3 x API 560 Nozzle Load Limits
API Standard 560 Fired Heaters for General Refinery Service
is referenced as the industry practice document to limit terminal loads
at tubes and manifolds. However, these loads are considered by some analysts to be
impractical for actual installations and, thus, a '3 x API 560' load limit
practice has developed in the industry.
The rote use of this practice should not be followed since it may
pose a safety and reliability risk for the facility if used without full comprehension. The
API 560 document is published as a standard; hence, this indicates that there is
broad industry consensus in its application. Arbitrary non-compliance should not
be undertaken without fully examining the requirements of the standard and
the potential impact of non-conformance.
API Standard 560 Terminal Loads
The terminal loads are provided in Tables 6 and 8 of the Standard where
tube and manifold terminals, both welded and flanged, are recognized.
The basis for these loads is not revealed; however, the Standard does present the
criteria for limiting the stress of tube supports.
These are similar in construct to the stress limits in the design of piping
and pressure vessels given in the ASME Codes such as ASME VIII Division 2 and B31.3. It would
be reasonable to consider these documents in formulating a rational assessment of terminal
loads using a stress based approach.
Table 1, above, listing the stress limits for fired heater structural parts use
higher stress allowables in the creep range compared to ASME Code pressure components which are
listed in Table 2.
The piping analyst needs to be aware of this and consider the industry practice
when evaluating nozzle overloads and considering the arbitrary use of the '3 x API 560'
The stress calculation for welded fired heater terminals can be readily
completed and evaluated against the temperature independent and creep dependent B31.3 Code limits.
A number of methods in the industry are available to evaluate flange loadings
such as the pressure equivalent, 'Peq' method.
A combination of stress and load rating approaches should be used and a follow on
risk assessment needs to be completed for thoroughness.