12. CENTRIFUGAL COMPRESSOR OPERATION

 12.1 INTRODUCTION

The operation of a compressor or of centrifugal compressor units means keeping the performance within the operating limits for which machines were specified in order to avoid any inefficiency in the plant and to obtain the highest economic profit.

The good operation of a compressor depends on the correct setting up of the auxiliaries (lube and seal oil systems, gas system, instrumentation). The reliability is also largely connected with the checks made during running and the evaluation of the data obtained; and also the execution of the inspection schedule.

During machine erection and initial operation a series of tests and checks will be carried out by Nuovo Pignone technicians who are to hand over the machines, together with the people in charge of the plant operation (see the following sheets).

The increasing number of plants more and more complex in which centrifugal compressors operate, and the increasing use of automation present users with the problem of finding sufficiently experienced personnel.

Nuovo Pignone offer their Customers many training courses, at different levels, thus contributing to develop the personnel in charge of maintenance and operation of machinery.

The people who will constantly attend the machine throughout its life have to know its operation and running data to be able to act in time before troubles occur.

Since the centrifugal compressors are largely employed in sundry industrial applications, the driving systems can be of different kinds; electric motors, steam turbines, conventional gas turbines or jet derivatives; therefore also the requirements of the motive force have to be considered among all operations necessary for the compressor operations. For lack of space the present paper will deal only with the procedure to be followed for a unit consisting of high-pressure centrifugal compressors driven by steam turbines. 

 12.1.1 Example of Compressor/Steam Turbine Operation

After completing the erection of the new or overhauled machines with their relevant systems and after checking the lists on which the values of inner clearances are recorded along with the setting values of instrumentation, as well as the alignment test certificates, the personnel in charge shall see that the following has been carried out before giving their approval to start: 

1. preparation, check and putting into operation of oil systems;
2. preparation and check of compressor gas systems;
3. preparation and check of turbine sets;
4. preparation of steam systems;
5. preparation and check of steam system at the turbine seals and condensing system.

The operations to be performed to start the machine set are indicated herebelow, as far as possible step by step:

1. insertion of turbine ratchet;
2. feeding of turbine seal steam system;
3. startup of condensing system;
4. turbine warming up;
5. purging of compressor gas system and refilling, by opening the suction valve, with gas having pressure and temperature values according to specification;
6. heating and pressurizing of live steam line;
7. check of pressure, temperature, oil, gas, steam and water levels, as well as check for lack of "stop" signals (except for "ratchet connected");
8. disconnection of ratchet
9. startup acc. to diagram on figure 12.1
10. turbine speed increase by the starting device seeing that the non-return valve on the steam extraction line opens properly and turbine "extraction" strats, until the min. control speed is reached;
11. speed increase from control room until the normal operating are gradually reaches, the compressor discharge valve is fully opened and the anti-surging valve is utterly closed.

The "full load" condition is thus achieved

During startup the following values have to be constantly checked:

• vibration level
• bearing lube oil temperature and pressure
• bearing temperature
• compressor and turbine casing expansion
• compressors  axial thrust

During the first startup or after restarting following a long standstill of the unit, the above values detected at every speed at which the machine is stopped for some time shall be indicated on the machine log along with startup times and methods.

When machines are normally running, although they do not need any particular assistance, they are subject to some checks which allow the machine operation and the need for overhaul to be assessed.

Herebelow the checks to be made at regular intervals during operation at normal speed are listed:

• pressure and temperature of inlet steam
• flow, temperature and pressure at extraction
• condenser vacuum
• temperature of the bearings
• seal oil/reference gas differential pressure at compressor inlet
• oil discharge temperature on the outside of every compressor seal
• axial thrust of compressors
• vibration level
• gas temperature, pressure and flow

The values not directly recorded shall be indicated on the machine log.

The operators must bear in mind that all data detected, both visually and automatically, are negligible, if not controlled and compared with those previously recorded and if no measure is taken, whenever necessary, to remove unsatisfactory service.

Generally two persons are required to carry out all operations necessary to start up the steam turbine and compressor unit; they have to be properly acquainted with the plant and the whole instrumentation installed therein, alarm and stop sequence included.

The control room operator is in charge of controlling all indicators and recorders, of starting the electric motors driving the pumps, actuating the electrically and pneumatically operated valves, controlling the operating data.

The second operator, in the machine room, is in charge of actuating the hand-operated valves, starting the steam turbines driving the oil and condensate pumps, putting the vacuum ejectors into operation, as well as all operations necessary to prepare machines for startup, including the initial steps for running until the minimum control speed is reached. He has also to drain the lines, check the sight glasses and see whether there are leaks and other Faults likelv to be indicated bv anomalous noise.

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