Middle-aged compressor marries virile young drive

As part of a modernisation project, chemical company Brunner Mond is now using variable-speed drives, instead of steam turbines to drive its centrifugal gas compressors.

As part of a modernisation project, chemical company Brunner Mond, based in north-west England, is now using variable-speed drives, instead of steam turbines to drive its centrifugal gas compressors.

The 50-year old compressors, now fitted with state-of-the-art drive technology, will remain to become an energy-saving, low-maintenance system with a high availability.

This has all been made possible by combining special high-speed motors, gearless couplings and medium-voltage drive convertors with intelligent HV IGBT (insulated gate bipolar transistor) power semiconductors.

In addition to supplying the drive system, Siemens carried out the computer-based vibration analysis, system integration, training of the operating personnel, commissioning and after sales service.

Brunner Mond Group is one of the leading European producers of alkaline chemicals.

The company has a total of four plants in Europe and Africa with its head office located in Northwich, in NW England.

The company produces mainly soda ash, a basic material for use in the glass industry and the production of washing powders, as well as industrial minerals.

Until September 2000 Brunner Mond owned and operated coal, oil- and gas-fired boilers in the production facilities at Winnington and Lostock, close to Northwich.

The boilers kept the plants supplied with process steam as well as high-pressure steam for the steam turbines driving the compressors.

Brunner Mond now has all of its steam raising carried out by Powergen Combined Heat and Power via a combination of five new gas boilers.

These central, gas-fired, boilers supply low-pressure steam, to the two facilities at Winnington and Lostock.

This can be used as process steam, but is not suitable for driving the compressors.

Brunner Mond therefore decided to keep the 50-year-old compressors, which were still fully operational, and replace the steam turbines with variable-speed electric drives.

At Lostock, this involved four units.

Compared with steam turbines, electric drives are distinguished by their high reliability and low service requirements.

As deposits continually reduce their efficiency, steam turbines have to be stripped down completely twice a year, cleaned and the seals replaced.

Modern electric drives, however, operate almost maintenance-free with the added advantages of significantly lower noise levels and cooler operation.

The speed of the motor is continually adapted to actual operational requirements, which guarantees accurate process control.

Energy is also saved because the system takes only the amount of power, which it actually requires.

In order to gain the advantages of variable-speed drive technology for the existing Brunner Mond compressors, Siemens selected a system comprising special high-speed motors, gearless couplings and water-cooled medium-voltage Simostart MV drive convertors.

The drive convertors are equipped with intelligent HV IGBT (insulated gate bipolar transistor) modules, which are optimally harmonised with each other.

These drives guarantee optimum reliable compressor operation over the complete speed range.

The motors, which have an operating range from 3800 to 6400rev/min, are an individually customised design from the Siemens special drives centre in Berlin.

Induction motors with laminated squirrel-cage rotors are used for the four drives, two machines require 1.5MW of absorbed power at the coupling and two smaller machines of 1.3MW, so that the motor dimensions allow the two existing compressors to be used without modification.

The motors have air-to-water heat exchangers, the internal cooling circuit using separately driven fans with counter current cooling at both ends.

The rotor has a ribbed surface for good cooling characteristics.

In order to eliminate vibration problems from the very beginning, the usual bending and torsion analysis of the rotor was supplemented by investigations into the strength and stiffness of the motor frame, which was optimised by computing the natural frequency.

In order to optimise the dynamic performance of the rotors, Siemens used special tilting-pad bearings.

Four additional balancing planes permit modal balancing and therefore a specific reduction in the bending vibrations in all of the natural forms.

In spite of the wide range of speeds, the application of each of these measures made it possible to locate all of the natural frequencies outside the speed control range.

Full-load system tests were carried out in the Berlin Dynamowerk factory, prior to installation, to ensure that all of the specifications were fulfilled.

For the Brunner Mond application, the motors are coupled to the compressors without using gearboxes.

This configuration has the advantages of a higher overall efficiency, low wear, and increased availability due to the elimination of all problems associated with gearboxes.

To guarantee the smooth running properties of the new motors, they were perfectly harmonised with the 50-year old compressors.

Siemens carried-out a torsion and vibration analysis for the complete mechanical transmission using a specially generated three-dimensional finite element model of the old compressor and the new motor.

The moment of inertia for the compressor was determined from documents, which were about 40 years old and using the FEM model, all possible operating situations and mutual interactions were simulated by computer.

From the information gained, optimum smooth running characteristics were obtained for the mechanical transmission up, to a maximum speed of 6400rev/min.

A Simovert MV medium-voltage convertor is used to control the speed.

This unit has an output of 2300 kVA at a motor voltage of 3.3kV and an output frequency of 107Hz.

As a result of its unique design, Simovert MV requires little maintenance and has a high availability.

This is particularly due to the fact that it is the first drive convertor in the medium-voltage range, which uses HV-IGBT power semiconductors.

In conjunction with a three level voltage DC link, the HV-IGBT modules permit a clearly structured, modular and space-saving design, which is both extremely reliable as well as service-friendly.

This could not be achieved when using other power semiconductors because the HV-IGBT is the only semiconductor element of its class which requires absolutely no snubber circuitry.

Together with the gating system, the IGBT modules form an intelligent switching unit which allows switching, independent of the load, and as a result unnecessary current peaks are eliminated.

In conjunction with the three-level technology and the high-performance transvector control, this guarantees sinusoidal motor currents.

This, in turn, reduces the motor losses and minimises the torque fluctuations, which reduces overstressing of the motor, mechanical transmission and especially the driven load.

Although Simovert MV units can be either water-cooled or air-cooled, it was only possible to use the water-cooled version for the Brunner Mond drives.

The ambient air was far too aggressive for air-cooled convertor technology, which is frequently the case in the chemical industry.

The water-cooled Simovert MV is part of a turnkey container solution.

It fulfilled the customer's demand to have a project, which did not involve any civil engineering modifications in the production hall and with minimum downtimes.

To meet this clear customer requirement, Siemens supplied climate-controlled containers, which accommodate the drive convertor, transformer, cooling system and control system for each drive train.

These were shipped to the site ready-wired and tested.

Special cranes were used to lift the containers onto their bases and it was only necessary to connect power cables to the mains supply and couple-up the raw cooling water pipework.

The final task was to connect the drive system to the supervisory control via Profibus DP and the system was ready to be commissioned.

As Karl Cleary, Brunner Mond's Northwich east plant manager commented recently, "The conversion of our compressors to high efficiency electric drives was a major operation.

Our main concerns were the potential impacts on both production and future drive train reliability.

Detailed planning, including containerisation of key components, ensured that installation was completed with minimal loss to production capacity.

Similarly, the careful harmonisation of individual drives and compressors has resulted in extremely smooth operation.

We are very pleased indeed with the quality that Siemens delivered at every stage of this project".

In addition to installation, system integration and commissioning, Siemens provided training support for operating personnel and also became responsible for after sales service.

Replacing out-of-date drive technology by state-of-the-art convertor systems, no matter whether it is for steam turbines or fixed-speed drives, provides many advantages, especially in the chemical industry.

In addition to compressors, this is particularly true for pump drives in the widest range of processes.

The reason is, as in the Brunner Mond case, that energy consumption increases as a function of the cube of the speed of the drive.

The DETRs Market Transformation Programme estimates that 12TW/h per annum of energy equivalent to 6Mt per annum carbon can be saved in the UK by 2010, if VSDs were installed in all appropriate applications.

Consequently, potentially billions of pounds can be saved, benefiting the environment and leading to higher profit margins.

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