ESD Valve Self Testing system

8/13/2019 ESD Valve Self Testing system

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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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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


SVM200

SVM200

SVM200

SVM200

SVMRACK

RS485


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

Smart

Valve

Monitor


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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.

Bar G

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11 12Seconds


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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

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11 12Seconds

Valve starts

to move

Bar G

1

2

3

4

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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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


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.

Bar G

1 2 3 4 5 10

1

2

3

4

Seconds6 7 8 9

1

2

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6

5

7


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Bar G

1 2 3 4 5 6

1

2

3

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


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


15/24


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


16/24

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


17/24


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


18/24

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.


19/24


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.


20/24

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


21/24


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


22/24

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


23/24


Notes


24/24

www.rotork.com

Electric Actuators and Control Systems

Fluid Power Actuators and Control Systems

Gearboxes and Gear OperatorsProjects, Services and Retrofit

Corporate headquarters

Rotork plc

tel +44 (0)1225 733200

fax +44 (0)1225 333467

email [email protected]

A full listing of our worldwide sales andservice network is available on our website.

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

Date post:	04-Jun-2018
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ESD Valve Self Testing system

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