This article discusses equipment and piping layout considerations for compressors handling hydrocarbon gases though most of these requirements will apply to other compression facilities as well. Layout of gas compression facilities shall take into cognizance the accessiblity, maintenance and safety requirements.
Compressors can be driven by electric motors, diesel engines, gas-fired engines, steam turbines or gas turbines. Gas turbine driven compressors are the most common types used in oil and gas industry. Compressors can be single stage compressors or multistage where high compression ratio is required. The other equipments associated with compression facilities are inlet separators, inlet scrubbers, knockout drums, heat exchangers, discharge scrubbers, lube oil and seal oil systems.
Compressor Equipment Layout
Compressors handling hydrocarbon gases are a source of hazard and should preferably located in the open to take advantage of natural ventilation and reduce the potential accumulation of hydrocarbons. Where the location is outdoors, compressors should be located near primary roads for easy access to mobile equipment for handling and maintenance.
If the compressors are required to be provided with shelter the building should be adequately ventilated. Ventilation should ensure that there is no danger to personnel under various operating conditions including unforeseen scenarios where there is leakage due to gasket blowout, leakage from packing glands etc. which can cause accumulation of hazardous concentrations of flammable or toxic vapors. Compressor shelter shall have access to fire fighting equipment from two opposite sides.
Compressors located inside buildings shall be provided with overhead travelling crane to facilitate maintenance. Provision shall be made for sufficient laydown areas/drop zones for handling large components of compressor during maintenance or replacement. The maintenance drop zones shall be sized such that heavy duty trucks or trailers can access the building to transport the heaviest removable component of the compressor. Local laydown areas shall be reserved for in-situ maintenance of equipment. Drop zones near the equipment and drop zones at the transportation areas shall be clearly identified on the layout to ensure that there are no obstructions.
Compressors in flammable service are considered as potential sources of hazards and hydrocarbon inventory and shall be located away and downwind from (recommended minimum 15m clearance) potential sources of ignition such as heaters and non-certified electrical equipments.
Building and operating platfoms shall have at least two means of escape located diagonally opposite. Operating platform shall have at least one primary exit through stairways and the second exit could be by means of fixed cage ladder. The maximum distance to the building or operating platform exit from any point shall not exceed the distance stipulated in the project safety philosophy document. In the absence of any guideline a maximum distance of 25m is recommended.
Manufacturers recommendation shall be followed for layout of compressors and the space required for operation and maintenance. Sufficient access shall be provided for valves that require frequent access and instruments that require frequent monitoring. Sufficient space shall be provided around the compressor rotor and driver for inspection and maintenance.
Compressors with bottom nozzles should be supported on elevated supports. Where compressors have top suction and discharge lines, piping layout shall ensure that adequate overhead clearances are available for maintenance requirements. Removable spool pieces shall be provided if required to facilitate maintenance.
Layout of compressor building/shelters shall take into consideration the space required for lube oil and seal oil systems.
Centrifugal compressors with horizontal split casings shall have maintenance access from both sides.
Centrifugal compressors with vertically split casings shall have maintenance access opposite the drive end for pulling of rotor during maintenance.
Compressors are a source of high hazard and hence no equipments shall be located directly over the compressors.
The suction knockout drum shall be located as close as possible to the compressor.
The spacing between two adjacent large heavy duty compressors may be dictated by the footprint of the foundations.
Centrifugal Compressors Piping Layout
Piping shall be designed to minimize vibration and noise.
Suction piping shall be designed to ensure that manufacturer recommended minimum straight lengths are available up to the suction nozzle inlet. The suction piping should be free of any restrictions or obstructions due to elements such as thermowells, flow elements etc. Suction piping shall be designed without any pockets between the knockout drum and the compressor inlet nozzle. Additionally the suction line should be sloped towards the knockout drum. If design without pockets is not possible due to other layout constraints, a drainage system shall be incorporated in the design. Top inlet compressors if selected can eliminate low points.
Where several compressors operate in parallel, the piping shall be laid symmetrical to ensure equal flow distribution.
Centrifugal compressor piping shall be sized to minimize pulsation, vibration, and noise.
Piping shall be designed such that the manufacturer ’s recommended nozzle loadings are not exceeded.
Piping around compressors including lube oil and seal oil piping shall not cause obstructions during operation, inspection and maintenance. Minimum operating and maintenance clearances around compressors for mechanical handling of casings, rotors, gearboxes, drivers etc shall be in accordance with manufacturers recommendations. Where dismantling of piping is required for maintenance, removable spool pieces shall be provided to facilitate maintenance access without having to disassemble the complete piping.
Temporary strainers shall be installed in the suction line to protect the compressor against damage due to entry of extraneous solid materials. These strainers are required mostly during pre-commissioning and shall be removed prior to start-up. Strainer shall be located as close as possible to the discharge nozzle. Provision for a spool piece should be included in the suction piping design for replacement of the temporary strainer. It is recommended to install temporary strainer as per vendor recommendations. In general, the recommended open area of the strainer will be 1.5 - 2.0 times the cross-sectional area of suction piping. All strainer screens shall have sufficient strength to prevent failure and entry into the compressor.
Lube and seal oil consoles shall be located close to the compressor building to minimize piping to and from the compressor. Return oil lines to the console should be designed to be self-draining back to the console.
Piping connected to compressors are prone to fatigue failure due to vibration and hence precaution shall be taken during design and fabrication to eliminate the possibility of fatigue failure.
During fabrication and installation stage, sufficient field welds and overlengths (fabrication allowances) shall be provided on last hook-up spools connected to the compressor nozzles to ensure proper fit-up without imposing any additional strains and facilitating proper machine alignment.
Reciprocating Compressors Piping Layout
Reciprocating compressors generate pressure pulses that can cause excessive vibration in piping systems.
Reciprocating compressor piping shall be designed to minimize pulsation, vibration and noise. Shock absorbing supports may be provided to reduce vibration to acceptable levels. Recriprocating compressors shall be equipped with pulsation dampers. High gas velocities in the discharge line can also cause flow induced vibration. The goal of piping design shall be to eliminate any flow induced vibrations.
Sufficient space shall be provided for withdrawal of compressor pistons. Additionally, sufficient space shall be provided for large lifts such as casing, rotor or motor removal.
To eliminate the possibility of fatigue failure due to vibration, small bore branches shall be avoided downstream of reciprocating compressors or located away from sources of vibration. It is recommended to follow Energy Institute guidelines and implement their recommendations such as bracing of small bore branches to reduce the likelihood of failure due to fatigue.
Relief valves shall be provided upstream of the first discharge isolation valve to protect the compressor from excessive pressure build up. The compressed discharge from relief valves will be at higher temperature and shall be cooled if it is required to be routed back to the suction line.
Check valves provided in reciprocating compressor discharge piping shall be suitable for pulsating flow conditions. Suggested quick-response check valve can be a disc type check valve. Valve shall be installed as close as possible to the nozzle to reduce potential damage to the compressor during surge conditions and to prevent backflow during an emergency. Anti-surge valves if required shall be located as close as possible to compressor discharge.
It is recommended to perform pulsation analysis of piping system connected to reciprocating compressors. Suction and discharge piping connected to reciprocating compressors shall be routed close to grade and secured held down at locations as recommended by analysis of the system to control the vibrations. Piping supports shall be spaced at irregular intervals to eliminate resonance effects. Compressor piping shall be supported on independent foundations and not from shelter structure to prevent vibrations getting transmitted to the structure.
An acoustical and mechanical analysis in accordance with API 618 shall be performed to avoid acoustic resonances which can produce excessive vibration and stress in the piping system.
If the compression is carried out in more than one stage, then the interstage lines shall have a pressure-temperature rating equal to that of the discharge line.
Layout shall ensure that manufacturer's recommended straight pipe lengths on suction and discharge are adhered to.
Other Considerations
Nitrogen compressors shall be located in open areas or adequately ventilated shelters to avoid the risk of asphyixation.
Air compressors should not be located downwind of cooling towers to avoid the possibility of water mist from cooling towers getting into the air intakes.
Utility stations shall be installed in compressor area.