Oct-2010

Revamping centrifugal compressors at an ethylene plant

Technical improvements and increased throughput justify the revamp of a Russian ethylene plant’s compressors

Gerald Oving
Thomassen Compression Systems

Article Summary

Many applications of centrifugal compressors may involve process conditions that are quite different from those for which the machine was originally designed. In some cases, these off-design conditions can be highly inefficient and increase overall energy consumption. In many instances, however, new internals — specifically designed for the revised operating conditions — can be installed in an existing compressor casing, thus achieving a significant improvement in the overall performance of the compressor. Most of the time, the reason for revamping is dictated by the need to increase plant production. This article describes the benefits of revamping existing equipment at the largest ethylene plant in Russia, illustrated by a recent project.

In 1995, Thomassen came into contact with the largest ethylene plant in the former Soviet Union. The ethylene plant is part of a conglomerate of petrochemical plants that also produce butadiene, ethylene oxide, butyl rubber, polystyrene and linear alpha olefins. In fact, the ethylene plant is a supplier of raw material to many of 
the other plants within the conglomerate.

The ethylene plant was put into operation in 1976. The plant underwent its first uprating in 1992 in a project under the control of contractor Toyo Engineering. At that time, the compressors were revamped by Ebara and new Mitsubishi Heavy Industries (MHI) steam turbines were installed.

Based on site inspections and discussions with maintenance and operational personnel, it became clear that the compressors  experienced mechanical and operational problems. In 2000, Thomassen took up the opportunity to carry out modernisation of the existing compressors to try to solve these problems. This modernisation process has involved several years’ work and is, in fact, a continuing project.

From 2001, technical and economical feasibility studies investigating various possibilities for capacity expansion were initiated by the ethylene plant’s management and ABB Lummus Global in Germany (see Table 1). These feasibility studies (termed “basic engineering studies”) were based on scenarios involving both the installation of new equipment as well as the revamping of equipment.

In December 2002, Thomassen was invited to quote for the revamp of the charge gas, propylene refrigerant and ethylene refrigerant steam turbines and compressors at the ethylene plant.

Modernisation
This ethylene plant is operated according to established practice in Russia. Every year, there is a complete plant shutdown in line with current state legislation, while maintenance methods tend to be rather conservative.

The modernisations are basically aimed at solving problems that date back to the previous uprate in 1992. In other words, they are troubleshooting activities implemented via root cause analyses. The modernisations are improvements that contribute greatly to increasing the reliability of the equipment and to establishing its more efficient 
operation. Examples of such improvements include:
• Greatly reducing seal oil leakage by replacing carbon seals with new mechanical oil seals (see Figure 1)
• Replacing conventional buffer gas labyrinths by installing positive seal barriers to prevent oil migration into the process and to eliminate pollution of the mechanical oil seal by process gas (see Figure 2)
• Supplying a new buffer gas system including stainless steel piping, valves, duplex filter sets, and so on
• Replacing outdated radial bearings with keys using customised spherical bearings (five pads - load between pads) in order to obtain stiffness and damping values for optimum rotor stability
• Installation of abradeable inter-stage seals to an improved design (impeller eye and shaft seals) to reduce secondary leakage flows
• Raising customer awareness by emphasising the importance of correct internal alignment of end walls to casings, bearing retainers to end walls and diaphragms to bearing retainers. Thrust bearing load, inter-stage seal losses, balance piston leakage and diaphragm split line leakages are all greatly influenced by the quality of internal alignment
• Improving the existing combined oil system for control-seal-lubrication oil supply to compressors and steam turbine by replacing outdated critical components in order to obtain a better-responding and therefore more reliable oil system
• Generally, supplying spare parts such as dry gas seals, compressor rotors, steam turbine parts and oil pump parts in order to enable control of overall quality and design integrity, and thus guaranteeing maximum reliability and availability.

Immediately after the first uprate in 1992, the compressors experienced high rates of seal oil leakage. As a consequence, the compressors had to be stopped three to five times a year, which meant significant loss of production.

The situation needed urgent attention. However, once the new mechanical oil seals were installed, the excessive oil leakage disappeared. Note that only a minor rotor modification was required for the 
compressors to enable the installation of the oil seals. The compressors operated in uninterrupted fashion between the yearly overhauls, meaning continuation of production and savings in investment and maintenance costs.

Revamp project
The results of the basic engineering studies of ABB Lummus Global were presented to the ethylene plant’s management in 2002. The next step — a detailed study — was never started. The customer decided to perform the complete ethylene plant revamp through a process of step-by-step debottlenecking using the results of the basic engineering study.

Thomassen was invited to quote for the revamp project involving the capacity increase for charge gas, propylene refrigerant and ethylene refrigerant compressor trains. Thomassen worked with MHI for all steam turbine-related issues. The quotation phase lasted from 2003 until 2004.