Pressure equipment hydrotesting reasons.

1. Pressure Equipment hydro-testing reasons
Across all pressure equipment codes (ASME B31.1/3, ASME SEC I, VIII DIV 1
etc.), there has always been a requirement of an important non-destructive
testing that is commonly referred in codes terms as “leak testing” which can be
done in number of ways either using air or liquid, and where the terminology
becomes more specific such a if water is used as a medium for pressurizing than
it is referred specifically as “Hydro-testing” in codes terminology.
In regards to the intent/purpose of the “hydro-test”, there has been always
varying range of opinions among the Subject matter experts of pressure
equipment in form of general discussion, codes interpretations, and available
books written by professional engineers experienced in field of pressure
equipment. The reasons for doing “hydro-testing” on pressure equipment are
cited are:
1) To check the leak tightness of the pressure equipment using pressure
greater than design pressure where general primary stresses in
components are below yield point. Here code terminology of “leak
testing” makes it clear that it is for leak testing.
If we agree with the above it means leak testing is to ensure that is no
leakage at one or all point, which includes
a) Welds (Welds most likely have defects such as cracks, lack
of fusion, porosity etc. and will become evident in hydro-
testing)
b) Main pressure component. (i.e. blind flanges which have no
welds etc. likely to have material defects and will become
evident during hydro-testing by leakage)
c) Flanged joints (where leakage can be attributed to poor
bold tightening, wrong gasket selection, etc. It may help
evaluate/test the choice of gasket selection, flanges surface
finish or bolt torqueing before the equipment is put to
service where leakage of a processing fluid will have severe
consequences)
Depending on if we agree with a), b) or c) or all above, it will impact how
to define hydro-test for replacement components of the pressure
equipment. Lets say as an example if some one has to replace a standard
or non-standard blind flange, and if we agree with a) than one might not
need a hydro-test but if we agree with a) and b), than the blind flange
without a weld will need to be hydro-tested.
Note: There are provisions in codes where due to non-practicality of a
situation/other hazards etc. hydro testing is deferred but instead
substituted with another non-destructive testing which could be a
combination of surface (MPT/PT) or volumetric examinations (UT/RT).

2. 2) The hydro-testing is a form of strength test of the pressure equipment
where the component is subjected to pressure greater than design
pressure by a factor of 1.3 to 1.5 as minimum.
Some Codes equations/formulas are relatively simplified with a quite a
factor of design margin in it. In most cases the general primary membrane
and primary bending stresses are calculated however other
local/secondary stresses at structure discontinuities are not calculated
and some how ignored in calculations due to the fact that primary
stresses are calculated with a margin great enough to provide adequate
conservatism so as to ignore the other types of stresses in calculations.
Further more hydro-test with a pressure greater than design pressure
will serve as a check/validation of the above approach. Lets say if there is
a unusual local or global geometry in the pressure equipment that has
been calculated using simplified approach. However during hydro-test it
failed either by gross distortion or local rupture. Here we can see the
benefit of hydro-test where the problem has been identified before it
becomes an HSE hazard on site/plant etc.
Note: Codes do acknowledge proof testing where components prototypes
are subjected to 3-5 times design pressure and are allowed to fail and the
design pressure is established using the proof testing pressure as a basis. It
is to be noted that proof-testing components thickness are not calculated
since these are complex components where no established code formulas or
equations are available, however for components subjected to hydro-test
the thicknesses are calculated using proper established code formulas.
3) From a fracture mechanics point of view hydro-test benefits in blunting
the crack tips and hence once equipment is put to service the crack will
not grow as it has become arrested due to blunt crack tip where stress
intensity factor is quite low compared to what was there before hydro-
test. Furthermore we know that there are imperfections in welding in
form of cracks, lack of fusion etc., in plates such as laminations, and in
forgings in form of internal cracks. Considering the scope and the
accuracy of the volumetric/surface NDE techniques, there is always a
potential to miss noticing one of these imperfections in materials/welds.
Had there been a crack unknown and the component is tested above
design pressure without any leakage/rupture, this kind of validates that
the unknown crack will not be a problem when component is subjected to
design pressure.
Now lets say in case of blind flange made of either plate or forgings, if we
agree with 3) than the blind flange will need to be hydro-tested.
4) Hydro-test to check gross error in calculations. Considering during design
calculations that is done/checked under a proper QMS, there is a
possibility of an error, for-example instead of 16 mm required thickness;
the calcs/drawings due to typo is showing 6 mm. The component is

3. fabricated and when hydro-tested it become evident that component
failed by gross distortion or rupture, and thus avoiding an HSE Hazard on
plant etc.
If we agree with this than we will need to be hydro-testing a blind flange.
My own personnel opinion based on experience and study of available literature
is that all pressure equipment design codes are used to construct equipment,
which can be operated safely within the design conditions stipulated for the
equipment. The word “safely” can be better explained from a risk point of view,
which means the risk is minimum to be accepted and so it is safe. Different codes
and similarly different jurisdictions have different level of risk tolerances. Leak
testing or hydro-testing is a form of risk mitigation (i.e. to reduce the risk of
failure of equipment in service). I am more of the opinion that lets say in case of
ASME SEC VIII DIV 1/ASME B31.3/ASME B31.1, hydro-testing is done to satisfy
as minimum only above points 1)-a), 1)-b), and 2) in lined with its risk tolerance.
However if we want to satisfy points 3) and 4) also that will give us a relatively
low risk compared to the minimum required by code.
Based on this as an example I consider blind Flange will need to be hydro-tested
as per ASME SEC VIII DIV 1/ASME B31.3/ASME B31.1.
Your Opinions?