Page 167 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Operational aspects of experimental liquid metal facilities 139

design. To put this into perspective, consider two identical 1in. schedule 40 steel pipes
with a valve at the outlet of each. One pipe contains LBE flowing with an initial veloc-
ity of 1m/s, and the other pipe contains water also flowing with an initial velocity of
1m/s. In the event of an instantaneous valve closure, the Joukowski expression esti-
mates a potential pressure surge rise of 156 and 14.6bar could be expected in the LBE
and water facilities, respectively.
Numerous techniques and technology such as pressure relief devices are available
and used to prevent or reduce the effects of pressure surges. However, the simple oper-
ational advice to be considered here is to ensure that valves close slowly. For example,
on the COMPLOT facility at SCK CEN, air flow restrictors were installed on the
l
pneumatic actuator air outlet to restrict air outflow. Check the expected valve closure
times with the valve supplier, and verify or adjust the closing time during commission-
ing of the facility.

3.4.9.3 Instrumentation

The properties of LBE and high operating temperatures usually dictate the need to use
remote seal diaphragm pressure transmitters. These transmitters utilize a thin-walled
flexible diaphragm, mounted in a chamber, to separate the process media from the
instrument. The space between the diaphragm and the instrument is filled with a pro-
cess media (normally a high-temperature resisting oil). When fluctuations in the pro-
cess media pressure occur, the change is transmitted across the flexible diaphragm
through the system fill fluid, which is hydraulically transmitted to the measuring
instrument.
Considering the above instrumentation setup, experience feedback suggests that
several aspects should be observed during operation when using these remote seal dia-
phragms, as follows:
l Prevent bubble entrapment inside pressure measurement system. The diaphragm chamber
must be completely filled with liquid metal during filling. Dedicated filling and venting lines
are recommended to allow filling/draining from the bottom and venting to the top. Refer to
Fig. 3.4.5 (left) that shows a diaphragm chamber with the filling/draining tube below and the
venting tube above. As shown in Fig. 3.4.5 (right), a network of tubing and valves can be
made to multiple pressure tapping points, to reduce the number of pressure transmitters
required. While this has some cost benefits, such a layout of small-diameter tubing is a
potential zone for gas-pocket accumulation. To ensure complete evacuation of potential
gas, liquid-metal flow could be established through the diaphragm chambers by keeping
the flushing valves open while the main LBE pump is running.
Employ separately controlled heat tracing for different components of the pressure measure-
l
ment system. The recommended filling/venting lines above and the connections from the
main process piping (or test section) to the pressure measurement diaphragm are mostly
made using small-diameter tubes (8–12mm). Layout of heat tracing and the grouping of
heating control on these small tubes are very important to avoid potential solidification
in these tubes, which could result in erroneous pressure measurements. For example, group-
ing a heater element of a larger component such as a valve or diaphragm flange with a heater
element of a small tube will likely result in different thermal responses and eventual