Increased process safety and plant availability in the chemical industry Partial Stroke Test siemens.com/chemical
Answers for industry.
Setting new standards: increased safety and efficiency in the chemical industry Safety in the process industry is especially important. We comply with the requirements of the chemical industry with a complete offering for safe applications called Safety Integrated. This comprises solutions which make a contribution toward increasing plant availability and reducing costs. The best example: the Partial Stroke Test (PST). Whereas regular manual testing of safety shutoff valves (Full Stroke Tests) is extremely expensive and time-consuming, the PST is an effective and simple method for testing safety valves, as a result of which the intervals between manual tests can be extended. The test is carried out without interfering with continuous plant operation. When integrated in the process control technology, it takes into account both safety and costeffectiveness requirements.
Increased efficiency for the maintenance of safety shutoff valves Safety Instrumented Systems (SIS) are used in industrial processes in order to minimize the dangers for people, the environment, and the plant. These safety-oriented control and protection systems intervene in the process in the event of a fault in order to transfer it to a safe state again. The systems thus make a significant contribution toward risk reduction. In particular, the reliability of the actuators in safety systems must be regularly tested, for example the safety shutoff valves. These represent the largest risk, but also the greatest potential for minimizing risks. On the one hand, shutoff valves are continuously exposed to flowing media, and on the other hand they remain in a fixed position during normal operation as pure safety equipment. In the past, these valves were usually subject to a manual test on site once a year (Full Stroke Test). This was usually associated with high organizational and financial overheads and could interfere with production. It was sometimes even necessary to remove the valve for testing in the factory. The Partial Stroke Test can be applied to detect and prevent any malfunctions early on – for significantly more safety in emergency situations. The PST increases the diagnostics coverage of the valves. This then enables plant owners to extend the test intervals, thus reducing maintenance costs and downtimes.
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Precautions which pay off If the Partial Stroke Test is carried out regularly, it is possible, for example, to double the maintenance interval for manual testing of safety shutoff valves. In other words: the costs for a Full Stroke Test are halved – as well as the time required for maintenance work. The following example illustrates the significant savings potentials for a single maintenance operation – even without considering the savings resulting from increased production times. • Plant with 60 valves
• Hourly rate 50 €
• Maintenance time: Without dismounting – 2 h; with dismounting – 10 h
• Doubling of maintenance interval
Maintenance
Valve not dismounted
Valve dismounted
Requirement per valve
2 h à 50 € = 100 €
10 h à 50 € = 500 €
Previous costs
60 à 100 € = 6 000 €
60 à 500 € = 30 000 €
With PST maintenance costs
3 000 €
15 000 €
Time saved per maintenance operation
60 h
300 h
Maintenance costs saved
3 000 €
15 000 €
Basics of functional safety A number of standards define the organizational and technical requirements placed on a Safety Instrumented System and its implementation. • IEC 61508 (basic standard) Defines the basics of functional safety. • IEC 61511 (application-specific standard for the process industry) Directive for planning, implementation, and operation of an SIS in process engineering plants. The requirements placed on the reliability of protective equipment result from risk analyses carried out during plant planning, and are divided in line with the abovementioned standards into four safety levels, the so-called Safety Integrity Levels SIL 1 to SIL 4. In the process industry, requirements up to SIL 3 can be achieved through use of a programmable Safety Instrumented System.
The SIL levels always apply to complete Safety Instrumented Functions (SIF) which basically consist of sensors, controllers, and actuators. Whereas controllers can mainly be considered as safe, sensors and actuators (which are exposed to special conditions since they come into contact with the medium) must be tested at regular intervals in order to comply with the required safety level. • NE 106 test intervals on protective equipment in process control systems The user association for automation technology in the process industry NAMUR (standards working group for measurement and control) has handled the topic covering extension of test intervals with PCS protective equipment in its recommendation NE 106. The basis for extending the test intervals is optimization of the diagnostics functions for sensors and actuators.
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Fewer downtimes: function test during operation No interference with plant safety and availability A Partial Stroke Test allows checking of functions during ongoing operation. To do this, a positioner or a special test device is mounted on the respective open-close valve. This valve is moved by approximately 5 to 15 percent during operation. This does not interfere with normal plant operation, but proves the mobility of the valve and thus excludes the most frequent passive faults. In addition, any deposits or incrustations are removed. The correct instrumentation as well as integration into the automation environment – as taken into account in our portfolio – are decisive factors for safe, automated valve startup tests. Our solutions for a Partial Stroke Test 1. Application of a switch box: with this rather simple test, control of the solenoid valve is interrupted by a binary signal until a set position has been reached, independent of the controller’s cycle time. Disadvantage: no expanded diagnostics possible. 2. Use of the intelligent SIPART PS2 positioner which can carry out the Partial Stroke Test independently of the control system. The protective function is guaranteed via the solenoid valve. 3. Use of the SIPART PS2 as an intelligent solenoid valve which handles the PST function in addition to the solenoid valve function.
Engineering station / maintenance station
Operator station
SIMATIC PCS 7 maintenance station (OK, no maintenance request)
PDM
4. Integration of the PST procedure into the SIMATIC S7-400F/FH safety system so that the test can be executed independently of the positioner used. Intelligent integration of PST in the SIPART PS2 positioner The intelligent SIPART PS2 positioner provides comprehensive valve diagnostics options. It reliably detects sluggishness, pneumatic leaks, wear, or even breakage of the valve plug. The Partial Stroke Test function is integrated in the positioner so that a valve startup test can be triggered on the positioner, per binary signal, using the SIMATIC PDM Process Device Manager, or also cyclically. The positioner measures the response time of the valve during the Partial Stroke Test and compares it with a saved reference value. If maintenance is required, this is indicated locally on the display or in the control system by means of a 3-level alarm concept in accordance with the NAMUR recommendation NE 107. To enable further diagnostics, SIPART PS2 generates a high-resolution PST response curve which can be visualized using SIMATIC PDM and compared with a reference curve. An expansion card allows separate testing of the solenoid valve.
Process Device Manager (SIMATIC PDM) • Parameterization • Diagnostics
Maintenance requirement Urgent maintenance request
SIPART PS2: • Partial Stroke Test
(Imminent) failure of fitting SIMATIC controller S7-400F/FH
Protective function
SIPART PS2 (SIL 2)
Optional connection to SIMATIC PCS 7
Safety shutoff unit with positioner
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Air supply
Since SIPART PS2 is suitable for safe venting up to SIL 2, it can also serve as a replacement for a solenoid valve. Only the SIPART PS2 is then fitted on the protection valve and handles both the solenoid function and the PST function. The 24V signal is sufficient for the auxiliary power supply to the positioner, making additional field wiring superfluous. The positioner can be integrated into the control system and the asset management system using HART, PROFIBUS PA or FF Foundation Fieldbus. Safety system with integral Partial Stroke Test functionality Independent of the positioner used, Siemens offers an integral PST functionality for the SIMATIC S7-400F/FH safety system in conjunction with the SIMATIC PCS 7 process control system. The preconfigured logic is responsible for carrying out the PST and the optional solenoid valve test.
Following expiry of the test time, the safety shutoff unit is returned to the operating position. As an option, the associated solenoid valve can be tested in a second step. The PST is automatically recorded on paper or electronically, including the curve and two reference values for any further diagnostics functions. The associated faceplate for the SIMATIC PCS 7 operator station provides a fast summary of all relevant information – valve status, configuration parameters, status and alarms of the last PST – and has a command interface for starting and aborting the valve test. All values are present in the control system, e.g. for integration into the asset management system. The safety function itself – in other words shutdown in case of an emergency – remains available during the PST.
Thus the PST can be carried out automatically at defined intervals or started manually as required. During the PST, a setpoint is output for the valve and compared with the actual value. The closing process must reach the setpoint including the set hysteresis within the defined test time.
SIMATIC PCS 7 operator station for: • Operation/monitoring • Alarm output • Archiving • Report printout
SIMATIC S7-400FH: • Partial Stroke Test • Solenoid valve test • Interface to operator station
Engineering station Operator station
SIMATIC controller S7-400F/FH
• Simple online function check: Faulty fittings can be detected early on • Increased plant availability: Function test during ongoing operation • Improved asset management/device safety: Following a successful PST, the fitting is “free” again, deposits and incrustations have been removed
Protective function
S
Setpoint for valve position
Partial Stroke Test – your advantages at a glance
Solenoid valve / control valve Air supply
Feedback of valve position
• Increased safety in emergency situations: The fitting is not blocked and can respond promptly • Cost savings: Reduced maintenance requirements, longer maintenance intervals, and fewer downtimes
Safety shutoff unit with positioner
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The best proof: Partial Stroke Test in real use Partial Stroke Test in use at INEOS Paraform
… with convincing results
The Partial Stroke Test integrated in the safety system has not only proven itself in the laboratory but also in a real industrial environment. INEOS Paraform, a company in Mainz/Germany which has manufactured organic chemicals for more than 150 years, investigated a two-month application of the new PST concept in the SIMATIC S7-400F/FH as a pilot customer. The pilot project was accompanied by the TÜV Süd certification company, and its assessment was unconditionally positive. The result: the valve startup test integrated in the safety system makes it possible to increase the test intervals between Full Stroke Tests without compromising plant safety.
The system responded perfectly in all test scenarios in which a PST was not to be carried out. The PST was not carried out either with the valve closed or if the process release was missing or if incorrect parameter settings or values were present. All fault scenarios such as reduced valve speed, sticking of the valve, or failure of the solenoid valve were detected. The corresponding messages and reports were generated reliably.
An innovative test … A safety shutoff unit present on a tank farm was equipped with an additional SIPART PS2 positioner controlled by a fail-safe SIMATIC S7-400 CPU. The valve startup test was triggered manually during ongoing operation several times a day. In addition, various possibilities for preventing the test and for fault detection were investigated. Continuous results were provided by both the faceplate on the operator station and the test report, including curve and two reference values.
Plant / control room unit GF1
PROFIBUS DP
0865.04
Hexa solution tank
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LZ+A LIS+A
0865.02 0865.01
SIMATIC ET 200 distributed I/O Safety monitoring (relay technology)
S
Fast venting
SIMATIC DP/PA-Link fieldbus coupler
S
PROFIBUS PA
SIPART PS2
The PST does not impair the safety level of the shutoff unit. There are no objections to an automatic valve startup test – even without maintenance personnel onsite – since the actual monitoring of the test takes place in the fail-safe section of the safety system. This makes the automatic PST an ideal supplement to the Full Stroke Test as a user-friendly solution which saves both time and costs and also increases plant availability – while simultaneously guaranteeing the greatest possible safety.
Building 192
SIMATIC PCS 7 SIMATIC S7-400F
Air supply
The conclusion:
3-way solenoid valve
Clear advantages – proven in industrial use • Complete integration with numerous functions: PST is integrated in the safety system and linked seamlessly to the SIMATIC PCS 7 process control system with faceplates, automatic archiving, and reporting • Reliable and low-cost detection of valve problems with PST and their immediate checking for several failure types • Simple fault detection using corresponding alarm outputs and visualization of the current valve state • Enhanced diagnostics for early detection of deterioration, such as the response time of the safety shutoff unit, through comparison of the analog position feedbacks
At INEOS, a test report with curves and two reference values was generated per PST.
• Automatic execution of the PST – results are displayed immediately using corresponding messages and documented in test reports • Reduced maintenance requirements and costs since the test intervals between Full Stroke Tests when using the PST are at least doubled • Increased plant availability resulting from function check with PST during ongoing operation • Continuous safety since emergency shutdown remains available during the test
Customer and TÜV statements:
Various faceplates and message displays are available for assessing the PST.
“The reduction in maintenance overhead is interesting for us, and we envisage excellent potential applications with this solution in continuous operations where downtimes are seldom and then only short.” Richard Frey, responsible for electrical engineering/I&C systems at INEOS Paraform in Mainz, Germany 02/2007
“The on-site test showed that the Partial Stroke Test always detects errors, such as failure of the safety shutoff unit due to sticking in the open position or control valve malfunctioning.” TÜV Süd Industrie Service GmbH, Electrical Engineering & Building Technology Dept., March 9, 2007
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Partial Stroke Test siemens.com/partial-stroke-test Safety Integrated Level (SIL) siemens.com/sil Positioners siemens.com/sipartps2
Siemens AG Industry Sector Industry Automation P.O. Box 48 48 90026 NUREMBERG GERMANY
Subject to change without prior notice Order No.: E20001-A180-T111-X-7600 Dispo 41513 WÜ/41007 MI.GC.VM.XXCH.52.2.05 WS 04122.0 Printed in Germany © Siemens AG 2012
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