Steam Turbine Low Pressure Inlet Valve

Introduction

In thermal power plants, the steam that is produced by boilers is sent via high pressure lines to the steam turbine(s) to generate electricity. There are three main ways of admitting steam into the turbine: via nozzle, throttle, or bypass governing.

In high-pressure (HP) turbines, steam enters the turbine admission sections via a set of control valves. These same valves are used to completely isolate the steam in emergency situations, or when the turbine is offline. With HP turbine designs, steam is admitted via nozzle groups, with each nozzle group being fed from one associated valve. In other turbine designs, especially those operating at lower pressures, cam or bar operated multi throttle valves are used; with this setup, a single valve feeds to multiple nozzles.

Control Valve Arrangement In A Nozzle-Governed Steam Turbine

 

How Cam-Operated Multi Throttle Valves Work

A typical cam-operated multi valve steam admission mechanism is illustrated below.

Cam Lift Valve Gear

In this arrangement, an operating cylinder (hydraulically actuated) is extended or retracted, which causes the torque bar to rotate. As the torque bar rotates, two lift rods are moved up or down in a linear direction, which causes a lift beam to move proportionally also. Multiple control valves are connected to the lift beam, with each valve stem passing through the beam, but not physically connecting to it. Sequential opening and closing of the valves occurs because each valve has a different stem length; poppet valves are typically used for this type of application.

 

A break-away linkage connects the torque bar to the high-pressure control valve linkage and thus the high-pressure control valve. Due to the break-away linkage’s design, the HP control valve will not begin to open until all valves in the low-pressure (LP) valve chest are in the fully open position. When all low-pressure valves are fully open, steam is admitted to the high-pressure turbine, but it is not physically possible for this to occur until all LP valves are fully open (built-in interlock design prevents HP steam being delivered without LP steam).

Within the steam chest, any number of control valves can be used, but five is the standard configuration (as illustrated in the design above). This type of steam control system requires accurate timing to ensure the turbine operates at high efficiency.

An inlet stop valve is installed prior to the steam chest; the stop valve admits steam during turbine start-up and acts as an emergency shut-off valve for the turbine.

The design discussed in this article uses a cam to operate the valves, but a bar lift method is also possible.  The bar method uses linear motion rather than rotary motion, to actuate the valves.

As cams can be externally adjusted, there is no need for the whole valve chest to be stripped when maintenance is required; the opposite is true for the bar lift valve design.

 

3D Model Annotations

Low Pressure (LP) Control Valve

Low pressure (LP) steam discharged to the low-pressure turbine passes through a stop valve and control valve. Steam is discharged to the turbine via a series of valves that can be opened incrementally. A mechanical linkage connects the LP control valve with the high pressure (HP) control valve; thus it is not possible to pass steam through the HP control valve without the LP control valve being sufficiently open.

Lift Beam

The lift beam is a metal bar with holes perforating through it. Each valve stem is passed through the lift beam and connected to its associated spherical valve disc.

Low Pressure Steam Chest

Steam from the low pressure stop valve flows to the low-pressure steam chest. The LP steam chest surrounds the poppet valves and lift beam.

Operating Cylinder

Hydraulic fluid is pumped to a servo valve, which controls fluid flow to the operating cylinder. The operating cylinder contains a piston, which moves linearly depending upon which side of the piston hydraulic fluid is discharged to. The linkage highlighted by this annotation connects to the operating cylinder.

Torque Bar

The torque bar mechanically connects the low-pressure linkages, break-away linkage, and operating cylinder linkage. Force applied to the bar by the operating cylinder causes it to rotate, which in turn causes linear motion to be transferred to the LP and HP control valve valves.

High Pressure Control Valve

The high-pressure control valve is connected to the torque bar via a break-away linkage. Mechanical actuation of the HP control valve only occurs once the lift bar has rotated enough to press against the body of the break-away linkage. A mechanical interlock of this kind ensures that all five low pressure steam chest valves are open prior to the HP control valve opening. A spring provides the tensile force needed to return the HP control valve to the closed position when the mechanical force from the torque bar is no longer present.

Break-Away Linkage

The break-away linkage connects the torque bar to the HP control valve. A hole within the body of the break-away linkage controls when force is transferred from the torque bar to the HP control valve. As the torque bar rotates, it presses against the body of the break-away linkage, which opens or closes the HP control valve.

Low Pressure Control Valve Operating Linkage

A mechanical linkage connects the LP control valve to the torque bar.

Lift Rods

Two lift rods connect the low-pressure control valve operating linkages to the lift beam.

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