24
Publication F903E Issue 10/10 Established Leaders in Valve Actuation Fluid Power Actuators and Control Systems Understanding The Smart Valve Monitor
Publication F903E Issue 10/10
Established Leaders in Valve Actuation
Fluid Power Actuators and Control Systems
Understanding The Smart Valve Monitor
2
In the more than 50 years since the company was founded, Rotork has become the standard for excellence in the field of valve and damper automation for the oil, gas, power, water and waste treatment industries around the world. As established leaders in actuation technology, we owe our success to a commitment to quality at every stage, and at every level, of Rotork's operations.
At the heart of the company is an exceptional workforce the highly trained, forward thinking engineers, technicians, and sales support staff who each play a crucial role in maintaining Rotork's unrivaled reputation for innovation, reliability and first class after sale support.
With several fluid power factories in Europe and the United States, and additional Rotork Centres of Excellence located around the globe, we are able to offer creative solutions and design systems for virtually any application — from sub-sea hydraulics to the most sophisticated yet simple fluid power control system.
Contact Rotork for your operational or safety application requirements. We will work with you from conception, to design, to manufacture, to installation, and finally to maintenance and service support.
Rotork. Established leaders in valve actuation technology.
Rotork – Established Leaders in Valve Actuation Technology
Established Leaders in Valve Actuation Technology 3
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 system for 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.
Contents
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
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
FallSVM Power
ESD Power
PassTest S
tatu
s
FallSVM Power
ESD Power
PassTest S
tatu
s
Com Reset Reset
Smart Valve Monitoring Unit
TAG No.
HPU/914
MODEL No.SERIAL No.TAG No.
Tel: +44 (0) 113 205 7278www.rotork.com
HPU/905
Com
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 available on 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 stroke testing 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 all 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 linear, pneumatic or hydraulic, spring-return or double-acting. SVM is compatible with 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 valve performance.
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.
SVM100 field unit for installation close to the valve in hazardous areas.
SVM200 rack mount unit for control rooms or other safe areas.
Established Leaders in Valve Actuation Technology 5
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.
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. Once the 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.
DCS
SmartValve
Monitor
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
6
Extended Full Closure Intervals
DCS
PT
SmartValve
Monitor
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 defined by 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: lD failure rates are measured in FITs (10-9failures/hour)
There are two components of the PFDavg calculation 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 TIFC is extended.
The DCPC for a PST is defined by IEC 61508 as follows:
Equation 2
DCPC = lDD / l TOTAL
lDD is the dangerous detected rate of the PST.
l TOTAL is the total dangerous failure rate.
SVM System Interaction
It is essential to maximise DCPC to give maximum weight to the 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 TIFC and 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 as shown below:
lD*(TIFC/2) = [DCPC*lD*(TIPC/2)]+[(1-DCPC)*lD*(TIFCNEW/2)]
TIFCNEW is 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 TIFCNEW gives:
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 TIFC
NEW and DCPC can be plotted. This will determine the effectiveness of any partial stroke testing system as shown in figure 4. The graph shows that TIFC
NEW
increases exponentially with the diagnostic coverage and that no significant gains are realised until the DCPC reaches 75%.
Established Leaders in Valve Actuation Technology 7
Extended Full Closure Intervals
DCPC
TIFC
(new
) (M
on
ths)
10%0
5
10
15
20
25
30
35
40
45
15% 22% 25% 35%35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 86% 90%
DCPC
TIFC
(new
) (M
on
ths)
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%
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
Fig. 4. Effect of DCPC on TIFC .
We can standardise DCPC for a PST system by taking data from exida's Safety Equipment Reliability Handbook (2nd edition) 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 final elements 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 DCPC on TIFC – Positioner vs. SVM.
The actual improvement to the closure interval can now be determined. Analysis shows that TIFC for 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.
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 incomparison to a partial stroke. Full details of the actual eventsrecorded in the graph are discussed on page 12.
Fig. 8. Full and partial stokes.
Additional Diagnostics
By 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 forpneumatic, hydraulic and electro-hydraulic actuated shutdownvalves.
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 is completed.
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.
Bar G
1
2
3
4
5
1 2 3 4 5 6 7 8 9 10 11 12Seconds
Established Leaders in Valve Actuation Technology 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 thecapabilities 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.
It’s 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 air pressure 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
1
2
3
4
5
1 2 3 4 5 6 7 8 9 10 11 12Seconds
Valve startsto move
Bar G
1
2
3
4
5
1 2 3 4 5 6 7 8 9 10 11 12Seconds
Valve startsto move
Valve fully closed
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 trip. 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. Exida’s 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
Established Leaders in Valve Actuation Technology 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 and reporting. 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
Rotork Smart Valve Monitoring Plant Report
Tag Number: SDV-1023 Details Original Full Closure Original Part Closure Current Part Closure Status: N/A N/A Fail Operation Date & Time: 24/09/08 12:16:01 24/09/08 12:18:06 03/11/08 16:09:50 Supply Pressure: 5.182 Bar 5.26 Bar 5.442 Bar Solenoid Changeover Response Time: 0.224 0.24 0.224 Emergency Valve Breakaway Pressure: 2.813 Bar 3.321 Bar 3.365 Bar Emergency Valve Movement Time: 5.76 Secs 1.84 Secs 1.936 Secs Solenoid Restore Response Time: 6.56 Secs 0.359 Secs 0.37 Pressure Stable at: 0.1465 Bar N/A N/A Emergency Valve Restore Pressure: N/A 2.217 Bar 2.31 Bar
12
Principles of SVM Fault Detection
Bar G
1 2 3 4 5 10
1
2
3
4
Seconds6 7 8 9
1
2
4
36
5
7
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 shutdown system. 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 SVM’s 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 a change 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 detail the 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.
Established Leaders in Valve Actuation Technology 13
Bar G
1 2 3 4 5 6
1
2
3
4
Seconds
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 in switching 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 seen here. 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 and actuator 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.
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.
14
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 stopped moving. 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 in the 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 as in 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.
Principles of SVM Fault Detection
Bar G
1 2 3 4 5
1
2
3
5
Seconds
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
Established Leaders in Valve Actuation Technology 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 valve since 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 doesn’t 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 a decrease 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 valve is behaving correctly up to point A where the actuator’s 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
ED
C
16
Application – Pneumatic Actuators
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
SmartValve
MonitorPT
Bar A
1 2 3 4 5 56 7 8 9 10
1
2
3
4
5
6
7
8
9
10
Seconds
Pvm
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 data accuracy. 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 tvm by 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 Pvm from which the tvm is 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 G
1 2 3 4 5 6
1
2
3
4
5
Seconds
Partial Stroke
Full Stroke
Established Leaders in Valve Actuation Technology 17
DCS
SmartValve
Monitor
PT
Bar A
1
1
2
2
3
3
4
5
6
Seconds
Valve StartsTo Move
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 in determining 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 + tps is 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 closures for 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 manner as a spring-return actuated valve.
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
18
Application – Hydraulic Actuators
DCS
SmartValve
MonitorPT
175
140
105
70
35
5 10 15 20 25 30
Bar
Seconds
Partial Closure
Full Closure
22
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
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
DCS
SmartValve
MonitorPT
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 data quality 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 particularly advantageous 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 below in 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.
Established Leaders in Valve Actuation Technology 19
DCSSmartValve
MonitorPT
1 2
22
3 4 5 6 7 8 9 10 11 12 13 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 still 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 maximum limit.
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.
20
Application – HIPPS Valve
Pipeline and Umbilical
Subsea IsolationValve
DCS
SmartValve
Monitor
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 operation these 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 <2 seconds duration.
• Intelligent pressure compensation to prevent overshoot in the event of decreased instrument pressure.
• Solenoid performance testing and quantification via intelligent software characterisation.
Configuration
Figure 35 shows the P&ID (Process and Instrumentation Diagram) of a typical HIPPS set up with SVM.
Fig 35. HIPPS P&ID.
Features of note:
• Twin solenoids, usually of different manufacture.
• In order to achieve the high stroke speeds, multiple quick exhaust valves may be employed, typically three or four.
• Pressure transducers located as close as possible to actuator for maximum performance.
• The SVM can be located locally or in a remote location.
Analysis of Smart Valve Monitor Data
Figure 36 shows actual SVM data for a 24” HIPPS valve. The blue and red curves indicate the full and partial stroke respectively. In order to achieve the required partial stroke for a fast-acting valve such as this, it is necessary to re-energise the solenoid in advance of the required position. In this case, it was necessary to offset this by 400ms to achieve a 30% partial stroke.
The performance of the solenoid valve can be easily assessed from the SVM data. The time and pressure for any section of the graph can be displayed by clicking on the appropriate graph as shown in the enlarged section.
Figure 37 shows additional data from an ultra-sonic position sensor. This was added during factory proof testing to show the actual position of the valve in relation to the pressure vs. time graph. It verifies the valve does indeed partial stroke by approximately 30%.
SVM also generates a quantitative report detailing many aspects of the systems performance. An example is shown in figure 38.
Established Leaders in Valve Actuation Technology 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
22
Application – Subsea Isolation Valves
Pipeline and Umbilical
Subsea IsolationValve
DCS
Platform Topside
SmartValve
MonitorPT
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 sufficient to 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.
22
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
44
66
88
110
132
154
178
198
Bar
Seconds
Partial Stroke
Full Stroke
Established Leaders in Valve Actuation Technology 23
Notes
www.rotork.com
A full listing of our worldwide sales and service network is available on our website.
Corporate HeadquartersRotork plctel +44 (0)1225 733200fax +44 (0)1225 333467email [email protected]
Fluid Power Actuators and Control Systems
Gearboxes and Gear Operators
Projects, Services and Retrofit
All Rotork Fluid Systems actuators are manufactured under a third party accredited ISO9001 quality assurance programme. As we are continually developing our products, their design is subject to change without notice.
The name Rotork is a registered trademark. Rotork recognises all registered trademarks. Published and produced in the UK by Rotork Fluid Systems. POWSH1010
Electric Actuators and Control Systems
