Categories: MICHELL Instruments

MICHELL Instruments XTC 601 Binary Gas Analyzer for Hydrogen Monitoring Instruction Manual

XTP/XTC 601
SIL Safety Manual
Note: Supplement to the instruction manual
l97587 Issue 1.2 September 2022
Michell Instruments 

For Michell Instruments’ contact information please go to www.michell.com 
© 2022 Michell Instruments
This document is the property of Michell Instruments Ltd and may not be copied or otherwise reproduced, communicated in any way to third parties, nor stored in any Data
Processing System without the express written authorization of Michell Instruments Ltd.
The contents of this safety manual shall not become part of or modify any prior or existing agreement, commitment or legal relationship. All obligations on the part of Michell Instruments are contained in the respective sales contract which also contains the complete and solely applicable warranty conditions. Any statements contained herein do not create new warranties or modify the existing warranty.
XTP & XTC SIL Safety Manual
NOTE:
This product must not be modified or altered in any way. Unauthorized change is not permitted and to do so would cause Functional Safety, as confirmed by the
IEC61508 assessment, to be null-and-void. The design of This product is strictly controlled and to do so would invalidate all approvals, certifications and warranties this product
holds. Please consult Michell Instruments Ltd directly for any functionality or service queries you may have.

Safety Guidelines

This manual relates only to the SIL aspects of this product.
For all other operation, installation & maintenance information refer to the product manual. The user must not use this equipment for any other purpose than that stated. Do not apply values greater than the maximum value stated.
This manual contains information relating to the SIL aspects of operating this product. Use competent personnel using good engineering practices for all procedures in this manual.

Qualified Personnel

This product should only be set up and used in conjunction with this documentation. Commissioning and operation of this product should only be performed by qualified personnel.

Abbreviations

The following abbreviations are used in this manual:

λ Failure Rate
λD Dangerous Failure Rate
λDD Dangerous Detected Failure Rate
λDU Dangerous Undetected Failure Rate
λs Safe Failure Rate
/hr Per Hour
ADC Analogue-To-Digital Converter
DAC Digital-To-Analogue Converter
DC Diagnostic Coverage
E/E/PE Electrical/Electronic/Programmable Electronic
EMF Electromotive Force
ESC Engineering Safety Consultants
EUC Equipment Under Control
FIT Failure in time
FREDA Failure Mode Effect and Diagnostics Analysis
FMR Failure Mode Ratio
FS Functional Safety
FSM Functional Safety Management
HFT Hardware Fault Tolerance
MDT Mean Down Time
MTTR Mean Time To Restoration
NPRD Non-Electronic Parts Reliability Data
O2 Oxygen
O/C Open Circuit
PFD Probability of Failure on Demand
PDF Average Frequency of a Dangerous Failure per Hour
PLC Programmable Logic Controller
PTI Proof Test Interval
QA Quality Assurance
RBD Reliability Block Diagram
S/C Short Circuit
SFF Safe Failure Fraction
SIF Safety Instrumented Function
SIL Safety Integrity Level
SR Safety Related
Tp Proof Test Interval

INTRODUCTION

1.1 General
This manual refers only to:
XTP601 Oxygen Transmitter.
XTP601 Oxygen Analyzer.
XTC601 Binary Gas Analyzer.
XTC601 Binary Gas Transmitter.
There are derivatives of each model as shown in the below table:

Analyzer Name Type
XTP601-GP1 General purpose analyzer with display
XTP601-GP2 General purpose analyzer with flame arrestors
XTP601-EX1 Hazardous area analyzer with display
XTC601-GP1 General purpose analyzer with display
XTC601-GP2 General purpose analyzer with flame arrestors
XTC601-EX1 Hazardous area analyzer with display

1.2 Required documentation
This document only applies in conjunction with the following documentation:

Analyzer Name Type Document No.
XTP601 Process Oxygen Analyzer User’s Manual (UK) 97313
XTP601 Binary Gas Analyzer User’s Manual (UK) 97400

NOTE: For each type, there are manuals with the same content translated into other languages.
This document contains SIL-related data that will be required when using the XTP601 & XTC601 products in safety-instrumented systems.
It is aimed at system planners, constructors, service and maintenance engineers and personnel who will commission the device.

SAFETY INSTRUCTIONS

These products are intended for use in safety applications.
All safety instructions relate exclusively to the analog output signal (4–20mA). The products are certified to SIL2 (IEC 61508). The product’s software is certified SIL2
(IEC61508). The use of these products integrated in to safety-related systems is therefore possible.
Definition: Safety-instrumented system
A safety-instrumented system executes the safety functions that are required to achieve or maintain a safe status in a system. It consists of a sensor, logic unit/control system
, and final controlling element. A Safety Instrumented System (SIS) could be made of an analyzer (e.g. XTP 02 Concentration), a Safety rated logic Solver (e.g. safety relay or safety-rated PLC) and a final element (e.g. valve, or alarm with defined response). Definition: Safety function
The defined function is executed by a safety-instrumented system with the objective of achieving or maintaining a safe system considering a defined dangerous occurrence.
Example: XTP O2 concentration above or below a defined threshold.
2.1 Safety Integrity Level (SIL)


The international standard IEC 61508 defines four discrete Safety Integrity Levels (SIL) from SIL 1 to SIL 4. Each level corresponds to the probability range for the failure in a
safety function. The higher the SIL of the safety-instrumented system, the higher the probability that the required safety function will work.
The achievable SIL is determined by the following safety characteristics:
  • Average probability of dangerous failure of a safety function in case of demand (PFDAvG)
  • Hardware fault tolerance (HFT)
  • Safe failure fraction (SFF)

Description: The following table shows the dependency of the SIL on the average probability of dangerous failures of a safety function of the entire safety-instrumented system (PFDAvG). The table deals with “Low demand mode”, i.e. the safety function is required a maximum of once per year on average.

SIL level PFDavg
SIL 4 10–4 > PFDavg ≧ 10–5
SIL 3 10–3 > PFDavg ≧ 10–4
SIL 2 10–2 > PFDavg ≧ 10–3
SIL 1 10–1 > PFDavg ≧ 10–2

Table 1 Safety Integrity Level
The “average probability of dangerous failures of the entire safety-instrumented system” (PFDAvG) is normally spilt between the entire SIL system.

The following table shows the achievable Safety Integrity Level (SIL) for the entire safety instrumented system for type B systems depending on the proportion of safe failures (SFF) and the hardware fault tolerance (HFT). XTP and XTC units are considered Type B due to their complexity. Type B systems also include sensors and positioners actuators with complex components, e.g. microprocessors (see also IEC 61508, Section 2).

SFF HFT
0 1 2
<60% Not allowed SIL1 SIL2
60 to 90% SIL1 SIL2 SIL3
90 to 99% SIL2 SIB SIL4
>99% SIL3 SIL4 SIL4

Table 2 Safety Integrity Level

DEVICE-SPECIFIC SAFETY INSTRUCTIONS

3.1 Applications
The Hardware assessment of the XTP601 & XTC601 shall provide the safety instrumentation engineer with the required failure data as per IEC 61508.
The hardware of XTP601 & XTC601 satisfies the requirements in terms of functional safety to SIL 2 in accordance with IEC 61508. The XTP601 & XTC601 is usable in safety
applications to monitor limits.
3.2 Safety Function
The XTP601 & XTC601 are mainly used for user-defined threshold monitoring.
The XTP601 Process Oxygen Analyzer was assessed against the following safety function:

  • Ability to detect oxygen presence within another gas stream and generate a 4–20mA output.
    The XTC601 Binary Gas Analyzer was assessed against the following safety function:
  • Ability to detect target gas in another gas stream and generate a 4–20mA output.

Warning
See the “Settings” and “Safety characteristics” sections for the binding settings and conditions. These conditions must be met to fulfill the safety function. When the safety function has been executed, safety-instrumented systems with no self-locking function should be brought to a monitored or otherwise safe status within the Mean Time To Repair (MTTR). The MTTR is 168 hours. For full product information refer to User Manuals 97313 & 97400.
3.3 Settings
After installation and commissioning (refer to User Manuals), the following parameter settings should be made for the safety function:
Safety parameters

Function
Analog Output Select 4–20mA (NAMUR)

Protection against configuration changes
After configuration, the menu access codes of XTP601 & XTC601 shall be changed so that the device is protected against unauthorized changes and operation.
Checking the safety function after installation After installation a safety function test must be carried out.
Using reference gas, i.e. N2 , 4mA must be measured at the analog output.


For the test of the safety function, it is fundamental to use a second reference gas with a defined proportion of oxygen. The results of the measurement must be within a range
of ±5% (full span) of the expected result.
3.4 In case of faults
Fault
The procedure in case of faults is described in the User Manuals.
Repair
The defective product should be sent to a Michell Instruments Service Department with details of the fault and the cause. When ordering a replacement product, please specify the serial number of the original product. The serial number can be found on the nameplate. Information regarding the location of Michell Instruments Service centres can be found at the following web address: www.michell.com
3.5 Maintenance/Calibration
We recommend that the functioning of the XTP601 & XTC601 is checked at regular intervals of one year.
Check at least the following: Test the basic functionality of the XTP601 & XTC601 as described in the User Manual.
Checking safety
You should regularly check the safety function of the entire safety circuit in line with IEC 61508/61511.
The testing intervals are determined during the circulation of each individual safety circuit in a system. The recommended prove interval depends on the application but it
should be at least once a year. To detect dangerous undetected faults, the XTP601 & XTC601 analog output shall be checked with the following test:
To execute the safety proof test both tests (1 and 2) must be performed. Proof test 1 consists of the steps described in the table below.
Step Action
1 Bypass the safety PLC or take other appropriate action to avoid a false trip.
2 Generate or simulate an alarm condition to force the product to go to the high alarm current output and verify that the analog current reaches that value.
3 Generate or simulate an alarm condition to force the product to go to the low alarm current output and verify that the analog current reaches that value.
4 Restore the loop to full operation.
5 Remove the bypass from the safety PLC or otherwise restore normal operation.

Proof test 2 consists of the steps described in the table below.

Step Action
1 Bypass the safety PLC or take other appropriate action to avoid a false trip.
2 Perform Proof Test 1.
3 Perform a 2-point calibration of the product.
4 Perform a reference measuring with at least one measuring point between min and max concentration. You must use a calibration gas with a well-known gas concentration. The expected result must have a tolerance of not more than 5%.
5 Restore the loop to full operation.
6 Remove the bypass from the safety PLC or otherwise restore normal operation.

This test will detect more than 90% of possible “du” failures in the product.
Should faults be detected, the product should not be used until completely rectified.
3.6 Safety Characteristics
The safety characteristics necessary for use of the system are listed in the SIL declaration of conformity (see Appendix A.1). These values apply under the following conditions:

  • The XTP601 & XTC601 are only used in safety-related systems with a low demand mode for the safety function.
  • The safety-related parameters/settings (see “Settings” section) have been entered by local operation and checked before commencing safety instrumented operation.
  • The XTP601 & XTC601 is blocked against unwanted and unauthorized changes/ operation.
  • The maximum operating temperature is +40°C for the XTC601 and +55°C for the XTP601.
  • All used materials are compatible with process conditions.
  • The MTTR after a device fault is 168 hours.
  • The logic solver (PLC) must be configured to detect over range (>21mA) and under range (<3.6mA) failure of the XTP601 & XTC601 (Fail High and Fail Low) and will recognize these as internal failures of the products and not cause a spurious trip.
    Also see the Settings section of this manual and Appendix below.

Appendix A
A.1 SIL Declaration of Conformity

ENGINEERING SAFETY CONSULTANTS


The Global Provider of Functional Safety Expense and Technical Consultancy
Random Hardware Reliability and Systematic Assessment Certificate
Functional Safety of Safety-Related Programmable Electronic Systems
The Michell Instruments UK Ltd, XTP601 Process Oxygen Analyser & XTC601 Binary Gas Analyser have been assessed and are considered capable for use in a low-demand Safety Function up to (and including) SIL 2 capability with regards to systematic, random hardware failures and architectural constraints.
The assessment was based on the assumptions, data provided, and recommendations given in:
  • Engineering Safety Consultants Ltd Report: H215_FM001 rev. 4.
    The products were assessed against the following failure modes:
  • XTP601: Ability to detect oxygen presence within another gas stream and generate a 420mA output;
  • XTC601: Ability to detect target gas in another gas stream and generate a 4-20mA output.
    The assessment was carried out to determine compliance with IEC 61508 (2010 Edition) with regards to:
  • Random Hardware Failure (Predicted PFD as shown in Table below) with a Mean Down Time (MDT) of 168 hours, a Proof Test Interval (PTI) of one year (8760 hours), a Proof Test Coverage of 95% or 90% and an Overhaul Interval of 10 years (87600 hours);
  • Random Hardware Failure with Achieved PFH:
    o XTP601 = 5.4E-08
    o XTC601 = 3.9E-08
  • Random Hardware Failure with Achieved DD:
    o XTP601 = 7.4E-07
    o XTC601 = 7.0E-07
  • Random Hardware Failure with Achieved DU:
    o XTP601 = 5.4E-08
    o XTC601 = 3.9E-08
  • Architectural Constraint (Type B, SFF >90%, <99%), HFT = 0;
  • Systematic SIL 2 capability against IEC 61508 (2010 Edition) Parts 1, 2 and 3.
Device Proof Test
Coverage
(PTC)
PFD Target
(20% of SIL
2 band)
AchievedPFD Estimated
Achieved
PFD
SFF Type Estimated
Achieved
SIL (Arch)
Estimated
Overall SIL
Capability
XTP601 95% 2.E-03 4.E-04 2 94% B 2 2
90% 5.E-04 2 2 2
XTC601 95% 2.E-03 3.E-04 2 96% B 2 2
90% 4.E-04 2 2 2

IMPORTANT: It should be noted that this assessment does not include confirmation of the response time of the device. For response times (along with any relevant assumptions) reference should be made to the Safety Manual of each device and the total SIF response time MUST be compared against the process safety time for the specific application.

Managing Director: Simon Burwood
Member of IEC 61508 (MT61808-1-2) & IEC 61511 (MT61511) Maintenance Committees Assessment Date: February 2020
Renewal Date: August 2022, valid to August 2024
Certificate: H215_CT001 rev. 3

A.2 Engineering Safety Consultants Limited. London, UK Test Report extract
2.1 General
This report provides a Prior Use Assessment of the Michell Instruments UK Ltd, XTP601 Process Oxygen Analyzer and XTC601 Binary Gas Analyzer, as defined in the Prior
Use requirements in IEC 61511 (2nd Edition) Clause 11.5.3 and 11.5.4 [2] including an estimation of Probability of Failure on Demand (PFD), Safe Failure Fraction (SFF) and a review of the systematic capability as supporting evidence for avoidance and minimization of systematic failures.
A Failure Mode Effects and Diagnostics Analysis (FMEDA) was conducted on the XTP601 & XTC601 to estimate the random hardware failure rate in order to assess suitability for use in a safety function with regards to the PFD and the architectural requirements in terms of Hardware Fault Tolerance (HFT) and SFF, using the approach detailed in Route
1H in IEC 61508-2 [1].
2.2 Hardware Reliability Verification
These devices will form part of the sensor element sub-system of a Safety Instrumented Function (SIF) and thus an assessment was conducted to demonstrate its capabilities in
terms of PFD. The remaining sensing, logic solver and final element sub-systems were excluded from the assessment, in order to allow for their PFD contributions, the devices
were assessed against 20% of Safety Integrity Level (SIL) 2 PFD band (e.g. SIL 2 band modified to 2.0E-03).
The analysis was based on the assumption that repairs would be carried out with a Mean Down Time (MDT) of 168 hours, a Proof Test Interval (PTI) of one year (8760
hours) and capable of revealing 100% of undetected failures.
The XTP601 Process Oxygen Analyzer was assessed against the following safety function:

  • Ability to detect oxygen presence within another gas stream and generate a 4–20mA output.
    The XTC601 Binary Gas Analyzer was assessed against the following safety function:
  • Ability to detect target gas in another gas stream and generate a 4–20mA output.
    Table 3 shows a summary of the results of the XTP601 & XTC601 based on the data provided and the assumptions given in this report. The full set of results for the hardware reliability verification is presented in Table 4.
Device PFD Target (20%
of SIL2 band)
PFD achieved PFD achieved
(SIL)
SFF Type Achieved SIL (Architecture HFT =0) Overall achieved
SIL
XTP601 2.E-03 4.E-04 2 94% B 2 2
XTC601 2.E-03 3.E-04 2 96% B 2 2

Table 3 SIL Results Summary

APPENDIX A

Device Reference XTP601 & XTC601
Function Specification XTP601 Oxygen Transmitter XTC601 Binary Gas Analyzer
Software Configuration/Settings As per customer order
Verson ion Firmware for XTP601: 36217 V1.09 Firmware for XTC601: 37701 V1.06
Hardware Diagram version XTP601: 80895/C V2.0 XTC601: 81003/C V1.0
Hardware Configuration/Settings As per customer order
Failure Mode(s) Definition Dangerous detected dangerous detected failure rate per hour
Dangerous undetected dangerous undetected failure rate per hour
Safe safe (or spurious) failure rate per hour
Estimated failure rate XTP601 7.0E-07, XTC601 5.9E-07
Dangerous Undetected Failures (ADU) XTP601 5.41E-08, XTC601 3.87E-08 (FIT/hr)
Dangerous Detected Failures (ADD) XTP601 7.39E-07, XTC601 7.00E-07 (FIT/hr)
Safe Failures (AS) XTP601 & XTC601 1.57E-07 (FIT/hr)
Probability of Failure on Demand (PFD) XTP601 3.6E-04, XTC601 2.9E-04
Safe Failure Fraction (SFF) XTP601 94% XTC601 96%
Hardware Fault Tolerance (HFT) 0
Classification (Type A or Type B) B
Demand (Low demand or High Demand) Low
Proof Testing Procedures See section 3.5
Installation Refer to User Manual 97313 (XTP) & 97400 (XTC)
Average lifetime of device (yrs) 5
Environmental Profile Max +50°C. 80%rh>31°C/50%>+50°C
Systematic/Proven in Use Safety Integrity Level 2
Assumptions Refer to User Manual
General Notes and applicable regulations This product complies with applicable standards and clauses of EU ATEX, EMC, PED Directives. Refer to the EU Declaration supplied with each product for full details of the latest versions.
Testing requirements See section 3.5

Table 4 Verification Results

NOTES……….

ENGINEERING SAFETY CONSULTANTS LTD
2nd Floor, Exchequer Court, 33 St. Mary Axe,
London, EC3A 8AA UK
Telephone/Fax: +44 (0)20 8542 2807
E-Mail: info@esc.uk.net Web: www.esc.uk.net
Registered in England and Wales: 7006868
Registered Office: 33 St. Mary Axe, London, EC3A 8AA
www.ProcessSensing.com
http://www.michell.com

Documents / Resources

MICHELL Instruments XTC 601 Binary Gas Analyzer for Hydrogen Monitoring [pdf] Instruction Manual
XTC 601 Binary Gas Analyzer for Hydrogen Monitoring, XTC 601, Binary Gas Analyzer for Hydrogen Monitoring, Hydrogen Monitoring, Gas Analyzer, Analyzer

References

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