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Increased process safety and plant availability in the chemical industryPartial Stroke Test

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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 cost-

effectiveness requirements.

Setting new standards: increased safety and efficiency in the chemical industry

Increased efficiency for the maintenance of safety shutoff valves

Safety Instrumented Systems (SIS) are used in industrial pro-cesses in order to minimize the dangers for people, the envi-ronment, 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 asso- ciated with high organizational and financial overheads and could interfere with production. It was sometimes even neces-sary 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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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 above-mentioned standards into four safety levels, the so-called Safety Integrity Levels SIL 1 to SIL 4. In the process indus-try, 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, con- trollers, and actuators. Whereas controllers can mainly be considered as safe, sensors and actuators (which are exposed to special conditions since they come into con-tact 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.

Precautions which pay offIf 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 consider-ing the savings resulting from increased production times.

• Plant with 60 valves•Maintenance time: Without dismount-

ing – 2 h; with dismounting – 10 h

•Hourly rate 50 €•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 mainte-nance operation

60 h

300 h

Maintenance costs saved 3 000 € 15 000 €

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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 dur-ing 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 instru-mentation as well as integration into the automation envi-ronment – 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. Disadvan-tage: 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.

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 comprehen-sive valve diagnostics options. It reliably detects sluggish-ness, 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 trig-gered 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 visual-ized using SIMATIC PDM and compared with a reference curve. An expansion card allows separate testing of the solenoid valve.

SIMATIC controller S7-400F/FH

Engineering station / maintenance station

PDM

Operator station

(OK, no maintenance request) Maintenance requirement Urgent maintenance request (Imminent) failure of fitting

SIMATIC PCS 7 maintenance station

Process Device Manager (SIMATIC PDM)•Parameterization•Diagnostics

SIPART PS2: •Partial Stroke Test

SIPART PS2 (SIL 2)Air supplyProtective function

Optional connection to SIMATIC PCS 7

Safety shutoff unit with positioner

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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 super- fluous.

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 pro-cess control system. The preconfigured logic is respon- sible for carrying out the PST and the optional solenoid valve test.

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

S

Engineering station Operator station

SIMATIC controller S7-400F/FH Protective function

Air supply

Solenoid valve / control valve

Safety shutoff unit with positioner

Setpoint for valve position

Feedback of valve position

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 electroni-cally, 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 sum-mary of all relevant information – valve status, configura-tion 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.

Partial Stroke Test – your advantages at a glance

• 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

• Increased safety in emergency situations: The fitting is not blocked and can respond promptly

• Cost savings: Reduced maintenance requirements, longer maintenance intervals, and fewer down-times

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The best proof: Partial Stroke Test in real use

Partial Stroke Test in use at INEOS Paraform

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 chemi-cals 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.

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 prevent-ing 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.

… with convincing results

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 sole-noid valve were detected. The corresponding messages and reports were generated reliably.

The conclusion:

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 on-site – 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 simulta-neously guaranteeing the greatest possible safety.

LZ+A

LIS+A

Hexa solution tank

0865.01

0865.02

0865.04

SIPART PS2Fast venting

Air supply

3-way solenoid valve

SIMATIC ET 200 distributed I/O

Safety monitoring (relay technology)

PROFIBUS PA

PROFIBUS DP SIMATIC DP/PA-Link fieldbus coupler

Building 192SIMATIC PCS 7 SIMATIC S7-400F

Plant / control room unit GF1

SS

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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 dete- rioration, such as the response time of the safety shutoff unit, through comparison of the analog position feedbacks

Various faceplates and message displays are available for assessing the PST.

At INEOS, a test report with curves and two reference values was generated per PST. Customer and TÜV statements:

• Automatic execution of the PST – results are dis-played 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

“The reduction in maintenance over-head is interesting for us, and we en-visage excellent potential applications with this solution in continuous opera-tions 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 er-rors, such as failure of the safety shut-off unit due to sticking in the open po-sition or control valve malfunctioning.”

TÜV Süd Industrie Service GmbH, Electrical Engineering & Building Technology Dept., March 9, 2007

Partial Stroke Testsiemens.com/partial-stroke-test

Safety Integrated Level (SIL)siemens.com/sil

Positionerssiemens.com/sipartps2

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

The information provided in this brochure contains merely general descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the prod-ucts. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of contract.

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