Wednesday, August 01, 2012
Steam Valves
Various types of valves are used in conjunction with
steam turbines to control or regulate the flow of steam
to and from the unit. Figure 1 shows a typical valve
arrangement schematically. In general, these valves
are either speed or pressure responsive. Their specific
functions, however, result in a wide variety of forms,
shapes and control requirements. In thinking of the principal valves used in turbines, it
will be helpful to group those having a common purpose. For example, the main stop valve, control
valves, intercept valve, reheat valve, and admission/extraction valves admit steam to the turbine for operation or interrupt the steam when overspeed
protection is needed. These valves are also interrelated to perform the other essential functions of starting and controlling speed and load. Other pressure and temperature sensing valves may
be grouped by system, such as those in the steam seal,
lube oil and hydraulic fluid systems. Still another
group might include individual single function devices such as the packing blowdown valve, ventilator
valve, and the diverter valves. Valves in the "systems" grouping are described in the
discussions dealing with the particular system of interest. The rest of the valves are described in the paragraphs which follow, beginning with those that
control the flow of steam to the turbine. (It will also be
helpful to consult a typical turbine control system diagram in conjunction with this discussion, if one is
available.)
Fig. 1 Typical Turbine Valve Line-up
CONTROL VALVES While the basic function of steam turbine valves is to
regulate the flow of steam to and from the turbine,
highly responsive control valves such as the inlet and
extraction configurations as shown in Figure 2 are required to control steam flow inside the turbine. Since
the main stop valve that admits steam to these control
valves (Figure 1) functions primarily to protect the
unit from the steam energy in the boiler in emergencies, it is described later with the emergency valves.
For our purposes here, it is sufficient to say that the
main stop valve also regulates steam flow during
warm-up by means of an inner bypass arrangement,
and when the unit is ready for loading it opens wide to
transfer control of the steam to the control valves. In
an emergency, however, the stop valve will slam shut
in response to the release of oil (or hydraulic fluid) from its operating cylinder, shutting off all steam flow
from the boiler. Figure 1 shows a typical steam path schematically, beginning at the boiler and moving through the main
stop valve into the high pressure turbine, under control of the inlet control valves and extraction valves.
Both the inlet control and extraction valves, Figure 2,
are tied together, so to speak, by means of the control
system, such that a change in one during operation
will cause compensating changes in the other(s). Inlet control valves are used to regulate the flow of
steam into the steam turbine in a very accurate and
precise manner. The valves are positioned in response
to signals from the control system so that the steam
flow through the turbine will produce exactly the
right amount of power to just match the turbine load at
the desired frequency or speed. Control valves must,
therefore, be able to operate at large or small pressure
drops, over a range of openings from just cracking to
wide open and be stable so as not to chatter and wear
or cause instability in the control system. Extraction valves are sometimes called "spillover"
valves because of the manner in which they function.
Each extraction valve, by controlling the amount of
steam that passes through it (or "spills over") into
succeeding stages downstream, indirectly controls the amount of steam being extracted at the stage immediately in back of it on the upstream side. That is,
instead of allowing all the steam to travel freely
through the turbine wheels, an extraction valve allows only a portion of the flow to pass, thus forcing
the "bottled up" portion immediately upstream to
leave through the extraction openings, as shown in
Figure 2. Thus, the amount of steam being extracted
for processing purposes depends indirectly, but surely, on the rate of flow permitted by the extraction
valves. To maintain efficiency, it is important to be able to
control the turbine steam flow with a minimum pressure drop through the control valves. Multiple control
valves and nozzle sections are used to achieve this.
Four to eight valves are normally used in modern turbines. The cross-sectional view of Figure 2, for example, shows a single inlet control valve, while
actually there will be from four to eight of these
valves at this location, with each feeding a segment of
the same nozzle plate as shown in Figure 3. Extraction valves, too, are similarly arranged. A brief review of the valve types and valve gear arrangement commonly employed in the control of
steam turbines will be beneficial here.
VALVE TYPES
Mechanical design considerations of the turbine,
steam pressure, steam flow, and manufacturing cost
considerations have resulted in a number of different
valve designs. Many of these designs fall into the following general group: 1.
Venturi valves
2.
Poppet valves
3.
Double lift valves
4.
Balanced valves
5.
Spool type valves
6.
Grid type valves
Each of these basic valve types has certain characteristics which suit it better than the other types for specific applications.
Fig. 2 Double Automatic Extraction Turbine Inlet and Extraction Control Valves
Fig. 3 Typical Multiple Control Valve Arrangement Venturi valves, Figure 4, are popular for single valve
or multiple valve application because the venturi seat
has a low pressure drop and a high flow coefficient, which permits using the smallest possible valve sizes to pass a given flow of steam. That is, the shape of this
valve is such that the steam loses very little pressure
(or power) in passing through it. The valve disk is
normally a ball or sphere arranged to be stable
through its effective lift to minimize or eliminate
valve chatter and vibration. The seat usually has a
short, conical, contact surface so that the spherical
ball will seat in the cone, forming a line contact for
tight seating. A ball valve also has the ability to maintain a tight contact with its seat even though the valve
disk may be tipped somewhat. Formerly, venturi
valves and seats were quite expensive compared to
poppet valves, but modern manufacturing techniques
have substantially reduced these differences.
Poppet valves come in a variety of shapes, but basically they have matching conical surfaces on the
valve and valve seats or in some cases a radius on the
valve (but not a spherical surface). The valve seats are
simple and relatively inexpensive as can be deduced
from Figure 5. Since this type of valve is not able to
seat tightly in a tipped position, it must be accurately
Fig. 4 Venturi Valve
Fig. 5 Poppet Valve
positioned for tight seating. The flow coefficient of a
poppet valve seat is somewhat less than for a venturi
seat, so a larger diameter poppet valve must be used to
obtain a given effective valve area. This means a larger contact diameter and higher lifting force are required for a given steam pressure unbalance
compared to a venturi valve of equal effective area.
The poppet type valve is often used on smaller turbines where valve lifting force is not critical.. In general, poppet valves are not as stable as venturi valves
and they have a tendency to chatter when applied in
sizes over four inches and in areas experiencing over
600 PSIG. Almost all new turbines larger than 5000
kW have venturi type valves rather than poppet
valves. Balanced valves are used for larger valves (from 4
inch to 20 inch sizes) which must open against full
steam pressure unbalance. The upper portion of the
valve disk fits into a balance chamber to form a piston. The advantage here is that the valve actuator has
to overcome only about 25 percent of the force which
would be required if the valve were not balanced. The
diameter of the piston and chamber is usually made
smaller than the valve disk contact diameter to create
a differential area which provides a stabilizing force
on the disk and stem. A pilot valve either inside the
valve disk or outside the valve body permits the steam
in the balance chamber to flow to the downstream
side of the valve disk and equalize or balance the pressure in the balance chamber with respect to the downstream steam pressure. This pilot valve is also called
an equalizer valve, bypass valve, or internal pilot
valve. These valves are relatively expensive, but again their cost is justified by virtue of the reduced
valve operating forces attained. Also, stable operation depends largely on provision of the proper unbalance forces. Alignment of the parts is critical to tight
seating and quiet, satisfactory operation.. Spool type valves, Figure 6, are a special form of balanced valves. Two valve disks are welded to a tubular
stem and two valve seats are mounted in a common
body to provide a balancing effect. Spool valves are
used for high flow, low pressure, and control valve applications. The valves are usually mounted on the
face of a control stage nozzle diaphragm rather than in
a steam chest. Up to eight valves are spaced around a
pitch circle on the diaphragm to permit supplying
steam to a full 360 degrees of nozzle arc, as compared
to many of the arrangements which supply steam to
only 180 degrees of nozzle arc (reference Fig. 7). Further, this type of valve will not usually seal quite tightly enough to prevent some degree of leakage.
Comparatively, these valves are also more expensive
than poppet or venturi valves, but they are capable of
handling a much greater volume flow capacity than
Fig. 6 Spool-Type Valve
Fig. 7 Typical Spool-Type Valve the other valve types. The spool valve gears are either
cam lift or bar lift, depending on the required accuracy and linearity of the valve flow lift characteristic Grid valves, one of the oldest valve designs in existence, operate much like the air damper in old cast-
iron stoves. The diaphragm and grid ring are flat on
their contacting faces and have sequentially overlapping ports as shown in Figure 8. As the grid ring is rotated relative to the stationary diaphragm, the ports in
the diaphragm are uncovered sequentially and steam
can flow through it to the nozzles. A grid valve requires less axial space in the turbine casing than does
a spool valve assembly, and its cost is comparatively
less. However, it has the drawback of requiring large
operating forces to slide one ring relative to the other
because of the friction between them: this results
from the steam pressure unbalance forcing the two plates together. Present day turbines usually do not
use grid valves except for very special cases because
the operating forces are so high that 400 or 600 PSIG
steam is generally used to operate the
grid.
Fig. 8 Grid-Type Valve Thus, the turbine designer has several possibilities to
keep in mind when he is choosing the turbine valves
and valve actuators or operators. Economical
construction requirements make the pilot poppet or
double lift valves attractive on small turbines because
the operating forces are smaller with this type valve,
and will not require excessive pump capacity or large
hydraulic cylinders. Very large, high pressure turbines use balanced type control valves to reduce the
valve operating forces to a level which can be handled
by reasonably sized hydraulic cylinders, utilizing
high pressure oil or hydraulic fluid.
VALVE GEAR ARRANGEMENTS Now that we have covered some of the design considerations for valves, it will be interesting to see how the
mechanical parts make up the different types of valve
gear.
Cam Lift Valve Gear The cam lift valve gear has broad application over a
wide range of steam pressures, turbine ratings, and
designs. Figure 9 presents a typical cam lift gear
found on a large number of medium steam turbines.
The valve stem is guided in a stem bushing, and the
small clearance between them acts as a labyrinth
packing to minimize the leakage of steam. Both the
stem and bushing surfaces are treated to achieve a
high resistance to wear, erosion, corrosion, and scoring by foreign material such as scale and boiler com¬
pound. The valve disk is pinned to the stem by a pin
fitted tightly in the disk and loosely in the stem. This
clearance between the pin and the stem permits the
valve disk to remain seated even though the stem and
valve seat alignment may vary due to thermal expansion, pressure loading, or mechanical misalignment. The valve stem's upper extremity is typically
threaded into a lever which has a fulcrum on one end
and a cam roller on the other, as shown in Figure 9.
Each cam roller, in turn, is in contact with a cam
mounted independently on a common camshaft
where shown. To turn the camshaft, a pinion and gear arrangement is generally employed. Typically, a pinion mounted on the end of the camshaft is driven by a
rack or segmental type gear, which is linked by means
of a rod directly to the operating cylinder. Usually, the cam is shaped to produce a governing
point lift of the valve in 15 to 40 degrees of cam rotation. The typical cam lift valve gear utilizes spring
force on the valve stem plus the valve disk steam load
to close the valves. The spring force is sufficient to
counteract the valve stem steam unbalance force plus
up to several hundred additional pounds bias at the
valve cracking point lift. Cracking point adjustments
for the arrangement shown in Figure 9 are made by
turning the upper collar and nut to raise or lower the
stem relative to the lever. Such adjustments will vary,
of course, depending on the design of the linkage utilized in any given application . Depending upon the application, steam turbines control valves will admit steam to the upper half shell,
only, however, many designs also incorporate a similar arrangement in the lower half to feed the bottom
180 degrees of nozzle arc. This lower half arrangement is essentially an upside down version of the upper arrangement. The use of control valves in the
lower half casing is necessitated by first stage bucket
and nozzle design considerations and introduces
somewhat more complex valve gears. In these cases,
the upper and lower valve gear camshafts are actuated
simultaneously by a common hydraulic servomotor.
Typically, mechanical linkage between the two valve
gears maintains the relative timing of the valves.
Linkage adjustments are also provided for adjusting
the relationship between the hydraulic piston and the
two valve gears.
Bar Lift Valve Gear Simple in concept, this type of valve gear features a
lifting beam, or bar, to manipulate the control valves.
Lift rods raise or lower the lifting beam in response to
signals from the control system. As the lifting beam is
raised, the valves are lifted one at a time in a sequence
related to the lengths of their stems. That is, the valve
with the shortest valve stem will be lifted by the beam
first.
Fig. 9 Control Valve Assembly Close fitting bushings minimize the amount of steam
that can leak along the lift rods, with any leakage being routed to a leakoff chamber away from the operating room. Cracking point adjustments are simple on
the direct lift valve gear where the lift rods are machined to finite lengths so that the beams will always
lift in a horizontal position. The lever operated bar lift
gear requires more care in that each lift rod can be
turned in its threaded clevis to raise or lower ends of
the beam. It is important that both lift rods be adjusted
so that the beam will maintain its horizontal position
as it passes through the effective valve lift for all of
the control valves.
Spool Valves Spool valves permit high volume steam flows and
efficient method of full arc control in extraction
applications. As described earlier, they are double-
seated valves mounted in separate bodies. The
bodies, in turn, are mounted in a circular arrangement
around the diaphragm. A major advantage of this type
of construction is that steam is supplied essentially
around a full 360 degree arc, thus resulting in more
uniform heating and less distortion. As shown, spool valves are mounted two to a stem,
and they lift sequentially, the lower one first. In most
cases, a cam type valve gear is used for purposes of
linearity, and, once again, control system signals are
typically transmitted to a camshaft by means of a hydraulic servomotor and a pinion-gear arrangement.
The double seated design provides pressure balancing of each valve, thus reducing the force needed for
opening and closing. Valve lift is usually determined by machining spacer
pieces for the inner two pairs of valves and by machining the length of the spool valve itself on the outer
two pairs. In practice, these valves are not expected to
be absolutely tight, but they can be made essentially
tight with negligible leakage by lapping to produce simultaneous contact on the upper and lower contact
surfaces for each spool valve. Although the mechanical construction of the spool valve assemblies may
also seem somewhat limber, this facilitates assembling the parts as well as minimizing any binding effects that the spool valve assembly might experience
over its range of
operation. This type of valve gear was developed primarily to replace the grid type valve gear discussed next. Although this spool valve arrangement has traditionally
been used in extraction applications, it is also being
used for inlet control purposes in cases where the effective valve area requirements exceed those available using poppet or venturi type valves.
Grid Valves Although this type of valve gear is not, in most cases,
being applied to newer units, many of them still exist
in the field, making a few words appropriate. The grid valve is essentially two rings which slide,
one relative to the other, in a rotary fashion. Each
plate has a series of ports, usually six or eight, which
are arranged so that as one plate rotates relative to the
other, the effective areas which permit steam to flow
through the ports are essentially linear with angular
degrees of rotation. The ports connect to sectional
nozzle-arc chambers in the diaphragm to provide the
flow path for the steam. While this type of valve takes up very little space as
compared to the spool valve arrangement, its limiting
factor is the amount of force required to rotate the
variable plate. Because of the high frictional forces
involved, steam operated actuators are often used in
place of hydraulic cylinders. Rotation of the variable
plate is accomplished through a ring and gear mechanism mounted on the top of the assembly.
EMERGENCY VALVES
STOP VALVE In addition to the safety aspects, controlling the degree of overspeed in a turbine is also very important in
maintaining reasonable design margins. That is, turbine machinery is limited to a specific amount of safe
overspeed operation consistent with a balance between economical turbine parts and good thermal efficiency.
Fig. 10 Main Stop Valve Assembly The main stop valve's primary function is to
provide a second line of defense (or back up protection) against the energy from the boiler in the event
that the inlet control valves fail. Moreover, the main
stop valve also closes upon routine shutdown or by
operation of certain boiler trips and other turbine devices that actuate the emergency trip system. This
valve, Figure 10, has been designed to provide extremely reliable control of the steam, under both routine and emergency conditions. Actually, the main stop valve (or valves) can be considered part of the emergency trip system. Its primary
function is to shut off, as quickly as possible, the flow
of admission steam to the turbine in case of an abnormal operating condition. The valve, therefore, is of
the quick closing type and can be tripped by means of
the mechanical trip on the turbine front standard, by
the action of the overspeed governor during an overspeed condition, by energizing a trip solenoid (which
reacts to such abnormal operating conditions as low
vacuum or low bearing oil pressure), or by any other
mechanism included for that purpose in the trip circuit. Except for a warm-up provision, the main stop valve
is not used as a throttle valve and has only two positions, wide open or fully closed. It cannot ordinarily be
opened unless the turbine control valves are closed.
However, a limited amount of throttling is accomplished by means of the stop valve in full-arc starting
units to facilitate warm-up and initial loading, as will
be discussed. Referring to Figure 10, it can be seen that the valve
body contains the steam inlet and outlet openings, the
above and below seat drains, the valve seat for the
main valve disk, a valve stem leakoff, and the valve
stem
bushing assembly. A removable cylindrical
steam strainer with its temporary fine mesh screen
surrounds the stop valve assembly to prevent boiler
and steam line contaminants from entering the turbine. The main valve disk is mounted on the valve stem and
contains a steam pilot valve. This pilot valve when
open allows steam to flow through the orifices in the main valve disk into the lower chamber of the valve
body. By building pressure beneath the disk in this
way, differential pressure across the disk is reduced, making it easier to open. Located in the base of the valve body just below the
valve seat is the valve stem bushing assembly, which
prevents steam leakage and resultant boiler deposits
along the valve stem during operation and provides
an intermediate stem seal leakoff. In the upper portion, opposite the steam inlet connection is a vertical
baffle which blocks off the annual space between the
outside of the steam strainer and the valve body. This
minimizes the effect of steam eddy flow or swirling
which is detrimental to the flow characteristic of the
valve and can unnecessarily increase pressure drop.
The baffle also stops solid particles such as dirt, metal
chips, shot blast, and welding bead which may be carried into the valve by the steam flow. Particles which
are too large to pass through the steam strainer are deflected around the outside of the strainer where they
pass into the annular space to the baffle. Having been
stopped by the baffle, they drop to the bottom of the
valve body on the above seat side. This section of the
valve should be inspected and any accumulation removed whenever the valve itself is opened for inspection or maintenance work. The hydraulic cylinder that moves the valve stem up
and down is shown coupled beneath it in Figure 12.
Both the valve stem and the cylinder piston are spring
loaded in the closing direction. Figure 11 illustrates
this section of the assembly for a MHC controlled unit - EHC units will utilize a somewhat similar construction with the dump valve contained within the actuator housing. Note that a manifold directs hydraulic
fluid between the hydraulic cylinder and dump valve.
During normal operation, hydraulic fluid from the
trip header flows through a passage in the dump valve
body and through the manifold to the underside of the
hydraulic cylinder piston: During a trip condition,
fluid drains from under the hydraulic cylinder piston
through the dump valve, from which it returns to the
hydraulic cylinder head and finally back into the cylinder above the piston. The dump valve, which makes
it possible for the stop valve to trip closed during an
emergency condition, contains a piston and spring loaded spool. After the trip condition has been corrected, the incoming fluid is again directed through an
orifice in the dump valve head where shown and continues through the manifold to the underside of the
hydraulic cylinder piston. As the piston is forced upward it lifts the main valve disk off its seat, returning it
once again to the open position.
Fig. 11 Hydraulic Cylinder and Dump Valve
Typically, the stop valve is designed as a 100 percent
unbalanced type, i.e., it cannot be opened against
rated steam pressure. To open the valve against full rated steam pressure, the control valves must first be
closed and the emergency trip system reset to route oil
to the hydraulic cylinder. As pressure under the hydraulic cylinder piston forces the piston to move in
the opening direction, the steam pilot valve begins to
raise. Raising the pilot valve allows steam flow
through the orifices of the main valve disk into the
lower valve chamber, thus building pressure under
the disk. When the differential pressure across the
disk drops to approximately 13 to 18 percent of rated
steam pressure, the main disk opens automatically Figure 12 also illustrates schematically the arrangement of the position signal switches that are used to
indicate when the stop valve is open, closed or at the
test position. The circuit breaker switch is also used to
sense that the stop valve is closed before the generator
circuit breaker is opened to disconnect the generator
from its load bus and the inherent speed limiting feature of the AC distribution system. Turbines placed in service at different times in the
past will also incorporate slightly different hydraulic
mechanism designs, since this hydraulic relay has
gone through several stages of evolution. Early valves were tripped directly by the overspeed
trigger and required all of the oil flow to pass from the
hydraulic piston through the dump valve of the emergency trip. Later designs employ a trip relay and bypass so that the emergency trip is only required to pass
the relay piston-oil displacement, and the dump
valve bypasses most of the main cylinder oil to the upper side of the stop valve cylinder. This type valve
also incorporates a test function to permit partial closing of the valve with the turbine under load so that the
valve action can be checked to determine whether or
not it will close when required. Another design employs a test function so that two stop valves can be
used in parallel, and each valve can be closed completely during the testing cycle. Many turbines now utilize a full arc starting stop
valve which employs a remotely operated valve positioner and a steam-pilot valve with sufficient bypass
capacity to permit warming, synchronizing, and partial loading of the turbine. Full arc starting, in simple
terms, means that the incoming steam passes through
all 360 degrees of the turbine nozzle plate or box during starting, to promote even heating and reduce
stresses in the heavy, high pressure shell cavities.
When used, the full arc admission feature of the valve
makes it possible to control (throttle) the flow of inlet
steam to the turbine during starting and initial loading. To do this, a bypass valve inside the main valve
disk is used to pass a portion of full throttle flow (up to
40 percent) with the turbine control valves wide open.
During full arc operation, steam flow is uniform and
velocities relatively low through critical shell passages. Heat transfer coefficients are small and, since
metal temperatures do not change rapidly, thermal
stresses are reduced.
COMBINED STOP AND CONTROL VALVES A number of modern, medium sized steam turbines
are now utilizing a combined stop and control valve,
that is, the control valve and stop valve are both contained in the same casing similar to the construction
found in combined reheat/intercept valves (Ref. Figure 13). The advantage of this configuration is obvious - cost of the valve casings is reduced and
piping is simplified. Currently, depending on the rating of the machine, one combined valve is used or
two valves may be combined in parallel. Note that
with this valve configuration, operation in partial arc
steam admission is not possible since only one (or
two) valve(s) actually controls steam admission to
the 1st stage nozzle area.
REHEAT STOP VALVES AND
INTERCEPT VALVES In the foregoing paragraphs, the role of the main stop
valve in protecting the unit from the steam energy in
the main boiler was discussed. Reheat turbines also
incorporate a reheat boiler, or reheater, in addition to
the main boiler. Since this reheater, too, is a powerful
source of steam energy, additional protective valving
is necessary in such units. Reheat stop valves and intercept valves are commonly used for this purpose. In the event of a sudden drop in generator load, the
steam flowing from the reheater and associated piping could drive the turbine to a dangerous overspeed
level. The intercept valve offers normal, or pre-emergency, protection against this by shutting off the
steam flow with the reheat stop valve acting as a backup or second line of defense in case the normal or pre-
emergency control devices fail. The intercept valve is usually controlled by a pre-emergency speed governor which typically goes into action when turbine speed increases to about 101 percent or more of rated speed.
Fig.13 Combined Stop and Control Valve
Fig. 14 Combined Reheat Stop Valve and Intercept Valve (However, the intercept
valve can also be tripped closed upon actuation of the
emergency trip system.) If speed continues to increase, typically between 110 and 112 percent of
rated speed the emergency speed governor will act
through the emergency trip system to close the reheat
stop valve. This valve will also close upon a routine
shutdown or by operation of certain boiler and electrical trips that actuate the emergency trip system. In terms of evolution, the intercept valve was originally located in the hot reheat steam line remote from
the turbine. Later, it was mounted on top of the turbine
shell in some units; still later, it was preceded by a reheat stop valve. More recently, the reheat stop and intercept valves have been integrated into a single valve casing, Figure 14, attached directly to the turbine
shell. By arranging this assembly in the hot reheat
lines, as close as feasible to the turbine inlet openings,
the entrained steam volume is reduced, thus limiting
potential overspeed. Although a common valve casing is utilized for these
combined valves, the reheat stop valve and intercept
valve provide different functions and have separate
operating mechanisms and control. Steam from the
reheat boiler enters the single inlet of each valve casing, passes through a strainer, continues through the
intercept valve and reheat stop valve disks, and discharges from a single outlet connected directly to the
reheat turbine section. The intercept valve, which is cylindrical, is located
above the reheat stop valve disk with its stem extending through the upper head. The reheat stop valve
stem extends vertically downward through the below
seat portion of the casing. Both valves share a common seat, however, the intercept valve is designed to
operate independently regardless of reheat stop valve
position, and vice versa. The intercept valves operate fully open for full arc admission starting, and remain fully open during the
transfer of steam flow control to the inlet control
valves and for all other periods of normal operation.
Upon deceleration of the turbine after a load rejection, the intercept valves are automatically positioned
to control speed during blowdown of the reheater before the control valve is reopened by means of the
speed governor. After being tripped closed, the intercept valves will reopen automatically when the emergency trip system is reset. The reheat stop valves also
open fully upon resetting the emergency trip system,
and they remain fully open for all normal and pre-emergency operation. Details of the hydraulic operating cylinders that move
the valve stems up and down can be seen in Figure 14.
Although the reheat stop and intercept valves share a
common seat, as indicated, each is actuated independently by its own operating cylinder as shown. In
units having two combined valves, however, the intercept valves will usually be operated in a master-slave relationship, with the operating cylinder of the
"master" intercept valve taking primary control; the reheat stop valves will continue to operate independently.
The combined valves are also equipped with solenoid
operated test devices which permit closing of the reheat stop and intercept valves to ensure that they are
free to close in the event of a trip signal. When two combined valves are used, the electrical test logic prevents one combined valve from being tested when the
other is in the test mode. This valve also incorporates a steam strainer to prevent foreign material from being carried through the
valve to the turbine. The strainer consists of a heavy
walled cylinder, over which is fastened two layers of
heavy wire mesh screen. The inner layer is a permanent coarse mesh screen which is always in place,
while the outer layer is a temporary fine mesh screen
which is used to trap small shot and other fine particles from being carried into the turbine at initial
startup or after boiler tube repairs.
EXTRACTION NON-RETURN
VALVES The extraction non-return valves are also known by
several other names such as non-return valves, bleedercheck valves or swing-check valves. Figure 15
shows a typical power actuated non-return valve. Basically, they are free-swing or air-operated check
valves. In general, the valve consists of a disk which
floats on the steam flow and is pivoted about a hinge
system in the upper part of the valve body. On larger
or power operated valves, the pivot or rockshaft
passes through a bushing so that the disk can be counterbalanced by an external weight. The rock shaft is
also linked as shown to the power actuated cylinder to
transmit a closing force to the valve disk. The location of the non-return valves is very important in limiting the amount of entrained steam energy
contained between the valves and the turbine casing.
If this entrained energy is too great, the turbine can be subjected to dangerous overspeeding even though all
the valves work properly.
Fig. 15 Extraction Non-Return Valve The power actuated cylinder is normally designed for
air operation and is controlled by the turbine emergency trip system through an oil-air relay. The air
cylinders are also generally equipped with a hand operated test valve so that operation of the piston and
rock-shaft can be observed periodically to assure that
it is capable of operating satisfactory in an emergency. Further, they may be sensor controlled to close
should a feedwater heater become flooded, to prevent
water induction into the turbine with the possibility of
forced outages, severe damage, or shortened life of
the various turbine parts. The power-actuated cylinder does not have sufficient force to close the valve
disk against more than a fraction of rated pressure; it
is only intended to supply enough closing force to
swing the valve closed if the steam flow approaches
zero or changes to a back flow. Even though construction of the non-return valve is
quite simple, they are still subject to some of the problems experienced with any valve, particularly because the disk "rides" the steam flow and is subjected
to constant buffeting by steam turbulence. In practically every incidence of a malfunctioning non-return
valve, the dangers of uncontrolled overspeed could
have been avoided by periodic inspection, maintenance, and testing to assure that they were in reliable
operating condition.
PACKING BLOWDOWN VALVES Many turbines, such as the opposed flow reheat unit
represented in Figure 1, require automatically operated
valves to divert or dispose of packing leakage
steam. For example, when a reheat turbine is tripped
out while carrying load, the closing of the main control valves and intercept valves bottles up a large volume of high pressure steam in between them,
particularly in the reheat boiler. With vacuum existing in the intermediate and low pressure sections of
the turbine, the high pressure steam will be invited to
throttle directly through the shaft packings between
the high and intermediate pressure sections, in the manner of a "short circuit," so to speak. If these packings are worn, it is possible that there may be sufficient steam flow leakage to drive the unit to
overspeed. To prevent this, an air operated blowdown
valve opens (at the same time the intercept valves
close) to divert most of the steam leakage from the
leakoff annulus of the shaft packing directly to the
condenser. The valve stem is guided in two bushings which provide high and low pressure stem leakoffs that are
piped to drain. A small bypass valve is loosely attached near the bottom of the valve stem by a pin that
travels in keyways in the main valve disk to prevent
any rotation of the bypass valve relative to the main
valve disk. As shown, the main valve guide bushing
guides the travel of the main valve disk, which has a
spacer and two piston rings to restrict steam flow
when the valve, is being opened. Both the main valve
disk and the bypass valve have inserted valve seats,
and all contact seating surfaces of the valves and their
seats have been hardened. In operation, air pressure above the power piston
overpowers the piston spring and holds the main
valve disk closed against the valve seat. With the
valve in the closed position, steam from the inlet side
will leak past the piston rings of the valve disk and establish full steam pressure on the top of both the main
valve and small bypass valve. Large drilled holes in
the valve cap help establish steam pressure above the
small bypass valve. When air pressure is removed,
the small bypass valve initially travels about 3/8 inch,
thereby blowing downstream pressure from the top of
the main valve disk in order to establish pressure on
both the bottom and top of the main valve disk. This
permits the valve to open and blowdown the high
pressure steam to condenser vacuum.
VENTILATOR VALVE In the event of a load rejection, or following a trip after carrying load, the high pressure section of an opposed flow turbine such as that shown in Figure 1 may
overheat due to windage losses: such losses can occur
as a result of being allowed to spin in the high pressure, high temperature steam, bottled up between the main stop valve and the reheat stop valve.
Fig. 16 Effects of Ventilator Valve on High Pressure Turbine Flow and Temperature (upon loss of Load and Trip) As the turbine increases speed above the rated value in this high
density steam, rotational losses quickly raise the temperature of the buckets and related parts. The combination of overspeed (higher stress levels) and
increased temperature (lower strength) may cause
damage to these parts. The design stress capability of materials typically
used for buckets and covers can decrease as much as
50 percent or more with a temperature increase from
700°F to only 1000°F. It is therefore apparent that in a very short time many parts and thin sections in the turbine could be experiencing some degree of distress if
there were no provision for ventilation. To alleviate this problem, a ventilator valve is incorporated in the turbine piping arrangement, as shown
schematically in Figure 1. When the ventilator valve
is automatically opened following a trip out, the high
pressure steam trapped in the reheater and high pressure turbine flows in a reverse direction through the
turbine to the condenser in response to the large pressure differential. When this happens, it is the relatively cooler steam from the reheater system that
maintains the high pressure turbine parts at reasonable
temperatures. The ventilator valve is of balanced design to allow the
use of a small operating mechanism. Once again, the
operator is an air piston which gets its signal from an
air valve on the speed relay. Thus, when the control
valves close, the ventilating valve opens, allowing
steam to flow to the condenser. Figure 16 illustrates what happens to the flows and
temperatures in a high pressure turbine when the ventilator valve opens. Upon loss of load and trip out, the
trapped steam would be quickly heated to excessive
temperature if there were no ventilator valve. However, with a quick opening ventilator valve, the reverse
flow keeps the high pressure turbine last stage at essentially normal exhaust temperature. As friction
losses increase the temperature of the steam as it
progresses toward the first stage, protection is provided against both excessive heating and abnormal
cooling.
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