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.

Outlines

00:00

🔧 Steam Turbine Operation and Control

This paragraph introduces the basics of steam turbine operation and control, focusing on how turbine speed and output are adjusted. The principle behind turbine control is straightforward: to increase power output, more steam is passed through the turbine, and to decrease it, less steam is admitted. The control valves, also known as admission valves, regulate steam flow into the turbine. In simple arrangements, a single control valve is used, but in more complex systems, multiple valves are employed, opening sequentially to manage steam flow efficiently. The hydraulic control system adjusts these valves based on the turbine's load. The paragraph also explains the use of a mechanical governor to sense turbine speed, which, through a series of mechanical and hydraulic actions, modulates the control valves to maintain the desired speed. The governor's set point can be adjusted manually or remotely using a speeder gear, which influences steam flow and, consequently, turbine speed and power output.

05:00

🛠 Governor and Control System Adjustments

This paragraph delves deeper into the governor mechanism, which is crucial for controlling the steam turbine. It describes a mechanical centrifugal governor that is connected directly to the turbine shaft and adjusts the control valve through a hydraulic servo mechanism. The governor's set point, which determines the turbine's speed, can be manually adjusted or controlled remotely using a speeder gear. When the turbine is not synchronized with the power system, adjustments to the governor's set point will change the turbine's speed. However, when synchronized, adjustments to the speeder gear primarily affect steam flow rather than generator speed, maintaining a constant output. The paragraph also hints at the discussion of different types of governors and their impact on the electric power system, which will be elaborated in the next module.

Keywords

💡Turbine Construction

Turbine construction refers to the physical assembly and design of a turbine, which is a rotary mechanical device that converts the energy from a fluid flow, such as steam, into mechanical work. In the context of the video, this term sets the stage for discussing the operation and control of steam turbines, emphasizing the importance of the turbine's design in achieving efficient energy conversion.

💡Turbine Speed

Turbine speed is a critical parameter that determines the power output of a turbine. The video explains that adjusting the turbine speed is essential for controlling the power output. For instance, to increase power, more steam must be passed through the turbine, which is achieved by adjusting the control valves.

💡Control Valves

Control valves, also known as admission valves, are integral to the operation of a steam turbine. They regulate the steam flow into the turbine, thereby controlling the turbine's speed and power output. The script describes how these valves are adjusted to manage the steam flow, with examples including single and multiple valve arrangements.

💡Throttling Losses

Throttling losses occur when steam is throttled or restricted, leading to a decrease in efficiency due to the reduction in steam pressure and temperature. The video mentions that using multiple control valves instead of a single large valve can reduce these losses, thus improving the turbine's overall efficiency.

💡Hydraulic Control System

A hydraulic control system is used to manage the operation of the turbine's control valves. It uses hydraulic pressure to adjust the position of the valves, which in turn controls the steam flow. The video describes how changes in turbine speed are sensed by a mechanical governor, which then adjusts the control valves through the hydraulic system.

💡Mechanical Governor

A mechanical governor is a device that automatically controls the speed of a rotating system by adjusting the steam flow. In the video, it is shown that a change in turbine speed causes the governor's centrifugal weights to move, which in turn adjusts the control valve and steam flow to maintain a constant speed.

💡Centrifugal Weights

Centrifugal weights are components of a mechanical governor that respond to changes in rotational speed. As the speed increases, these weights move outward due to centrifugal force, which is used to adjust the control valve and maintain a constant turbine speed, as described in the video.

💡Power Cylinder

The power cylinder is part of the hydraulic control system and is responsible for the physical movement of the control valves. It converts the hydraulic pressure into mechanical force that opens or closes the valves. The video explains how the position of the power piston within the power cylinder is adjusted by the governor to control the steam flow.

💡Steam Chest

A steam chest is a pressure vessel that collects and distributes steam to the turbine. The video describes arrangements where multiple control valves are located in each steam chest, which is connected to the turbine shell. This setup allows for precise control of steam flow to different sections of the turbine.

💡Speeder Gear

The speeder gear is a small motor drive used to adjust the set point of the governor remotely. The video explains that adjusting the speeder gear changes the governor's set point, which in turn affects the steam flow and turbine speed. However, when the turbine is synchronized with the power system, the speeder gear primarily adjusts the steam flow without significantly altering the generator's output.

Highlights

The principle of turbine control is to adjust steam flow to increase or decrease power output.

Turbine steam flow is controlled by adjusting admission valves or control valves.

In simple arrangements, one control valve regulates steam flow into the turbine.

Multiple control valves are used in most turbines to reduce throttling losses.

Control valves open in sequence according to the position of the cross-arm.

Large machines often have two steam chests, one above and one below the turbine shell.

Each steam chest in large machines has multiple control valves.

Stop valves are fitted at the entrance to each steam chest and remain open during normal operation.

The opening of control valves is determined by the position of the operating lever, adjusted by the hydraulic control gear.

A mechanical governor senses turbine speed and adjusts the control valve through a hydraulic servo mechanism.

Centrifugal weights in the governor move in response to changes in turbine speed.

The governor's set point can be adjusted by manipulating the compression on the spring.

Adjustments to the governor setpoint can be made manually or via a remote speeder gear.

When the turbine is on load, adjustments to the speeder gear affect steam flow but not generator speed.

The governor's effect on the electric power system will be discussed in the next module.

Different types of governors are used on various machines for turbine control.

Transcripts

[Music]

in the previous module we looked at the

main features of turbine construction

and the essential support systems now

it's time for us to turn our attention

to steam turbine operation and control

and we'll begin by looking at the means

of adjusting the turbine speed or

turbine output the principle of turbine

control is very simple if we need to

increase the power output of the turbine

we have to pass more steam through the

turbine conversely in order to decrease

power output it is necessary to decrease

the amount of steam admitted to the

turbine turbine steam flow is controlled

by adjusting the turbine admission

valves or control valves as they are

often called in the most simple

arrangement shown here we have one

control valve which depending on its

setting allows more or less steam to

flow into the turbine from the steam

chest when the turbine stop valve is

open the steam chest is charged with

steam directly from the boiler in most

turbines multiple control valves are

used as shown here in this particular

arrangement the steam chest is located

above the high-pressure end of the

turbine shell we can see here eight

valves and these are opened in sequence

according to the position of the

cross-arm which is itself adjusted by

the hydraulic control system these

control valves are set in such a manner

that only one valve at a time is

actually throttling steam while the

others are either fully open or closed

depending on the actual load the result

is that the throttling losses are

smaller than in the case where only one

large control valve is used in large

machines say greater than 100 megawatts

it is more common to have to steam

chests one above and one below the shell

or located one on either side of the

turbine in this case multiple control

valves are located in each steam chest

with steam lines connect

to the turbine shell at the

high-pressure end in most arrangements

of this type a stop valve is fitted at

the entrance to each steam chest during

normal operation the stop valve remains

in the wide open position while the

control valves are modulated to adjust

steam flow the actual opening of the

control valves is determined by the

position of the operating lever and this

in turn is adjusted by the power

cylinder of the hydraulic control gear

this schematic shows us a simplified

version of a hydraulic control scheme in

this particular arrangement a mechanical

governor is used to sense turbine speed

other types of Governors are also used

as we'll see in a moment in this

configuration a change of turbine speed

causes the centrifugal weights to move

for example a decrease in speed causes

the centrifugal weights to move inwards

and lower the pilot valve this in turn

allows more high pressure oil to enter

the power cylinder and raise the piston

against the compression spring this

movement of the power piston opens the

steam control valve and allows more

steam to enter the turbine which causes

its speed to increase also known as the

power piston Rises the reset lever lifts

the pilot valve back into the neutral

position again on large turbines with

multiple control valves a considerable

amount of power is required from the

power cylinder in this case its usual to

employ a double relay type of pilot

valve in this arrangement we have high

pressure oil above and below the piston

now when the position of the governor

spindle changes say for an increase in

speed the pilot valve lifts and allows

high pressure oil to flow into the space

above the power piston at the same time

the pilot valve exposes the space below

the piston to the oil drain and returns

the oil to the tank the high pressure

above the piston pushes it downward and

consequently moves the control valve in

the close direction at the same time it

also moved the reset lever to bring the

pilot valve back to neutral position so

prevented further movement of the

control valve now let's take a closer

look at the governor itself this

mechanical type centrifugal governor is

driven directly from the turbine shaft

through a gear drive as speed increases

the weights fly outward due to

centrifugal force and lift the sleeve

against the compression spring the

sleeve itself is connected to the pilot

relay which in turn adjusts the control

valve through the hydraulic servo

mechanism we can adjust the set point of

the governor by adjusting the

compression on the spring the adjusting

nut moves up or down on the cording to

rotation of the screw thread this action

can be performed manually at the turbine

or as is more usual well from a remote

position by operation of a small motor

drive known as the speeder gear this

name derives from the fact that when the

turbine generator is not synchronized to

the power system any adjustment of the

governor setpoint

will indeed alter the speed of the

turbine however when the turbine is on

load and the generator is synchronized

to other machines any adjustment of the

speeder year produces an imperceptible

change in generator speed instead this

action causes a change in steam flow

through the turbine with the constant in

generator output but we'll be talking

more about characteristics of the

governor and its effect on the electric

power system in the next module

another type of governor used on many

machines is