Ashcroft Pressure Switch User Manual

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

Safety Manual

A-Series Pressure Switch

Document: I&M009-10210

REV A - Release 4/19/2013

Table of Contents:

Section pg.

1. Introduction……………………………………………… 2

2. Device Description……………………………………… 4

3. Designing A SIF Using a Manufacturers Product…… 4

4. Installation and Commissioning……………………….. 7

5. Operation and Maintenance…………………………… 7

6. Start-up Checklist………………………………………. 9

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

1 INTRODUCTION

This Safety Manual provides information necessary to design, install, verify and
maintain a Safety Instrumented Function (SIF) utilizing the A-Series pressure
switch. This manual provides necessary requirements for meeting the IEC 61508
or IEC 61511 functional safety standards.

1.1 Terms and Abbreviations

Safety Freedom from unacceptable risk of harm

Functional Safety The ability of a system to carry out the actions necessary to

achieve or to maintain a defined safe state for the equipment
/ machinery / plant / apparatus under control of the system

Basic Safety The equipment must be designed and manufactured such

that it protects against risk of damage to persons by
electrical shock and other hazards and against resulting fire
and explosion. The protection must be effective under all
conditions of the nominal operation and under single fault
condition

Safety Assessment The investigation to arrive at a judgment - based on evidence

- of the safety achieved by safety-related systems

Fail-Safe State State where solenoid valve is de-energized and spring is

extended.

Fail Safe Failure that causes the valve to go to the defined fail-safe

state without a demand from the process.

Fail Dangerous Failure that does not respond to a demand from the process

(i.e. being unable to go to the defined fail-safe state).

Fail Dangerous Undetected Failure that is dangerous and that is not being

diagnosed by automatic stroke testing.

Fail Dangerous Detected Failure that is dangerous but is detected by

automatic stroke testing.

Fail Annunciation Undetected Failure that does not cause a false trip or prevent

the safety function but does cause loss of an automatic
diagnostic and is not detected by another diagnostic.

Fail Annunciation Detected Failure that does not cause a false trip or prevent

the safety function but does cause loss of an automatic
diagnostic or false diagnostic indication.

Fail No Effect Failure of a component that is part of the safety function but

that has no effect on the safety function.

Low demand mode Mode, where the frequency of demands for operation made

on a safety-related system is no greater than twice the proof
test frequency.

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

1.2 Acronyms

FMEDA Failure Modes, Effects and Diagnostic Analysis

HFT Hardware Fault Tolerance

MOC Management of Change. These are specific procedures often

done when performing any work activities in compliance with
government regulatory authorities.

PFDavg Average Probability of Failure on Demand

SFF Safe Failure Fraction, the fraction of the overall failure rate of

a device that results in either a safe fault or a diagnosed
unsafe fault.

SIF Safety Instrumented Function, a set of equipment intended

to reduce the risk due to a specific hazard (a safety loop).

SIL Safety Integrity Level, discrete level (one out of a possible

four) for specifying the safety integrity requirements of the
safety functions to be allocated to the E/E/PE safety-related
systems where Safety Integrity Level 4 has the highest level
of safety integrity and Safety Integrity Level 1 has the lowest.

SIS Safety Instrumented System – Implementation of one or more

Safety Instrumented Functions. A SIS is composed of any
combination of sensor(s), logic solver(s), and final element(s).

1.3 Product Support

Product support can be obtained from:

Ashcroft Inc., 250 East Main St., Stratford Ct. 06614

www.ashcroft.com

[phone]

1.4 Related Literature

Hardware Documents:

• Ashcroft A-Series Switch Installation, Operation and Maintenance

Instructions

Guidelines/References:

• Safety Integrity Level Selection – Systematic Methods Including Layer of

Protection Analysis, ISBN 1-55617-777-1, ISA

• Control System Safety Evaluation and Reliability, 2nd Edition, ISBN 1-

55617-638-8, ISA

• Safety Instrumented Systems Verification, Practical Probabilistic

Calculations, ISBN 1-55617-909-9, ISA

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

1.5 Reference Standards

Functional Safety

• IEC 61508: 2010 Functional safety of electrical/electronic/ programmable

electronic safety-related systems

• ANSI/ISA 84.00.01-2004 (IEC 61511 Mod.) Functional Safety – Safety

Instrumented Systems for the Process Industry Sector

2 DEVICE DESCRIPTION

The A-Series Pressure switch is an electrical switch which is actuated via inlet
pressure. The switch will change state from normally closed (NC) to normally open
(NO) as pressure increases. It will change state again from NO to NC as pressure
decreases.

The switch is available in several configurations. The unit can be purchased as a
factory set model (APS) where the switch point is set at the time of manufacture
and is not adjustable, or a field adjustable model (APA) which can be adjusted by
the end user. Each of these styles is available in a watertight housing or an
explosion-proof housing. There are a variety of female or male pressure inlets
available. There are also several electrical connectors available (18 awg wire leads,
½” conduit fitting, cable, spade terminals, Micro Din Form C connector). The
switch can be purchased as a single-pole double throw switch (SPDT) or a double
pole double throw switch (DPDT). Electrical ratings range from .1amps at 125Vdc
to 5amps at 250Vdc depending on the type of switch ordered. Pressure ranges are
available from -15psi through 7500psi with setpoints no greater than the
maximum range of the product.

3 DESIGNING A SIF USING A MANUFACTURER PRODUCT

3.1 Safety Function

The A-Series switch will change states with changes in inlet pressure. Once an
actuation pressure is achieved the switch will change state as described in Section
2 of this document. The designer of the SIF must consider if the alarm condition is
on fall pressure or increasing pressure and adjust the switch accordingly. If a
factory set switch is desired the product must be specified to change state at a
predefined pressure and in the desired direction. When using the A-Series product
with dual switches only one setpoint pressure can be used. That pressure will
actuate both switches.

The A-Series switch is intended to be part of final element subsystem as defined
per IEC 61508 and the achieved SIL level of the designed function must be verified
by the designer.

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

3.2 Environmental limits

The designer of a SIF must check that the product is rated for use within the
expected environmental limits. Refer to the temperature limits labeled on the
product, or the A-Series pressure switch datasheet available at www.Ashcroft.com.

3.3 Application limits

The materials of construction of an A-Series switch are specified in the Ashcroft A­Series pressure switch datasheet. It is especially important that the designer check
for material compatibility considering on-site chemical contaminants and air
supply conditions. If the A-Series pressure switch is used outside of the application
limits or with incompatible materials, the reliability data provided becomes invalid.

3.4 Design Verification

A detailed Failure Mode, Effects, and Diagnostics Analysis (FMEDA) report is
available from Ashcroft Inc. This report details all failure rates and failure modes
as well as the expected lifetime.

The achieved Safety Integrity Level (SIL) of an entire Safety Instrumented Function
(SIF) design must be verified by the designer via a calculation of PFDavg
considering architecture, proof test interval, proof test effectiveness, any automatic
diagnostics, average repair time and the specific failure rates of all products
included in the SIF. Each subsystem must be checked to assure compliance with
minimum hardware fault tolerance (HFT) requirements. The exida exSILentia® tool
is recommended for this purpose as it contains accurate models for the A-Series
pressure switch and its failure rates.

When using an A-Series pressure switch in a redundant configuration, a common
cause factor of at least 5% should be included in safety integrity calculations.

The failure rate data listed the FMEDA report is only valid for the useful life time of
an A-Series pressure switch. The failure rates will increase sometime after this
time period. Reliability calculations based on the data listed in the FMEDA report
for mission times beyond the lifetime may yield results that are too optimistic, i.e.
the calculated Safety Integrity Level will not be achieved.

3.5 SIL Capability

3.5.1 Systematic Integrity

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

The product has met manufacturer design process requirements of Safety Integrity
Level (SIL) 3. These are intended to achieve sufficient integrity against systematic
errors of design by the manufacturer. A Safety Instrumented Function (SIF)
designed with this product must not be used at a SIL level higher than the
statement without “prior use” justification by end user or diverse technology
redundancy in the design.

3.5.2 Random Integrity

The A-Series pressure switch is a Type A Device. Therefore based on the SFF
between 60% and 90%, when the A-Series pressure switch is used in low trip
applications, and as the only component in a final element subassembly, a design
can meet SIL 2 @ HFT=0. When used in high trip applications the SFF is <60%;
therefore the architectural constraints are SIL 1 @ HFT=0 & SIL 2 @ HFT=1.

When the element assembly consists of many components the SIL must be verified
for the entire assembly using failure rates from all components. This analysis must
account for any hardware fault tolerance and architecture constraints.

3.5.3 Safety Parameters

For detailed failure rate information refer to the Failure Modes, Effects and
Diagnostic Analysis Report for the A-Series pressure switch.

3.6 General Requirements

The system’s response time shall be less than process safety time. The A-Series
pressure switch will change state in less than 1 S under specified conditions.

All SIS components including the A-Series pressure switch must be operational
before process start-up.

User shall verify that the A-Series pressure switch is suitable for use in safety
applications by confirming the A-Series pressure switch’s nameplate is properly
marked.

Personnel performing maintenance and testing on the A-Series pressure switch
shall be competent to do so.

Results from the proof tests shall be recorded and reviewed periodically.

The useful life of the A-Series pressure switch is discussed in the Failure Modes,
Effects and Diagnostic Analysis Report for the A-Series pressure switch.

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

4 INSTALLATION AND COMMISSIONING

4.1 Installation

The A-Series pressure switch must be installed per standard practices outlined in
the Installation Manual.

The environment must be checked to verify that environmental conditions do not
exceed the ratings.

The A-Series pressure switch must be accessible for physical inspection.

4.2 Physical Location and Placement

The A-Series pressure switch shall be accessible with sufficient room for pressure
and electrical connections and shall allow manual proof testing.

The A-Series pressure switch shall be mounted in a low vibration environment. If
excessive vibration can be expected special precautions shall be taken to ensure
the integrity of pneumatic connectors or the vibration should be reduced using
appropriate damping mounts.

4.3 Pressure Connections

It is the responsibility of the designer of the SIF to ensure that the pressure tubing
and connections used when installing the switch are rated for the operating
pressure of the system, and do not restrict the pressure to the switch.

5 OPERATION AND MAINTENANCE

5.1 Proof test without automatic testing

The objective of proof testing is to detect failures within an Ashcroft Switch that are
not detected by any automatic diagnostics of the system. Of main concern are
undetected failures that prevent the safety instrumented function from performing
its intended function.

The frequency of proof testing, or the proof test interval, is to be determined in
reliability calculations for the safety instrumented functions for which an Ashcroft
Switch is applied. The proof tests must be performed more frequently than or as
frequently as specified in the calculation in order to maintain the required safety
integrity of the safety instrumented function.

The following proof test is recommended. The results of the proof test should be
recorded and any failures that are detected and that compromise functional safety
should be reported to Ashcroft.

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

Table1: Recommended Proof Test

Step

Action

1

Bypass the safety function and take appropriate action to avoid a false trip.

2

Adjust pressure to the switch and verify that switch trips under designed
conditions.

3

Inspect the A-Series pressure switch for any visible damage or contamination.
Note that the white housing vent is still in place. (A missing vent plug is
indication of a possible pressure leak in the switch)

4

Record any failures in your company’s SIF inspection database.

5

Remove the bypass and otherwise restore normal operation.

This test will detect >90% of possible DU failures in the A-Series pressure switch.

The person(s) performing the proof test of an A-Series pressure switch should be
trained in SIS operations, including bypass procedures, switch maintenance and
company Management of Change procedures. No special tools are required.

5.2 Repair and replacement

An A-Series switch is adjustable (APA version only) but is not repairable. If a failure
has occurred the switch must be replaced. The person(s) replacing an A-Series
pressure switch should be trained in SIS operations, including bypass procedures,
switch maintenance and company Management of Change procedures.

5.3 Useful Life

The useful life of the A-Series pressure switch is 10 to 15 years, or 10,000 cycles

5.4 MANUFACTURER Notification

Any failures that are detected and that compromise functional safety should be
reported to Ashcroft. Please contact Ashcroft customer service.

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

6 START-UP CHECKLIST

The following checklist may be used as a guide to employ the A-Series pressure
switch in a safety critical SIF compliant to IEC61508.

#

Activity

Result

Verified

By

Date

Design

Target Safety Integrity Level and PFDavg
determined

Correct valve mode chosen (Fail-closed, Fail­open)

Design decision documented

Pneumatic compatibility and suitability verified

SIS logic solver requirements for valve tests
defined and documented

Routing of pneumatic connections determined

SIS logic solver requirements for partial stroke
tests defined and documented

Design formally reviewed and suitability
formally assessed

Implementation

Physical location appropriate

Pneumatic connections appropriate and
according to applicable codes

SIS logic solver valve actuation test
implemented

Maintenance instructions for proof test released

Verification and test plan released

Implementation formally reviewed and
suitability formally assessed

A-SERIES SWITCH SAFETY MANUAL

I&M009-10210
REV A - 4/19/2013

#

Activity

Result

Verified

By

Date

Verification and Testing

Electrical connections verified and tested

Pneumatic connection verified and tested

SIS logic solver valve actuation test verified

Safety loop function verified

Safety loop timing measured

Bypass function tested

Verification and test results formally reviewed
and suitability formally assessed

Maintenance

Tubing blockage / partial blockage tested

Safety loop function tested