of aerosol mists from the gas and not intended for solid removal. Internal filter media, operating in parallel, is utilized to capture gas contaminants. The filter media are sized based on the capacity rating of individual elements resulting in a X number of elements required to properly treat the gas. Solid particles and liquid droplets are captured in the filter elements and any liquid released from the gas is collected in the liquid sumps. As the elements lose their efficiency, a differential pressure across the filter will indicate when they need to be replaced. The filter separator is designed with a quick opening closure to access, remove, and replace the spent filter elements/cartridges. The minimum instrument package includes a level controller and integral drain valve, and high and high-high level switches that control the sump level in the filter separator. Carbon steel material should be specified for the vessels and piping all the way to the final stage filter separator. At that point, the gas will be at its cleanest. Stainless steel piping would be suggested downstream of the filter/separator/coalescer to prevent any unnecessary carbon steel contamination downstream to process.
There may be considerable liquid accumulation collected in the sumps of the operating equipment. The liquid dropping out from the knock out drum, scrubber, heater, and filter/separators is pipped to a common drain tank where it is collected and stored. This small atmospheric horizontal carbon steel vessel will include a local level gauge and high-level switch as a minimum. The vents from the vessel(s) and heater may also be discharged to the drain tank, so a tank mounted flame arrestor will be utilized to protect against collection of hazardous hydrocarbons.
An ancillary step may require the gas flowrate to be monitored by a flow metering station. The station will compromise one or two runs of piping spool(s) each with a calibrated metering orifice and manual or automated valves. The accuracy of the application will dictate the type of meter to be utilized (metering orifice, ultrasonic, or turbine flow meter) for measurement and recording purposes.
The sourcing of equipment can be an important step in the setup of a fuel gas system. Gathering individual components alone can be a time-consuming effort. Skidding the individual components on a common skid provides single source accountability and removes the burden of working with multiple vendors. The fabricator/vendor will skid mount the equipment, interconnecting piping, associated electrical, assemble, and shop test on a common skid. This will reduce installation costs and minimize field erection times associated with handling multiple components. The skidded system can be simply be dropped in the field and hooked up to the necessary utilities.
The successful design and operation of a fuel gas system includes the proper selection of components and the overall system layout. This could prevent hardware damage, reduce maintenance, impact plant availability, and create safer working conditions for plant personnel. Incorporating a combination of these fuel gas conditioning components can be a means of improving overall plant efficiency.
“Author Claude Ramos”
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