Centrifugal Gas Compressor Product

Overview

Centrifugal compressors move most of the world's natural gas. Pipeline stations every 100–200 km restore the pressure lost to friction; offshore platforms use the same machines to reinject gas or export it to shore; LNG plants chain them into refrigeration strings absorbing over 100 MW. The machine described here is a barrel-type unit per API 617: six impellers on a single shaft inside a forged pressure barrel, compressing 4–7 MPa pipeline gas toward 15 MPa at 10–30 MW absorbed power.

Compression path

Gas enters the Suction Nozzle, is distributed by the Inlet Guide, and meets the first Impeller. Each impeller flings the gas outward at tip speeds near 300 m/s, converting shaft work into velocity; the surrounding Stage Diaphragm diffuser recovers that velocity as pressure, and a vaned Return Channel straightens the flow into the next stage eye. Six stages at 1.3–1.5 ratio each compound to the overall ratio. Labyrinth Seal at every impeller eye and along the Balance Piston limit internal recirculation, which is the main efficiency leak in a high-pressure machine.

The diaphragms stack inside a Bundle Shell to form a cartridge. For overhaul, one End Cover is unbolted and the entire bundle, rotor included, slides axially out of the Barrel Shell — the barrel itself never leaves its foundation. Vertical splitting is what allows the casing to be a seamless forging rated for very high pressure; horizontally-split casings are limited to roughly 7 MPa because their long flange becomes impossible to seal against light gas.

Rotor dynamics

The Compressor Shaft runs well above its first bending critical speed, so rotor dynamics dominate the mechanical design. Five-pad Tilting-Pad Journal Bearing units suppress the oil-whirl instability that plain sleeves would suffer at these speeds. Axial gas thrust from six unbalanced impellers would crush any thrust bearing, so the balance piston cancels most of it, leaving the Thrust Bearing to handle the residual through the Thrust Collar. Pad temperatures are watched by embedded Bearing Pad RTD elements, and an API 670 suite — X-Y Vibration Probe at each bearing, Axial Position Probe on the thrust collar, a Keyphasor Probe phase reference — feeds the Machinery Protection Rack, which trips the train on voted limits.

Dry gas seals

Where the shaft penetrates the casing, tandem Dry Gas Seal Cartridge seals replace the oil film seals of older machines. Spiral grooves machined into a rotating silicon-carbide ring pump gas inward as it spins, lifting the stationary carbon face on a 3–5 µm gas film: no contact, no wear, and primary leakage of only 1–5 Nm³/h routed to flare. The second seal of the tandem is a backup holding full pressure if the primary fails. A nitrogen-buffered Separation Seal keeps bearing oil mist away from the faces, and the Seal Gas Conditioning Panel feeds the seals with gas filtered to 1 µm and heated above dewpoint — liquid droplets or fine solids destroy seal faces, and seal gas conditioning failures cause more DGS losses than the seals themselves. A small Seal Gas Booster keeps the supply alive during pressurized standstill.

Surge and its prevention

Below a minimum flow the impellers stall and flow reverses violently through the machine — surge. Cycles repeat several times per second, hammering the thrust bearing and seals; a few seconds of deep surge can destroy a rotor. The defense is the Anti-Surge System loop: a Suction Flow Element in the suction and paired pressure transmitters let the Anti-Surge Controller compute the operating point's distance from the surge line every 25–50 ms. Approaching the control margin, it opens the Anti-Surge Valve — full stroke under two seconds — recycling discharge gas through the Recycle Cooler back to suction to hold flow up. The Discharge Check Valve stops the pipeline blowing back through the machine on trip.

Drive and lubrication

A gas turbine or VFD motor drives the rotor through an API 671 dry Disc Pack coupling with hydraulically fitted Coupling Hub and a drop-out Spacer Tube sized so seal cartridges can be pulled without moving either machine. The API 614 lube console supplies the bearings — Lube Oil Pump in duty/standby, duplex Duplex Oil Filter at 10 µm, twin Oil Cooler units — with an elevated Overhead Rundown Tank gravity-feeding oil through coastdown if all pumps die. Trains of this class routinely log availability above 99% with 5–6 year intervals between bundle pulls.