KS3-Steam Turbine Over Speed Trip Systems

8/14/2019 KS3-Steam Turbine Over Speed Trip Systems

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Prepared By: Arul Kumaran Kothandapani

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Numerous failures and near failures of rotating equipment throughout history can be attributed to themalfunctions of over-speed protective devices. This can be due to lack of preventative maintenance oroperators not having a clear understanding of the devices.

There is a need for everyone; associated with rotating equipment to have a working knowledge of the over-speed protective device; we know and have today.

In this knowledge sharing; let me share, the over-speed trip devices and their operation. Though its a smallexperience & a brief knowledge, I do believe it may be small light in a major path.

INTRODUCTION

In addition to operating speed governors, Steam Turbines are fitted with a shutdown system. Without proper

control and adequate over-speed protection, catastrophic machinery failures can and do occur. The principalproblems lie in the trip throttle valves; however, the entire system must be considered before any greatimprovement can be achieved.

HOW THE SYSTEMS WORK

The governor and over-speed systems vary from machine to machine and may be mechanical, hydraulic,electrical, and pneumatic or combinations. Governor control systems consist of three basic elements. Theseelements are sensing, transmitting and correcting. Sensing elements may include fly ball weights, electricgenerator, and positive displacement pumps. Transmitting elements may be mechanical linkage, hydraulic or

pneumatic pressure, electrical signals or, as is most common, a combination.

Sometimes an amplifying device such as a pilot, converter, or servomotor is necessary to boost the signal to apoint where it can do useful work. The correcting element of the governor system is the valve or valves thatcontrol the flow of steam to the turbine. The valve for general purpose turbines is usually a single, doubleseated design, characterized by relatively high flows with low lift and low unbalance forces.

The desirable characteristics of a governor system are:

Respond promptly to a small change in speed. Adjust the throttle valve with a minimum of overshoot. Have sufficient power to overcome friction losses and unbalance forces in the throttle valve.

Permit very little speed fluctuation under constant load and steam conditions.

There are several basic types of governors utilizing the above principles:

Mechanical shaft - The familiar fly weight type. A hand adjustment permits speed regulation at themachine.

Direct acting orifice - This consists of a shaft driven positive displacement type oil pump whichdelivers pressure to a spring diaphragm connected to the governor valve stem. Since the delivered oilpressure is directly proportional to shaft speed, control is accomplished. Hand or automatic speedregulation is possible.


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fxt fxt fxt fxt gu|x bx gu|x bx gu|x bx gu|x bx@ @@ @fxxw g| fxfxxw g| fxfxxw g| fxfxxw g| fx Oil relay - Built to utilize lube oil pressure or a separate governor oil pressure, a double acting oil relay

piston permits more precise control of the governor valve. It is integral to the turbine and usually designed by the turbine manufacturer.

Precision oil relay - A separate shaft-driven oil relay offers more precise control. Utilizing its own oilsystem, this type governor is not made by the turbine manufacturer. If more governor valve operatingforce is needed, a second double acting servo-motor may be utilized.

The electronic governor usually provides more precise and reliable speed control. The speedmeasurement signal can be generated in two ways. One method is by utilizing a magnetic pickup inproximity to a toothed wheel/gear mounted on the turbine shaft. Another method is to utilize a shaft-mounted permanent magnet generator where the poles rotate and produce an electronic pulsemeasured by a microprocessor. In the first case, only two pairs of wires connect to the unit. Onesupplies 48 VDC that is the required operating voltage; the other connects to a magnetic pickup on

the turbine shaft. In the second case, no external power is required, as the unit is powered from theturbine shaft rotation. In the first case, output air (normally 3-15 lbs.) goes to the diaphragm of astandard control valve in the inlet steam line to the turbine. In the second case, hydraulic pressuredrives a pre-piloted servo-motor that operates the governor valve.

Three internal adjustments are provided to set the operating speed and the gain and reset response of the unit.Electronically, the device is straightforward, consisting of a frequency to voltage converter providing the speedmeasurement that is compared to the internal speed set control. The difference is applied to the controllersection where it will move the steam valve to hold the speed as desired.

OVER-SPEED TRIP SYSTEMSIn addition to a speed control system, steam and gas turbines are fitted with a shutdown system to preventdamage to the machine. In the event the speed governor fails to control the speed, the over-speed trip actuatesto shut down the machine. When shaft speed exceeds a desired safe level, generally 10% over-speed, a latchingdevice or oil dump mechanism is actuated to close a special emergency stop valve. This system is totally independent of the governor

There are two primary types of trip actuation systems; The Mechanical type and The Electronic type

Mechanical system is completely separate from the speed governing systems. A trip pin or plunger is mountedin the turbine shaft with its center of gravity slightly off center. In the event the speed regulating governor failsto control the speed, the unbalanced plunger overcomes a spring force at a preset trip speed. As it movesoutward, it strikes the trip-lever, causing release of a spring dump valve that releases the trip circuit oilpressure. This unbalances a piston-spring combination and causes the trip and throttle valve to slam shut by the force of a spring and the steam pressure above the valve disk. A few high-speed machines use a weighteddisk and a dished washer to accomplish the tripping action. The remainder of the action is identical.

In the Electronic trip , speed is sensed similar to the system described in the governor section. When over-

speed reaches the set point, an action is initiated to shut the emergency stop valve. This action is usually


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fxt fxt fxt fxt gu|x bx gu|x bx gu|x bx gu|x bx@ @@ @fxxw g| fxfxxw g| fxfxxw g| fxfxxw g| fx through an electric solenoid or mechanical valve that dumps the hydraulic oil on a trip throttle valve (largeturbines) or releases a mechanical link to the emergency stop valve (small turbines).

In addition to over-speed, a solenoid valve can be made to shut down the turbine in response to low oilpressure, remote push buttons, or abnormal process conditions.

BASIC TRIP-THROTTLE VALVE DESIGNS

The design concept of the standard trip throttle valve is basically that of a globe valve with a stem nut that ismounted in a frame or bracket that is free to move. There are four design variations:

Two concerning the direction of the closing action, and Two involving the method of holding the movable stem nut in its operating position.

Direction of Closing Action:

The basic designs of the trip throttle valve with respect to direction of travel can be placed in two categories:

(a) Those where the valve plug is pushed onto the seat by the closing force, and(b) Those where the valve plug is pulled onto a seat by the closing force.

Because of the dual functions required of the valve - the tripping action and the throttling action - the stemmust be in two pieces in both designs. The stem of the steam shutoff part of the valve does not rotate; it only slides to fulfill the tripping action needed. The actuator assembly stem has rotary motion so that it can be

positioned within the spring-loaded, hydraulically positioned stem nut to permit throttling. Therefore, theremust be a change of direction and rotation within the split coupling. A hardened steel button, commonly called a thrust bearing, separates the ends of the two stems. Maintenance of alignment between the two stemsis difficult.

Disk Is PUSHED onto Seat Design:

In the larger valve sizes, the closing force on the valve stems and split coupling is not adequately designed toaccommodate the impact load generated by this high closing force and any misalignment. This closing forcemust function in less than one-half second upon turbine over-speed, loss of oil pressure, etc. Frequently,

damage occurs to stems or the split coupling with the plug pushed onto a seat design.

Disk Is PULLED onto Seat Design:

The design that "pulls the plug onto the seat without a lower guide is the preferable design within thestandard designs since the two stems and the split coupling operate in tension. This seems to limit themechanical damage to the valve during the closing action.

Methods of Holding Stem Nut:

Latch Type Stem Nut Holder Design - In this design, the bracket is spring loaded to push it in one direction,and has a knife edge latch mechanism to hold it and the stem nut in the proper position. When the valve iscalled upon to act as a trip throttle valve, the stem is latched in place and operates in a conventional manner,


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fxt fxt fxt fxt gu|x bx gu|x bx gu|x bx gu|x bx@ @@ @fxxw g| fxfxxw g| fxfxxw g| fxfxxw g| fx permitting rising and lowering of the plug. When the valve is called upon to act as a trip valve, the stem nutand bracket are released from their operating position on the knife edges by a small hydraulic piston and thespring pushes the stem nut downward so as to close the valve. "Hang up of the hydraulic release piston isdifficult to predict or prevent and is a major problem.

Piston-type Stem Nut Holder Design

Another type of trip throttle valve dispenses with the knife edge latch device and substitutes a larger oilcylinder. This is a globe type valve of inverted construction with the operating mechanism below the disk anda semi-balanced disk arrangement. In this valve, the force for closing the valve is provided by a main springabove the oil piston and the steam pressure above the disk

After the valve has been tripped shut, turning the hand wheel clockwise resets it. The rotation of the screw spindle will raise the main piston and compress the spring. The hand wheel will be turned until the piston

comes to rest against the cover and stops in an upward direction.

This valve has oil admitted through an oil inlet connection and orifice to the main oil cylinder with a relay valve. When the oil supply pressure is less than that required resetting the valve (generally about 50% of tripheader pressure), the relay valve is unseated and the chamber below the main piston is opened to drain.

When the oil pressure is increased to the reset pressure, the oil pressure on the relay piston overcomes theforce of the relay spring, thus seating the relay valve and closing the passage from below the piston to thedrain, permitting pressure to build up in the main cylinder.

To open the valve, the hand wheel is turned counterclockwise. The oil pressure will hold the main pistonagainst the cover and the rotation of the screw spindle will lift the pilot valve off the seat. After the pilot valveis moved its full stroke, it contacts the disk flange and further movement unseats the disk. The valve shouldbe backed off about two turns from the wide-open position.

When the oil supply pressure drops below the trip pressure (45 to 50% of normal pressure), the relay springunseats the relay valve below the main piston to drain in the passage to the area above the piston. The springand steam pressure closing forces will then trip the valve shut.

Some valves are equipped with an "exerciser to check freedom of movement of parts. The exerciser is designedto limit the travel in the closing direction to a specified distance, normally less than 1/4 of the valve travel.

Operating Problems

The original designs of all trip throttle valves were based on the premise that the valve would be exercisedthrough its full travel on a relatively frequent basis (2-3 month intervals). Most plants do not operate in thismode. Serious doubts exist if any of these present trip throttle device designs can remain on the line forextended periods and be free to operate in an emergency. Most of the failure-to-trip conditions can beattributed to five basic problems.

Steam deposits on the valve stem (or stems). Lubrication deposits (i.e., soaps, dirt, detergents, etc.) in the top works of the valve exposed to the

elements.


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fxt fxt fxt fxt gu|x bx gu|x bx gu|x bx gu|x bx@ @@ @fxxw g| fxfxxw g| fxfxxw g| fxfxxw g| fx Mechanical failures of the valve resulting from bent stems, either in the valve proper or the upper works, damaged split couplings, etc., all within about a 6" area near the center of the valve mechanism.

Galling of the piston in the hydraulic latch cylinder. Jamming of the screw spindle in the larger cylinder-type valve design due to forcing by operations

personnel.

Steam deposits present a problem either in the design that pulls the valve plug on the seat or the pushing force valve design. The latter quite often has an upper and lower guide bushing with tight fitting clearances. Bothdesigns are then subject to movement retardation due to collection of steam deposits on the stem, which mustenter into a tight fitting hardened bushing. In addition, the guide sleeve for both designs' steam valve tends to

warp and offers a restraint to plug movement.

Because of the extensive sliding and pivoting actions needed to release the stem nut for the tripping action,lubrication of the valve upper works is necessary. This tends to either attract dirt and grit or retain moisture

near tight fitting components. Exposure to high steam temperature also "cooks" out the lubricant, leaving"soap" or base material. All of these problems can cause binding of the linkages.

Experiences with various turbine installations indicate the following are good design practices.

The "pull on" plug design is better than the "push on" type. A built-in "exerciser" is preferable. This feature is available only on the piston style valve. A significant

percentage of valve hang-ups take place in the last one third of the plug travel so that any exerciserdevice might not move the plug far enough to clear the area of obstruction. However, even limitedmovement is better than ignoring the valve for extended periods. Manual exercising also has thislimitation.

Most of these valves have less than 20 to 24 trips available before the valve must be disassembled forrepairs. Often, all of these "good trips" are expended in setting the over-speed trip and other testingbefore the unit is placed in service. Trip repeat tolerance limits should not be too tight.

ELECTRONIC TRIP SYSTEM

Electronic trip systems are highly desirable in higher speed units. The spring-loaded plunger becomes unstableat higher trip speeds and requires hydraulic relay valves to complete the tripping action.

The mechanical system requires extra shaft overhangs (detrimental to vibration on high-speed machines) anduses hydraulic fluid in long piping runs. An electronic system measures pulses from a toothed wheel on theturbine shaft and puts those pulses through a frequency to voltage converter. That output goes to acomparator that is switched at the set point and is coupled to a power amplifier, which trips an electric relay.The relay dumps a solenoid valve, which dumps trip header pressure. From here, the system is the same as themechanical system.

The advantages of the electronic system are that(a) it requires only short shaft overhangs,

(b) it is fast, accurate, and easily adjustable,(c) it allows great flexibility of system arrangement,


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fxt fxt fxt fxt gu|x bx gu|x bx gu|x bx gu|x bx@ @@ @fxxw g| fxfxxw g| fxfxxw g| fxfxxw g| fx (d) it can be easily tested without running the machine, and(e) it gives a high degree of repeatability.

The disadvantages of the electronic system are that(a) it requires an additional reliable power source and(b) it requires explosion-proof classification in many applications.

TEST RUNNING OF THE TURBINE

The ease of operation of the electronic governor systems makes it very convenient for the operator to use andto bring the turbine up to speed. Under normal operating conditions, this poses no problems, but when slow rolling, test running or over-speed testing the turbine while uncoupled from the load, control should be by theTRIP THROTTLE VALVE ONLY.

The slightest movement of the governor valve in an uncoupled situation can result in tremendous changes of speed. A stuck governor or linkage can be very dangerous. If it is necessary to prove out the governor system,limit the maximum speed with the hand wheel of the trip throttle valve and control the speed downward withthe governor system.

Lube Oil Analysis

Most analysis carried is convinced that oil analysis is a vital predictive and preventive maintenance tool andexpects to see it in use. Periodic turbine oil analysis may be performed monthly or quarterly. Component wear

can be verified by the presence of certain contaminants in the oil, such as wear metals like copper, chrome,aluminum, and iron, nickel, lead and tin.

Most bearing or gear failures occur after their condition has slowly deteriorated over the course of months oreven years. Quarterly sampling can provide a more subtle indication of oil or component deterioration, or theslow beginning of oil contamination. Long-term monitoring of oil condition (over several months or quarters)can reveal improper repair or maintenance practices, such as the failure to conduct effective system flushesafter repairs, or the improper handling of lubricants, which can introduce dirt or even water to the oil.

High Energy Piping A periodic piping inspection that incorporates the routine inspection (usually during major overhauls) andcharacterization of anomalies in the piping welds is necessary for the piping to and from all the turbine valvesand other components. Responsibility for inspection of this piping lies with the turbinemaintenance/inspection group or the boiler group.

Vibration Monitoring and Phase Angle Checks

State-of-the-art turbine bearing vibration monitoring systems should be installed and operating properly.

Vibration monitoring is a particularly important condition monitoring tool, considering the extendedoverhaul intervals now in vogue (see sidebar), and its use is expected by insurers to ameliorate their risk


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fxt fxt fxt fxt gu|x bx gu|x bx gu|x bx gu|x bx@ @@ @fxxw g| fxfxxw g| fxfxxw g| fxfxxw g| fx exposure. Phase angle monitoring of the bearing vibrations is also an important component of this conditionmonitoring and should be incorporated into a good vibration monitoring program.

Bearing vibrations can be monitored using one of several methods. Displacement probes measure shaftmovement directly. Some models contact the shaft directly, using shaft riders; others are non-contacting types,called proximity probes. Conversely, velocity pick-ups do not measure shaft displacement directly, but quantify the energy transferred from the shaft to the bearing housing. To measure absolute shaft vibration, a proximity probe and a velocity pick-up are generally installed together at the bearing housing. This arrangement providesboth absolute shaft vibration levels as well as vibrations relative to the bearing measurement. Displacementprobes are usually used on turbines and generators that have a high rotor-to-casing weight ratio, or on turbinegenerators greater than 100 MW. Rotating equipment that has a high casing-to-rotor weight ratio can use

velocity pick-ups with success.

Over-speed Trip Testing

To guard against catastrophic failure from an uncontrolled over-speed by a steam turbine and its drivenequipment, protection is provided in the turbine trip system to close the steam valves.

Conducting the annual over-speed trip test on steam turbines is, and will continue to be, a contentious issue with insurance carriers. With overhaul intervals and the time between routine boiler outages increasing,scheduling the steam turbines over-speed test may be extended or overlooked. Specific concerns with themechanical integrity of the turbine or generator field may also engender reluctance to conduct the test.

Generally, over-speed prevention techniques have centered on the over-speed trip mechanism. However, the

over-speed trip checks should be viewed as a system verification that comprises more than the mechanical orelectronic over-speed trip device. Many uncontrolled over-speed events are the result of valves failing to close,even when the over-speed trip device operates. Further, nearly all uncontrolled over-speed failures arecatastrophic, resulting in blade failures, shaft breakage and retaining ring bursts.

Over-speed protection should be a combination of the following: Proper functioning of mechanical or electronic over-speed trip mechanisms and system Positive closing of the main steam and control valves Positive closing of the reheat inlet valves Proper functioning of the extraction system non-return valves Proper functioning of the reverse power trip on the generator.

Mechanical/Electronic Trip Mechanism

The mechanical or electronic trip mechanism is the last line of defense for protecting the steam turbine anddriven object. To reach the trip point for this device, all other means of controlling the energy input into theturbine have already operated or not functioned. If the valves and devices work properly, the likelihood of theturbine going to severe over-speed is much less. If the valves do not fully shut, and the other devices do not

work properly, even though the trip mechanism actuates, the turbine may still over-speed because the steamsource is still present and uncontained.


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fxt fxt fxt fxt gu|x bx gu|x bx gu|x bx gu|x bx@ @@ @fxxw g| fxfxxw g| fxfxxw g| fxfxxw g| fx Most actual over-speed of the turbine are tested annually. In some cases, its carried after 18 months betweentests to accommodate extended outage schedules. The simulated electronic trip devices can be tested morefrequently, as often as monthly, because they have no direct effect on the steam turbines operation.

Now-a-days annual mechanical tests are restricted because they can place additional stresses on steam turbineand/or generator components, such as last-row turbine blades or generator end turns. Most carriers believethat the proper operation of the over-speed devices revealed by testing is more important than whateverminimal stress the turbine components may experience from the test.

When the mechanical device is tested, all automatic turbine steam valve operation to the turbine should be verified by visual inspection. All requisite alarms and indicators should also be observed for proper operation.Some original equipment manufacturers (OEMs) have relaxed their requirements for testing over-speed tripdevices.

Main Steam and Control Valves

Unfortunately, the OEMs provide differing arrangements for admitting steam to the turbine and the ability totest the valves is sometimes compromised. The desired testing method is to stroke a valve from fully open toclosed, thus checking the valves capability to operate through its entire range. Build-up on the valve stem,excessive stem run-out, deposits and deteriorated components can prevent the valve from operating properly or not at all.

These valves should be exercised on a weekly basis, at a minimum. It is common, however, for these valves tobe tested daily to deter a valve from sticking in a fixed position and to verify each valves ability to fully shut.

KS3-Steam Turbine Over Speed Trip Systems

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