Piping Layout: Compressor Piping And General Arrangement Drawing

Compressors are normally located inside a permanent shelter or building (Compressor House) for weather protection. The compressor house can be fully covered by side cladding to grade level if handling non-hazardous materials e.g. air.

For compressor, handling flammable materials, ventilation and weather protection is assured by significant openings upto 2.5m ht. at grade level together with roof ventilators.

Except for lighter than air gases, trenches, pits and similar gas traps should be avoided within gas Compressor House. This will eliminate chances of suffocation or explosion risk due to accumulation of heavy gases in pits.

For open compressor house, the side cladding on all sides should be provided upto 1m below crane level.

The general arrangement of compressor house shall consider the vendor drawings and vendor recommendation, if any, for space and location of auxiliary units.

For compressor house where a number of installations from multiple vendors are to be accommodated, a thorough discussion should be held among the engineers of Piping, Process and Civil discipline to finalize the detail plot plan of the unit.

The clear space between compressors shall be minimum 1.5m or half width of the compressors. The clearance between rows of compressor and at the end of each compressor shall be also 1.5m.Built-in maintenance equipment viz. travelling gantry with overhead crane / monorail with hoist and chain-pulley blocks as well as the drop-out areas shall be provided in the compressor house.

The clearance above the compressor should be at least 3m more than the longest internal part to be removed.

The substantial space required for lube oil and seal oil consoles shall be taken into consideration to prepare unit plot plan.

RECIPROCATING COMPRESSORS

Reciprocating compressor generates considerable vibrations due to unbalanced forces, pulsation etc. For this reason, the reciprocating compressors should be located as close as possible to the grade level.

The building foundation and the compressor foundation should be separate to avoid transmission of vibrations from compressor to the building structure.

The pulsation dampeners are used to eliminate pulsation in suction and discharge piping and to separate the source of vibration from the piping system.

The piping arrangement around the reciprocating compressor should be planned at grade level for ease of supporting with minimum changes in direction

The grade supports should be spaced unevenly to reduce harmonic motion in the piping.

The piping routed simply with short run is less prone to vibration, but at the same time the line should be checked for the flexibility and the compressor nozzle loadings within the allowable limits furnished by the vendor.

The piping shall remain clear of the cylinders and the withdrawal space at cylinder heads.

CENTRIFUGAL COMPRESSORS

The general considerations for centrifugal compressor layout is same as the reciprocating compressor, exception being that for centrifugal compressor, the pipeline size is larger, temperatures can often be higher and nozzle loadings on compressor casing is lower.

The knockout pots, interstage exchangers can be located at grade outside the compressor house with auxiliary equipment consisting of lubricating, seal and control oil systems be placed adjacent to the machine.

The centrifugal compressor inside a building normally have foundations separate from the building foundation.

The centrifugal compressor with drive are generally mounted on the concrete table supported on RCC column.

The maintenance facilities like overhead crane or monorail at the centre of the compressor bay and the drop-out area at one of the building or shed is the usual practice.

If the building is having installation of several compressors, the height of the travelling crane is to be carefully estimated so the machine components and rotors can be lifted over the adjacent equipment.

The compressor suction lines must be free of any foreign particles that could damage the internals of the machine. Strainers are installed in the inlet line between the isolation valve and the compressor inlet nozzle.

ASME PTC code recommends a minimum 3 times diameter of straight run piping between elbow and the inlet nozzle.

The designer shall ensure that all connections shown on the vendor piping and instrumentation diagrams are properly taken care in the piping layout. All valves shall be arranged in such a way that they are accessible from the operating floor around the machine.

AUXILIARY EQUIPMENT

Centrifugal and reciprocating compressors and their drives require a variety of auxiliary equipment to support their operation. The equipment for these compressors are discussed below.

Lube Oil Consoles: Compressor bearings receive lubricating oil from the lube oil console. These consoles may be either stand alone or be mounted directly onto the compressor frame. The console consists of lube oil reservoir, oil filters, oil coolers and lube oil pumps.

Seal Oil Consoles:

The hydraulic seals located at the outer ends of the compressor shaft receive oil from the seal oil console. The seal oil console consists of seal oil reservior, oil filters and main seal oil pumps.

Inlet Filters:

The inlet filters for air compressors are installed outside the building /shed at a level suitable for clean air suction without any obstruction in the air flow. The vendor drawing of the filter shall be reviewed for correct inlet/outlet ducting and the supporting arrangement.

Suction drum / knockout pot :

As compressors require dry gas free of foreign particles, it is necessary to pass inlet gas through the suction drum or knockout pot. This vessel removes moisture and particles from the gas by passing it through a demister screen located just below the outlet nozzle.

Pulsation dampener / volume bottles:

The negative effects of vibration on the life of reciprocating compressors and associated piping can be minimized by the use of pulsation dampeners (Refer Fig.CHP6). The pulsation dampeners are sized by the compressor vendor and are mounted directly on the cylinder nozzles.

Volume bottles are used to reduce vibration. They are located downstream of the discharge pulsation dampener and are similar to snubbers without internal baffles or choke tubes.

PIPING ARRANGEMENT

The compressor house piping consist of suction / discharge piping, auxiliary equipment piping and utility system piping. The main suction line with its components shall be as short and direct as possible. The discharge line with its main components shall be routed clearing the compressor and its driver and supported independent of compressor foundation or building column foundation. This will minimise the transmission of damaging vibrations to the building structure / frame.

The vendor furnishes that P&ID for the compressor with its auxiliary equipment. These drawings should be reviewed fully for the provisions of vents and drains requirement of the installation.

For reciprocating compressors, API 618 provides the acceptance criteria for nozzle loads. For centrifugal compressors, API 617 provides the acceptance criteria for nozzle loads.

Reciprocating compressor piping arrangement should be finalised after analog study which identifies potentially damaging accoustic or pulsation problems during design phase itself.