MAINTENANCE MONITOR SYSTEM FOR STEAM TURBINE
VALVES
TECHNICAL FIELD
This invention relates generally to steam turbines and more
particularly to a monitoring system for steam turbine valves.
BACKGROUND
It is important for the safe and reliable operation of a steam
turbine that valves and associated actuators open and close to control
steam flow as required by the control system. Due to the severe
operating conditions in high pressure and high temperature steam
applications, it is necessary to inspect these valves frequently for
wear, excessive friction, oxidation of sliding parts, etc. in order to
insure the required reliability. These inspections sometimes require
costly shutdowns of the turbine for disassembly and inspection of the
valves.
At the present time, steam turbine valves are also "tested" by
closing and reopening them on a regular basis while the turbine is
operating online. The valve is observed visually to determine that the
valve stem motion is smooth and that full stroke is obtained. Present
online valve testing methods, however, do not provide quantitative
data on the operating condition of turbine valves, thus the
maintenance schedule is set using average experience, and must be
set conservatively for safety and reliability reasons. As a result, little
indication of an impending problem is given by the present tests, and
this may result in very costly unscheduled shutdowns. As a result,
valve maintenance requirements are often a limiting item, dictating a
conservative and therefore costly maintenance schedule.
It is also known to measure valve stroke vs. force required to
move the valve in a laboratory setting, prior to valve shipment. Such
one-of-a-kind tests, however, merely determine the acceptability of the
valve as assembled, and do not in any way enable, or even relate to,
online valve testing after installation within the turbine.
DISCLOSURE OF THE INVENTION
It is the overall objective of this invention to provide an improved
valve monitoring system, which can optimize the operating time
between valve inspections, consistent with the requirements of safety
and reliability.
In accordance with an exemplary embodiment of this invention,
a monitoring system is provided which will record on an ongoing basis
hydraulic pressure vs. valve stem stroke as a quantitative measure of
valve performance. The hydraulic pressure is a measure of the force
required to stroke the valve and thus also a measure of valve
performance. When this system is coupled to a computer, future poor
performance of the valve can be anticipated more precisely than with
prior testing techniques.
More specifically, the monitoring system includes a valve stroke
measuring device and a pressure transducer to measure hydraulic
pressure while stroking the valve. During routine valve testing in the
exemplary embodiment, the computer also monitors parameters such
as rate of valve opening or closing, and hydraulic pressure difference
between opening and closing at various stroke points. The instrument
outputs are sent to a signal processor and a computer system which
records and compares the output signals with predetermined limits,
design values and/or historical data stored in the computer, and which
can signal the need for present or future maintenance.
Accordingly, in its broader aspects, the invention provides in an
operating steam turbine, a valve monitoring system for a valve
assembly having a valve, a valve stem supporting the valve and
adapted to move linearly (or rotationally, depending on the specific
valve construction) in valve opening and valve closing directions; a
valve actuator for causing the valve stem to move in either of the
opening and closing directions; a first sensor for measuring stroke
length of the valve stem in the opening and closing directions; a
second sensor for measuring hydraulic pressure required to move the
valve in the opening and closing directions; and means for recording
signals from the first and second sensors and comparing the signals to
corresponding reference values to thereby enable identification of
abnormal valve behavior.
In another aspect, the invention relates to a method of testing a
steam turbine valve, the valve having a valve stem extending through
a housing, the stem having a piston fixed to a distal end thereof inside
the housing comprising the steps of:
a) measuring valve stem stroke length in opening and closing
directions;
b) measuring hydraulic pressure required to move the valve;
c) generating electrical output signals from steps a) and b);
d) sending the signals to a signal processor and then to a
computer; and
e) comparing the signals to data stored in the computer
including normal valve stroke length and hydraulic pressure
measurements, and to thereby enable identification of abnormal valve
behavior.
Other objects and advantages will be apparent from the detailed
description which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a side elevation, partly in section, illustrating a
steam turbine valve and actuator, along with a monitoring system in
accordance with this invention;
Figure 2 is a graph illustrating normal valve operation in terms
of opening and closing strokes and hydraulic pressure; and
FIGURES 3-5 are schematic illustrations of the monitoring
system of this invention with alternative valve constructions.
BEST MODE FOR CARRYING OUT THE INVENTION
In Figure 1, an exemplary embodiment is illustrated wherein a
valve and actuator assembly 10 includes a steam turbine valve 12
engageable with an annular seat 14 (only partially shown). The valve
12 includes an elongated valve stem 16 which extends in an
downstream direction (i.e., in the direction of flow through the seat 16)
and into an actuator housing 18. The latter includes a relatively larger
diameter spring enclosing section 20 and a relatively smaller diameter
piston enclosing section 22. Within section 20, there is a coil spring
24 (or Bellville or other suitable spring) which is supported on a flange
26 secured to the stem 16 and which also abuts against an upper wall
of the housing section 20, thus biasing the valve 12 in a direction
toward engagement with the seat 14.
The stem 16 continues beyond flange 26 into the smaller
section 22, terminating at a piston 28. In other words, valve 12 and
piston 28 are at opposite ends of the stem 16. The piston 28 divides
the housing section 22 into chambers 30, 32, respectively, below and
above the piston. The lower chamber 30 (as viewed in the Figure) is
pressurized via a hydraulic pressure input shown at 34, and a pressure
transducer 36 communicates with chamber 30 to provide an electrical
output corresponding to the pressure in that chamber. It will be
appreciated that the spring 24 is calibrated so that movement of the
valve 12 toward or away from the seat is determined by pressure
within the cavity 30 below the piston 28. In this regard, there are also
steam forces acting on the valve stem, which during turbine operation
may be much larger than the spring force.
Above the housing 18, there is a linear, variable differential
transformer (LVDT) stroke measuring device 38 fixed to the stem 16,
the device also providing an electrical output corresponding to stem
movement.
The electrical outputs of the transducer 36 and stroke
measuring device 38 are sent to a signal processing device 40 and
then to a microprocessor (e.g., a PC) 42 which records and compares
the outputs with predetermined stored values representing normal and
calculated valve behavior. With reference to Figure 2, a normal valve
output is plotted on a stroke vs. hydraulic pressure graph. During
routine valve testing, the computer 42 will also monitor the rate of
valve opening or closing as well as pressure difference between
opening and closing at various points along the stroke, etc. The solid
lines show a plot of hydraulic pressure vs. valve stroke for a normally
operating valve, showing both opening and closing directions. The
dotted line between the solid lines shows the calculated hydraulic
pressure required to balance the spring and steam forces acting on the
valve stem. The pressure difference Pd is a measure, for any given
stroke, of frictional forces which, if excessive, will stall the valve. Thus,
when recorded values fall outside the area W enclosed by the solid
lines in Figure 2 by more than a predetermined amount, maintenance
may be indicated, and the computer would display an appropriate
signal to this effect on a monitor. In other words, depending on the
particular application, valve and valve experience, readings within a
larger window may be acceptable (see the dotted lines A and B in Fig.
2), but readings outside this larger window may be so abnormal as to
indicate the need for immediate maintenance. Of course, all relevant
valve information and historical data is stored in the computer memory
and is used as necessary to provide the desired display output.
The computer may also analyze readings within the window to
determine the rate at which they approach the solid lines and/or the
extended window defined by dotted lines A and/or B and thereby
identify abnormal valve behavior indicating possible future
maintenance, including projections as to when the valve will stick, etc.
The monitoring system in accordance with this invention is not
limited to use with the specific valve construction illustrated in Figure
1. In fact, the system may be used in connection with various rotary
and linear type valves including butterfly valves, and overhead
actuator-type valves. For example, and with reference to Figure 3, an
overhead actuator-type valve is illustrated, using reference numerals
similar to those used in Figure 1 for corresponding components, but
with the prefix "1" added. In this embodiment, the actuator assembly is
upstream of the valve 112 (in the flow direction), but the monitoring
system is otherwise similar to that described in connection with Figure
1. The annular seat 114, with the valve stem 116 extending in a
upstream direction and into the actuator housing 118. As in the earlier
described embodiment, the housing 118 includes a relatively larger
diameter spring enclosing section 120 and a relatively smaller
diameter piston enclosing section 122. Within section 120, there is a
coil (or other suitable type) spring 124 which is supported on a flange
126 secured to the stem 116. The other end of the spring abuts
against the upper wall of the housing 120, thus biasing the valve 112
in an downstream direction toward engagement with the seat 114.
Valve stem 116 terminates at a piston 128 located in the smaller
diameter housing section 122, with the piston dividing the housing
section 122 into chambers 130 and 132, respectively. A pressure
transducer 136 communicates with the chamber 132 and provides an
electrical output corresponding to the pressure in that chamber. Below
the housing 118, there is a variable differential transformer stroke
measuring device 138 fixed to the stem 116, providing an electrical
output corresponding to stem movement. As indicated above, the
operation of the monitoring system in this arrangement is otherwise
similar to the arrangement illustrated in Figures 1 and 2.
In Figure 4, a double acting type actuator is illustrated, with
hydraulic pressure applied to either side of the actuator piston 228 (via
inputs 234, 234'), depending on whether the valve is opening or
closing. Here again, reference numerals used in the figure are similar
to those used for corresponding components in Figure 1, but with the
prefix "2" added. In this embodiment, the valve arrangement is similar
to the Figure 3 embodiment, but here, two pressure transducers 236
are employed, in communication with the respective chambers 230,
232 on either side of the piston 228 to monitor the hydraulic pressure
in both chambers. This type valve assembly eliminates the need for a
spring such as 24 (Figure 1) or 124 (Figure 3). Otherwise, the
operation of the monitoring system is as previously described.
In Figure 5, another double actuator-type valve assembly is
shown, but in combination with a spring. More specifically, and using
reference numerals similar to those used for corresponding
components in Figures 1, 3 and 4, but with the prefix "3" added, a pair
of pressure transducers 336 are located on either side of the piston
328, in communication with chambers 330 and 332, respectively.
Hydraulic fluid is supplied to chamber 330, 332 by means of input 334,
334'. At the same time, a spring 324 continuously biases the piston
flange 326 in a valve closing direction.
In both arrangements illustrated in Figures 4 and 5, the signal
processor takes the output from the pair of pressure transducers
above and below the piston and combines the pressures to calculate
an effective net opening or closing pressure which would become the
horizontal axis of Figure 2.
Those of ordinary skill in the art will understand that still further
valve constructions may benefit from the monitoring system in
accordance with this invention. Those skilled in the art will also
understand that the data generated in accordance with this invention
may be utilized in various additional ways to monitor, record and
predict valve behavior.
While the invention has been described in connection with what
is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is intended to
cover various modifications and equivalent arrangements included
within the spirit and scope of the appended claims.
WE CLAIM
1. In an operating steam turbine, a valve monitoring system
for a valve assembly having a valve, a valve stem supporting said
valve and adapted to move in valve opening and valve closing
directions; a valve actuator for causing said valve stem to move in
either of said directions; a first sensor for measuring stroke length of
the valve stem in opening and closing directions; a second sensor for
measuring hydraulic pressure required to move said valve in said
opening and closing directions; and means for recording signals from
said first and second sensors and comparing said signals to
corresponding reference values to thereby enable identification of
abnormal valve behavior.
2. The system of claim 1 wherein said first sensor comprises
an LVDT stroke measuring device.
3. The system of claim 2 wherein said second sensor
comprises a pressure transducer.
4. The system of claim 3 wherein electrical outputs from said
first and second sensors are sent to a signal processor and then to a
microprocessor.
5. The system of claim 1 wherein said valve stem extends
through a first housing and into a second housing, said valve stem
having a piston on a distal end thereof, separating said second
housing into upper and lower chambers.
6. The system of claim 1 wherein said valve stem extends into
a housing, said valve stem having a piston on a distal end thereof,
separating said housing into upper and lower chambers.
7. The system of claim 5 wherein said second sensor
communicates with said lower chamber.
8. The system of claim 7 wherein said first housing encloses a
spring engaged at one end with a flange on said valve stem, such that
said stem is urged in the closing direction by the spring and in the
opening direction by pressure in said lower chamber.
9. The system of claim 5 wherein said first sensor is attached
to said valve stem outside said first housing.
10. The system of claim 8 wherein said first sensor is
attached to said valve stem outside said first housing.
11. The system of claim 6 wherein hydraulic pressure is
applied to said upper and lower chambers depending on direction of
movement of said valve stem.
12. A method of testing a valve in an operating steam turbine,
the valve having a valve stem extending through a housing, said stem
having a piston fixed to a distal end thereof inside said housing, the
method comprising the steps of:
a) measuring valve stem stroke length in opening and closing
directions;
b) measuring hydraulic pressure required to move the valve;
c) generating electrical output signals from steps a) and b);
d) sending said signals to a signal processor and then to a
computer; and
e) comparing said signals to data stored in said computer
relating to normal valve stroke length and hydraulic pressure
measurements, and to thereby enable identification of abnormal valve
behavior.
13. The method of claim 12 and including the step of:
f) generating a warning in the event the signals exceed by a
predetermined amount said normal valve stroke length and hydraulic
pressure measurements.
14. The method of claim 12 wherein step (b) is carried out by
measuring pressure on opposite sides of said piston, depending on the
direction of movement thereof.

In an operating steam turbine, a valve monitoring system is
provided for a valve assembly having a valve, a valve stem supporting
the valve and adapted to move in valve opening and valve closing
directions; a valve actuator for causing the valve stem to move in
either of the directions; a first sensor for measuring stroke length of the
valve stem in opening and closing directions; a second sensor for
measuring hydraulic pressure required to move the valve in the
opening and closing directions; and means for recording signals from
the first and second sensors and comparing the signals to
corresponding reference values to thereby enable identification of
abnormal valve behavior.