Optimisation of hydraulic tests, technical certification and planned–preventative maintenance - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 255

reliability of the pressure vessel or pipeline achieved in HT and its changes
over time. Therefore, despite carrying ot hydraulic (pneumatic) tests pressure
vessels or pipelines can fail prior to the next HT and its operation can be
aborted because of leaking or damage that usually leads to great economic
losses and, possibly, loss of human life.
Technical results achieved using the technology described below are
based on the correct choice of the frequency of HT conditions, pressure
and test temperature which in turn will lead to guaranteed reliability and
safe operation of pressure vessels and piping.
These technical results are achieved by subjecting the pressure vessels
(pipelines) to periodic loading with the pressure of liquid or gas which
exceeds the working pressure, followed by holding the vessels (pipelines)
under pressure. The pressure then drops and the vessels (pipelines) are
inspected and they are considered to have passed the hydraulic (pneumatic)
test if testing and examination reveal no leaks and ruptures of tmetal in
holding and the pressure drop does not exceed the established limits in the
regulations and no new residual strains are detected after testing. The test
conditions are selected to ensure that the size of a family of critical size
cracks, corresponding to the appropriate test regime, is smaller than that
of the family of the critical size cracks, corresponding to the operating
mode ( )

cr

( )

c r

( )

cr

ac ac a < is the size of the family of the critical
size cracks corresponding to the test conditions, ( ) c w

, , , where ,

HT w HT

ac is the size of the
family of critical size cracks corresponding to the working conditons.
The frequency of such tests is selected so that the time between hydraulic
(pneumatic) tests does not exceed the duration of increase (growth) of the
size of the family of critical size cracks, corresponding to the conditions
of the hydraulic (pneumatic) tests, to the size of the family of the critical
size cracks corresponding to the working conditions (temperature, pressure
and other influences): ( )



cr

( )

s



→   .
The method for selecting the optimum pressure and periodicity of HT
is shown in Figs. 8.1-8.15.

T ac , ac ,

HT w

￿ ￿ ￿ ￿ ￿

8.1 Diagrams of loading of a pressure vessel: 1) vessel; 2). pump.