Md1 0 v 111-09-00008 a Extraction System Requirements

8/10/2019 Md1 0 v 111-09-00008 a Extraction System Requirements

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A 26-04-12 Issue for information H.D.KIM K.T.NAM J.W.CHOI

REV. DATE DESCRIPTION DSGN CHKD APPD

SUB-CONTRACTORS DOCUMENT NUMBER: MD1-0-V-111-09-00008

A 26-04-12 Issue for information J.A.SEO M.K.LEE J.Y.KIM

REV. DATE DESCRIPTION DSGN CHKD APPD

PROJECT :

TWO(2) x 500 MW MONG DUONG 1 THERMAL POWER PLANT

EMPLOYER :

CONSULTANT :

CONTRACTOR : SUB-CONTRACTOR :

DESIGNED BY DATE TITLE :

H.D.KIM 26-04-12

EXTRACTION SYSTEM REQUIREMENTSCHECKED BY DATE

K.T.NAM 26-04-12

APPROVED BY DATE PROJECT NUMBER DOCUMENT NUMBER REV.

J.W.CHOI 26-04-12 ADB/MD1-TPIP/EPC150911 MD1-0-V-111-09-00008 A

FOR INFORMATION

APPROVED A

Approved without exception

APPROVED WITH COMMENTS AC

Approved Subject to Incorporation of comments

RETURNED FOR CORRECTION RT Insufficient Information/ Detail

Resubmit for Approval

REJECTED RJ

Complete redesign required

RECEIVED FOR INFORMATION I

Returned without comments

RECEIVED FOR INFORMATION IC

Returned with comments

any obligations covered under contract

Engineer:

Discipline:

Date:

Note: Approval or comments does not relieve the Contractor of

Pyry Energy

TWO(2) X 500MW

MONG DUONG 1 THERMAL POWER PLANT

Soumen MitraSenior Mec Engineer27-Jul-12


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TABLE OF CONTENTS

I. GENERAL 4

II. BACKGROUND 4

III. CHECK VALVE DESIGN REQUIREMENTS 5

A. General Specifications of Check Valves Used for Overspeed Protection 5

B. Specifications for the Free Swing Portion of the Check Valve 6

C. Specifications for the Power Assisted Portion of the Check Valve 8

D. Specifications for the Performance of the Air System 9

IV. BASIC RULES FOR DETERMINING NUMBER AND TYPES OF CHECK VALVES 11

V. PROCEDURE TO DETERMINE THE NEED AND LOCATION OF CHECK VALVES 12

VI. OVERSPEED PROTECTION AGAINST EXTRACTION SYSTEM

AUXILIARY STEAM SUPPLIES 13

VII. TYPICAL ARRANGEMENTS 16

A. Arrangement No. 1 16

B. Arrangement No. 2 17

C. Arrangement No. 3 and 4 17

D. Arrangement No. 5 17

VIII. MISCELLANEOUS SOURCES OF OVERSPEED 17

IX. PRESSURIZATION OF FEEDWATER HEATERS 21

X. PROBLEMS OF LOWPRESSURE (LP) HEATER APPLICATION 21

XI. WET EXTRACTIONSNUCLEAR UNITS 22

XII. WATER INDUCTION 22

XIII. INFORMATION NEEDED FROM CUSTOMER 23


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LIST OF FIGURES

Figure 1. Pressure Switch Activation of Non-Critical Positive Asset Cylinders 14

Figure 2. Air Relay Dump Valve Activation of Non-Critical Power Actuators 14

Figure 3. 3-Way Solenoid Valves in the Air Lines to Each Cylinder 15

Figure 4. 3-Way Solenoids in Air Lines to the Spring Side of the Air Piston 15

Arrangement No. 1 19





Figure 5. Typical Heater Shells and Internals 24

Figure 6. Baffling with Special Drip Arrangement 25Figure 7. Baffling with Drip Arrangement 25

Figure 8. Dry Heater by Means of External Drain Receiver Section 26

Figure 9. Multiple Strings of Feedwater Heaters 26


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I. GENERAL

The following describes Doosans aims and requirements pertaining to the extraction system

and should answer the majority of questions that arise on this subject. The enclosed typical data

form (ATT. #1) will aid in the study of a particular extraction system and overspeed situation. It

will also help determine the requirement for and the proper location of check valves in the

extraction system. Form (ATT. #1) issued for the specific unit must be returned to Doosan

completely filled in for the review of the overspeed potential by the responsible engineering

section. This document, together with DST00132A, Water Induction in Large Steam Turbines

Design Recommendations, should be used in designing the extraction system and controls.

II. BACKGROUND

A wellknown fact among consultants and power companies is that excessive overspeed of a

turbinegenerator shaft can be disastrous. A lesser known fact is that the energy contained in

the feedwater heaters of a steam turbinegenerator power cycle is often sufficient to contribute

significantly to the magnitude of the turbinegenerator rotating speed upon an electrical load

rejection or tripout and must be prevented from doing so by some means, the most common of

which is by the use of check valves. This energy is in the steam contained in the piping from the

turbine to the feedwater heaters and in the heater shells, in the water contained in the feedwater

heaters, as well as in the metal parts.

After a load rejection, the steam admission valves will close, causing the pressure of the steam

already in the turbine to decay. This decay allows the steam in the extraction piping and heater

shell to flow back into the turbine giving its energy to the rotor. This will cause the heater

pressure to decay so that the water which was saturated at the heater pressure under normal

operating conditions will become superheated momentarily, flash into steam, and flow through

the turbine giving up its energy to the rotor. The heat of the metal components provides some of

the necessary latent heat of vaporization.

In applications where steam volumes and water volumes are large, check valves in the

extraction piping are considered necessary to protect the turbine from this energy. The number

of check valves recommended by Doosan depends upon the steam and water volumes andinclude the free swing and power assisted types. Two power assisted valves in series (with

proper maintenance and testing) are considered to afford the maximum protection necessary.

Doosan steam turbine design rules and code requirements specify that the turbine controls will

be capable of preventing the turbine speed from rising above a certain maximum value after a

full load rejection or trip. Under this condition, the speed will rise due to the delays in getting

the steam admission valves closed and due to the energy contained in the bottled up volumes of

steam within the turbine. Part of the latter is the extraction system volume. The amount of speed

increase contributed by the extraction system must be held below a certain maximum value by


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the use of check valves or baffles.

The consequences of check valve failure are considered when determining how many and what

type check valves are needed. The turbine valve control system is made up of two separate

systems such that if the first fails, the second should operate. Each part of the control system is

extremely reliable so that this redundancy results in an exceptionally reliable protection against

excessive turbine overspeed. This philosophy of extremely reliable protection must be

employed in the selection of extraction check valves as well. For example, if the energy of a

particular extraction point is sufficient to cause a speed rise exceeding the maximum allowable

limit, then one check valve must be backed up with another in series, and both are to be the

power assisted type actuated by the turbine emergency trip system.

Therefore, it can be seen that considerable importance is attached to the protection of the

turbine rotor from experiencing excessive speed, and the cost of protecting the turbine against

the energy of the extraction system by the use of extraction check valves is minute compared to

the cost of repairing turbine or generator parts damaged by overspeed.

III. CHECK VALVE DESIGN REQUIREMENTS

With this background, it should be understandable why the design requirements of the

extraction check valves, listed below, mustbe adhered to.

A. GENERAL SPECIFICATIONS OF CHECK VALVES USED FOR OVERSPEED

PROTECTION

1. The valve specifications sent to valve vendors should include all, but not be limited to,

the specifications outlined below. These specifications are intended to provide an

extraction system that has reliable, adequate protection against overspeed.

2. The valve vendors recommendations as to maximum velocity through the valve should

be observed to maintain reliability. Too small a velocity will cause the valve to remain

partially open causing continuous fluttering. Too large a velocity may prevent closure of

the valve during testing. Typical ranges of maximum velocity could be between 125

ft./sec (40 m/sec) and 200 ft./sec (60 m/sec) at rated load. The valve vendor should be

supplied with the maximum design flow through each ex-traction check valve, the

maximum pressure and temperature (the maximum pressure and % moisture if the

extraction is wet) of the steam, and the system air pressure supplied to the air relay

dump valve on the turbine.

3. The check valve must be located in the extraction pipe such that allowable overspeed

criteria discussed later are met. The valve should also be located with consideration of

the vendors recommendations to minimize vibration problems on the valve. For


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example, the vendor may recommend locating the valve in a straight run of pipe 10 pipe

diameters downstream from any elbow in the pipe and 5 pipe diameters upstream from

the next elbow.

B. SPECIFICATIONS FOR THE FREE SWING PORTION OF THE CHECK VALVE

1. The free swing portion of the check valve must be such that it will freely close with no

flow in the extraction line.

2. The pivot shaft should be a single shaft.

3. On check valves with a circular disc attached to a swing arm there should be a means of

preventing rotation of the disc.

4. On check valves with separate check valve discs and swing arms, there should be twodifferent positive means of assuring the disc will not come off the swing arm. Just

pinning the nut to the disc threads is not satisfactory. It is preferred that the disc and

swing arm assemblies be secured by the valve vendor such that field removal can only

take place by pulling the pivot shaft and removing the whole assembly rather than by

allowing internal removal of the disc off the swing arm.

5. Field tests have shown that on a turbine load rejection small diameter high pressure

check valves stroke from full open to closed position in 0.1 seconds. As the valvesincrease in diameter and the steam pressures decrease, the stroke times increase to

approximately 0.6 seconds. The check valves should be capable of these repeated

closures without sustaining permanent deformation that would prevent shutoff of steam

on reverse flow.

6. Disc to seat sealing shall be accomplished with metal to metal seals. There shall be no

seal ring screwed into the disc, unless there are two provisions for locking the screws in

place, for example, bottoming screw torqued and staked secured in place. There shall be

no screwed in seats, unless they are seal welded in place.

7. The disc must be designed such that it will cause minimum pressure drop when open.

8. The use of a counterweight on a lever, external to the valve body, to balance the weight

of the disc for the purpose of reducing pressure drop is permissible, but only if the

following conditions are met.

a. The lever should be short enough so that it is impossible to develop more than 50% of


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the torque required to open the valve under no flow conditions no matter where the

weight is positioned on the lever.

b. The weight should be secured positively on the lever. For example, the weight could

be pinned in place on the lever, then the pin should be staked.

c. The shaft seal should meet the criteria set forth in the next items. This will probably

lead to applications where external counterweighting would not be permitted.

9. Shaft sealing has a significant impact on the reliability of the check valve to perform

satisfactorily. Based on field experience the following restrictions are placed on the

type of shaft sealing.

a. To avoid a potential water source of water induction to the turbine, and to prevent

quenching and distortion of the valve, water sealing the shaft is not permitted.

b. To avoid binding of the free swing check valve disc due to overtightening of packing

material, the valve must be designed with shaft seals such that it is not possible to

overtighten packing to restrain free swing motion.

10. Possible means of sealing the shaft include:

a. Hardened steel bushings with gasketed end caps where the shaft does not protrude

through the valve body

b. Hardened steel bushings with an intermediate steam leak off to the turbine steam seal

system, with no soft packings

c. Packing arrangements that cannot be overtightened enough in the field to restrain

motion

d. Soft packing, but with the lost motion device internal to the valve so that the packing

does not restrict free swing motion. This implies that any counterweighting required

must be internal to the valve.

11. On check valves with double D swing plates.

a. The plates should not open to an angle of greater than 85 perpendicular to the flow,

so that re-verse flow will force the plates closed.


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b. There should be 11/2 in. (40 mm) dia. hole drilled through one plate to ensure positive

drainage.

c. The spring should be selected so that it is stressed below its endurance limit.

C. SPECIFICATIONS FOR THE POWER ASSISTED PORTION OF THE CHECK

VALVE

1. The power assisted portion of the check valve shall consist of an actuator designed such that,

on tripping the air system by the turbine air relay dump valve (ARDV), a spring will cause the

actuator to close the check valves.

2. Upon supplying air to the cylinder, the air should compress the spring and allow the check

valve to open. It must not restrict the free swing motion of the check valve disc in the closed

direction.

3. To help assure a fast response when the air system is tripped, the air cylinder and spring are to

be sized so that with the cylinder connected to the valve and with no steam flow:

a. Approximately 30 psig [206.9 kPa (gauge)] [2.1 kg/cm2(gauge)] air pressure is required

to lift the piston from its closed end stop.

b. Approximately 60 psig [413.7 kPa (gauge)] [4.3 kg/cm2(gauge)] air pressure is required

to hold the piston against its open end stop. [Assuming air supply pressure to be in the 60

to 100 psig(413.7 to 689.5 kPa (gauge)] [4.3 to 7.1 kg/cm2(gauge)] range.]

4. To help assure an adequate force level to overcome valve friction, the air cylinder and spring

are to be sized so that:

a. With the cylinder in the closed position, there must be enough spring force to exceed, by a

factor of at least four, the force required to overcome the combined normal friction levelsof the air piston, linkage and valve disc.

b. With the power assist actuator hooked up to the valve, with the check valve disc free to

move, with full steam flow down the extraction line, and with normal friction levels in the

valve and actuator, there will be enough spring force to close the disc to the point where

the disc will have closed off 10% of the cross sectional flow area. If the 10% flow point is

not known, it is accept-able to size the spring to shift the disc from 100% open to 90%

open position of angular travel against the above conditions of full resistance force. At the

90% position reverse steam flow is to be a positive closing force on the disc.


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5. The power assist actuator should have a stroke such that it would move the check valve to the

point where the check valve disc would have closed off at least3/4 of the cross sectional flow

area.

6. The power assist actuator should be designed with a convenient means of local daily testing

that the free swing portion of the check valve is still free to move. The testing feature should

be located such that the operator testing the valve can be sure that the valve moves freely

through stroke. Because rod end seals in the top of conventional air closing cylinders tend to

develop excessive leakage with time, testing force margin can be lost. To overcome this

maintenance problem, the heater high level solenoid can be used to accomplish the daily check

valve test. This option is shown in figure 3.

D. SPECIFICATIONS FOR THE PERFORMANCE OF THE AIR SYSTEM

1. The extraction air piping from the turbine air relay dump valve (ARDV) to the individual

power assist actuator must be designed so that the air in the cylinder will depressurize

rapidly. With full rated air pressure in the system and check valve discs in the closed

position, this rate of decay should be fast enough for the air pistons to go through full stroke

in the check valve closing direction within two seconds of tripping the air relay dump valve.

Careful attention to the size and routing of all the air piping is required to minimize the

volume of air contained in the system and yet maximize the response time of the critical airpistons. In general the following has been used successfully in the field. The main manifold

run from a single ARDV at the turbine end standard down to the first or second tee for the

closing cylinders has worked well when sized to 1 in. (27 mm) inside diameter. Note that for

two ARDVs tied together the main header flow area could be doubled. The individual runs

to single air closing cylinders have worked well with the equivalent of1/2 in. (13 mm)

tubing. Sub manifolding lines were sized in between the main header and individual runs.

Minimizing the number of elbows is important. Because of the variety of field installations

the above information is only intended as a guide. The architect engineer and customer are

responsible for determining their optimum station layout which will meet the two sec-ond

timing requirement.

It is common practice to provide check valves of the power assisted type even when it has

been deter-mined only a free swing check valve is required for adequate overspeed

protection. The extra air piping and volume involved can make it very difficult for those

valves requiring power assist for overspeed reasons to have a satisfactory response time. The

following two subsections describe possible alternatives for arranging the critical and non

critical power assists. Subsection a. is for a single air relay dump valve (ARDV) and


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subsection b. is for a system with two ARDVs.

a. On units with only one air relay dump valve (ARDV), use the arrangement shown in

Figure 1. Run the air piping from the ARDV (again giving careful attention to the size

and routing) only to those critical check valves on which power assist actuators are

required for overspeed protection. Install or utilize a pressure switch (used for opening

extraction drains) in the outlet of the ARDV (again minimizing size and routing). Run

an air supply line directly to the remaining power assist actuators. This air supply will

not go through the ARDV. Dumping of the air from these cylinders will be by means of

an automatic three way valve in each individual air line. The pressure switch on the

outlet of the ARDV should provide a signal to these automatic valves so that they will

dump the air to these noncritical cylinders on activation of the ARDV. If the critical

check valves which are to be connected to the ARDV are also tripped by this pressure

switch, then a modified timing requirement is imposed. The pressure switch trip has to

meet the two second closure; and with the pressure switch trip temporarily disconnected,the critical air cylinders are to close in less than five seconds by action of the ARDV.

b. On units with two air relay dump valves (ARDV 1 & ARDV 2) use the arrangements

shown in Figure 2 or 4. Run the air piping from ARDV 1 (again giving careful attention

to the size & routing) to only those valves in which the positive assist feature is required

for overspeed protection. Run the air piping from ARDV 2 to the remaining valves. If

the air volume in the runs of critical lines exceeds the noncritical lines, then

manifolding the lines together as shown in Figure 3 is acceptable since the trip times ofthe critical valves will be reduced.

As a guide in analysis of the system response time, the turbine front standard contributes a

loss of about 7.5 velocity heads per extraction relay dump valve based on a 1.063 in. (27 mm)

ID pipe.

2. No more than twelve critical power assisted check valves should be connected to a single

ARDV. On most units shipping after 1980, two ARDVs are supplied. With two ARDVs this

rule can be easily met.

3. There must be no possibility of restricting air decay from air cylinders on power assist check

valves required for overspeed protection by the location of a valve in the air line. This means

that there must be no 2way shutoff valves in the air lines from an ARDV, which has critical

positive assist check valves hooked to it, through to the individual air cylinders on any check

valve.

4. Any 3way solenoid valves in the air lines from the ARDV to each air cylinder hooked up to


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an ARDV with critical positive assist check valves must not pose any problems in restricting

flow on tripping the air system from the ARDV. See Figure 3 for the arrangement. Thus the

solenoid valve must be of the direct acting type, and must have been selected for fast dumping

of the cylinder through the cylinder to inlet port direction. Fast dumping through the cylinder

to vent connection is not as important as through the cylinder to inlet connection. This is

because the cylinder to vent connection is used on heater level as a protection against water

induction. Water from a leaking feed-water heater is a relatively slow process. Flow rates for

such an event have been given in ASME TDP1 for use in timing when shutoff valves have to

close. This has resulted in time typically in the order of1/2 to 1

1/2 minutes.

5. Bypassing any solenoid valve in the air lines with another solenoid valve and/or a check valve

to improve the response time in dumping the cylinders through the inlet port of the solenoid

valve via the ARDV is not permitted if that ARDV has any critical positive assist check valves

hooked up to it.

6. The above restrictions on the solenoid valve do not apply if the solenoid valve is used to

equalize pressures around the air cylinder as in figure 4. The solenoid valve is now in the air

line to the spring side of the air piston rather than the nonspring side. In this position it has no

influence at all on dumping the cylinder via the ARDV valve.

Note this is still fail safe, as loss of air still always calls for valve closure.

IV. BASIC RULES FOR DETERMINING NUMBER AND TYPES OF CHECK VALVES

The basic rules for determining the type and number of check valves in each extraction line to

guard against overspeed are as follows:

A. The omission of a check valve is permitted if the energy storage in the line and heater is

negligible.

B. One freeswing check valve is required when failure of the check valve to stop reverse flow

on loss of maximum load would result in no more than a moderate rise in speed.

C. One springclosed check valve, controlled by the emergency trip system, is required when

failure of the check valve to stop reverse flow, on loss of maximum load, would result in a

turbinegenerator speed rise approaching the maximum speed considered allowable for

repeated occurrences.

D. Two springclosed check valves in series, actuated by the emergency trip system, are

required if reverse flow, on loss of maximum load would result in a turbinegenerator speed

rise exceeding the maximum allowable limit.


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V. PROCEDURE TO DETERMINE THE NEED AND LOCATION OF CHECK VALVES

The item numbers referred to in the following correspond to the item numbers on the enclosed

typical Extraction Data Form (ATT. #1).

Item 1 should be determined and includes all crossties and branch line volumes entering the

extraction line on the turbine side of check valves. This item should include all volumes to theheater and heater steam volume, if and only if, no check valve is to be provided.

Item 2 should include all crossties and branch line volumes entering the extraction line after

the check valve.

This item should be omitted, if and only if, no check valve is to be provided.

Item 3 should be filled in, if and only if, a check valve is provided.

Item 4 should be filled in, if and only if, no check valve is provided.

Items 5, 6 and 7 do not apply to all extraction lines but should be filled in where applicable.

Items 8 and 9 are previously filled in by the Doosan. The quantities given are the available

energy of steam and water in each extraction line/heater combination.

Items 10, 11, 12 and 13 are determined from the previously filled in information. That is the

total volume of steam or water times its available energy. It is important here to ensure that

items 1, 2, 3 and 4 have been entered correctly, depending on check valve provisions.

Item 14 is the summation of items 10, 11, 12 and 13 and represents the total amount of

available energy per extraction. The summation of all item 15 extraction entries represents the

total unrestrained energy in the whole extraction system. This must be below the value given on

ATT. #1. If the value is above this allowable, the check valves should be relocated nearer the

turbine. Check valves if not installed should be installed.

The volume of water in heaters unprotected by reverse flow check valves has to be minimized

or the heaters run dry. If unrestrained energy is still a problem, the final alternative is to submit

data to the Doosan and we will attempt to size heater baffles which would reduce the effective

volume of water in the heater by restricting the flashing rate of the water during the time of

turbine acceleration. As the baffle flow area is reduced, significant water could collect on top of

the baffles. Sometimes relocating the cascaded drains to enter below the baffles will keep the

water volume above the baffles at an acceptable low level. Also higher pressure heaters have

more difficulty using baffles because of the large increase in available energy of a cubic foot of

water. If a suggested baffle size is issued by Doosan, the designer is to determine if the baffle

size can be installed and is to supply Doosan with the volume of water above the baffles. The

entries in the ATT. #1 form may thus require some iteration before final data is acceptable to


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the Doosan.

Items 16 and 17 are to be filled in to show what is being done to meet the individual extraction

requirements.

Items 18 through 26 should be filled in for each different check valve that is used in the

extraction system. Note these items are not under the column headings of items 1 through 17.The information requested is to assist in assuring that the reliability and performance of the

check valves will be satisfactory. They are grouped in four major areas.

Items 18, 19, and 20 relate to general information about the check valves.

Items 21 and 22 relate to the free swing performance of the check valve.

Items 23, 24, 25 and 26 relate to solenoid valve restriction of flow. Additional sheets may be

needed to fill in the data for all types of valves.

To eliminate late changes and corresponding higher costs the form should be returned to the

Doosan by the date shown on the top of ATT. #1 in partial satisfaction of schedule item ATT.

#1. It is appreciated the final data (to meet partial satisfaction of schedule item ATT. #1), might

not be obtained by this date, but good estimates would indicate whether a problem exists.

VI. OVERSPEED PROTECTION AGAINST EXTRACTION SYSTEM AUXILIARY

STEAM SUPPLIES

Should the unit undergo a load rejection, it is subsequently free to accelerate in speed at a rate

directly proportional to any energy input to it (minus rotation losses) and inversely

proportional to the inertia of the rotors. From the worst case point of view, it must be assumed

that no auxiliary load remains on the generator.

The form of energy input, which we are concerned with here, is due to externally unlimited or

auxiliary sup-plies of steam which enter a feedwater heater, an extraction pipe, a branch of an

extraction pipe or any auxiliary turbine or vessel equipment associated with an extraction pipe.

Interconnections between extractions on the same unit may also come under the above

category depending upon whether or not they constitute a by-pass path around normalprotective devices (i.e. main stop valves, control valves, combined valves, etc.). If such an

auxiliary supply, upon a worst case failure of regulators, reducers, various types of valves, etc.,

could have an open backflow path through the extraction piping to the main turbine wheels

and to the con-denser at full vacuum, the location of that supply must then receive special

consideration. Typically, because of the low condenser pressure, sonic flow generally would

exist somewhere in the backflow path. If the power of the resulting backflow can overcome

rotation losses, the unit will accelerate towards an unsafe speed as determined by the available

power then being equal to the losses at that speed. Any such source of supply is then defined

as being externally unlimited.


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The regulators, reducers or valves mentioned above, which in an emergency would require

manual operator action, are not acceptable as turbine protective devices. Two (2) power

assisted check valves (PACV) must be situated in series in some way between the auxiliary

supply and the turbine extraction wheel space. Both PACVs must be positively closed by

action of the turbine emergency trip system through the extraction relay dump valve. This is

consistent with the two in series valves in the more common turbine flow paths (main

stop/control valves, crossover administration stop/control valves, speed matching valves and


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combined valves) which also receive emergency trip signals.

Freeswing check valves (FSCV) may be required somewhere in the same extraction system

for other reasons, but they cannot be accepted as performing the function of one or both of the

above PACVs.

VII. TYPICAL ARRANGEMENTS

Arrangements 1 and 2 typify a popular BFP turbine design wherein the primary BFP turbine

supply is branched from a main turbine extraction pipe. An auxiliary supply, separately

admitted, enables normal operation of the BFP turbine while the main turbine is operating at

low flows. The backflow path, which must be guarded against, is through the BFP turbine

itself. It would take the path from the auxiliary supply, through the first BFP turbine stage, out

of the primary admission, through the primary supply branch and into the main turbine

extraction pipe. Depending upon the plant cycle, there are five possible arrangements as

follows.

A. ARRANGEMENT NO. 1

For the cycles in which the BFP turbine primary supply line branches from an extraction

pipe, which also supplies a closed feedwater heater, the first PACV should be installed in

the extraction pipe at some point between the main turbine and the primary supply branch

line (preferably as close to the main turbine as possible). This valve will also protect the

main turbine from the overspeed contribution of the steam and to some extent water in the

closed feedwater heater. The second PACV could be installed in the primary supply

branch, close to the primary supply stop valve. BFP turbine manufacturers generally

require a FSCV in the primary supply branch, close to the primary supply stop valves.

This FSCV would prevent backflow to the feedwater heater, while the auxiliary supply is

admitting, and loss of BFP turbine efficiency. However, since the second PACV would

provide for this function, the additional FSCV would not be required. The second PACV

would also provide the required protection of the main turbine against any other process

systems which may also have some form of auxiliary supply or contain unusually large

columns of steam.


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B. ARRANGEMENT NO. 2

For plant cycles in which the BFP turbine primary supply branches from an extraction

pipe which also supplies a deaerator, it would not be necessary to install two additional

PACVs. These will already be provided in the extraction pipe to protect the main turbine

from the overspeed hazard presented by the large amount of water in the deaerator. The

requirement here is that the BFP turbine primary supply should branch from the

extraction pipe at some point between the PACVs and the deaerator. In this way the

same PACVs will afford the required protection of the main turbine against either the

deaerator or the BFP turbine auxiliary supply. However, for the reasons outlined under

Arrangement No. 1, the BFP turbine manufacturer may require the FSCV. Again, the two

PACVs also provide the required protection against any other process systems.

C. ARRANGEMENT NO. 3 AND 4

These arrangements are simply generalizations of 1 and 2, respectively, just discussed. It

is recognized that there may be several other types of BFP turbine cycles and designs.

There are also a number of other plant process systems which might take their primary

supply from a main turbine extraction pipe, but also require an auxiliary source of steam

into the same branch. Discounting any auxiliary supplies of steam for the moment, the

volume of a process system, such as might be in a plant heating network, could become

so large as to constitute a nearly unlimited source of steam. All such volumes should be

taken into account by the customer when filling out Form ATT. #1, shown in this bulletin.

D. ARRANGEMENT NO. 5

Upon occasion, the volume of steam and water in a closed feedwater heater will require

only a simple FSCV, or possibly no valve at all, for protection of the main turbine. In

determining and approving protective FSCV or PACV(s) requirements, for heaters and

deaerators, the Doosan will use the data supplied on Form ATT. #1. Since such

determinations could result in some liberties in choosing the numbers and sizes of FSCV

or PACVs, the customer should complete and return the Form ATT. #1 at the earliest

possible date. The PACV is generally the more costly valve with size and it is desirable to

have the optimum arrangement of valves in an extraction system, from the standpoint of

normal flow head losses, and still meet the main turbine protective requirements. Form

ATT. #1 also provides space in which to describe the auxiliary steam supplies we have

mentioned.

VIII. MISCELLANEOUS SOURCES OF OVERSPEED


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The following list gives the possible energy sources which could result in excessive

overspeed if a load rejection occurred.

A. The deaerator and feedwater steam and water storage

B. Evaporator vapor combined with boiler feed pump turbine exhaust, or others

C. Pegging lines or startup steam connection from the boiler

D. Interconnection from the cold reheat line with some lower extraction

E. Interconnection from an extraction stage of another turbine


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F. Station header fed from boilers

G. Pressurization of heaters during startup or low load operation to maintain feedwater

temperature at the inlet of the steam generator.

The overspeed potential of these energy sources can be determined by considering the available

energy storage and the maximum possible rate at which the energy can be applied to the


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turbine rotor if reverse flow occurs. In some cases the piping may restrict the flow to such an

extent that the excessive overspeed will never be reached even though the total energy storage

is very large.

IX. PRESSURIZATION OF FEEDWATER HEATERS

On some fossil units, for example oncethru boilers of the Bensen or Universal design, it hasbeen deter-mined that there may be a severe overspeed problem even during the startup phases.

Consequently, it is a requirement to have two (2) PACVs between any heater receiving

pressurizing steam or pressurizzing water and the turbine. Also, it is a requirement to have two

(2) PACVs between the turbine and any heaters receiving pressurizing energy through

cascaded drains of higher point heaters.

Nuclear units are considerably more complicated in the feedwater cycle than fossil units.

Pressurizing feed-water heaters on nuclear units makes the opportunity greater for equipment

failure, control system failure and operator failure to cause excessive turbine overspeed or

water induction as compared to fossil units. Doosan Large Steam Turbine Department

recommends against pressurizing feedwater heaters on nu-clear units.

X. PROBLEMS OF LOWPRESSURE (LP) HEATER APPLICATION

Experience has shown that it is permissible, from an overspeed protection point of view, to

eliminate check valve protection in the last one or two LP stage heaters. This requires: first, the

proper location of the check valves of the other extraction lines and second, actual steam and

water volume reduction in the LP heaters by changing pipe routing, relocating heaters,

maintaining a lower water level in the heater, etc. or by the addition of a baffle plate below the

LP heater tube bundle but above the heater water volume to restrict the rate of flashing and

back flow.

In many cases, the LP heaters have been drained completely dry with the water level

maintained in separate drain receivers, standpipes, etc. A heater water level control adjusted to

hold the amount of saturated water storage at a minimum in the heater shell itself has been

possible in the majority of cases where it is necessary to do so. Such arrangements in heater

design and drain system have become increasingly important with the increasing popularity of

LP heaters located in the condenser neck where it is difficult to provide check valves in the

extraction lines.

Figures 5 through 8 show some typical methods of water volume reduction in heaters. They do

not cover all the possible methods or means of accomplishing this reduction but are

representative of some in use today.

Some gains can be made by considering the case shown in Figure 9, where there are X number

of extraction lines coming from the number Z extraction stage of LPA shell going to heater YA


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and there are X number of extraction lines coming from the number Z extraction stage of LPB

shell going to heater YB. If these steam extraction lines or heaters are not intertied,they may

be treated as separate extractions to separate heaters. We suggest that the data requested on

DST00173A be provided for each separate heater and extraction such as YA & YB in this

example.

XI. WET EXTRACTIONSNUCLEAR UNITS

It should be recognized that the wet extraction steam associated with saturated steam cycles will

introduce variable quantities of saturated water in the extraction piping system.

The extraction lines, therefore, must be adequately pitched and drained to prevent or minimize the

accumulation of water in the extraction piping.

Where feedwater heaters are located such that the extraction line does not continuously pitch

downward or a check or isolation valve is not present between the turbine connection and the heater

shell, special attention must be given to providing adequate drainage. This could be accomplished by

use of a drain pot with a level control, high level alarm and a bypass drain dumping directly to the

condenser.

When a check or isolation valve is located in an extraction line, the extraction piping between the

turbine connection and the valve must be adequately drained. This is done to prevent accumulation of

water when the valve is closed.

For full details of the extraction drain recommendations see the Turbine Steam Drain

Recommendations.

XII. WATER INDUCTION

The introduction of water into any part of the turbine can cause serious damage to the shells, rotors,

buckets and thrust bearings. The problem has reached serious proportions. The Doosan has revised

and reissued design recommendations to prevent water induction in large steam turbines. These

recommendations are outlined in DST00132. These recommendations closely follow the

Recommended Practices for Prevention of Water Damage to Steam Turbines Used for Electric

Power Generation, issued by the American Society of Mechanical Engineers, ASME, Turbine Water

Damage prevention Committee.

Design of an extraction system consistent with the safety and reliability required of a large steam

turbine must be undertaken by reviewing the above two publications along with this publication. This

publication is now used only to consider the extraction system from an overspeed viewpoint and to

transmit the Doosans requirements to protect the unit from overspeed.


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XIII. INFORMATION NEEDED FROM CUSTOMER

A. Forward a diagram illustrating how the emergency trip system controls the power assisted check

valves. Include high water level controls if used in conjunction with this system.

B. Forward one copy of flow diagram of steam piping and heater drain piping to assist in the review.

C. Forward a filled in copy of ATT. #1 with as many extra sheets as required.

D. The above three items should be submitted at the earliest possible time.

E. Item 3 should be resubmitted, along with any changes in items 1 and 2, in final design stages for

final engineering approval.

F. A ATT. #1 form employing Standard International (SI) units is available for use by customers

who so desire. Upon notification, Doosan will issue a form with appropriate SI units for volume. A sample

form is included in this document.


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ATT. #1 EXTRACTION SYSTEM DATA

INITIAL RETURN BY CUSTOMER ___________________

REVISION # ___________________

PURCHASER ___________________ STATION ___________________ TB. NO. ___________________

TO ENSURE NO LATE CHANGES OCCUR THIS FORM SHOULD BE RETURNED TO

DOOSAN CO. BY __________________

DATE ___________________

EXTRACTION STAGE NO./ HEATER NO.

1. VOLUME OF PIPING FROM TURBINE TO CHECK VALVES (FT.3 ) OR FROM TURBINETO HEATER PLUS HEATER STEAM VOLUME (FT.3 ) IF THERE ARE NO CHECKVALVES.

2. VOLUME OF PIPING FROM CHECK VALVE TO HEATER PLUS HEATER STEAMVOLUME. ITEM 2 ISNOT TO BE ENTERED IF THERE ARE NO CHECK VALVES.

3. VOLUME OF SATURATED WATER IN HE ATER SHELL OUTSIDE SUB COOLINGSECTION OR IN THE STORAGE TANK OF A DEAERATOR (FT.3 ) IF THERE ARECHECK VALVES.

4. VOLUME OF SATURATED WATER IN HE ATER SHELL OUTSIDE OF SUB COOLINGSEC TION (FT.3 ). ITEM 4 ISNOT TO BE ENTERED IF THERE ARE CHECK VALVES.

5. VOLUME OF WATER IN HEATER SUB COOLING SECTION OR IN SEPARATE DRAIN

COOLER, OR IN SEPARATE DRAIN TANK (FT.3 ).

6. FLOW AREA (IN.2 ) OF OPENING BETWEEN SHELL AND SUBCOOLING SECTION,OR PIPE SIZE BETWEEN HEATER SHELL AND DRAIN COOLER, OR PIPE SIZE

BETWEEN HEATER SHELL AND SEPARATE DRAIN TANK IF THERE ARE NO CHECKVALVES.

7. AUXILIARY STEAM SUPPLY TO HEATER, IF ANY. GIVE MAXIMUM CAPACITY(#/HR) PRES SURE, TEMPERATURE AND SOURCE. DESCRIBE SOURCE SO THAT ITWILL BE KNOWN WHETHER OR NOT THIS SUPPLY WILL BE SHUT OFF UPON

CLOSING OF THE THROTTLE STEAM TO THE TURBINE. ALSO, INCLUDE ANYINTERCONNECTION BETWEEN TWO EXTRACTION STAGES AS IN AUXILIARYSTEAM SUPPLY TO THE LOWER STAGE.

8. AVAILABLE ENERGY OF STEAM, BTU/PER FT.3

9. AVAILABLE ENERGY OF WATER, BTU/PER FT.3

10. TOTAL AVAILABLE ENERGY OF STEAM UNRESTRAINED BY CHECK VALVES

BTU(#1) X (#8)

11. TOTAL AVAILABLE ENERGY OF STEAM RESTRAINED BY CHECK VALVESBTU(#2) X (#8)

12 TOTAL AVAILABLE ENERGY OF STEAM UNRESTRAINED BY CHECK VALVESBTU(#4) X (#9)

13. TOTAL AVAILABLE ENERGY OF STEAM RESTRAINED BY CHECK VALVES

BTU(#3) X (#9)

14. TOTAL AVAILABLE ENERGY OF STEAM AND WATER, BTU (#10) + (#11) + (#12) +

(#13)

15. TOTAL AVAILABLE ENERGY OF STEAM AND WATER UNRESTRAINED BY

CHECK VALVES, BTU(#10) + (#12)

LIMITATIONS INDICATES PRELIMINARY DATA SUBJECT TO CHANGEIF: TOTAL AVAILABLE ENERGY PER EXTRACTION LINE ____________ BTU, BUT < __________BTU, NEED ONE (1) FSCVIF: TOTAL AVAILABLE ENERGY PER EXTRACTION LINE >____________ BTU, BUT < __________BTU, NEED ONE (1) PACVIF: TOTAL AVAILABLE ENERGY PER EXTRACTION LINE >____________ BTU, NEED TWO (2) PACV

TOTAL AMOUNT OF UNTRSTRAINED ENERGY MUST NOT EXCEED____________ BTU, REFER TO DST 00132A FOR ALTERNATES:(SUM TOTAL OF ALL AMOUNTS ENTERED IN LINE 15) NOTE USE OF BAFFLES EFFECTIVELY REDUCES THE VOLUME OF

WATER ENTERED IN ITEM 4 & USED IN ITEMS 12 & 15. IT DOES NOTELIMINATE THE WATER.

16. NUMBER OF CHECK VALVES IN SERIES PER EXTRACTION LINE.

17. STATE TYPE OF CHECK VALVE, PACV OR FSCV. IF PACV, STATE WHETHERCONTROLLED DIRECTLY BY ARDV OR HEATER HIGH LEVEL OR BOTH. STATE

BAFFLE AREA PER HEATER IF USED.

UPON RECEIPT OF THIS FORM SEE DST 00132 Doosan DATE OF INITIAL ISSUE __________________

NOTE: APPROVAL AND/OR COMMENTS OF THE SUBJECT DATA WILL BE RECEIVED BY LETTER

FROM Doosan REVISION AND REISSUANCE OF THIS FORM DOES NOT IMPLY APPROVAL.

REVISION __________________ DATE __________________


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Purchaser ____________________ Station Date Returned By Customer _________________________

Turbine No. ___________________ Revision No. __________________________


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INITIAL RETURN BY CUSTOMER

PURCHASER STATION TB.NO. REVISION #

TO ENSURE NO LATE CHANGES OCCUR THIS FORM SHOULD BE RETURNED

TO DOOSAN CO. BY

DATEEXTRACTION STAGE NO./ HEATER NO.

1. VOLUME OF PIPING FROM TURBINE TO CHECK VALVES (M3 ) OR FROM TURBINE TOHEATER PLUS HEATER STEAM VOLUME (M3 ) IF THERE ARE NO CHECK VALVES.

2. VOLUME OF PIPING FROM CHECK VALVE TO HEATER PLUS HEATER STEAM

VOLUME. ITEM 2 ISNOT TO BE ENTERED IF THERE ARE NO CHECK VALVES.

3. VOLUME OF SATURATED WATER IN HE ATER SHELL OUTSIDE OF SUB COOLINGSECTION OR IN THE STORAGE TANK OF A DEAERATOR (M3 ) IF THERE ARE CHECK

VALVES.

4. VOLUME OF SATURATED WATER IN HE ATER SHELL OUTSIDE OF SUB COOLINGSECTION (M3 ). ITEM 4 ISNOT TO BE ENTERED IF THERE ARE CHECK VALVES.

5. VOLUME OF WATER IN HEATER SUB COOLING SECTION OR IN SEPARATE DRAINCOOLER, OR IN SEPARATE DRAIN TANK (M3 ).

6. FLOW AREA (CM2 ) OF OPENING BETWEEN SHELL AND SUBCOOLING SECTION, OR

PIPE SIZE BETWEEN HEATER SHELL AND DRAIN COOLER, OR PIPE SIZE BETWEEN

HEATER SHELL AND SEPARATE DRAIN TANK IF THERE ARE NO CHECK VALVES.7. AUXILIARY STEAM SUPPLY TO HEATER, IF ANY. GIVE MAXIMUM CAPACITY

(KG/HR) PRES SURE, TEMPERATURE AND SOURCE. DESCRIBE SOURCE SO THAT ITWILL BE KNOWN WHETHER OR NOT THIS SUPPLY WILL BE SHUT OFF UPON CLOSINGOF THE THROTTLE STEAM TO THE TURBINE. ALSO, INCLUDE ANYINTERCONNECTION BETWEEN TWO EXTRACTION STAGES AS IN AUXILIARY STEAM

SUPPLY TO THE LOWER STAGE.

8. AVAILABLE ENERGY OF STEAM, KILOJOULES/M.3

9. AVAILABLE ENERGY OF WATER, KILOJOULES/M.3

10. TOTAL AVAILABLE ENERGY OF STEAM UNRESTRAINED BY CHECK VALVES

KILOJOULES (#1) X (#8)

11. TOTAL AVAILABLE ENERGY OF STEAM RESTRAINED BY CHECK VALVESKILOJOULES (#2) X (#8)

12 TOTAL AVAILABLE ENERGY OF STEAM UNRESTRAINED BY CHECK VALVESKILOJOULES (#4) X (#9)

13. TOTAL AVAILABLE ENERGY OF WATER RESTRAINED BY CHECK VALVESKILOJOULES (#3) X (#9)

14. TOTAL AVAILABLE ENERGY OF STEAM AND WATER, KILOJOULES (#10) + (#11) +(#12) + (#13)

15. TOTAL AVAILABLE ENERGY OF STEAM AND WATER UNRESTRAINED BY CHECKVALVES, KILOJOULES (#10) + (#12)

LIMITATIONS INDICATES PRELIMINARY DATA SUBJECT TO CHANGEIF: TOTAL AVAILABLE ENERGY PER EXTRACTION LINE _____________ KJ, BUT _____________ KJ, BUT _____________ KJ, NEED TWO (2) PACV

TOTAL AMOUNT OF UNTRSTRAINED ENERGY MUST NOT EXCEED_____________ KJ, REFER TO DST 00132A FOR ALTERNATES:(SUM TOTAL OF ALL AMOUNTS ENTERED IN LINE 15) NOTE USE OF BAFFLES EFFECTIVELY REDUCES THE VOLUME OF

WATER ENTERED IN ITEM 4 & USED IN ITEMS 12 & 15. IT DOES NOT

ELIMINATE THE WATER.

16. NUMBER OF CHECK VALVES IN SERIES PER EXTRACTION LINE.

17. STATE TYPE OF CHECK VALVE, PACV OR FSCV. IF PACV, STATE WHETHERCONTROLLED DIRECTLY BY ARDV OR HEATER HIGH LEVEL OR BOTH. STATE BAFFLE

AREA PER HEATER IF USED.

UPON RECEIPT OF THIS FORM SEE DST 00132 Doosan DATE OF INITIAL ISSUE________________

NOTE: APPROVAL AND/OR COMMENTS OF THE SUBJECT DATA WILL BE RECEIVED BY LETTER

FROM Doosan REVISION AND REISSUANCE OF THIS FORM DOES NOT IMPLY APPROVAL.

REVISION __________________ DATE ___________

Md1 0 v 111-09-00008 a Extraction System Requirements

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