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Apr-2002

Understanding centrifugal compressor performance

In a connected process system, expensive changes to the compressor and driver can be avoided with system debottlenecking modifications

Scott W Golden, Scott A Fulton and Daryl W Hanson
Process Consulting Services
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Article Summary
Wet gas compressor capacity limits feed rate or unit conversion in many FCC and delayed coker units. Understanding compressor performance and its interaction with the connected process systems is critical when revamping FCC and delayed coker units. Unnecessary changes are frequently made to the compressor and driver. Alternatively, lower cost process system modifications can be used to debottleneck a compressor limit. Figure 1 is a block diagram of a compressor and the connected process system components.

The connected process system and compressor performance must be thoroughly evaluated as a single system to determine the most cost effective way to increase compressor capacity. Conventional process design approaches use several equipment disciplines to evaluate piping, heat exchange, and distillation systems independently. Thus, the opportunity to debottleneck the compressor with lower cost process system changes my go unnoticed.

Reducing system pressure drop to increase suction pressure or decrease discharge pressure allows more gas to be compressed through the compressor without modifications, as outlined below.
Process equipment pressure drop:
- Main column internals
- Piping/nozzles
- Control valves
- Fin-fans
- Shell and tube exchangers
- Flow metering

However, the impact of suction and discharge system changes on compressor capacity is not the same. Suction pressure changes have a much stronger influence on compressor capacity due to their effect on overhead receiver condensation, gas density, and compressor head.

Process system operating pressure and system pressure drop strongly influence wet gas compressor capacity. Compressor discharge and suction pressure are variables and should be manipulated whenever possible to raise compressor capacity. Increasing compressor suction pressure and reducing discharge pressure will increase compressor capacity. Finding cost effective solutions always starts with field measurements of the current operation to identify high pressure drop components. Distillation column internals, process piping, heat exchangers, control valves and flow metering in the connected process system must be modelled together with the compressor to quantify compressor capacity increases resulting from equipment modifications.

In an FCCU, feed rate, reactor/ regenerator differential pressure and system pressure drop set compressor suction pressure. In a coker, coke drum constraints and the system pressure drop set suction pressure. Discharge pressure is controlled by the gas plant operating pressure and system pressure drop. Practical changes to consider include process flow scheme, tower internals, heat exchangers, piping/nozzles, control valves, and orifice plate modifications.

These components all generate pressure drop. Process flow scheme changes may include adding a pumparound to the main column or bypassing absorber bottoms liquid around the high pressure condenser to reduce pressure drop.

System pressure drop between the main column inlet nozzle and the compressor inlet will vary from a low of 5psi to over 25psi. High pressure drop components need to be identified and cost-effective and reliable changes made. In some instances, replacing main column trayed internals with structured packing will be the low-cost solution. At other times, condenser system pressure drop will control compressor suction pressure. Therefore, piping, fin-fan, shell and tube exchanger, control valve, or flow metering modifications will need to be considered.

Absorber operating pressure and system pressure drop set the compressor discharge pressure (Figure 1). Lower discharge pressure reduces compressor head and driver power, which increases compressor capacity. Discharge pressure should be minimised without reducing gas plant performance. Absorber pressure controls C3 recovery, assuming other process variables have been optimised.

In a few instances, reducing absorber operating pressure will not materially change C3 recovery. In most cases, however, propylene recovery drops as pressure is reduced and it is not a cost effective way to increase compressor capacity. If the existing compressor discharge system has high pressure drop, then equipment changes may be an effective means to debottleneck the compressor. Typically, compressor discharge pressure will need to be reduced by at least 20psi to have a meaningful effect on compressor capacity and driver power.

Compressor fundamentals
Most FCC and delayed coker wet gas compressors have an inter-cooler system that improves compressor efficiency and reduces the gas temperature rise through the stages of compression. Inter-cooled compressors will have a low-stage curve defining performance upstream of the inter-cooler and a high-stage curve for the downstream portion. In reality, the low and high-stages will have three to four actual wheels, each with their own individual performance curves.

These low and high-stage performance curves are a composite of the individual stage curves. Usually these low and high-stage curves are sufficient to evaluate compressor performance and the connected process system’s influence on compressor capacity.

Centrifugal compressors have performance curves similar to pumps. The major difference is that a compressor moves gas which is compressible, while the pump moves liquid that is not compressible. The compressor curve flow term is always based on inlet conditions. Consequently, inlet gas density influences volumetric flow.
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