Specifying coupling and gear units for compressors in high-pressure service

Published on: 

Successful vertically split centrifugal compressor models developed by competent vendors usually come in 10 to 14 frame sizes to allow proper selection and optimization based on process requirements. Flow range is usually from around 3,000 m


/h. Each frame size increases around 40-50% compare to previous one. Vertically-split compressor models feature a completely modular bundle assembly. Bundle concept allows easy assembly and disassembly of compressor internals. Famous compressor models offered extensive modular design concept, introduced significant advancements in efficiency, manufacturing and ease of maintenance while reducing life-cycle costs.

Gear unit

Epicyclic gear unit is generally not recommended. The epicyclic unit may be considered as special case if result in a critical reduction in the overall size, weight, or both, of a machinery train. Care needed when pinion speeds higher than 3000 rpm, pitch line velocities more than 20 m/s, or journal velocities above 7 m/s are used. Generally critical centrifugal compressors are ‘un-spared’ and it impose higher expected reliability on every equipment in train particularly gear unit. Ample rating is always encouraged. Gear units located next to the driver should be capable of continuously transmitting the rated driver power, multiplied by an application factor (minimum 1.3). In other words, the gear unit rated power is identified by driver nameplate rating multiplied by a suitable application and service factors. Requirement for a break-in period is not acceptable for un-spared critical services (API 613 gear units).

Pinion and gear wheel hardness have considerable effects on trouble-free and smooth operation. Correct hardness need to be defined considering the API recommendations, ductility of the gear teeth, future upgrading and finish requirements.

When gear pitch line velocities exceeding 120 m/s, special design considerations are given such as deep sump (let say 600 mm depth), etc. For gears at pitch line velocities above 150 m/sec, additional requirements are recommended to ensure smooth and high quality gear operation such as: 1- Material cleanliness, usually ANSI/AGMA ISO 6336-5 Grade ME and MX requirements or higher. 2- Superior material mechanical properties (hardness, strength, fracture toughness, etc). 3- Extra ultrasonic and wet magnetic (magnaglo) inspections. 4- More conservative ratios and factors.

Special purpose coupling

Preferred coupling type is almost always metallic flexible-element coupling known as high torsional stiffness coupling. The coupling is selected mainly based on the train loading and required misalignment. Correctly selected coupling is capable of transmitting the maximum steady-state torques, cyclic torques, and the maximum transient torques under all conditions of angular misalignment, axial displacement, speed and temperature variation, simultaneously, to which train will be subjected in service. The maximum angular misalignment is important for coupling selection. It is specified with respect to expected misalignment during start-up, normal operation and shut-down of the coupled machines, based on known effects from thermal, pressure and dynamic forces. The coupling steady-state angular misalignment capability is usually not less than 0.2°.

The coupling service factor is very important and allows for various modes of off-design operation that can result from such factors as a change in gas (molar-mass, temperature or pressure variation), unequal load sharing, fouling, torsional oscillations and driver output at maximum conditions or possible future up-rating. A good example of future up-rating is augmentation of the power output of a gas turbine by water injection. For metallic flexible-element couplings, minimum recommended service factor is 1.5.


For initial coupling selection, a large cyclic torque requirement is typically assumed until all conditions are known so that the torsional response analysis can be completed. Sufficient spacer length (known as shaft separation) is required for removal of coupling. It allows for maintenance of adjacent bearings and seals without removal of the shaft or disturbance of the train alignment. Distance between shaft-ends of around 450 mm or more is preferred for trouble-free installation and maintenance. The potential unbalance as per ANSI/AGMA 9000-C90 is specified in API 671 for various couplings. As a rule of thumb, for low speed (around 1500 rpm) and high speed (above 5000 rpm) couplings, consider potential mass-centre displacement of 50 μm and 15 μm respectively. Metallic, deformed-thread, self-locking fasteners are only acceptable fasteners for special purpose couplings. A number of spare coupling fasteners usually consumed during installation and pre-commissioning period (let say 10-20%). Keyless connection methods (coupling hub type) are always preferred. Best option: suitable flange connection, second option: hydraulic fitted with 1:24 taper.

Oil spray to control temperature is not recommended. A useful hint for operation inspection is to ask coupling manufacturer to study and indicate which component(s), should be inspected or replaced following the occurrence of torque greater than the peak torque rating. Some metallic-flexible couplings, such as single element convoluted diaphragm styles, can exhibit an un-damped response to forced axial vibration. Multi-disc, multi-diaphragm and non-convoluted single element flexible couplings typically do not exhibit such axial vibration behavior.

o compare to 0.20

Axial movement and thrust load 

1- Does train rotor make excessive force on the axial end-stop(s) of the thrust bearing(s)?

Axial alignment is critical in complex trains. It ensures that during operation, when operating temperatures have been reached and all final thermal expansions have taken place, the magnetic centre of the rotor is in proper position. This is done by proper coupling selection and giving couplings defined pre-stretches. Train design and installation using mentioned coupling pre-stretches and correct alignment procedure, should assure that motor-rotor stay in magnetic centre position during cold operation (installation temperature) as well as hot operation (normal operation temperature).

1- Cold scenario: Considering situation after assembly, used to determine alignment.

3- Transient scenario: During speed-up or speed-down, shaft is floating on the oil film while it is gradually heated or cooled. If temperature is in cold (installation temperature) to hot (normal operation temperature) range, there is no need to consider this scenario. However there are some trains which present extreme transient temperature which is critical in axial load study.

5- Stand-still cool-down scenario: In this situation one bearing holds the shaft in its place, so the shaft will shrink to another direction.

Case study is presented for a double-end motor driven train. The compressor train consists of a both-ends drive motor (motor output at both ends of the shaft) installed in the centre of the train, two speed increasing gears provided at outer ends, and two compressors (LP and HP) outside them. Table 1 shows axial movement study for this compressor train. Axial displacement values are normalized based on LP coupling pre-stretch (around 2 mm).


Coupling pre-stretch




Cold-starting-up scenario




Hot scenario