Flowserve MX Electronic Actuator SIL Safety IOM - Manual (Page 17)

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Limitorque MX Electronic Actuator FCD LMENIM2350-01 – 9/13
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In other words, using an MXa in an SIL-2 SIF will absorb 52.4% of the available bandwidth for SIL-2. In order to
maintain a SIL-2 capability for the entire SIF, the total of the PFDavg values for all of the components that comprise
the SIF must sum to no more than 0.01. Therefore the total of the PFDavg values for all of the other components that
comprise the SIF control loop must be less than 0.01-0.00524 = 0.00476, or 47.6% of the SIL-2 bandwidth.
Note that available bandwidth for other components can be easily increased for the identical actuator simply by
performing a monthly Partial Stroke Test [PST].
Continuing with the previous example, by performing a monthly Partial Stroke Test [PST] in addition to a yearly Full
Proof Test, the PFDavg for the MXa is reduced to only 0.00347, which is only 34.7% of the SIL-2 bandwidth, The
available bandwidth for all of the other components is now 0.01 - 0.00347 = 0.00704. That is, 70.4% of the SIL-2
bandwidth is available for all of the other components that comprise the SIL control loop.
The MXa is able to perform PST because SIF and PST functions do not bypass the normal controls and position
feedback mechanisms.
6.2 Basic plus any optional board add-on
Optional functions may be added to the basic functionality of the basic actuator by the addition of various printed circuit
boards. (See Table 1 – Available Functional Add-ons on page 5)
None of these add-ons will contribute to operation of the Safety Function [SIF], but may be used for additional control
and/or monitoring of the actuator for non SIF operation.
Inclusion of these add-ons will minimally affect the Probability of Failures on Demand [PFDavg) values shown in Table
3 and Table 4, above. To determine the actual PFDavg for the optional add-ons to be deployed, the user should refer to
the failure rate data as shown in FMEDA assessment reports and noted in section 8 Safety Instrumented System [SIS]
Assessment on page 23.