lesson 14: steam turbine protection

Steam Turbine Protection Mechanisms

Main Steam Stop Valve Functionality

• The main steam stop valve is crucial for protecting the steam turbine; it must be closed during abnormal operating conditions.

• In reheat turbines, both the main steam stop valve and the reheat stop valve are tripped simultaneously to ensure safety.

Trip Relay Operations

• The closure of the stop valves is actuated by a pilot or trip relay that drains oil pressure from the hydraulic system.

• Multiple trip relays may exist in the hydraulic system, each triggered by different protection devices to initiate protective actions.

Overspeed Conditions

• Overspeed is identified as a critical hazard that can occur during load rejection or accidental generator breaker trips.

• If a generator breaker trips while loaded, steam flow continues, potentially causing rapid speed increases if control valves fail to close.

Consequences of Overspeed

• Failure of governor mechanisms or control valves can lead to turbine speeds rising dangerously high, risking mechanical failure.

• Historical incidents have shown turbine blades can break apart and pose risks to personnel and equipment due to overspeed events.

Overspeed Protection Devices

• To mitigate overspeed risks, one or two separate protection devices are installed, set at 10% and 12% overspeed thresholds.

• A traditional mechanical device uses centrifugal force on a bolt within the shaft to trigger a trip relay when overspeed occurs.

Monitoring Back Pressure in Turbines

Importance of Back Pressure Monitoring

• Normal back pressure in condensing turbines should be between one to three inches of mercury; excessive back pressure can lead to severe operational issues.

Risks Associated with High Back Pressure

• Increased back pressure raises steam density at exhaust, leading to overheating and potential damage to low-pressure blades if not monitored properly.

Alarm Systems for Back Pressure

• A monitoring system triggers alarms at five inches of mercury and operates trip relays at ten inches of mercury for operator intervention before critical failures occur.

Preventing Exhaust Hood Damage

Risks During Startup Procedures

• Starting up without established cooling water flow can cause steam buildup in condensers, risking exhaust hood rupture due to pressure accumulation.

Relief Diaphragm Functionality

• A relief diaphragm fitted on the exhaust hood acts independently as a safety measure against overpressure by releasing excess pressure when necessary.

Thrust Bearing Failure Monitoring

Thrust Bearing Position Measurement

Turbine Safety Mechanisms and Operations

Thrust Bearing Failure and Trip Relay Activation

• The thrust collar's movement is critical; failure of the thrust bearing can lead to contact between rotating and stationary blades, necessitating immediate turbine shutdown via trip relay activation.

Loss of Lube Oil Pressure

• A significant condition for turbine stop valve tripping is the loss of lube oil pressure, which can cause rapid overheating and potential catastrophic failures in bearings.

Hydraulic Control Systems

• While hydraulic systems are designed to operate safely without a trip relay, they will close both the stop valve and control valve if hydraulic oil pressure fails due to spring pressure.

Manual and Remote Trip Options

• Operators have manual trip options located on the turbine pedestal, with remote solenoid trips available from the control room for emergencies like generator or boiler faults.

Major Protection Devices in Steam Turbines

• Key protective devices include main/reheat stop valves, non-return valves on extraction lines, over-speed trips, low vacuum trips, loss of lube oil pressure trips, manual trip levers, remote solenoid trips, and inter-tripping mechanisms for generator/boiler protection. Regular testing of these devices is essential for operational safety.

Testing Procedures for Turbine Safety

Importance of Regular Testing

• In some plants where turbines remain loaded for extended periods, simulated tests may be conducted to ensure that protective measures function correctly without actual tripping.

Exercising Stop Valves

• Daily exercises of turbine stop valves are crucial to confirm their functionality; failure to close when commanded could render all protective devices ineffective.

Familiarity with Protective Equipment

• Operators must be well-acquainted with testing procedures and functions of all protective devices specific to their turbines to ensure effective operation during emergencies.

D Loading Device Functionality

Role in Preventing Damage

• The D loading device protects against falling steam pressures from the boiler by monitoring steam conditions; it closes control valves if pressure drops below 80% nominal to prevent water carryover into the turbine.

Operational Context

• This device is bypassed during startup at low steam pressures but becomes active once normal operating conditions are established.

Control Room Operations

Operator Responsibilities

• Most turbine operations are managed remotely from a central control room equipped with various controls and indicators. Operators monitor steam conditions such as pressure and temperature closely through dedicated instruments.

Alarm Conditions Monitoring

• An annunciator panel provides real-time alerts about alarm conditions within the plant environment.

Turbine Operation Insights

Understanding Turbine Conditions

• The operation of turbines is influenced by various factors, including water conditions, lube oil temperatures, vibration levels, and the mechanical condition of the turbine as indicated by supervisory instruments.

• This module covers only the most common features of turbine operation; operators are encouraged to familiarize themselves with their specific machines.

• Each turbine may exhibit unique characteristics or idiosyncrasies that experienced operators will come to recognize over time.

• Mastery of these operational nuances is essential for effective turbine management and maintenance.