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Steam turbines are used in most cases as drivers for centrifugal pumps or compressors. In certain cases they may be used to operate reciprocating equipment by transmission of power through a counter shaft and crank shaft gear assembly.
A turbine consists of single or multi stage wheels with blades mounted on the periphery fixed to a shaft in a casing. Steam enters from a ‘steam chest’, fitted on the base, through a nozzle where it expands, (transforming pressure into velocity), impinges on the blades causing the wheel (rotor) to revolve. Steam may be directed tangentially, in which case the rotor is equipped with ‘bucket’ blades or axially when the rotor is fitted with ‘fan’ blades.
In a single stage turbine, the steam after passing through the blades, goes to a reversing chamber where it is directed back on to the blades a second time.
In a multistage turbine, steam passes through the first impeller blades and then is redirected by fixed or ‘stator’ blades on to the next set of blades and ultimately to the exhaust system.
In multistage turbines the steam usually exhausts into a vacuum condenser for maximum efficiency. The condensate is pumped back into the boiler feed water system.
All turbines are equipped with governors for controlling speed at desired conditions.
If the governor does not hold the desired speed, the maintenance machinist will make the necessary adjustment.
Operating personnel are not authorized to alter the governor settings, unless the turbine is equipped with a built-in speed control valve specifically designed for this purpose.
If the speed regulator or governor fails, the turbine, unloaded or lightly loaded could overspeed and fail. Overspeed ‘trips’ are, therefore, fitted to prevent this.
OVER SPEED TRIPS
All turbines are equipped with overspeed trips which release a cut-off valve for shutting down the turbine in the event of the operating speed exceeding a safe maximum. Conditions which may cause the overspeed mechanism to trip and shutdown the turbine are:
1. Manual release of the trip.
2. Sudden excessive speed due to loss of suction at pump end.
3. Excessive speed due to mechanical failures, ie., broken shaft, loose impeller at pump end, etc.
4. Bringing turbine up to speed too rapidly.
3 RESET OVERSPEED TRIP
To reset the overspeed trip:
1. Close the valve on steam to the turbine, allow turbine to slow down.
2. Bring the manual release lever back to reset position and set the trigger catch to hold position.
It will be noted that tripping mechanisms vary considerably in design and attention must be given to the operating principles of each type.
In starting a steam turbine, the following procedure must be observed:
Check all bearings to be certain they are properly lubricated.
Start cooling water to bearings where supplied. Adjust cooling water inlet valve for sufficient cooling water.
Check the overspeed trip mechanism to be certain the overspeed cut-out valve on the steam inlet is open.
Open the bleeders on the steam to the turbine and exhaust steam from the turbine and bleed off any accumulated condensate.
Slowly open the exhaust steam valve wide.
Crack open steam valve to the turbine sufficiently to allow case to warm up and blow any condensate from the case.
When turbine case is warm, close the drain valves.
Continue opening steam valve slowly, until turbine speed governor starts controlling at desired speed. Then open steam valve wide.
Check for any abnormalities such as noise, vibration, bearing temperature etc.
To shutdown a steam turbine, observe the following general procedure.
1. If practicable, stop the turbine by tripping the overspeed trip. Stopping by this method assures proper operation of this mechanism.
2. Close the steam valve to the turbine.
3. Close the exhaust steam valve.
4. Open the turbine case bleeder valves.
5. After shutting down, reset the overspeed trip for normal operation.
Routine checks must be made similar to that outlined for centrifugal pumps. In addition, when possible, the overspeed trip should be checked by a machinist using a tachometer.
AUTO START OPERATION
On all steam turbine driven pumps, which are standby and set for automatic cut-in operation, valves should be set as follows:
Pump suction and discharge valves wide open.
Exhaust steam valve open.
Steam to the turbine, valve open.
The warm up bypass valve, around the steam control valve, cracked open sufficiently to keep the turbine case warm.
If turbine case is not equipped with a steam trap, keep the bleeder cracked open.
The automatically controlled steam valve on the turbine is set to remain closed if the main pump is operating correctly.
PREPARATION FOR REPAIRS
When repairs are necessary which involve opening the turbine case, both the valves on the steam to the turbine and the exhaust must be positively isolated and the coupling with the pump disconnected.
Gas turbines are used as drivers for centrifugal compressors, generators and large centrifugal pumps.
Gas turbines are internal combustion engines following the cycle of compression, combustion, expansion and exhaust which is continuous.
Refer gas turbine manuals
Gas turbines are specialized equipment manufactured by only a few companies in the world.
Detailed information and operating instructions are available in the manuals provided by the respective manufacturers.
Single and two shaft machines
Gas turbines can be classified as single shaft or two shaft types. Single shaft are used on machines where constant speed is essential like electric generators.
Typical two shaft gas turbine
A typical two shaft gas turbine consists of a gas generator and a power turbine.
The gas generator section is made up of the air inlet system, the compressor, the combustors and the stator & rotor assembly of the high pressure turbine.
The power turbine comprises the LP turbine stator assembly, exhaust collecter assembly and power turbine rotor assembly from which the drive is transmitted to the driven machinery.
5 Operational principle
By the use of a starter, the compressor is brought up to firing speed delivering air at partial capacity. Fuel gas is then admitted to the combustors and fired to expand the gases. Due to the increased velocity at the stator nozzles the speed increases. At a certain point of speed when the generator is self sufficient the starter is disengaged and the speed increased further to normal maximum.
The LP turbine (power turbine) starts rolling within a certain range of the HP turbine speed and picks up speed to its normal minimum.
The final speed of the LP turbine will depend up on the load conditions of the driven equipment.
Start-up and shutdown sequences
Modern gas turbines have very little of manual operations to be done for starting and shutting down operations.
For safety of the equipment and process suitability the start-up and shutdown of gas turbines are carried out by electronic automation on the basis of a predetermined sequence.
Operating personnel must be fully trained for each type of gas turbine used in a plant. They should be familiar with the various systems of the turbine as well as the driven machinery such as the fuel system, air system, lubrication system, exhaust system, seal oil system, air conditioning system etc.
The machine will be stopped automatically for a number of
conditions that might damage the driver or the driven equipment, such as,
low lube oil pressure, flame failure, high exhaust temperature, high
suction drum levels, high process gas temperature, high vibration etc.
Re-starting of the machine will be done only after ascertaining the cause of trouble and rectifying the situation.
The turbine can be stopped manually by remote switches for emergency.
Built in fire protection
All gas turbines are provided with an in-built CO2 discharge system in the event of a fire.