Page 166 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

138 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

moves, the bellow maintains a seal and significantly reduces leakage probability com-
pared with conventional valve designs with gland packing.
However, the bellows used in these valves have to be relatively thin, such as a few
tenths of a millimeter, to allow flexibility. Due to the volume change during LBE
solidification, freezing liquid metal in the confined space of a valve body/bonnet
should be avoided to prevent damage to the delicate bellow. If solidification is
unavoidable for some reason, particular precautions should be taken not to operate
the valve so that the bellow is not overstressed. Automated valves should be inter-
locked not to actuate below a set point close to the melting point. This limit could
be 150°C for LBE and 380°C for lead.

3.4.9.2 Pressure surge
Pressure surge, or fluid hammer, is a pressure transient caused by a sudden change in
fluid velocity, that is, fluid momentum change. The phenomenon of pressure surge is
not unique to liquid metals but applicable to many fluid network systems. Yet, as will
be shown, the high density of LBE (and all other HLM) makes the phenomenon of
fluid hammer an important consideration in the design and operation of HLM facil-
ities. Fluid hammer can occur when a valve closes too quickly or a pump starts with an
empty discharge line or stops suddenly during a pump trip. The result is the establish-
ment of pressure waves that are potentially significantly higher than the normal oper-
ating pressure, which can cause severe vibration, piping strain, and even pipe or
component rupture.
The maximum possible pressure rise (potential surge) associated with a fluid ham-
mer event is simply determined from the well-known Joukowski equation, which
assumes an instantaneous valve closure in frictionless pipes (Thorley, 2004). The
Joukowski equation is given by Eq. (3.4.2):

ΔP ¼ ρ c ΔV (3.4.2)

where
ΔP¼surge pressure rise (Pa)
3
ρ¼fluid density (kg/m )
c¼pressure wave speed (m/s)
ΔV¼change in fluid velocity (m/s)
The assumption of instantaneous valve closure is conservative with respect to the
surge pressure rise. For more accurate estimations, more analysis is required. The
pressure wave velocity and hence the magnitude of the surge pressure wave also
depend upon the piping restraint characteristics, the modulus of the pipe material,
and the modulus of elasticity of the liquid. While the pressure wave speed (c) is depen-
dent on the pipeline elasticity characteristics, the pressure rise is directly proportional
to the fluid density. Considering the high density of LBE, an instantaneous valve clo-
sure in an LBE facility would produce a potential surge pressure rise approximately
10 times greater than a water facility with the same initial flow velocity and piping