Proper compressor layout In a process plant is essential for ideal operation, maintenance and construction requirements. compressors are used to compress gases to high pressures. High pressures are required for various reasons such as particular chemical reaction requirements, or to condense the gas such as LPG, LNG etc. Variety of compressors are used based on type of gas to be compressed, pressure requirements, economy and availability.
Types of Compressors
Following types of compressors are used in process plants:
- Centrifugal compressors
- Reciprocating compressors.
- Liquid ring compressors
- Screw compressors
- Diaphragm compressors
Parts of a Compressor System
Compressor system includes
- Lube oil console,
- Seal oil console,
- Surface Condensers,
- Condensate Pump,
- Air Blowers and Filters,
- Waste Heat System,
- Compressor suction/knockout pot,
- Pulsation dampener.
- Vertical Split case compressor,
- Horizontal Split Case Compressor.
Compressor Location Requirements
Compressor house should be located near the battery limits to facilitate ease in maintenance and operation. Compressors shall be located to keep suction lines as short as possible. The gas compressors shall be located downwind side of furnace so that leaks are not blown towards furnace. In general compressors are to be located under shed. When compressors are located under shed, sides to be kept fully open for the low shed or shed, however, in case of hydrogen compressors located under the sheds provision for top venting from compressor sheds shall be provided. Gas compressors should have roofing and open from sides to avoid accumulation of heavier vapours/gases on the floor of compressor house.
In case of a turbine driven compressor, if exhaust steam is condensed, turbine and compressor need to be located at an elevated level and condenser to be located below turbine.
Large centrifugal compressors may be elevated sufficiently for piping and auxiliaries to be located below main operating platforms. Smaller compressors may be grade mounted
with piping and auxiliaries arranged for convenient access and maintenance.
A major consideration in centrifugal compressor location is the lube and seal oil console. It must be accessible from a road, must be lower than the compressor to allow gravity drain of oil to the consoles oil tank. Intercoolers & Knockout pots may be kept within/outside the Compressor house shed, but, shall be kept near compressor house.
Lube and seal oil consoles shall be located adjacent to, and a minimum distance from, the compressor.
Compressor Maintenance Requirements
- Electrically Operated Travelling Crane to handle heaviest removable piece shall be provided for each compressor house.
- Maintenance bay for compressors shall be provided. Maintenance bay shall be accessible from road to facilitate unloading & loading on to truck etc.
- For removal of bundles of exchangers located within building monorail arrangement shall be provided.
- Compressor manufacturer may be consulted for recommended layout and additional requirement for maintenance.
- Licensor’s requirement, if any, shall also be taken into consideration.
- Drop out area should be provided for maintenance.
- Lube oil cooler space shall be provided in a way so as to facilitate tube bundle removal.
Compressor Piping Layout
As compressors induce lot of vibrations in connected piping and structure, careful design of compressor piping to absorb vibrations and minimize fatigue impact on piping components is must.
- Volume bottles shall be provided, as required, at the suctions and discharges.
- Suction piping shall have adequate flanged joints for ease of erection and maintenance.
- Where the line between knockout drum and the compressor cannot be routed without pocket, low point drain shall be provided to remove accumulation of liquid. The system should be approved by process.
- A minimum straight length of suction pipe shall be provided as per manufacturer’s recommendation.
- All operating valves on main suction and discharge piping shall be lined on one side as far as possible.
- Low points in the discharge line from an air compressor shall be avoided because it is possible for lube oil to be trapped and subsequently ignited. If low points are unavoidable, they shall be provided with drains.
- The piping drawings shall show the limits of compressor suction piping where special internal cleaning is required.
- Temporary screens or strainers shall be provided which comes with a differential pressure indication to allow monitoring of the screen during start-up.
- Removable spool pieces shall be provided at compressors, where needed, to permit maintenance without major piping dis-assembly.
- Crankcase and distance piece vents shall be piped to the outside of the compressor building.
- Distant piece drains and lube oil drain should be piped to an underground slop tank.
- Engine exhaust and starting motor exhaust shall be piped to outside of the compressor building.
- Compressor engine exhaust shall be insulated inside the compressor building.
- Non-slam check valves are required on the discharge line of each compressor downstream of the pressure relief valve and upstream of the nearest discharge block valve.
- Oil return lines from compressor and driver shall have a minimum slope of 1 :25 to inlet connection at oil reservoir.
- Oil component arrangement shall be reviewed for
access and operation.
Compressor Piping Stress Analysis and Supporting
Piping shall be designed so that forces and moments imposed on the compressor do not exceed the manufacturer’s recommendation. In case of reciprocating compressor, piping shall be suitably supported to avoid vibrations due to pulsating flow. Unless requirements of no pockets are specified by the licensor, all the piping shall run at 500mm above grade level so that proper supports can be provided to minimise vibrations.
Analog study shall be carried out for complete compressor piping including suction / discharge piping as per P&IDs and the study recommendations if any, shall be implemented. Reciprocating compressor piping shall be provided independent supports from grade and shall not be supported from compressor platform structure. The small bore branches shall be checked for requirement of braced supports.
Suction and discharge lines shall be securely clamped.
Besides providing support, the supporting system must also maintain within allowable values the stresses on the piping deriving from vibrations transmitted by the machine and from the mechanical vibrations induced by the pressure pulsations. Thus the supporting structures will be designed taking into account both static and dynamic loads.
The support system will be realized in accordance with calculations results of stress analysis and analogue simulation as per company practice.
As a general rule, individual supports will be :
- Independent from any building and with their own foundations, to avoid the transmission of vibrations to other plant components.
- Rigid, with foundations of mass adequate to the loads.
- Constructed in such a way to ensure that their components are subject to tension and compression stresses only, but not to bending stresses.
- The guide must be designed with zero clearance; this can be achieved with the use of adjustable devices, with the insertion of spring washer in the ‘hold down’ type support, or with the insertion of rubbery materials between pipe and support/clamp (neoprene or equivalent).