lesson 1: steam turbine operation and control with mechanical governor

Summary

This module delves into steam turbine operation and control, focusing on adjusting turbine speed and output. It explains that power output is controlled by modulating steam flow through admission valves. The script describes various valve arrangements, highlighting the benefits of multiple control valves for reducing throttling losses. It also introduces hydraulic control systems, mechanical governors, and their role in sensing turbine speed and adjusting control valves. The discussion includes governor set points, speeder gear adjustments, and their impact on turbine operation, promising more on the governor's effect on the electric power system in subsequent modules.

Takeaways

• 🔧 The principle of steam turbine control is to adjust the power output by varying the steam flow through the turbine.
• 💧 Turbine steam flow is controlled by adjusting admission valves or control valves, which can be a single valve or multiple valves in sequence.
• 🚀 In large turbines, multiple control valves are used to reduce throttling losses compared to using a single large control valve.
• 📍 Steam chests can be located above, below, or on either side of the turbine shell, with multiple control valves in each chest.
• 🛠 A stop valve is typically fitted at the entrance to each steam chest, remaining open during normal operation while control valves modulate steam flow.
• 📊 The opening of control valves is determined by the position of the operating lever, which is adjusted by the power cylinder of the hydraulic control gear.
• 🔄 A mechanical governor senses turbine speed and adjusts the control valve through a hydraulic servo mechanism in response to changes in speed.
• ⚙️ The governor's set point can be adjusted manually or remotely using a speeder gear, which affects steam flow and generator output when the turbine is on load.
• 🔄 A double relay type of pilot valve is often used in large turbines with multiple control valves to manage the significant power requirements.
• 🔧 The centrifugal governor is driven directly from the turbine shaft and adjusts the control valve through a pilot relay, influenced by the centrifugal force on weights.

Q & A

What is the primary method for controlling steam turbine output?

The primary method for controlling steam turbine output is by adjusting the amount of steam admitted to the turbine through the turbine admission valves or control valves.

How does increasing the power output of a steam turbine relate to the steam flow?

To increase the power output of a steam turbine, more steam must be passed through it, which is achieved by opening the control valves wider to allow more steam flow.

What is the purpose of having multiple control valves in a steam turbine arrangement?

Multiple control valves are used to reduce throttling losses by opening them in sequence according to the turbine load, ensuring that only one valve is throttling steam at a time while the others are either fully open or closed.

Why are steam chests often positioned above and below the turbine shell in large turbines?

In large turbines, steam chests are positioned above and below the shell, or on either side, to facilitate the use of multiple control valves and to manage the steam flow more effectively across the turbine.

What is the role of the stop valve in the steam chest during normal operation?

During normal operation, the stop valve in the steam chest remains in the wide open position, allowing the steam chest to be charged with steam directly from the boiler while the control valves modulate the steam flow.

How does the hydraulic control system adjust the position of the control valves in a steam turbine?

The hydraulic control system uses a power cylinder that is adjusted by the position of the operating lever, which in turn is controlled by the power cylinder of the hydraulic control gear, to open and close the control valves.

What is the function of a mechanical governor in a steam turbine control system?

A mechanical governor senses turbine speed and adjusts the control valve through a hydraulic servo mechanism to maintain a constant speed by modulating the steam flow into the turbine.

How does the centrifugal governor respond to changes in turbine speed?

The centrifugal governor responds to changes in turbine speed by moving the centrifugal weights inwards or outwards, which in turn lowers or lifts the pilot valve, allowing more or less high-pressure oil to enter the power cylinder and adjust the steam control valve.

What is the purpose of the speeder gear in adjusting the governor set point?

The speeder gear allows for manual or remote adjustment of the governor set point, which influences the speed of the turbine when not synchronized to the power system or the steam flow when synchronized, maintaining constant generator output.

What is the significance of the double relay type of pilot valve in large turbines?

In large turbines with multiple control valves, a double relay type of pilot valve is used to manage the significant amount of power required from the power cylinder, ensuring efficient and precise control of the control valves.