Hydrocarbon Processing - May 2021 - 40

Maintenance and Reliability
In radial trimming, the outlet of the impeller will be trimmed
in the radial direction by reducing the diameter. Normally, radial trimming is preferred where the reduction in pressure rise
across the impeller is required. This is simple and does not require modification in shroud; however, it will change the fluid
exit angle, which often calls for diffuser redesign. This option is
not preferred for high-speed wheels since it will result in lower
efficiency, whereas in low-specific speed wheels, it improves the
specific speed value closer to optimum value, and radial trimming does not alter the flow coefficient at the inlet.
Axial trimming involves the reduction of impeller blade
height at the outlet of the impeller without changes in the inTABLE 5. Major factors considered during the
decision-making process
Capital cost

Driver cost
Equipment cost
Installation cost
Auxiliary equipment/piping/software cost
Foundation cost
Operating cost

Plant life
Service cost
Maintenance cost
Utility/chemicals cost
Efficiency
Cost of spares
Suitability for future modiﬁcation

Logistics

Operating experience
Supply chain (single/multiple location)
Installation equipment/utilities
Temporary facility/resources
Packing and delivery
Payback period

Economy
and safety

Major process variables

Field/location accessibility

Reliability, availability and maintainability (RAM)
Shutdown requirements: production loss,
impact on customers
Opportunity cost
Plant and personal safety
Noise

Environmental

Vibration
Emissions, gas and vapor
Liquid discharge
Toxicity
Hazardous waste management
Drain and vent management
Regulatory requirements

Other

Reusability of existing spares/equipment
Material compatibility
Training requirement
Operability
Schedule

MAY 2021 | HydrocarbonProcessing.com

ducer. This is preferred for high-specific speed impellers. Reducing the outlet height will lower the specific speed of the impeller.
Trimming to control the losses due to tip gap (clearance between
impeller and shroud) should be checked and revisited. It should
be noted that the tip gap for the given operating speed is generally based on material distortion, which is greatest at the outer
diameter where centrifugal forces have the strongest effect. It is
normally expected that axial trimming will result in an efficiency
decrease and narrow choke margin but will have no major effect
on the flow range of the compressor in the initial stages.
Control of losses. The following losses are normally encoun-

tered in the compressors, affecting performance. By controlling
these losses, the compressor system can be optimized.
* Disk friction loss due to adhesive forces between
rotating disk and fluid. The shear force acting between
the impeller back face and the stationary surface is to be
overcome by cost of power. Generally, it increases with
rotational speed and impeller exit radius.
* Skin friction loss due to adhesive forces between
the channel surfaces and the fluid. Channel surfaces
includes the hub, blades and shroud.
* Incidence loss caused by the direction of the gas flow
diffusing from blade angle. The deviation between the
relative inlet angle of the gas and the actual blade angle
causes the gas to change its direction, resulting in
(energy loss) incidence loss. Incidence loss occurs
when the relative fluid flow angle of fluid entering the
impeller deviates from the actual blade inlet angle.
This loss can be minimized by adjusting the blade angle
closer to the relative flow angle, curving of the blade
in the direction of the entering flow, and by improving
the inducer (initial part of the impeller) design.
* Blade loading loss due to a pressure difference between
blade to blade. Blade loading loss is due to the growth of
the boundary layer in the impeller and is highly
dependent on the diffusion of working fluid to the
impeller. Blade loading loss is a function of the diffusion
factor and the tangential impeller velocity. It can be
optimized by increasing the rotational speed or by
increasing the flowrate. Changing the relative flows
(inlet and exit) and the tangential impeller exit speed
will result in a change in blade loading loss.
* Recirculation loss caused by the back flow of fluid
to the impeller, which requires additional power
to overcome the backflow.
* Clearance loss due to significant flow leakage through
the clearance between the impeller and casing due to
pressure difference. This will form a small vortex on the
suction side (low-pressure side) of the impeller vane outer
tip. It affects both the suction side and discharge side.
* Leakage loss caused by the leakage of fluid through
compressor seals. Seal loss decreases the energy available
to convert into pressure head due to internal recirculation
inside the compressor.
* Vaneless diffuser loss in the vaneless diffuser space as a
result of friction and the absolute flow angle. Whether
the compressor has a diffuser or not, a vaneless space is
always directly flowing through the impeller where the

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Hydrocarbon Processing - May 2021

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