Steam turbines and their operation

A steam turbine is a type of heat engine that works by converting steam into a working mechanism

Professional diagnostics and maintenance of steam turbines minimizes the likelihood of breakdown or emergency. When diagnosing steam equipment, one of the important points is knowledge of its correct operation and purpose.

Types of turbines

A steam engine consists of two parts: movable and non-movable. As a rule, they are divided into groups based on the types of process occurring in the unit:

• condensing;
• heat-extraction;
• special-purpose turbines.

1. Condensing.

They are used to convert the maximum achievable dose of steam heat into mechanical work, they operate with the exhaust of the waste into the condenser, where a vacuum is necessarily created. These devices are divided into two types: stationary and transport.

1.1. Stationary.

In other words, turbo generators are created on the same shaft with alternating current generators. Such turbines are mainly used at power plants for the generation of electricity, where they are used for different purposes. Thus, a division can be made into:

• Base, which bear the main load of the power plant;
• Peak, which are used only at times of the highest load;
• Turbines for auxiliary needs, due to which the operation of the power plant is ensured.

1.2. Transport.

This type of device found its purpose on ships, acting as an engine.

2. Cogeneration.

A type of device that achieves two goals at once: obtaining heat and electrical energy. It has also become widespread at power plants (CHP).

Features:

• The presence of back pressure, where all the waste can be used for technological needs;
• The ability to regulate steam extraction, where steam can be partially removed from intermediate stages, and the remainder sent to the condenser;
• The presence of extraction and back pressure, where steam can also be partially removed, and the waste sent to heating.

3. Special-purpose turbines.

They are actively used in metallurgy, mechanical engineering and the chemical industry.

Types:

• Muffled steam turbines. They operate using waste from piston machines, presses and steam hammers, in which the pressure slightly exceeds the atmospheric pressure;
• Two-pressure turbines. They can operate both on waste from steam mechanisms and on new ones;
• Pre-switched turbines. Initially, they have a high level of pressure and back pressure. The waste can be sent to other turbines with lower pressure;
• Drive. The highest drive power is needed here.

Operation

To avoid accidents during operation, it is necessary to organize the operation of the device in full compliance with the requirements of the manufacturer, the instructions for use, fire and safety precautions during maintenance. It is important to remember that this is a technically complex mechanism and only highly qualified specialists should work with it. Taking into account all the above points, it is necessary to develop an internal operating manual, which must include all the most important aspects of working with steam turbines.

1. Before starting, it is necessary to eliminate all existing faults that are incompatible with starting the machine. These include:

• Faulty pressure gauges, thermometers, vibrometers, tachometers and other devices that monitor the thermal process;
• Faulty lubrication system;
• Faulty control system;
• Faulty shaft-turning device;
• Faulty protection system that interrupts steam supply in an emergency.

2. Before starting the turbine, it is necessary to carry out preparatory work.

Start-up stages:

• Preparation;
• Turning over and bringing to maximum speed;
• Synchronization and load.

During the preparation period, it is necessary to check the condition of the equipment, the completion of the work, the operability of the alarms and devices. Warm up the steam line and bypass pipes for an hour and a half, check the oil pumps, protection and control systems.

3. The turbine is started or brought into operation by starting the first control valve. At this stage, all systems are tested in parallel, and only after a successful test is the turbine gradually loaded.

4. Backpressure turbines are started in two ways:

• The backpressure pipeline is turned off, and the turbine is started with the condition of steam emission into the atmosphere, until the pressure level in the outlet pipe is higher than the steam pressure in the backpressure steam pipeline;
• After finishing warming up the steam pipeline, the unit is warmed up from the exhaust side.

The main feature of starting such turbines is the absence of a condenser.

5. The machine operation includes: start-up, normal operation and shutdown. During the operation of the device, the rotor extension, its axial shift and vibration state must be under special control. Since deviations in these indicators above the permissible standards will not allow the entire system to operate reliably. It is also necessary to constantly check the standards of fresh steam, oil in the lubrication system, in order to prevent bearing heating. Among other things, it is necessary to continuously check the units and elements of regulation, protection, steam distribution bodies. Particular attention should be paid to oil leaks on fasteners and stopper devices.

6. Shutting down turbines

It is important to maintain the highest metal temperature when stopping the turbine. However, it is allowed to cool it when transferring to a long-term reserve or to perform repair work of any type.

Before the start of the shutdown, the turbine is unloaded to 10-15% and only after that the steam supply is stopped. After this, make sure that all valves on the selection line are closed and check that the power on the wattmeter has dropped to negative values. Only if all these rules are followed can the turbine be disconnected from the network. If the valves are not tight and steam continues to flow into the turbine, the generator must not be turned off under any circumstances. Then the rotor starts to coast, its rotation speed is gradually reduced to zero. Immediately after this, the VPU is turned on and operates during metal cooling.

7. Emergency shutdown of turbines.

During emergencies, special attention should be paid to the following indicators:

• Load;
• Indicators of reheating and fresh steam;
• Condenser vacuum condition;
• Vibration level;
• Rotor axial shift;
• Oil pressure in the lubrication and control systems.

An emergency shutdown is carried out due to an immediate stop of the steam supply, after which the generator is turned off. Then work is carried out on a normal shutdown of the turbines.