hydrogen concentration in the dissolver offgas and provide a signal to
shutdown the dissolving operation before explosive concentrations of
hydrogen are reached." The specific accidents associated with the safety
function should be identified.
This subsection provides a description of the safety-class SSC and the basic
principles by which it performs its safety function (e.g., sensor and interlock
for hydrogen detector discussed in section 4.3.X.1). Describe its boundaries
and interface points with other SSCs relevant to the safety function.
Identify SSCs whose failure would result in a safety-class SSC losing the
ability to perform its required safety function. These SSCs would also be
considered safety-class SSCs for the specific accident conditions for which the
safety-class designation was made originally.
When describing the SSC, provide a basic summation of the physical
information known abo ut the SSC, including Process and Instrumentation
Drawings (P&IDs), or a simplified system drawing with reference to P&IDs.
If known, abstract and reference pertinent aspects of manufacturer's
specifications. Pertinent aspects are considered to be those that directly relate
to the safety function (e.g., diesel generator load capacity, time to load if
critical) as opposed to general industrial equipment specifications that fall out
from these capabilities (e.g., starting torque, motor insulation, number and type
of windings). Such lower tier details should be implicitly included only by
reference to the overall specifications.
This subsection identifies requirements that are specifically needed to fulfill
safety functions. Such functional requirements are specified for both the safety
class SSC and any needed support safety-class SSCs.
Limit functional requirement designation to those requirements necessary for
the safety function. Functional requirements are provided for safety-class
SSCs for the specific accident(s) where the safety-class SSC must function
(e.g., if that accident is not initiated by an earthquake, the functional
requirement does not involve seismic parameters).
Functional requirements specifically address the pertinent response parameters
or nonambient environmental stresses related to an accident for which the
safety function is being relied upon. In the hydrogen detector example, one
obvious parameter would be maintaining hydrogen concentration below the
explosive limit. If the offgas temperature was significantly above ambient
temperatures, operation at that temperature would be a functional requirement