The Total Impact
The Total Impact
In a comparison study by exida the two designs
show a significant difference in PFDavg, a measure of safety
integrity and MTTFS, mean time to failure spurious (false trip).
The ad hoc solution uses conventional design techniques including
three transmitters, a SIL3 logic solver and remote actuated valve.
The Yokogawa integrated solution utilizes a single EJX certified
pressure transmitter, a ProSafe-RS SIL3 certified logic solver and
a single remote actuated valve with the SVI II digital positioner
configured for automatic partial valve stroke testing every week
(168 hours).
| Yokogawa integrated solution | Ad-hoc solution | |
| Description | Single EJX pressure transmitter, ProSafe-RS SIL3 logic solver, single remote actuated ball valve with SVI II. | 2oo3 Pressure transmitters, SIL3 logic solver, single remote actuated ball valve with 3 way solenoid. |
| MTTFS | 39 years | 20 years |
| PFDavg / RRF | 8.98E-3 / 111 | 2.68E-2 / 37 |
| Proof test interval (valve) |
5 years | 6 months |
| Capital cost | Lower | Higher |
| Lifecycle cost | Low | Very High |
| MTTFS | : | mean-time-to-failure spurious |
| PFD | : | probability of failure on demand |
| RRF | : | risk reduction factor |
As compared to the ad-hoc solution, the
Yokogawa integrated solution doubles safety loop availability,
triples safety and extends valve proof test interval by ten times.
The results show significant differences. When one considers
the lower capital cost of the integrated solution, the differences
look even better. When life cycle cost is considered (more frequent
proof testing), the integrated solution looks even more superior.
