ML20211E290

Rev 0 to GE Turbine Overspeed Protection. Supporting Documentation Encl

ML20211E290
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Site:	Seabrook
Issue date:	04/09/1975
From:	GENERAL ELECTRIC CO.
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ML20211E267	List: ML20211E255 ML20211E269 ML20211E284 ML20211E290
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TOP-4-9-75, NUDOCS 8606130306
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' T O P 4/9/75 GDTERAL ELEM TURBINE OVERSPEEI PROTECTION

- Modern electrohydraulic control systems for large nuclear steem turbinas provida protection against shaf t overspeed with two essentially separate and redundant

. systems. This paper describes these basic systems and also shows how additional signals are connected to provida protection against other risks. It includes a discussion of testing features and the benefit they bring and finally a discussion of some potential common failure modes and measures taken to prevent them from happening. The appendix describes technical deemile of some of the major components.

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It is shown that the overspeed protection systems are essentially separate and redundant and offer a very high degree of protection due to the fail-safe design and testing provisions.

I. Basic Systems There are two separate basic systems protecting a nuclear turbine against overspeed: -

The normal overspeed protection system and The emergency overspeed protection system.

The normal overspeed protection system is shown on Fig. 1. It uses two speed signals (pulses) generated by magnetic pickups opposite a toothed wheel in the turbine front standard as feedback to control cpeed including slowly increasing speed above synchronous speed. It implements FP 7 361Fol

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- a I. Basic Svstems (continued) its control through proportional position signals to the control and intercept valves in the main and reheat steam lines respectively.

Under some conditions, it is desirable to move C7 & IV rapidly. More rapid action is desired in case of large. generator load rejections in order to limit the turbine shaf t overspeed to a level below the set-

, point of the emergency overspeed protection system. This permits the ~

unit to have a load rejection, yet remain running under control of the speed governor at or near synchronous speed. Thus the unit can, if desired, continue to carry station auxiliary load and also be in a position for prompt resynchonizing with the system. To keep the turbine shaft speed below this setpoint during all normal operation of the turbine

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l including voluntary and involuntary load reductions or rejections up l through ==r1== guaranteed load, the normal overspeed protection system is equipped with a power-load unbalance sensing system.

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The power-load unbalance system receives as input two signals; one is an electrical signal indicative of generator electrical power output and the other is a signal indicative of the mechanical power produced by the turbine. These two signals are continously compared. When the turbine mechanical power exceeds the generator electrical power by a fixed amount, it is an indication of imminent rapid speed rise. Detection of a rapid occurrence of such a difference by the power-load unbalance system will -initiate ismediate fast ' closing of turbine staan valves through j special fast closing input devices on the valve actuators. Any input by i

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I. Basic Systems (continued) the proportional system will be completely overridden by the fast closing devices. Once the steam valves have been closed fast the proportional positioning system will again take over control of the speed. The power-load unbalance system will reset automatically once the initiating condition has disappeared.

The power-load unbalance signal is a very early signal indicative of imminent overspeed. Its performance is very superior to the fastest practical acceleration sensing devices.

To prevent steam entry into the turbine from extraction points all such

{ sources of serious overspeed potential are equipped with two positive assisted swing check valves in series, each capable of preventing steam reentry. During performance of the normal overspeed protection system these check valves will be forced close by bednning reverse flow through these lines thus shutting off these possible contributions to turbine overspeed.

The normal overspeed protection system consisting of a combination of proportional position for control of slow speed changes'and power-load unbalance for fast closing has permitted full load rejection on the largest turbines built to date without the turbine shaft speed rising to the point of activating the emergency overspeed protection system. 1 t.

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1. Basic Systems (continued)

The normal overspeed protection system is electrical from the sensing devices to the input dsvices on the steam valves. The two speed input channels are used in a fail-safe one-out-of-two arrangement; more details of the design can be found in the appendix.

The emereency oversneed erotection svstem is shown on Fig. 2. It is a mechanical hydraulic system. An overspeed trip device (OST) is mounted .

at the front and of the turbine shaf t. This device is a mechanical ring which will remain concentric with the shaft as long as the shaf t rotational speed is below a certaic level. Above this level (the setpoint of the l trip device) the ring will move to an eccentric position and actuate a trip mechanism shown in Fig. 2. TRIP and RESET. This mechanima which is a latching mechani== converts the trip signal to a hydraulic signal (pressure) in the mechanical trip valva (mrv) which passes the signal along through a pipe system to the steam valve actuators. This signal is termed emergency trip signal (ETS).

The design of the hydraulic system is such that the ETS is high (1600 PSIG) '

I when the OST and trip and reset mechanisms are reset, and low (less than 50 PSIG) when these' are tripped. When the trip and reset mechanism has been actuated it must be reset by the operator through pushbutton ac tion. .

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The ETS signal is connected to the , fast closing devices (disk dump valves) i l on the actuators of Main Stop Valves (MSV) and Reheat Stop Valves (RSV) 1 -

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I. Basic Systems (continued) ~ ,

as shown on Fig. 2. Relwase (dumping) of the ETS signal will cause fast closing of all MSVs sad RSVs. .

In addition the ETS signal is connected to a relay dump valva (RDV) also shown on Fig. 2. The RDV converts the ETS signal to an air signal which is conducted to positive closing features on the non-return valves in the turbine extraction lines described earlier. When ETS is high the output of the RDV*is high and the posi.tive closing of the extraction valves is removed; when EIS is low the air pressure is low and the positive closing feature is applied.

( The setpoint of the OST is normally M of rated speed above the overspeed reached by the turbine following full load rejection controlled by the normal overspeed protection system. Typical values for an 1800 RPM nuclear turbine are: normal overspeed 109% of rated, setpoint range for I the OST 110-111% of rated.

As already stated the emergency overspeed protection system is entirely mechanical hydraulic. The OST takes its energy for actuation from the turbine shaft, the trip and reset mechanism has energy for tripping

, stored in springs. The only power supply required is a hydraulic pressure (1600 PSI) and it is inherent in the design that failure of the pressure vill cause a turbine trip which is a safe failure mode.

More on this can be found in the appendix.

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. 6 I. Basic Systems (continued)

Because of this safe failure mode the pressure supply for the emergency overspeed protection system can be taken from the same scurce which powers the valve actuators used both for NSV and RSVs as well as for CVs and IVs.

From the above description it can be seen that the nomal overspeed protection and the emergency overspeed protection system constitute two automatic, and essentially redundant and independent systems". The only exception to both redundancy and independence is the common hydraulic pressure with the design being such that loss of hydraulic pressure causes all valves to close. Use of a common hydraulic pressure source is discussed in more detcil later.

There is another feature which detracts in a small amount from complete independence of the two systems, namely, a " crossover" between the two systems; a protective action by the emergency overspeed protection system will also be initiated on the turbine speed control system, by

arbitrarily closing the turbine CYs and IVs. This will occur because the ETS hydraulic signal is connected to the actuators of the CYs and and IVs (in addition to the MSVs and RSVs) and dumping of this pressure will cause rapid clostre of these valves. More on " crossovers" will be found in Section III.

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.s_ - -- T I. Basic Systems (continued) _, ,

• ewed in this simple form, the separate major elements of the two systems are clear.

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1 Normal Oversneed Emereenev Oversneed

1. Electrical sensing and logic 1. Mechanical sensing and logic (OST) '

2. Control and Intercept Valves 2. Main Stop Valves and Reheat Stop Valves

3. One line of Extraction Non- 3. Second line of Extraction Non-Return Valves where needed Return Valves where moeded Additions to these systems to marief se probability of overspeed protection.

and for other purposes are described in the following.

is II. Additional Protection In almost all cases of a turbine approaching some kind of distress (including overspeed) the best protective action is to " trip the turbine,"

i.e. , close all steam admission rapidly. Because the emergency overspeed protection system is designed b implementi such rapid closing of MSVs and RSVs it is a very convenient input for additional protection functions.

These functions have been added to Fig. 2 to form Fig. 3; (note that Fig. 3 also.shows ETS connected to CVs and IVs). The added functions are:

Mechanical Trip Handle, MIH l

Mechanical Trip Solenoid Valve, MISV, and trip piston Electrical Trip falve, ETV, with two elec. trip solenoid s valves (2)ETSV

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l II. Additional Protection (continued) i l

1 Two trip bus systems, one energized to trip, one de- '

energized to trip l Master Trip Button input to these busses l

Other turbine trip inputs to these bussess  !

l These functions are discussed briefly below. '

The mechanical trip handle is located on the turbine front standard and is a very simple and reliable means of tripping the turbine by actuating the same trip finger as the OST in a menner which cannot  ;

physically interfere with the proper operation of the OST. The principle of the MTH is shown on Fig. 3; here it would work by being pulled to the ,

i left and engaging the trip finger. Should the handle not return to .

l reset position, it would prevent resetting of the trip and reset mechanism.

The HTH can be used by an operator at the front standard any time he wishes to trip the turbine.

The MTSV and trip piston can activate the trip finger in an interference '

free maar; in principle a. shown in Fig. 3 by pushing the finger to the right. The MTSV must be energized to trip.

The ETV and (2) ETSV pilot valves are mounted in the turbine front standard and can interrppt the ETS signal and connect it to drain and thereby implement closure of all steam valves. The (2) ETSVs are energized continuously during normal turbine operation and both nust be de-energized to trip the turbine. The Erv contains no closed center position; it is either reset and passes the signal from MIV undisturbed on to ETS, or it is tripped and causes EIS to'go low. ,

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II. Additional Protection (continued) _. .

The two trip busses each use their dedicated input device as shown on Fig. 3. Electrical cross-tripping (not shown on Fig. 3) is provided between the two busses such that a trip from one will also cause a trip on the other. Of course, the continuously energized bus together with

.0 the (2) ETSVs provide tripping protection on loss of electrical power to the systems.

i The master trip button is located in the control room on the turbine control panel in front of the operator. It contains contacts which directly input a trip to each of the two electrical busses.

( The other trips cover a long list of protective functions classified i

as vital trips, important trips, or operational trips. Examples of the three categories are loss of bearing oil pressure, low condenser vacuum, and high bearing vibration respectively. These trip signals are all generated with appropriate sensors and logic systems- and input to the two trip busses fot implementation.

It should be noted that the three input devices added to the emergency overspeed protection system, namely MIH, MTSV, and trip piston, and ETV '

with two (2) ETSVs are all designed not to interfere with the reliability of the basic system. The first two only engage extensions of the

trip finger by push contact, the third which is inserted into the path of the ETS signal will either pass it on unchanged or itself trip the g ETS signal.

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10 III. Testing of the Overspeed l'rstection Af ter control devicas have been manufactured, casted successfully, and operated briefly, most devices will for a 1:ng period be subject to failures which occur at random inte:vals, the twas of which its termed mean time between failures, m'37. This failura pactarn will continua as long as the devices are not operated beyond a certain length of time .

which is often thought of as their useful life. Towards the and of this  ;

period the failure rate will iscrease rapidly as wear cut, aging or I

a similar mechanism begins. Protection devices should never be operated f or so long a period that serious wear-cut begins. .

For a control device exhibiting random failure rate, experience shows that a complace exercise, or testing, of the device will restore it to '

essentially new condition as regards its reliability, i.e. , the probe- I bility of correct function immediately after a test is essentially 100f, from which value it begins to decay as time passes.

Therefore, if a protection system or its components are tested regularly ]

at intervals that are short compared to the MI3F for the system or ]

I t component a very major improvement can be achieved in the reliability,,  ;

i.e., in the probability of correct function at any given time. Expressed I i

in other terms: Long experience with turbine protective devices has '

shown that only if they can be regularly tested (and from a practical 1

s tandpoint, regular testing requires ability to test under load) can they be relied on to function properly when needed.

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, III. Testing of the oversneed Protection (continued)

The overspeed protection system for all modern G.E. turbines is desigced

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to take advantage of such an improvement by providing test capabilin for all components whose failure would significantly increase the pro-bability of a serious overspeed incident (without exception) while the turbine is carrying load.

The test intervals are selected on the basis of operating experience to be practical for the operators, provide.a worthwhile improvement in reliability and not wear out devices by too frequent testing.

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In order te test a protection system "on line," without major disturbance

( of the turbine load, it is necessary either to isolate or " lock-out" j certain portions for test or provide para 11al devices. For example, the latter principla is used for the main steam valves, see Fig. 5, l taa fermer generally' for other parts of the systems.

i Ta provide isolation of compor.ents lock-out equipment must be incorpor-ated; it is cbvious chas this equipment must be designed such that it does act subtract from the overall reliability cf protection so much that its purpose is defeated. Also, it nurt not seriously affect the i

operating reliability by causing unwarranted prctective action. The

{ latter state =nt holds very tr2ch for the protection system too where l

ex;assive zeal in providing protection :r.sy lead to an unacceptable number of ucwarranted potective actions.

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III. Testing of the Overspeed Protection (continued) l Fig. 4 shows a schematic of the complete overspeed protection systam. j It is the combination of Figs. I and 3 with the addition of devices needed to provide complete test capability of all components. The added devices are:

a. Electrical Lockout Valve, ELV

b. Mechanical Lockout Valve, MLV

c. Backup Overspeed Trip, BUOT

a. The electrical lockout valve (ELV) permits on-line testing of the '

electrical trip valve (ETV) and its solenoid valves (ETSV) by interrupting the signal from the ETV to FIS and substituting for it a signal bypassed directly'from the mechanical trip valve (MTV) around the MLV and ETV as shown in Fig. 4. Therefore, while the ETV is exercised and monitored for correct function full operating capability of the emergency overspeed protection system is maintained since a trip can be implemented by the MTV. The ELY has no " closed center" i

position; i.e., it goes essentially directly from normal tc locked-out position. Its position during and after a lockout operation can be monitored on the operator's test panel,

b. The mechatical lockout valve (MLV) will similarly.in its lo'cked-out position isolate the MIV and permit testing on line of the OST, MIK, or, MTSV actuating the Mrv. Position and pressure monitoring is provided to verify correct performance. When the test is completed the MIV is again shifted to normal position and this can also be

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monitored. While the PC.V is locked out emergency overspeed protection is provided by the backup overspeed trip (BUOT) also shown on Fig. 4 and described below.

In order not to reduce the reliability of the protection by the introduction of the lockout valves for testing purposes, these valves are, as already mantioned, designed not to have a closed center which might lease them in neither " normal" nor " locked out" position; they are also equipped with position monitoring devices signaling -

their state at the operator's panal; finally, it is noteworthy that '

I due to the hydraulic circuit design even if one or both of the lockout

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A. valves are accidentally left in the " locked out" positien af ter a test, there is still protection against emergency overspeed by either the OST or the BUOT. The aforementioned crosstripping between 24 VDC .

bus and 125 VDC bus can now be seen in its full importance because '

it assures that trip inputs always arrive at both MTV and ITV. During normal condition this just means redundant inputs to the ETS system; however, during testing or even accidental lockout it assures that a trip signal will arrive at a device capable of d.mplementing a t

l trip of the ETS signal. ,

c. The bachup overspeed trip system (BUOT)-See Fig. 4-is an electronic system which monitors the turbine shaft speed and will initiate a turbine trip with rapid closing of all main and extraction steam I g .

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III. c. (contieved) valves if the system setpoint is exceeded. The setpoint is normally 1% of rated speed above the setpoint of the overspeed tri$ syntem.

W BUOT system utilizes three identical speed signals (pulses) ,

, genSrstad by magnetic pickups opposite the toothed wheel in the

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turbine front standard separate of normal speed pickups. Each' signal is conditioned independently to a DC voltage pronortional to speed and each level is again monitored by ladependent voltage level sensors (voltage comparators).

Each of these three voltage comparators is equipped with output ,

contacts, the state of which will change as tha voltage being monitored increases above the setpoint of the comparators, i.e., as the shaft speed increases above the setpoint.

The output of the thsse speed channels are combined in a "2-out-of-3 logic irystes," i.e., a system in which at least two out of the three voltage comparator idputs must change state in order for a trip to be implementad, See Appendix for a description of this system.

The BtDT system as, indicated on Fig. 4 implements a trip by signals both to the MISV and the (2) ETSVs and thereby transmits a trip signal to all main and axeraction steam valves by changing the ETS pressure to lov which will make these valves close fast, as explained ea.rlie r. l

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III. (continued)

A trip imp?.amented by the BUOT is latched mechanically by the trip and reset mechanism shown on Fig. 4. *In addition there is an ,

electric lockup of the 24 VDC trip system and the 125VDC trip bus.

Sinea the three backup overspeed sensing channals are connected in a 2-out-of-3 logic arrangement to trip, each channal can be indivi-dually tested without causing any tripping action and thus, this testing can be performed with the turbine on the line. The testing consists of temporarily lowering the setpoint of one channel at a time below operating speed, i.e. , to about 99% of rated speed, and verifying that this caused tripping action by the ' channel.

k In the turbine speed control system there are test provisions with indicating lights that allow the simulation of a power-load t

unbalance condition such that the actuation circuits can be tested.

The output signal to the fast closing devices on control and intercept valves arc blocked during this test.

From the above description it should appear that all comeponents.in

( the overspeed protection system up to the steam valves can be tested "on line" for correct function. In addition, there are test provisions that allow test closing of each steam valve while the i

turbine is carrying load. For testing of main stop, reheat stop, and intercept valves there is a very minor disturbance of the

. turbina power during a test. For testing of control valves there e

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A 16 III. c. (continued) will be a small temporary reduction in load only if the load carried is so high that there is not enough compensating stroke lef t on valves not being tested. It is, therefore, a very practical and feasible procedure to test steam valves on an operating turbine. The test of each valve includes complete ,

closure with the last portion being a fast closing actuated by the fast closing and tripping devices.

The positive closing' feature on non-return extraction valves is testable locally where partial movement of the valve disk and shaft can also be observed in most cases. -

We have now accounted for testing of all major portions of the overspeed protection system.

The 'value of the testing of elements of the overspeed protection system has been analyzed carefully by Reliability Engineering ,

techniques to show that a worthwhile improvement is obtained when the additional equipment is considered. We have found testing at intervals which are practical but short relative to the M1'BF for a device to be the most powerful way of improving the protection I reliability; with improvements of up to several orders of magnitude being achieved.

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III. c. (continued) .. .

The BUOr is used under two special conditions not described above.

The normal testing of the overspeed trip, OST, is performed by introduction of oil into,it to create sufficient unbalance in it to be actuated at rated speed. However, about once per year it is recommended that the turbine be brought alowly to true overspeed to test the setting of the OST. During this test the BUCT also serves as protection being set about 1% of rated speed above the OST. Since there is no device with normal setpoint above that of the BUOT it can only be tested safely by temporarily lowering its setpoint.

l During operation with the turbine normal overepeed protection out of service, which occurs when the " standby" mode is selected to permit test and servicing of major portions of the control system, the BUOT setpoint is automatically lowered to about 105% of rated. With this setpoint the BUOT and the OST still provide two lines of protection against turbine overspeed.

IV. Common Mode Fdlures M.

i Inspite of apparent independence and redundancy in protection systems it is conceivable to suffer simultaneous or nearly simultaneous failures if the systems have certain features in common which all are vulnerable to a certain effect or logical series of effects. These common features may be in design concept, materials, physical location, signal path, s'

' power supply, manufacturing source and others.

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18 IV. Coimnon Mode Failures (continued)

There are probably very few protection systems that do not have some potential for e.ommon mode failures. Below is a discussion of some conceivable common failure modes in the turbine oversneed protection system.

The hydraulic newer sunoly which provides hydraulic pressure for the ETS signal of the emergency overspeed protection system is also used to supply pressurized fluid for the actuators operating the control and intercept valves (CV and IV) belonging to the normal overspeed protection as well as for the actuators operating the main stop and reheat stop valves (MSV and RSV) belonging to the emergency overspeed protection system; see Fig. 4. The common hydraulic power supply operates at 1600 psig with phosphate ester type fire resistant fluid. (More details may be found in the appendix.)

All steam valve actuators are designed to be single acting, spring l

closed, with hydraulic pressure to open valves; this means that all steam valves will go closed, forced by eteam and spring forces if the hydraulic pressure is lost. The design of the ETS system and the actuating devices on the steam valve actuators also provides for inherent and automatic tripping on loss of the ETS pressure. (More details of the steam valve actuators may be found *in the appendix.)

Therefora, the use of a comunon hydrislic power supply does not jeopardize the overspeed protection by common loss of all hydraulic pressure because such a loss will automatically close all s' team vcives.

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IV. Common Modes Failures (cont.inued) .

It is, however, possible that the common fluid could change properties in such a manner that it would adversely affect components throughout the protective system to such a degree that several of them might simultaneously become disabled. (Examples of such effects are:

a corrosion through increased water content or chemical decomposition of the fluid; particle conr==4n= tion of the fluid.)

This failure mode has been acespred after provision of the following safeguards.

Testing interval of all hydraulic devices is ahort compared to the time required to degrade the hydraulic components.

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Fluid maintenance program which includes periodic saspling for chemical analysis and contamination particle count at intervals that are short compared to tien needsd for fluid degradation.

Non-collapsible full flow filters of 5 micron filtration rating in the pressure supply line of the hydraulic power unit. These filters are qualification testad against collapse at a differential of about twice normal total operating pressure.

I No bypasses around the full flow filters but alarm provisions

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to detect excessive pressure drop across them. The plutging of filters automatically closes steam valves and shuts the turbine down.

A redundant device that detects beginning contamination of the hydraulic fluid and begins shutdown of the turbine before over-speed protective devices become inoperative,

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I l IV. Conanon Mode Failures (continued)

All hydraulic valves in the emergency and backup overspeed protection system shall either be of positive seating type or if of sliding spool type shall have shifting forces available in the order of the hydraulic pressure times the main cross-sectional area of the spool.

j No sliding seals shall have pressure drop across them during normal turbine operation. All valves shall be made of non-corroding meterials. All this to ensure marimum resistance to particle cont ==4 nation and rust or other corrosion attack on the valves.

Ra11 ability analysis has indicated that with the above provisions the s

probability of safe performance of the hydraulic system is not the controlling factor in the overall probability for overspeed.

The ETS hydraulic lines are connected both to the steam valves (MSV & RSV) belonging to the emergency overspeed protection system and to the valves

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(CV ~& IV) belonging to the normal overspeed protectica systas. These latter connections increase the exposure of the ETS lines to damage.

Rupture or serious leak would cause depressurization of the ETS linee which is a safe faflure mode which causes instant tripping and shutdown of the turbine. Pinching or complete blockage by " =aical damage on i .

l the connections to CVs and IVs might prevent these valves from being i

actuated by the depressurization of ETS but MSVs and RSVs could still

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be actuated ~by the emergency overspeed protection system, and the CVs and IVs could still be actuated by the normal overspeed protection system.

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L 21 IV. Comunon Mode' Failures (continued) - -

Of course pinching of the EIS lines connected to the MSVs and RSVs might prevent actuation of one or more of .these valves although complete cutoff to a degree sufficient to prevent actuation is very unlikely

, since virtually no flow is required through the ETS lines but only a decompression signal. This latter type of pinching is, however, not a i cousson failure mode since the normal overspeed protection system can still actuate its steam valves - CVs and IVs by its normal means - the electrical fast closing signal - See Fig. 1.

Similarly, it can be seen on Fig. 4 that the emergency overspeed protection syster and the backup overspeed trip (BUOT) have a piece

( of their trip channels in common, namely, from the electrical trip valve, ETV, to the MSVs and RSVs. Therefore, interruption of this portion of the ETS line might disable both these trips. Since the normal use of the BUOT is to permit testing of the emergency overspeed protection, this kind of failure is not a common mode failure because it does not affect the normal overspeed protection. Only during standby operation do the BUOT and emergency overspeed protection systems constitute the two channels of overspeed protection. The unobserved pinching of the ETS line causing interruption without leakage has been accepted as a most unlikely event. for the very short periods of standby operation that are likely to occur.

The cpeed sensinz devices for all overspeed protection systems are _in cousson location, namely, inside the front standard. This fact has the 1

s.

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22 IV. Common Mode Failures (continued) potential for a cotamon mode failure in case of a) shaf t breakage between turbine and speed sensors or b) physical damage to the front standard which would render all speed sensors inoperative.

a) The shaf t breakage possibility has been accepted for several reasons:

The front standard is the nearest practical point to the primary energy inlet to the shaf t -(at first high pressure turbine stage). The farther away the speed sensors are the greater the probability of isolating a driven portion of the turbine t shaf t from the speed sensors.

Breakage of the shaf t Wich carries the speed sensors has always led to automatic tripping because there is sufficient orbital l movement of the shaf t to activate the OST.

The shafa which carries the speed sensors also carries the main lube oil pump and the loss of output pressure from this pump will cause a turbine trip because it is among "0THER T3 TRIPS" shown on Fig. 4.

Upon loss of both speed signals to the normal overspeed protection '

a turbine trip is automatically implemented by monitoring circuits.

i b) Some of the causes that have been considered are mechanical damage by falling or moving objects or damage by fire. These risks have been accepted because the front standard is a very rigid and strong sr-*ucture and no muchanical damage has ever been experienced which disabled the speed sensors while the turbine remained in tact.

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23 IV. Comon Mode Failures (continued) -

b) (continued)

All G.E. turbines are equipped with guarded lube oil piping in l

the neighborhood of the high temperature parts; this very greatly

, reduces the risk of asjor fires outside of the front standard.

Even in case of fire the heavy structure will protect the speed sensing and tripping devices sufficiently long to allow operators to trip the turbine off.

Egerience and lab tests show that with turbine lube oil having flash and fire-point temperature 250-300*F above the operating tenperature in the front standard there is essentially no possi-

} bility for a fire to originate or propagate inside the front standard.

General Electric Co.

3 Large Steam Turbine-Generator Dept.

J. Kure-Jensen, Manager-Cont.is Acc. Dev. Eng'rg April 9, 1975 i, *

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24

, - APPENDIX - ~t

1. Turbine Steam Valves

2. Hydraulic Power Supply

3. Electrical Speed Control Unit

\

4. Two-out-of-three Logic System E

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25

• ^ ~

APPENDIX

1. Turbine steam valves are provided, two in se' ries, in all major steam lines which have a steam supply potential capable of driving the turbine to dangerous speed levels. On Fig. 1 is shown schematically a main steam lead equipped with a main stop valve (MSV) followed by a control valve (CV), .

as well as a reheat steam line equipped with an intercept valve (IV) followed by a reheat stop valve (RSV). The two latter valves are normally built into a common casing to form a combined intermediate valve (CIV).

A large nuclear turbine will normally be equipped with four MSVs in parallel, followed by four C7s, as shovn' in Fig. 5. To permit testing of individual

. valves during service, an equalizer is provided betwesen the MSVs and the CVs. Two CIVs are provided to each low pressure turbine, as also shown on Fig. 5, to permit testing of one CIV without significant reduction in steam flow to a low pressure turbine.

Due. to the parallel arrangement of valves, it is necessary for all valves of a specific group, CVs, IVs, etc., to close completely in order to interrupt the steam flow from a source. The valves are described in more t

detail below. ,

Extraction lines from the turbine are each sinilarly equipped with two valves - positive closing non-return valves - in series to interrupt backflow of steam into the turbine.

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.-.- ..: . . . w :- - a . -)

26

1. (continued) .

Fig. 6 shows a cross-section of an angle body main stop and control valve combination for a nuclear turbine. Four such valves are arranged in parallel as a.hown in Fig. 5; the equalizer below the stop valve seat can be seen on Fig. 6.

Each MSV has two normal positions: fully open and fully closed. The use of stem sealing permits relatively large stem-to-bushing clearance, '

which minimizes the possibility of stou sticking. Special attention has been given to design features which promote reliable closing and tight seating throughout the life of the valve. Ste111ted contact areas are used on main <md aur114 sry valve disks which are spherical so that perfect contact is made even if they are not precisely aligned with their seats. ' '

Valve disks are operated from the downstream side so that steam forces on the valve will tend to close the valve and so that stem and stem clearsace areas are not subjected to steam pressure when the valve is closed. A strainer is provided upstream of the stop valve disk to protect the valve and turbine from foreign matter.

The stop valve actuator is located below the valve. It consists of a large spring to close the valve and a single acting high pressure hydraulic servosocor to open the valve. The hydraulic servomotor has at the lower end a large pilot opersedd disk dump valve which can very quickly drain the hydraulic fluid from the servonocor. An electrahydraulic servovalve or a small three-way valve is used to'hdmit hydraulic fluid to the servomotor when the pilot pressure for the dump valve has closed this valve.

1

- m _ _ -

27

3. (continued) ,

Fig. 7 is a schematic representation of a main stop valve and its actuator. The pilot pressure (ETS) for the siisk dump valve is derived in the overspeed protection system. If this pressure is released the

, actuator will close rapidly; depending on design the closing time is

.1 .3 seconds.

The control valves (CVs) - See Fig. 6 - are partially balanced valves lif ted from the side of the higher steam pressure. To prevent b1wout of the valve due to the stem force in the open position of the valve, a spring is provided above the valve directly in line with the stem. Other detailed design features are similar to those mentioned for the MSV.

( The actuator is of a design identical to that used for the MSV except it is mounted on the side and actuates by means of rods and levers. The electrohydraulic servcastor is used for position control of the valve, the disk dump vahe held by the pilot pressure (ETS) can be used to fast close the valve.

Fig. 8 shows a combined intercept valve (IV) and reheat stop valve (RSV).

The IV portion is the upper, partially balanced valve actuated from above; 9

the RSV is the lower, solid disk actuated from below. It can be seen that these valves use the same design elements as the MSV and CV except designed for the much lower pressure level at which the combined valve operates. The valve actuators are of a design identical to those for the MSV and CV.

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28 l

1. (continued) i Although the reheat stop and intercept valves are combined into one pressure vessel, it should be noted that they are otherwise very inde-pendent with their actua*:ing mechanisms entering from opposite ends.

2. Hydraulic Power Suonly The hydraulic, supply is consson to all steam valves and the Emergency TripSystem(ETS). A schematic is shown on Fiz. 9. The hydraulic power unit operates with phosphate ester type fire resistant fluid pressurized to 1600 PSIC. Two identical variable displacement pumps can pump into a common supply manifold. Normally, only one pump is required, with the other in active standby ready to start automatically on a preset drop in manifold pressure. Hydraulic accumulators help support a transient flow demand which exceeds the delivery capacity of a pump.

The hydraulic power unit is equipped with full flow filtration (5 micron rating) in the high pressure discharge line from each pump. The filters are equipped with collapse-proof cartridges (design pressure 3000 PSIG differential) and warning devices for ,ezcess pressure drop due to clogging

. of filters. There is no bypass around a filter. Filters in a piamping systes can be serviced while the other pumping system is operating.

A bypass filtering syste$a operates continuously to condition the fluid through Fuller's Earth filters and very fine particle filtration. The conditio~n 'of'the fluid is monitored through a prescribed sampling i *

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2. (continued) .

schedule to keep the characteristics of the fluid (particle count,  !

i neut. number, 2H O content, etc.) within specified limits. '

j The entire overspeed protection system is designed such that loss of fluid pressure will cause automatic shutdown of the turbine. This is achieved as follows.

If the eparating pump fails to maintain the hydraulic pressure at a level sufficient for normal and accurate control of the turbine the standby pump is started automatically. Should this puisp also fail to maintain pressure a further decay will initiate an electrical turbine .

i trip included in "OTHER TURBINE TRIPS" on Fig. 3. Finally, on radical loss of hydraulic pressure the steam valves will be forced closed by steam and spring forces and loss of ETS pressure will initiate valve rapid closure by release of the disk dump valves on the individual valve actuators, see Fig. 7.

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3. Elcetrient Speml Control Unit The speed control unit, Fig.10, produces the speed error signal that is determined by comparing the desired speed with the actual speed of the turbine at steady-state conditions, or the desired acceleration with the actual acceleration during startup.

When the desired speed signal is increased in a step, the acceleration control will take over and accelerate the unit at the set rate up to the value of the new speed reference, where the speed control will take over again automatically.

Upon decrease of the speed reference, the unit will coast down with the 9

valves closed. They will re-open only when the new set speed has been reached; there is no limit in deceleration.

During normal operation at rated speed, the speed error signal is zero, regardless of load.

Because of the extreme importance in safeguarding against overspeed, the speed control unit has two redundant chann=1=.

l The low value gate will put out a signal requiring the lowest steam

• valve opening commanded by either of these two speed channels.

Loss of. output from a single channel 1s alarmed and loss of output from l

i both channels will automatically initiate a turbine trip through "0THER TURBINE TRIPS" shown on Fig. 4.

4

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4 31

4. Two-Out-Of-Three Lonic Sys tem In turbine protective systems it is of ten found that use of a single sensor, subsystem or channel is unsatisfactory because failure of the device into the protective mode will cause unwarranted protective action,

, (turbine tripping); also, use of a single device does not permit mainte-nance while it is in service without interference with the protection desired.

To overcome these difficulties a two-out-of-three logic system is used.

It is shown in electrical schematic on Fig.11 in two versions. The pair of contacts (-1 and -2) shown may belong to a sensor (e.g. , pressure switch), subsystem (e.g. , relay) or channel (e.g. , speed channels of the r

t, BUOT). It can easily be seen that two of the devices must change state in order for the system to change state. The advantage of such a system is:

much better protection than with a single device greatly reduced probability of unwarranted protective action i

ability to test and service devices while maintaining essentially unreduced degree of protection 1

To implement a two-out-of-three system with three different sensors, each sensor must have a pair of independent cot tsets. For easy wiring G.E. has l

developed a terminal board which will establish the circuit shown on Fig.11 I when the contacts are wired into designatad terminal points. The terminal i

I board is further desi;gned to facilitate testing of a single device without

!1 l

danger of interfering with the two others such that prottetion is maintained l and chances of faulty tripping during test essentially eliminated.

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