Last week at the Emerson Exchange conference, I caught up with Emerson’s Riyaz Ali. You may recall Riyaz from many safety instrumented system-related posts. We discussed some of the trends in integrated positioners + solenoid valves + limit switches + valve position transmitters.
Riyaz Ali Senior Business Development Manager
For these devices with an integral solenoid-operated valve (SOV), the pneumatic path is only a single path and the requirement for a redundant path will not be met.
This will affect the PFDavg calculations as per ISA TR84.00.02-2002 part 2 using the simplified equation for a one-out-of-one (1oo1) arrangement:
λdu x T /2 (Note: λdu is dangerous undetected failure rate of equipment under control (EUC) and T is test interval.)
For solutions with external SOVs in series with smart positioners, this 1oo2 approach has a PFDavg:
λdu x T2 /3
A 1oo2 arrangement provides an improved PFDavg over a 1oo1 single box arrangement.
Riyaz notes that going to external SOV will improve safety reliability, which means either the SOV or smart positioner are capable of taking a valve to safe state. With integral SOVs with smart positioners, only one pneumatic path is available, which means there is no redundancy. Project teams may have to re-visit the HAZOP analysis to evaluate new safety integrity level (SIL) conditions.
Referring to the global safety standard, IEC 61508, Riyaz makes the following points:
Type A and Type B definition is listed from IEC61508 part 2.
IEC 61508 part 2 – clause 7.4.3.1.2 define Type A. A subsystem (see 7.4.2.11, note 1) can be regarded as type A if, for the components required to achieve the safety function:
IEC61508 part 2 – clause 7.4.3.1.3 defines Type B as a subsystem (see 7.4.2.11, note 1) shall be regarded as type B if, for the components required to achieve the safety function:
This means that if at least one of the components of a subsystem itself satisfies the conditions for a type B subsystem then that subsystem must be regarded as type B rather than type A. See also 7.4.2.11, note 1.
Hence two devices pneumatically in series will have MTTFs for a single device (SOV Type A device only). A smart positioner and SOV mounted externally pneumatically in series will be ideal from safety reliability and plant availability.
You can connect and interact with other safety experts in the Safety Instrumented Systems track of the Emerson Exchange 365 community.
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