esd valve self testing system
TRANSCRIPT
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Understanding theSmart Valve Monitor
Established Leadersin Flow Control
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Contents
Section Page
Product Overview 4
Extended Full Closure Intervals 6
Full Stroke Monitoring 8
Monitoring Additional Parameters 9
Interaction with the DCS to SOV Connection 10
Server Software 11
Principles of SVM Fault Detection 12
Sticking Solenoid Valve 13
Valve Obstruction / Damaged 14
Actuator Cylinder
Seized Valve / Spring Failure 14 Stem Shear / Disconnected Valve 14
Stiff Valve (increased torque demand) 14
Damaged Valve Seat / Internal 15
Cylinder Corrosion
Exhaust Restriction 15
Increased Breakout Torque Requirement 15
2
Section Page
Application Specifics 16
Pneumatic Spring-Return Actuators 16
Pneumatic Double-Acting Actuators 17
Hydraulic Spring-Return Actuators 18
Hydraulic Double-Acting Actuators 18
HIPPS Valve 20
Subsea Valve 22
Rotork is the global market leader in valve
automation and flow control. Our products and
services are helping organisations around the
world to improve efficiency, assure safety and
protect the environment.
We strive always for technical excellence, innovation and
the highest quality standards in everything we do. As a
result, our people and products remain at the forefront of
actuation technology.
Uncompromising reliability is a feature of our entire product
range, from our flagship electric actuator range through to
our pneumatic, hydraulic and electro-hydraulic actuators, as
well as gear boxes and valve accessories.
Rotork is committed to providing first class support to
each client throughout the whole life of their plant, from
initial site surveys to installation, maintenance, audits and
repair. From our network of national and international
offices, our engineers work around the clock to maintain
our position of trust.
Rotork. Established leaders in flow control.
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Established Leadersin Flow Control 3
Introduction
This brochure provides a rudimentary overview
of the capabilities and workings of the Rotork
Smart Valve Monitor (SVM), the most versatile
and comprehensive partial stroke test systemfor fluid power actuated valve actuators on
the market.
It's data logging capability facilitates strategic maintenance by
providing diagnostic data for all final elements including the
valve, actuator, and unique to PST devices all related
solenoid and exhaust valves. SVM is suitable for use at any SIL
level rating and can greatly improve performance verification
and compliance with any applicable standards, including
those from IEC and ISA.
SVM200 rack mount unit for control rooms or other safe areas.
SVM100 field unit for installation close to the valve
in hazardous areas.
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4
Smart Valve Monitoring SVM Product Overview
Reliable, Non-intrusive Design
The Rotork Smart Valve Monitor is the most versatile and
comprehensive partial stroke testing (PST) system for
hydraulically or pneumatically actuated on/off valves availableon the market.
Partial stroke testing is a technique that allows an operator
to perform a diagnostic test on a valve without the need for
a plant/process shutdown. The majority of faults associated
with on/off valves relate to the valve becoming fixed in
position from long periods of inactivity. an operator can verify
operation by moving the valve by only a small percentage of
its travel.
A Problem
Because of the many process applications within a plant, there
is always a diverse range of valves, actuators, and control
systems. One of the main issues associated with partial stroketesting systems is the need to test all this equipment from
multiple manufacturers in a variety of configurations. Testing
systems supplied by valve and actuator manufacturers tend
to be specific to the particular manufacturer and are not
flexible enough to function in other configurations. Therefore,
within a plant, several different test systems and protocols are
often in use. This results in increased costs for procurement,
installation, commissioning and user training.
The Solution SVM
The Smart Valve Monitor system is unique in partial stroke
testing systems in that no additional components are fitted
directly to the valve, actuator or associated controls. Any
changes to the configuration of these components will not
affect the manner in which tests are conducted.
Compatible With All Valve Types
The SVM system can test al l valve types and has successfully
been used on ball, globe, gate, butterfly and HIPPS valves in a
variety of applications including ESD, blowdown, and subsea
isolation.
Compatible With All Fluid Power Actuators
The Smart Valve Monitor can be used with virtually any fluid
power valve actuator quarter-turn or l inear, pneumatic or
hydraulic, spring-return or double-acting. SVM is compatiblewith ESDV systems with any number of solenoid valves (SOVs)
in either normally energised or normally de-energised states.
Variation in quick exhaust valve (QEV) configurations has
no effect upon the operation of the SVM. The equipment
supplied is identical in each of these cases with varied
configuration conducted during setup. It is only necessary
that the system is equipped with a pressure transmitter with
the proper range to monitor the instrument supply. SVM can
also be configured for use with manual reset solenoids and IS
barriers.
The SVM System
The system consists of a control unit that connects to
the power supply, the solenoid valve, and also a pressure
transmitter to provide feedback for the analysis of valveperformance.
The SVM is available in two variants: one for mounting in a
hazardous area near the valve and the other for mounting
more remotely in a safe area, e.g., a control room cabinet. In
either case, installation is facilitated by the fact that the SVM
does not mount directly to the actuator. This design feature is
unique among partial stroke testing techniques. The SVM has
no direct interaction with either the valve or actuator and has
no possible effect upon the normal operation of the valve.
A simple installation is shown in figure 1. This is a pneumatic
spring-return actuator with three quick exhaust valves and
two redundant SOVs. The components of the SVM system are
shown in red.
Figure 2 shows a much more sophisticated example where
the DCS/safety system can automate tests via a MODBUS
interface to the SVM server. The server is controlling the SVM
units via a fibre optic and RS485 link. In addition, Ethernet can
be utilsed for remote access to SVM test data stored on the
server.
Fig. 1. Typical SVM installation schematic.
DCS
Smart
Valve
Monitor
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Established Leadersin Flow Control 5
Valve Testing
The SVM is powered by the ESD signal to the solenoid
and performs a test by switching the signal. The analysis is
conducted by fitting a pressure sensor to the instrument
supply and observing characteristics during the partial stroke
In conducting a test, the SVM will change the state of the
solenoid valves (and subsequently any quick exhaust valves)
for a fixed time and monitors the pressure transmitter. Oncethe required time is reached, the solenoid valve is switched
back and the valve under test returns to its original position.
The fixed time for which the state of the solenoid is changed
is set during the commissioning process and is related to the
percentage of valve movement desired. Upon completion
of the test, the SVM analyses the pressure data and returns
either a pass or fail result to the operator.
Fig. 2. Sophisticated SVM system installation.
MODBUS
Local Equipment RoomSafe Area
SVM200
SVM200
SVM200
SVM200
SVMRACK
RS485
Fibre Optic
Office PCEthernetMODBUSTM SVM ServerDCS/Safety System
Hazardous Area
Safe Area
Local Equipment RoomSafe Area
SVM200
SVM200
SVM200
SVM200
SVMRACK
RS485
Local Equipment RoomSafe Area
SVM200
SVM200
SVM200
SVM200
SVMRACK
RS485
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6
Extended Full Closure Intervals
Partial Stroke Testing and Probability of Failure
on Demand (PFD)
One of the primary uses of partial stroke testing techniques is
to extend full closure test intervals. These intervals are definedby evaluating the required SIL level and the average Probability
of Failure on Demand (PFDavg) in conjunction with the following
equation:
Equation 1
PFDavg= [DCPC*lD*(TIPC/2)]+ [(1-DCPC)*lD*(TIFC/2)] DCPC= Diagnostic Coverage of the PST.
lD= Total Dangerous Failure Rate.
TI = Test Interval of Full or Partial Closure.
Note: lDfailure rates are measured in FITs (10 -9failures/hour)
There are two components of the PFDavgcalculation that relate
to the test intervals of the partial closure (TIPC) and full closure
(TIFC). The weight of each component is directly dependent
upon the diagnostic coverage of the PST. As the DCPC
increases, the weight of the full closure component decreases
and the TIFCis extended.
The DCPCfor a PST is defined by IEC 61508 as follows:
Equation 2
DCPC= lDD/ lTOTAL
lDDis the dangerous detected rate of the PST.
lTOTALis the total dangerous failure rate.
SVM System Interaction
It is essential to maximise DCPCto give maximum weight tothe partial closure side of equation 1. To achieve this, the SVM
system was designed to meet the following criteria:
Have zero contribution to the PFD of the valve.
Never prevent the valve from closing on demand.
Test all final elements, (i.e., valve, actuator and
control mechanisms).
Test at the designed operating speed.
Specifically, the SVM system is completely non-invasive to the
normal operation of the valve system. During test, valve control
is conducted by switching the supply to the solenoid and
diagnostic analysis is conducted by monitoring the pneumatic
or hydraulic instrument supply as shown in figure 3. This simple
connection method facilitates installation in both new and
retrofit applications.
Fig. 3. SVM Interaction.
Existing Plant
We will examine an example of an existing plant with an
accredited SIL rating and approved full closure testing regimes
and see what improvement partial stroke testing can make to
the established full closure intervals. Positioner based systems
and the Rotork SVM will both be examined.
The PFD equations can be applied to the plant to allow an
extension of the TIFCand maintain the required SIL rating.
When the plant has no partial stroke testing equation 1 is
simplified to:
Equation 3
PFDavg= lD*(TIFC/2)
Since the SIL rating of the plant cannot be changed, the
PDFavg must remain the same both before and after the PST
has been applied. This means that equation 1 = equation 3 asshown below:
lD*(TIFC/2) = [DCPC*lD*(TIPC/2)]+[(1-DCPC)*lD*(TIFCNEW/2)]
TIFCNEWis the extended full closure regime when PST is applied.
Equation 3 can be rationalised to yield equation 4 since "lD"
and "/2" are present in each part of the equation.
Equation 4
TIFC= [DCPC*TIPC]+ [(1-DCPC)*TIFCNEW]
Solving for TIFCNEWgives:
Equation 5
TIFCNEW= TIFC [DCPC* (TIPC)]
1-DCPC
If the example plant has a current testing regime of TIFC= 6
months, and we choose to conduct a PST every 2 months, the
relationship between TIFCNEWand DCPCcan be plotted. This
will determine the effectiveness of any partial stroke testing
system as shown in figure 4. The graph shows that TIFCNEW
increases exponentially with the diagnostic coverage and that
no significant gains are realised until the DCPCreaches 75%.
DCS
PT
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Established Leadersin Flow Control 7
Fig. 4. Effect of DCPCon TIFC.
We can standardise DCPCfor a PST system by taking data from
exida'sSafety Equipment Reliability Handbook (2ndedition)for
a generic ball valve, actuator and generic SOV and applying
it to calculate a system DCPC, as shown in figure 5. Note
that functional safety standard IEC 61508 states that all finalelements must be tested.
lDD is the dangerous detected rate of the PST.
lDU is the dangerous undetected rate of the PST.
lSD is the safe detected rate of the PST.
lSU is the safe undetected rate of the PST.
Fig. 5. Diagnostic coverage compared.
In figure 6, diagnostic coverage of a positioner PST system is
compared with that of the SVM system. It is evident that the
gains are vast with SVM but only marginal with a positioner.
Fig. 6. Effect of DCPCon TIFC Positioner vs. SVM.
The actual improvement to the closure interval can now be
determined. Analysis shows that TIFCfor positioner type PST
systems is increased from 6 to 8 months. This reduces the
number of full closures required over a 10-year period by only
25%, from 20 to 15, eliminating only 5 full closures.
With the SVM system, the full closure period is extended from
6 to 31 months more than a 5-fold increase! Only 4 full
closures are required over a 10-year period. This gives the
operator very significant savings in plant downtime.
Diagnostic coverage can be further analysed by looking at
potential failure modes and analysing whether a partial stroke
test will detect these failures. In figure 7 fault detection for
positioner systems (POS) and Smart Valve Monitor (SVM) are
compared. Note that a question mark in the chart indicates
that the fault can only be detected if the speed of valve
movement is monitored.
It is easily concluded that with the SVM system the
vast majority of final element failure types are now not
only detectable, but in most cases, by utilising strategic
maintenance, preventable. The SVM system clearly improves
maintenance activities and greatly extends full closure intervals.
Fig. 7. Fault detection compared.
DCPC
TIFC(new)(M
onths)
10%
0
5
10
15
20
25
30
35
40
45
15% 22% 25% 35%3 5% 4 0% 4 5% 5 0% 5 5% 6 0% 6 5% 7 0% 7 5% 8 0% 8 6% 9 0%
DCPC
TIFC(new
)(Months)
Smart Valve Monitor
Positioner
10%
0
5
10
15
20
25
30
35
40
45
15% 20% 25% 29% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 86% 90%
FAILURE PST USING POSITIONER PST WITH SVM
RATE
Valve Actuator SOV Total Valve Actuator SOV Total
lDD 810 426 1236 810 426 2400 3636
lDU 540 34 2400 2974 540 34 574
lSD 1650 919 2569 1650 919 3600 6169
lDD 3600 3600
DCPC= 29% DCPC= 86%
FailureRate
PST USING POSITIONER PST WITH SVM
Valve Actuator SOV Total Valve Actuator SOV Total
lDD 810 426 1236 810 426 2400 3636
lDU 540 34 2400 2974 540 34 574
lSD 1650 919 2569 1650 919 3600 6169
lDD 3600 3600
DCPC= 29% DCPC= 86%
FAULT POS SVM
VALVE
Valve Seized YES YES
Increased Breakout Torque YES YES
Polymerisation of Valve Seat ? YES
Increased Packing Friction ? YES
Valve Stem Shear NO YES
Valve Seat Leakage NO NO
ACTUATOR
Broken Spring YES YES
Cylinder Damage YES YES
Internal Cylinder Corrosion NO YES
SOLENOID AND/OR QEV
Exhaust Blockage NO YES
Solenoid Mechanism Wear NO YES
Damaged Tubing NO YES
HIPPS SYSTEMS
Individual QEV Failure NO YES
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8
Full Stroke Monitoring Black Box Function
Automatic Shutdown Monitoring
In addition to partial stroke testing the SVM system is capable
of automatically monitoring all full strokes of the valve
whether they are: Planned (a maintenance shutdown)
Unplanned (a spurious trip)
An actual demand
Figure 8 below shows an example of a full stroke in
comparison to a partial stroke. Full details of the actual events
recorded in the graph are discussed on page 12.
Fig. 8. Full and partial stokes.
Additional DiagnosticsBy using the full stroke monitoring Black Box function, the
operator can now detect a number of additional valve failure
modes to give the most comprehensive diagnostics possible.
This includes data relating to the following events:
Dynamic torque during full travel
Torque required to close the valve
Time to complete a full stroke
This provides the user unparallel diagnostic capability for
pneumatic, hydraulic and electro-hydraulic actuated shutdown
valves.
Planned Shutdowns
Utilising the Black Box function during a planned
shutdown, the operator is able to automatically gain full
stroke signatures for all monitored valves by simply tripping
the safety system. The SVM system will then upload all
the data and indicate any valves that have failed the full-
stroke test. This ensures that the operator maximises the
efficiency of operations during the shutdown and no essential
maintenance will be overlooked.
Unplanned Shutdowns / Spurious Trips
Spurious trips can be incredibly expensive and have a
detrimental affect on plant efficiency. This is particularly so
if a trip occurs only a short time after a planned shutdown iscompleted.
By using the SVM Black Box function on all critical valves
the operator can prove the functionality of the valve and in
some circumstances use this as a credit for the compulsory
shutdowns required by regulation. This can yield very
significant financial benefits to the operator.
In addition, because the operator now has comprehensive test
data for all valves, spare parts can be ordered in advance of
maintenance shutdowns.
Actual Demands
In the event of an actual demand, it is essential that the
operator is able to trace the root cause of the demand and
thus initiate any design or procedural changes to mitigate
against any potential repeat events. The Black Box function
allows the operator to detect which valves operate within the
required parameters during such a demand and help indicate
where systematic failures may have occurred.
Manual Testing
In addition to automatic testing the operator can also initiate
manual full stroke tests as required.
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Established Leadersin Flow Control 9
Monitoring Additional Parameters
Automatic Shutdown Monitoring
In the Smart Valve Monitor system instrument pressure is used
as the primary parameter for performing valve diagnostics
but the SVM can also monitor up to two additional inputs.This can provide the operator even more data to diagnose the
performance of safety system shutdown valves. The following
additional parameters can be monitored:
Position Indication
A second pressure sensor (for double-acting actuators)
Torque
Strain gauge
Temperature
Operators can gain an even more comprehensive picture of
the full performance of all final elements well beyond the
capabilities of measuring position alone.
Position Indication
The most useful additional parameter to monitor is position.
Most users are more familiar with analysing position data than
pressure data and therefore this is a very useful diagnostic
addition. To facilitate this the actuator must be fitted with
either a 4-20 mA or 0-10V position transmitter.
Figure 9 shows a partial stroke example of a pneumatic
actuator/valve indicating both instrument pressure (green) and
position (red).
Fig. 9. Instrument pressure and position partial stroke.
Its easily seen that both graphs identify the same point at
which the valve starts to move, the position data further
corroborating valve/actuator performance. Figure 10
represents a complete close stoke of the same hypothetic
valve and actuator depicted in figure 9. In the full stroke
the position at which the valve is fully closed is also readily
identified.
Fig. 10. Instrument pressure and position full stroke.
A Second Pressure Sensor
In double-acting actuators it is possible to monitor the airpressure on both sides of the piston giving the operator
diagnostics relating to the flow rate of air required to close
the valve. This allows determination of whether there is any
damage to the SOV or associated tubing to the actuator.
Torque
If torque sensors are available, monitoring torque will allow
operators to see how the force required to operate the valve
changes over time.
Strain Gauge
On linear actuators it is often possible to attach a strain gauge
to the valve stem to provide data relating to the force required
to operate the valve and, as with torque sensors, monitor
changes in force required.
Temperature
By monitoring the temperature at the valve (either process
or ambient) during each test, operators can detect whether
changes in valve performance are attributable to changes
in temperature, as opposed to actual wear and tear, thus
preventing unnecessary maintenance. This can be of particular
use in climates that experience large changes in seasonal
conditions.
Bar G
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1 2 3 4 5 6 7 8 9 10 11 12Seconds
Valve starts
to move
Bar G
1
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5
1 2 3 4 5 6 7 8 9 10 1 1 12Seconds
Valve startsto move
Valve fully closed
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10
SVM Interaction with the DCS To SOV Connection
SVM System Functionality
The SVM system was designed so that it is impossible for it to
prevent the valve from closing upon demand. Figure 11 shows
how the unit interacts with the SOV supply from the DCS.The SVM switches only the -ve supply to the solenoid. If the
supply from the DCS is dropped the solenoid will lose power.
Therefore, whatever the operation of the SVM unit, it cannot
prevent the ESDV from closing.
Fig. 11.
SVM Availability and Reliability
Rotork has made the SVM system as reliable as possible. No
system can remain 100% reliable for years on end but Rotork
has used best practices to ensure that the SVM system will
provide the lowest possible chance of causing a spurious tr ip.
Figure 12 illustrates how the system switches and monitors
the valve under test including how the control and monitoring
circuit is kept isolated from the switching circuit.
Fig. 12.
Under normal circumstances, the relay contact is closed and
the PLC input is passed directly to the SOV. During set-up or
testing of the valve, the relay is opened and the -ve line of the
supply is switched by the MOSFET transistor.
Proven Dependability
An independent study by exida has shown that the only
possible failure modes for the SVM system are fail-safe
failures. These include diagnostic annunciation and fail-safe (both detectable and undetectable). With a diagnostic
annunciation failure the operation of the valve would be
unaffected. With a fail-safe failure, the SVM system could,
depending upon the source of failure, cause a safe but
undesired closure. But, whether running a test or in an idle
state, it cannot lead to an unsafe situation.
The probability of an SVM system failure causing a safe but
spurious closure can be expressed in FITs. A FITs is a unit
of probability of failure 1x109 failures per hour. Exidas
evaluation determined the total failure probability of the SVM
to be 76.9 FITs. This equates to 0.00067 failures per year or
a spurious trip rate of once per 1,500 years. Some positioner
type systems have, according to exida data, exhibited a
spurious trip rate of once every 75-150 years.
Conclusion
The SVM system has an insignificant probability of causing a
spurious plant trip and is ideally suited to the application of
partial stroke testing of emergency shutdown valves.
Control &Monitoring
Circuitry
PT
SOV 1
SwitchingCircuitry
Input fromexisting ESDControl System
+ve
-ve
Control &Monitoring
Circuitry
Switching Circuitry
OptoIsolator
MOSFETTransistor
SOV 1Input fromexisting ESDControl System
ClosedRelayContact
+ve
-ve
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Established Leadersin Flow Control 11
SVM Server Software
SVM Server Software
At the heart of the SVM system is the server software. This
package performs all the functions required by the SVM
system including commissioning, testing, diagnostics andreporting. It also has the capability to interface with other
control systems.
Graphic Display
Figure 13 is an example of the default graphical display. It
shows the pressure profile curve for the selected valve. All set-
up and testing functions are performed using this display. The
result of the last test is the default view.
Fig. 13.
The user can overlay multiple graphs of historic tests in order
to assess performance over time as shown in figure 14.
Fig. 14.
Tabular Display
In addition to the graphical display, there is also a tabular
report that allows the user to view physical data relating to
valve operation.Figure 14 demonstrates that physical parameters such as
solenoid response time and the pressure at which the valve
starts to move can be quantified.
Fig. 15.
Interface
The SVM server software can utilise either MODBUS or OPC
for interaction with other control systems.
Bar A
SOV Open
ESDV Moves
1 2 3 4 5 6
1
2
3
4
5
Seconds7
SOV Closed
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12
SVM Interaction with the DCS To SOV Connection
Introduction
The Smart Valve Monitor system uses a pressure transmitter
connected to the actuator's fluid power supply to monitor
the performance of all of the final elements of a shutdownsystem. Figure 16 below shows a basic P&ID for a spring-
return pneumatic system. Note that there are no components
fitted directly to either the actuator or valve and that there is
no direct position feedback.
Fig.16. Basic SVM P&ID.
The basic principle behind the SVMs ability to detect all final
element faults lies in Newton's 3rd Law: Every action has an
equal and opposite reaction. Each final element component
will exert forces upon others and therefore any change in
performance will result in a change in force applied.
If we will consider the valve, any fault here will result in achange in force applied to the actuator. The majority of valve
faults will result in the actuator seeing an increased torque
demand. If the torque demand increases, the actuator will
not move as quickly and the rate of change in volume in the
cylinder will change. This subsequently results in a change
in the measured instrument pressure and can therefore be
detected by a pressure transmitter. Positioner based systems
that rely on a position indicator as their primary means of
monitoring are unable to diagnose a detached or broken valve
stem or any issues relating to SOV performance. The SVM
system can monitor up to 2 additional inputs and a position
indicator can be added as an optional extra if desired.
The SVM system monitors and stores instrument pressure data
during a stroke test. With the use of the included software,a user can review this data both graphically and in tabulated
reports. Analysing this data, particularly in comparing tests
over time, degraded performance is readily apparent. Often
the compromised component can be identified, facilitating
remedial action.
Basic Understanding of Test Data
The following section will examine in detail how the SVM
system can detect the various faults within the final elements
of a shutdown system. This section will demonstrate in detailthe ease with which SVM can monitor performance over time
and be used to identify various valve faults by interpretation
of graphic display data.
Figure 17 shows both full (blue) and partial (green) strokes
for an ESD valve equipped with a pneumatic spring-return
actuator. The text opposite explains what the various points
on the graph represent.
Fig.17.
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1 2 3 4 5 10
1
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Seconds6 7 8 9
1
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Established Leadersin Flow Control 13
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4
Seconds
Identifying Faults by Analyzing PST Data
A series of subsequent graphs will delineate how typical
final element faults are identified in the test results from the
SVM system. Each of the graphs has a red curve representing
proper operation and a black curve representing a particular
fault. Analysis of the test curve and comparison with previous
test results help identify deteriorating performance or a failed
component.
a. Sticking Solenoid Valve
The graphs illustrate a system where the SOV is operational
but performance has degraded.
The black graph is offset to the right of the red graph because
the horizontal section relating to the switching of the solenoid(as shown within the blue oval) is extended. The SOV has
taken longer to switch state than it had previously. The SVM
system measures this to a millisecond, allowing the user to
easily set a threshold for the required performance of the
solenoid before a fault is reported.
In addition to the solenoid, if there are any pilot, shuttle or
quick exhaust valves, these will also be monitored in this
section of the graph.
Point Description
1 The point at which the solenoid is switched. The time taken for the solenoid to react is shown in the short horizontal section and any change inswitching time will be shown here.
1 to 2 Venting of the over-pressure in the actuator before the valve starts to move. Any change in flow rate through the SOV or the exhaust will be seenhere. This curve is an exponential decay as this is a fixed volume venting through a fixed orifice.
2 The point where the spring force is equal and opposite to the air pressure. At this point, the difference between the operation of the valve andactuator can be seen during the take up of the mechanical slack in the actuator assembly. This is the point where the valve starts to move.
2 to 3 The movement of the valve and actuator during the partial stroke.
3 This is the pre-determined end point of the partial stroke.
3 to 4 The valve is re-opening after the partial stroke.
4 The valve is now fully open at the end of the partial stroke.
4 to 5 The re-pressurisation of the actuator once the valve is fully open.
2 to 6 The movement of the valve and actuator during the full stroke.
6 The valve is fully closed.
6 to 7 Final venting of the residual pressure in the actuator after the valve has full closed. Again, this is an exponential decay.
7 The actuator is now fully de-pressurised.
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14
SVM Interaction with the DCS To SOV Connection
b. Valve Obstruction or Damaged Actuator Cylinder
The black and red graphs are identical to the point within the
blue oval. At this point the black graph exhibits an exponential
decay indicating that the valve and actuator have stoppedmoving. This could be caused by either an obstruction within
the valve body or damage to the actuator.
c. Seized Valve or Spring Failure
In this example the valve and actuator fail to move. The black
curve shows continual exponential decay that indicates that
there is no change in volume in the cylinder and therefore
no valve/actuator movement. This type of failure could be
caused by internal damage to the actuator or damage to the
valve that prevents the valve from moving. In this particular
case, the data identifies an actuator problem as there is no
indication of the take-up of mechanical slack in the actuator
that would be seen if the problem was a seized valve.
d. Stem Shear or Disconnected Valve
Stem shear can be a dangerous failure because most partial
stroke techniques are unable to detect it. This is true for any
technique that relies upon measuring the position of the
actuator to determine valve performance. This is because the
position indication will show that the actuator has moved and
therefore report a Pass for the test.
Because the SVM system does not measure position, and also
moves the valve at the designed operating speed, this failure
is easily detected. In the following graph, the section of the
black graph relating to initial movement is at in increased
pressure than in the original partial stroke. This is due to the
fact that if the valve is not connected to the actuator, it exerts
no load upon it. The actuator therefore starts to move sooner
and faster. Since the actuator is moving faster, the volume inthe cylinder decreases more rapidly and, due to the fixed CV
of the SOV, the pressure increases.
This fault could be caused by simple human error the valve
was not connected/re-connected to the actuator. Or, much
more seriously, the stem is sheared off.
e. Stiff Valve (increased torque)
In this graph the torque required to keep the valve moving
has increased. At the point at which the valve starts to move,
the black curve drops below the red curve. This is because the
actuator has an increased load and is not moving as quickly asin the previous test. Therefore, the rate of change of volume
in the cylinder is decreased and the pressure drops more
quickly.
The SVM system can monitor this over time, document
degrading performance, and assist in predicting when the
valve should be serviced.
Bar G
1 2 3 4 5
1
2
3
5
4
Bar G
1 2 5 6 8 10
1
2
3
Seconds
4
3 4 7 9
Bar G
1 2 3 4 5 6
1
2
3
4
Seconds
SOV Open
Bar G
1 2 5 6 8 10
1
2
3
Seconds
4
3 4 7 9
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Established Leadersin Flow Control 15
f. Damaged Valve Seat or Internal Cylinder Corrosion
In this example, in addition to the red partial stroke, we
have also shown a full stroke in blue to further exemplify
the fault. It is evident that there is a problem with this valvesince it should start to close at approximately 3 bar (point A).
However, at point A, the curve flattens before returning to
exponential decay. This flattening indicates a small amount
of movement as the actuator moves off its end stop. It then
encounters the resistance of the valve and then briefly stops
moving. This indicates the problem is likely with the valve and
not the actuator. Internal actuator corrosion would result in a
similar graph but without the flat section at point A.
At point E, the valve begins to move but it jumps from the
seat rather than moving smoothly. Further examination
of the partial stroke reveals that the actuator doesnt start
to reopen until pressure reaches 8 bar (point B) and that
it takes in excess of 10 bar (point C) to fully reopen the
valve. Examination of the full stroke after point E shows the
valve juddering as it continues to close (point D), further
documenting the problem.
g. Exhaust Restriction
Exhaust restriction is a common final element problem. This
is a very simple fault to detect yet many partial stroke testing
systems do not test for it. Common causes for this fault are
dust or sand, ice in cold environments, insect nests, or salt
growth in offshore applications.
The black curve shows a slower decay than the previous
red curve during the initial depressurisation. This is due to adecrease in the CV of the SOV caused by a restriction in the
exhaust.
It is also possible for restrictions in air flow to be caused
by damaged supply tubing but that is not the case in this
example. This is determined by the fact that the re-opening
portion of the curve is almost identical to that of the
previous test, clearly indicating that there is no restriction of
airflow into the actuator and therefore the problem is with
the exhaust.
h. Increased Breakout Torque
The final failure mode we will examine is an increased
breakout torque requirement a fairly common problem
in valves that have been in service for a long time. The valveis behaving correctly up to point A where the actuators
mechanical slack is taken-up. After this point, the black curve
shows continued exponential decay indicating that there is no
movement of the valve or actuator. This continues until point
B, where the spring force is finally sufficient to move the valve
from its seat and it now moves normally.
As with many other faults, the SVM system can monitor the
degrading performance and assist in predicting when the
valve should be serviced.
Bar G
1 2 3 4 5 6
1
2
3
4
Seconds
Bar G
1 2 3 4 5 6
1
2
3
4
Seconds
A
B
Bar A
5 10 20 25 35 40
1
2
6
Seconds
9
10
7
8
3
4
5
15 30
A
B
E
D
C
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16
Application Pneumatic Actuators
Spring-Return
A typical layout for a spring-return system is shown in figure
18. Generally, the pressure transducer is located as close to
the actuator as possible. This is desirable since it increases dataaccuracy. The compressible nature of gases can adversely affect
data quality if the transducer is located a considerable distance
from the actuator.
Fig. 18. Typical pneumatic spring-return system.
Figure 19 is an example of the full and partial closure curves
produced by a properly operating installation of this type.
Fig. 19. Typical spring-return full and partial stroke curves.
When controlling pneumatic spring-return actuators for a partial
stroke test, consideration must be given to the unique operating
characteristics of actuator. With a spring-return actuator, a
certain minimum amount of air pressure is required to keep
the spring compressed. The pressure supplied to the actuator
in excess of this minimum amount may vary. Since the excess
air must be evacuated before spring force will start to close the
valve, its volume will affect the amount of time the valve will
take to close. PST solutions based on timing alone have the
possibility of over-stroking the valve. The SVM system overcomes
this problem and ensures that the valve always strokes to the
same position by using an "intelligent" pressure compensation
algorithm. Figure 20 below shows the factors that SVM
addresses when calculating a compensated partial stroke time.
Fig.20.
The abbreviations in figure 18 indicate the following:
Pvm= Pressure at which the valve starts to move.
tvm= Time at which the valve starts to move.
tps= Time for the partial stroke movement.
ttot= Total time for the partial stroke test (ttot= tvm+ tps).
To perform the pressure compensation, SVM calculates the
variable tvmby performing a pre-test. This pre-test de-energises
the solenoid for a short period of time to generate a curve. This
curve is then extrapolated to point Pvmfrom which the tvmis
calculated. This is shown in figure 21.
Fig. 21. Pressure compensation curve extrapolation.
In figure 22, the blue curve shows the original full closure;
green, the original partial closure; black, a test at a lower
pressure without pressure compensation; and red, a test at the
same pressure with pressure compensation.
Fig. 22. Compensation vs. no compensation.
The test reflected in the black curve was conducted at a lower
pressure than the original test shown in green. Since there is
no pressure compensation, the valve closes by a larger than
desired percentage. The red curve shows a test conducted at
the same lower pressure as the test shown in black, but in this
example the SVM pressure compensation algorithm has been
used. Note that the timing for the partial closure has been
reduced. This ensures that the valve will only close the
desired amount.
Bar A
1 2 3 4 5 6 7 8 9 10
1
2
3
4
5
6
Seconds
Pvm
tpstvm
ttot
Bar G
1 2 3 4 5 6
1
2
3
4
5
Seconds
Pvm
DCS
Smart
ValveMonitor
PT
Bar A
1 2 3 4 5 56 7 8 9 10
1
2
3
4
5
6
7
8
9
10
Seconds
Pvm
Bar G
1 2 3 4 5 6
1
2
3
4
5
Seconds
Partial Stroke
Full Stroke
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Established Leadersin Flow Control 17
Double-Acting
A typical layout for a double-acting system is shown below in
figure 23 where the pressure transducer is located on the high-
pressure to open side of the actuator.
Fig. 23. Typical pneumatic double-acting system.
Figure 24 is an example of the full and partial closure curves
produced by a properly operating installation of this type.
Fig. 24. Typical double-acting full and partial stroke curves.
The abbreviations in figure 25 indicate the following:
Pvm= Pressure at which the valve starts to move.
tvm= Time at which the valve starts to move.
tps= Time for the partial stroke movement.
ttot= Total time for the partial stroke test (ttot= tvm+ tps).
Fig. 25.
With a double-acting actuator, the motive force for the valve
is the air pressure in the cylinder. This force is not constant but
is dependent upon the instrument supply pressure which is
affected by demand. In addition, it is the differential pressure
ratio across the actuator piston that is the defining factor indetermining how the actuator moves. The rate of change in the
differential pressure ratio is constant; it is determined by the
fixed CV of the tubing. This means that the pressure at which
the valve starts to move (Pvm) is not constant, but the time at
which the valve start to move (tvm) is constant. Therefore, the
timing equation ttot= tvm+ tpsis now constant. The result
is that pressure compensation is no longer needed in the
equation.
The analysis of the data for fault diagnostics is very
different from that of a spring-return pneumatic actuator. With
a double-acting actuator, the graph may vary considerably
without there being any change in the performance of the
system. The graph in figure 26 shows a series of full closuresfor a pneumatic double-acting actuator at various instrument
pressures.
Fig. 26. Affects of varied supply pressure.
Even though the pressure at which the valve starts to move
varies linearly with instrument pressure, the time taken for the
valve to start to move remains constant. The overall time taken
for the valve to close will increase with decreasing instrument
pressure, but note that the pressure changes shown here are
extreme for the sake of illustration and it is highly unlikely
that a variation of 4 bar would happen in the field. Therefore,
the movement of the valve can practically be considered to
be independent of the instrument supply pressure. All other
potential component failures are evaluated in the same manneras a spring-return actuated valve.
DCS
Smart
Valve
Monitor
PT
Bar A
1
1
2
2
3
3
4
5
6
Seconds
Valve Starts
To Move
Bar A
1
1
2
2
3
3
Seconds
Partial Stroke
Full Stroke
Bar A
1
1
2
2
3
3
Seconds
Pvm
tpstvm
ttot
Partial Stroke
Full Stroke
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18
Application Hydraulic Actuators
DCS
Smart
Valve
MonitorPT
175
140
105
70
35
5 10 15 20 25 30Seconds
Partial Closure
Full Closure
22
5 10 15 20 25 30 35 40 4 5 50 55 60 65 70 75 80 85 9 0 9 5 100
44
66
88
110
132
154
178
198
Bar A
Seconds
Bar
Seconds
22
5 10 15 20 25 30 35 40 45 50
44
66
88
110
132
154
178
198
DC
Smart
Valve
Monitor
PT
Spring-Return
A typical layout for a spring-return system is shown below in
figure 27. Unlike a pneumatic system, the power supply fluid
(hydraulic oil) is non-compressible so there is no loss in dataquality if the transducer is located a considerable distance
from the actuator.
Fig. 27. Typical spring-return system.
The following graph, figure 28, depicts typical full and partial
close curves produced by a properly operating installation
of this type. In this case a 26-litre hydraulic spring-return
actuator operating a 28 ball valve.
Fig. 28. Typical spring-return full and partial stroke curves.
Note the smooth action with good linear valve movement.
The linear section is slowly falling as the spring force decreases
towards the end of travel.
Due to the fact that no equipment is fitted to the valve or
actuator, a unique advantage is realised being able to test
a valve without requiring direct access to it. A particularlyadvantageous application of this capability is the testing of
subsea valves. Subsea applications are delineated elsewhere in
this publication.
The graph in figure 29 shows data from a valve in the North
Sea. The valve is on the seabed 150m deep, 500m adjacent
to the platform and the transducer is mounted topside.
This graph shows full reopening. This was of particular
interest to the user since it provides more diagnostic data for
this completely inaccessible valve.
Fig. 29. Full and partial stroke curves from a subsea valve.
Double-Acting
A typical schematic for a double-acting system is shown belowin figure 30, where the pressure transducer is located on the
high-pressure to open side of the actuator.
Fig. 30. Typical double-acting system.
This set-up produces a graph as shown in figure 31.
Fig 31. Typical double-acting full and partial stroke curves.
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Established Leadersin Flow Control 19
Smart
Valve
Monitor
PT
1 2
22
3 4 5 6 7 8 9 10 11 12 1 3 14 15 16 17 18Seconds
Bar
Partial Closure
Full Closure
4
532
1 9 9
876
SmartValve
MonitorESD
Open
Close
PT
DCS
ESDSOV
ESDPilot
ClosePilot
OpenPilot
CloseSOV
OpenSOV
Electric
Pneumatic
Hydraulic
Although this double-acting graph may initially look
somewhat similar to that of the spring-return actuator, there
are important differences. Of note, the linear section for the
valve movement tends to be much flatter.
With hydraulic double-acting actuators applications, it is
not always possible to locate the transducer in the optimum
position; access is often restricted in older plants. The
Rotork SVM system can overcome this obstacle by using an
alternative position for the transducer, locating it between the
SOV and the pump as shown below in figure 32.
Fig. 32. System with alternate pressure transducer location.
This set-up produces very different curve characteristics, as
shown below in figure 33, but sti ll provides full diagnostic
capability.
Fig. 33. Full and partial stroke curves produced by an alternate
pressure transducer location.
The numbered events depicted in figure 33 are
described below:
1. Solenoid switches.
2. Valve starts to close.
3. Valve closing.
4. End of partial closure.
5. Valve opening (part closure only).
6. Valve fully open (part closure only).
7. Valve closing (full closure only).
8. Valve fully closed.
9. Pump returns hydraulic pressure to maximum.
When analysing this data, note that the SVM is recording
the pressure that is being applied by the pump. Whenever
the valve is moving the pump has to move a volume of fluid,
therefore the pressure drops. Once the valve has stopped
moving, the pump restores the pressure to the maximumlimit.
Multiple Solenoid Systems
Many manufacturers use more than one SOV to control the
movement of the valve. This is particularly true of hydraulic
double-acting systems. Figure 34 below shows a complex
arrangement for a double-acting actuator with three pairs of
solenoid and pilot valves; one pair to open the valve, a second
pair to close the valve, and a third pair to perform the ESD.
Fig. 34. Example of a complex SOV system.
In this installation, SVM is configured to switch both the
Open and ESD solenoids. Otherwise, the valve will fail to
re-open after the partial closure. The fact that pilot valves are
the final component that actually initiates valve movement
is transparent to the SVM system. All components that are
required to perform an ESD are tested. The pressure vs. time
curve generated for this arrangement will be similar to that for
the double-acting examples shown previously; once the valve
has started moving, the solenoid arrangement has no effect
on the pressure curve.
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20
HIPPS (High Integrity Pressure Protection Systems)
Valves utilising pneumatic spring-return actuators
A HIPPS valve characteristic is that they are very fast-acting
in order to provide the required level of safety protection.They are placed in critical locations on systems that may be
subject to over-pressurisation. These valves isolate the source
of the over-pressure as opposed to conventional systems
where the pressure is relieved. In order to perform this task,
these valves are often required to have fast closure times of
less than 1 second.
Closing of a valve for testing purposes is often undesirable
because of the impact upon the production process.
Therefore, partial stroke techniques are utilised that verify
valve operation yet have little or no impact on the process.
Many conventional partial stroke systems use limit switches
to indicate the point of closure at which to return the valve
to the fully open state. Due to the high speed of operationthese valves have a very high inertia. Conventional systems
can cause the valve to overshoot the partial closure point and
potentially fully close.
In addition, due to the fast acting nature of these valves, the
performance of the solenoid valves is critical. Proportional
control or limit switch systems cannot test this final element.
The Rotork Smart Valve Monitor has several features that
compensate for fast speed of operation and ensure that the
SOV is performance is properly evaluated.
Direct control of the valve allows the test to be
conducted in real time and in the same manner as
during an emergency. Solenoid timing can be controlled in milliseconds to
provide a precisely controlled pneumatic pulse. This
enables partial closure tests of tc
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Established Leadersin Flow Control 21
Fig. 36. HIPPS valve full and partial stroke curves. Fig. 37. HIPPS valve full and partial stroke curves with
position sensor data.
Original Full Closure Original Part Closure Current Part Closure
N/A
08/02/0513:05:13
5.073 Bar
0.27 Secs
3.648 Bar
0.59 Secs
0.25 Secs
N/A
N/A
N/A
08/02/0513:09:58
5.025 Bar
0.276 Secs
3.253 Bar
0.808 Secs
1.086 Secs
N/A
2.916 Bar
PASS
08/02/0513:46:42
5.027 Bar
0.252 Secs
3.194 Bar
0.77 Secs
1.044 Secs
N/A
2.877 Bar
Fig. 38. SVM quantitative report.
Status:
Operation Date & Time:
Supply Pressure:
Solenoid Changeover Response Time:
Emergency Valve Breakaway Pressure:
Emergency Valve Movement Time:
Solenoid Restore Response Time:
Pressure Stable at :
Emergency Valve Restore Pressure:
Details
Tag Number: 24 inch HIPPS
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22
Application Subsea Isolation Valves
The Problem
Subsea isolation valves (SSIV) have historically not utilised
partial stroke testing because the benefit provided by the
limited scope of available testing techniques was not sufficientto offset the installation costs associated with shutdown
and diving to attach control and monitoring equipment
directly to the valve/actuator. This is a major hindrance for
operators because the failure of an SSIV presents a significant
maintenance task. Ideally, operators would be able to
diagnose potential failures well in advance to allow for more
strategically planned preventative maintenance activities.
Figure 39 below shows a typical SSIV installation with an
umbilical providing all hydraulic and control signals to
the SSIV.
Fig. 39. Typical SSIV installation configuration.
The Solution
The Smart Valve Monitoring System connects only to the
hydraulic instrument supply and the SOV supply with nothing
fitted to either the valve or actuator. This ensures that all test
equipment can be located topside allowing operators easy
installation to existing SSIVs. Use of SVM on SSIVs is facilitated
by the fact that most SSIV actuators are hydraulically
operated. Since hydraulic fluid is non-compressible there is no
loss of resolution of data by monitoring topside. Figure 40 is
representative of this type of installation.
Fig. 40. Typical SSIV with SVM system.
The graph below in Figure 41 shows full and partial strokes
curves for a SSIV. The re-opening cycle of the valve is also
shown to give the operator higher diagnostic capability. In
this case, the valve is fully closed after 43 seconds and the
partial stroke is conducted for 14 seconds a partial stroke of
approximately 33%.
Fig. 41. SSIV full and partial stroke curves.
SVM Benefits
With the Smart Valve Monitor, subsea valves, whether
shutdown or otherwise, can now be partial stroke tested to
provide key performance data essential to strategic planning
of maintenance activities. SVM also facilitates compliance with
any applicable safety standards. Simple topside installation
makes SVM a cost effective solution for both new and existing
installations no expensive diving activities are required.
Pipeline and Umbilical
Subsea IsolationValve
DCS
Platform Topside
Smart
Valve
MonitorPT
22
5 10 15 20 25 30 35 40 4 5 50 55 60 65 70 75 80 85 9 0 95 1 00
44
66
88
110
132
154
178
198
Bar
Seconds
Partial Stroke
Full Stroke
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Established Leadersin Flow Control 23
Notes
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PUB026-002-00
Formerly F903E. As part of a process of on-going product development, Rotork reserves theright to amend and change specifications without prior notice. Published data may be subjectto change. For the very latest version release, visit our website at www.rotork.com