Quick guide to api 570 4 131 terminology api and asme

Quick Guide to API 570
4

1.3.1 Terminology
API and ASME documents use terminology that occasion-
ally differs from that used in European and other codes.
Non-destructive examination (NDE), for example, is nor-
mally referred to as non-destructive testing (NDT) in Europe
and the API work on the concept that an operative who
performs NDE is known as the examiner rather than the term
technician used in other countries. Most of the differences are
not particularly significant in a technical sense – they just
take a little getting used to.
In some cases, meanings can differ between ASME and
API codes (pressure and leak testing are two examples). API
codes benefit from their principle of having a separate section
(see API 570 section 3) containing definitions. These
definitions are selective rather than complete (try and find
an explanation of the difference between the terms approve
and authorize , for example).
Questions from the ICP examination papers are based
solely on the terminology and definitions understood by the
referenced codes. That is the end of the matter.
1.3.2 Calculations
Historically, both API and ASME codes were based on the
United States Customary System (USCS) family of units.
There are practical differences between this and the
European SI system of units.
SI is a consistent system of units, in which equations are
expressed using a combination of base units. For example:
Stress S ð Þ ¼
pressure p ð Þ ± diameter d ð Þ
2 ± thickness t ð Þ
In SI units all the parameters would be stated in their base
units, i.e.
Stress: N/m
2
(Pa)
Pressure: N/m
2
(Pa)
Diameter: m
Thickness: m
Interpreting ASME and API Codes
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Compare this with the USCS system in which parameters
may be expressed in several different ‘base’ units, combined
with a multiplying factor. For example, the equation for
determining the minimum allowable corroded shell thickness
of storage tanks is
t
min
¼
2 : 6 ð H ² 1 Þ DG
SE
where t
min
is in inches, fill height ( H ) is in feet, tank diameter
( D ) is in feet, G is specific gravity, S is allowable stress in psi
and E is joint efficiency.
Note how, instead of stating dimensions in a single base
unit (e.g. inches) the dimensions are stated in the most
convenient dimension for measurement, i.e. shell thickness in
inches and tank diameter and fill height in feet. Remember
that:
. This gives the same answer; the difference is simply in the
method of expression.
. In many cases this can be easier to use than the more
rigorous SI system – it avoids awkward exponential (10
6
,
10
-6
, etc.) factors that have to be written in and
subsequently cancelled out.
. The written terms tend to be smaller and more convenient.
1.3.3 Trends in code units
Until fairly recently, ASME and API codes were written
exclusively in USCS units. The trend is increasing to express
all units in dual terms USCS(SI), i.e. the USCS term followed
by the SI term in brackets. Note the results of this trend:
. Not all codes have been converted at once; there is an
inevitable process of progressive change.