2682510-steam-turbine-over-speed-trip-systems

8/7/2019 2682510-Steam-Turbine-Over-speed-Trip-Systems

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Steam Turbine Over speed Trip Systemsby Boyd Davis

ABSTRACT:

Numerous failures and near failures of rotating equipment throughout history can be attributed to themalfunctions of overspeed protective devices. This can be due to lack of preventative maintenanceor operators not having a clear understanding of the devices.

There is a need for everyone associated with rotating equipment to have a working knowledge of theoverspeed protective device we know and have today.

In this presentation, the various overspeed trip devices and their operation will be discussed so thatwe all may have a better understanding of their purpose.

INTRODUCTION:

In addition to operating speed governors, steam turbines are fitted with a shutdown system. Withoutproper control and adequate overspeed protection, catastrophic machinery failures can and do occur.

The principal problems lie in the trip throttle valves; however, the entire system must be consideredbefore any great improvement can be achieved.

HOW THE SYSTEMS WORK:

The governor and overspeed systems vary from machine to machine and may be mechanical,hydraulic, electrical, pneumatic or combinations. Governor control systems consist of three basicelements. These elements are sensing, transmitting and correcting. Sensing elements may includefly ball weights, electric generator, and positive displacement pumps. Transmitting elements may bemechanical linkage, hydraulic or pneumatic pressure, electrical signals or, as is most common, acombination. Sometimes an amplifying device such as a pilot, converter, or servomotor is necessary

to boost the signal to a point where it can do useful work. The correcting element of the governorsystem is the valve or valves that control the flow of steam to the turbine. The valve for generalpurpose turbines is usually a single, double seated design, characterized by relatively high flows withlow lift and low unbalance forces.

The desirable characteristics of a governor system are:

1. Respond promptly to a small change in speed.

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generally 10% overspeed, a latching device or oil dump mechanism is actuated to close a specialemergency stop valve. This system is totally independent of the governor

There are two primary types of trip actuation systems, the mechanical type and electronic type.Figure No.1 shows a mechanical system that is completely separate from the speed governing

systems. A trip pin or plunger is mounted in the turbine shaft with its center of gravity slightly offcenter. In the event the speed regulating governor fails to control the speed, the unbalanced plungerovercomes a spring force at a preset trip speed. As it moves outward, it strikes the trip-lever, causingrelease of a spring dump valve that releases the trip circuit oil pressure. This unbalances a piston-spring combination and causes the trip and throttle valve to slam shut by the force of a spring and thesteam pressure above the valve disk. A few high-speed machines use a weighted disk and a dishedwasher 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. Whenoverspeed reaches the set point, an action is initiated to shut the emergency stop valve. This action isusually through an electric solenoid or mechanical valve that dumps the hydraulic oil on a trip throttle

valve (large turbines) or releases a mechanical link to the emergency stop valve (small turbines).

In addition to overspeed, a solenoid valve can be made to shut down the turbine in response to lowoil pressure, 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 nutthat is mounted in a frame or bracket that is free to move. There are four design variations: twoconcerning the direction of the closing action, and two involving the method of holding the movablestem 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 twocategories: (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 functionsrequired of the valve - the tripping action and the throttling action - the stem must be in two pieces inboth designs. The stem of the steam shutoff part of the valve does not rotate; it only slides to fulfillthe tripping action needed. The actuator assembly stem has rotary motion so that it can be positionedwithin the spring-loaded, hydraulically positioned stem nut to permit throttling. Therefore, there mustbe 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 thetwo stems is difficult.

Disk Is PUSHED onto Seat Design:

In the larger valve sizes, the closing force on the valve stems and split coupling is not adequatelydesigned to accommodate the impact load generated by this high closing force and anymisalignment. This closing force must function in less than one-half second upon turbine overspeed,

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loss of oil pressure, etc. Frequently, damage occurs to stems or the split coupling with the plugpushed 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 limitthe mechanical 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 (See Figure No.2). When the

valve is called upon to act as a trip throttle valve, the stem is latched in place and operates in a conventional

manner, permitting raising and lowering of the plug. When the valve is called upon to act as a trip valve, the

stem nut and bracket are released from their operating position on the knife edges by a small hydraulic piston

and the spring pushes the stem nut downward so as to close the valve. "Hang up

of the hydraulic release pistonis difficult 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 oil cylinder. This is a globe type valve of inverted construction with the

operating mechanism below the disk and a semi-balanced disk arrangement. In this valve, the force for closing

the valve is provided by a main spring above the oil piston and the steam pressure above the disk (See Figure

No.3).

After the valve has been tripped shut, turning the hand wheel clockwise resets it. The rotation of thescrew 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 arelay valve. When the oil supply pressure is less than that required to reset the valve (generally about50% of trip header pressure), the relay valve is unseated and the chamber below the main piston isopened to drain. When the oil pressure is increased to the reset pressure, the oil pressure on therelay piston overcomes the force of the relay spring, thus seating the relay valve and closing thepassage from below the piston to the drain, 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 mainpiston against the cover and the rotation of the screw spindle will lift the pilot valve off the seat. After

the pilot valve is moved its full stroke, it contacts the disk flange and further movement unseats thedisk. The valve should be 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 relayspring unseats the relay valve below the main piston to drain in the passage to the area above thepiston. The spring and steam pressure closing forces will then trip the valve shut.

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

Operating Problems:

The original designs of all trip throttle valves were based on the premise that the valve would beexercised through its full travel on a relatively frequent basis (2-3 month intervals). Mostpetrochemical plants do not operate in this mode. Serious doubts exist if any of these present tripthrottle device designs can remain on the line for extended periods and be free to operate in anemergency. Most of the failure-to-trip conditions can be attributed to five basic problems.

1. Steam deposits on the valve stem (or stems).

2. Lubrication deposits (i.e., soaps, dirt, detergents, etc.) in the top works of thevalve exposed to the elements.

3. 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.

4. Galling of the piston in the hydraulic latch cylinder.

5. Jamming of the screw spindle in the larger cylinder-type valve design due toforcing by operations personnel.

Steam deposits present a problem either in the design that pulls the valve plug on the seat or thepushing force valve design. The latter quite often has an upper and lower guide bushing with tightfitting clearances. Both designs are then subject to movement retardation due to collection of steamdeposits on the stem, which must enter into a tight fitting hardened bushing. In addition, the guidesleeve 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 trippingaction, lubrication of the valve upper works is necessary. This tends to either attract dirt and grit orretain moisture near tight fitting components. Exposure to high steam temperature also "cooks" outthe 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.

1. The "pull on" plug design is better than the "push on" type.

2. 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 onethird of the plug travel so that any exerciser device might not move the plug farenough to clear the area of obstruction. However, even limited movement isbetter than ignoring the valve for extended periods. Manual exercising also hasthis limitation.

3. Most of these valves have less than 20 to 24 trips available before the valve mustbe disassembled for repairs. Often, all of these "good trips" are expended insetting the overspeed trip and other testing before the unit is placed in service.

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Trip repeat tolerance limits should not be too tight.

AP1-6 17 (Centrifugal Compressors for General Refinery Services) calls for trip speeds of 115% of RATED

speed for compressor drives. This is 110% of maximum continuous speed. Tolerances are +2%. This gives trip

ranges on the order of:

3,600 3,960+70 rpm (spread 140)

6,000 6,600+120 rpm (spread 240)

9,000 9,900+180 rpm (spread 360)

The trip itself and the latching mechanism are not designed for greater accuracy than this. Two or three repeats

within these ranges are adequate. A great number of trips tend to damage the bolt and/or linkage and reduce its

reliability.

ELECTRONIC TRIP SYSTEM:

Electronic trip systems are highly desirable in higher speed units. The spring-loaded plungerbecomes unstable at higher trip speeds and requires hydraulic relay valves to complete the trippingaction.

The mechanical system requires extra shaft overhangs (detrimental to vibration on high-speedmachines) and uses hydraulic fluid in long piping runs. An electronic system measures pulses from atoothed wheel on the turbine shaft and puts those pulses through a frequency to voltage converter.That output goes to a comparator 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 the mechanical system.

The advantages of the electronic system are that (a) it requires only short shaft overhangs, (b) it isfast, accurate, and easily adjustable, (c) it allows great flexibility of system arrangement, (d) it can beeasily 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 sourceand (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 touse and to bring the turbine up to speed. Under normal operating conditions, this poses no problems,but when slow rolling, test running or overspeed testing the turbine while uncoupled from the load,control should be by the TRIP THROTTLE VALVE ONLY. The slightest movement of the governorvalve in an uncoupled situation can result in tremendous changes of speed. A stuck governor orlinkage can be very dangerous. If it is necessary to prove out the governor system, limit themaximum speed with the hand wheel of the trip throttle valve and control the speed downward withthe governor system.

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CONCLUSION:

Due to the speeds and demands that rotating machinery is subjected to today, there is a demand fora new design concept. The present mechanical hydraulic system should be evaluated for itsreliability and replaced with a hydraulic system that minimizes all of the mechanical functions as much

as possible. The entire trip system needs to be reviewed and made more reliable.

Serious consideration should be given to eliminating the dual trip and throttle valve system on largerturbines operating (500 hp and up) and installing a separate overspeed trip valve that is direct acting,unbalanced and has one stem. The balanced type throttle valve has the tendency to slow down theclosing time of the valve required to prevent the rotor from accelerating beyond the set trip speed thatcould result in mechanical damage.

Every trip valve should be equipped with an exercise so that the valve stem can be periodicallystroked to approximately one third of the closed position, and any deposit built up on the valve stemdue to solids in the steam can be removed. This travel should be controlled by some type of limiting

travel relay to prevent over-travel.

Mechanical Hydraulic Control System has proven to be a highly reliable and effective means ofcontrolling steam turbine speed and load. It will provide years of trouble free operation if it is properlyadjusted and maintained. If not maintained properly, it can be the leading cause of forced outages forthe steam turbine. The reliability and availability of the control system can be a matter of skilledroutine operations and maintenance activities. Efficient maintenance outages are often the result ofthorough job planning and a complete understanding of the necessary skills and procedures.Furthermore, availability is often a function of the plant being able to get the unit "on-line" by resolvingobstacles quickly.

Lube Oil Analysis

Most insurance carriers are convinced that oil analysis is a vital predictive and preventive

maintenance tool and expect to see it in use at its insured locations. Periodic turbine oilanalysis 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, iron, nickel, lead and tin.

Most bearing or gear failures occur after their condition has slowly deteriorated over thecourse of months or even years. Quarterly sampling can provide a more subtle indicationof oil or component deterioration, or the slow beginning of oil contamination. Long-term

monitoring of oil condition (over several months or quarters) can reveal improper repairor maintenance practices, such as the failure to conduct effective system flushes after

repairs, or the improper handling of lubricants, which can introduce dirt or even water to

the oil.

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High Energy Piping

All major insurance carriers require periodic piping inspection. A program that

incorporates the routine inspection (usually during major overhauls) and characterizationof anomalies in the piping welds is necessary for the piping to and from all the turbine

valves and other components. Responsibility for inspection of this piping lies with theturbine maintenance/inspection group or the boiler group.

Vibration Monitoring and Phase Angle Checks

State-of-the-art turbine bearing vibration monitoring systems should be installed andoperating properly. Vibration monitoring is a particularly important condition monitoring

tool, considering the extended overhaul intervals now in vogue (see sidebar), and its useis expected by insurers to ameliorate their risk exposure. Phase angle monitoring of the

bearing vibrations is also an important component of this condition monitoring and

should be incorporated into a good vibration monitoring program.

Bearing vibrations can be monitored using one of several methods. Displacement probes

measure shaft movement directly. Some models contact the shaft directly, using shaft

riders; others are non-contacting types, called proximity probes. Conversely, velocitypick-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 proximityprobe and a velocity pick-up are generally installed together at the bearing housing. This

arrangement provides both absolute shaft vibration levels as well as vibrations relative

to the bearing measurement. Displacement probes are usually used on turbines andgenerators that have a high rotor-to-casing weight ratio, or on turbine generators

greater than 100 MW. Rotating equipment that has a high casing-to-rotor weight ratiocan use velocity pick-ups with success.

Overspeed Trip Testing

To guard against catastrophic failure from an uncontrolled overspeed by a steam turbine

and its driven equipment, protection is provided in the turbine trip system to close thesteam valves.

Conducting the annual overspeed trip test on steam turbines is, and will continue to be,a contentious issue with insurance carriers. With overhaul intervals and the timebetween routine boiler outages increasing, scheduling the steam turbines overspeed

test may be extended or overlooked. Specific concerns with the mechanical integrity ofthe turbine or generator field may also engender reluctance to conduct the test.

Generally, overspeed prevention techniques have centered on the overspeed trip

mechanism. However, the overspeed trip checks should be viewed as a system

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verification that comprises more than the mechanical or electronic overspeed trip device.Many uncontrolled overspeed events are the result of valves failing to close, even when

the overspeed trip device operates. Further, nearly all uncontrolled overspeed failuresare catastrophic, resulting in blade failures, shaft breakage and retaining ring bursts.

Overspeed protection should be a combination of the following:

Proper functioning of mechanical or electronic overspeed 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 thesteam turbine and driven object. To reach the trip point for this device, all other means

of controlling the energy input into the turbine have already operated or not functioned.

If the valves and devices work properly, the likelihood of the turbine going to severeoverspeed 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 overspeed

because the steam source is still present and uncontained.

Most insurance carriers require that the mechanical device be tested annually by an

actual overspeed of the turbine. In some cases, however, insurance carriers will allow upto 18 months between tests to accommodate extended outage schedules. The simulated

electronic trip devices can be tested more frequently, as often as monthly, because theyhave no direct effect on the steam turbines operation.

Some owners have resisted annual mechanical tests because they can place additional

stresses on steam turbine and/or generator components, such as last-row turbine bladesor generator end turns. Most carriers believe that the proper operation of the overspeed

devices revealed by testing is more important than whatever minimal stress the turbine

components may experience from the test. If there is concern that a specific componentmay be damaged by the test, this can raise a flag to the insurance company that there

is higher risk of catastrophic loss. This raised flag could lead to restricted coverage of theunit or other insurance penalties.

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When the mechanical device is tested, all automatic turbine steam valve operation to theturbine should be verified by visual inspection. All requisite alarms and indicators should

also be observed for proper operation.

Click here to enlarge image

Retaining ring burst due to a steam turbine overspeed

event.

Some original equipment manufacturers (OEMs) have relaxed their requirements for

testing overspeed trip devices. In direct contrast with the insurance carriers, at least oneOEM has allowed the testing interval of the overspeed trip device to be extended to the

units major inspections. For some units, this could mean more than 10 years between

tests.

Main Steam and Control Valves

Unfortunately, the OEMs provide differing arrangements for admitting steam to theturbine and the ability to test the valves is sometimes compromised. The desired testing

method is to stroke a valve from fully open to closed, thus checking the valvescapability 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 to be tested daily to deter a valve from sticking in a fixedposition and to verify each valves ability to fully shut.

Machinist Struck and Killed by Fragments from Ruptured Steam Turbine Housing

http://www.cdc.gov/NIOSH/face/stateface/mi/01mi011.html

Conclusion

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http://www.cdc.gov/NIOSH/face/stateface/mi/01mi011.htmlhttp://www.cdc.gov/NIOSH/face/stateface/mi/01mi011.html


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Because of their age, design, and lack of maintenance, todays steam turbinegovernor control systems are operating in a manner far different than the originalmanufacturers designed them. They are also operating in an order of magnitude lessefficient in performance than that of similar sized turbines operating with the latestdigital controls. Typically these older steam turbines drift, control has a sluggish

response caused by multiple individual servo motors connected by antiquated linkagesand connectors, or does not operate in a coordinated fashion. These problems arecosting your operation and maintenance teams time and money better spent onimproving the operation rather than just keeping it running. Find a vendor that canimplement an innovative solution for modernizing your steam turbine controls and thatcan integrate this upgrade into your current or future plant systems using a scalable,standardized, proven platform. Upgrading to a state-of-the-art turbine control system willgenerate true long-term benefits for operations, maintenance, and service of your steamturbine.

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2682510-steam-turbine-over-speed-trip-systems

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