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4
NOZ-CHEK
®Cryogenic
Installation, Operation & Maintenance
(e) Welded End Valves
(Bevelled per ASME B16.25 figure 2 (a) or 3 (a) depending
on wall thickness).
Valves with end connections prepared for welding into a
pipeline should be lowered into position between mating
pipe ends after removal of any debris or foreign matter.
Ensure proper alignment of valve and pipe.
Temperature rise due to heat conduction along inlet end
of valve in proximity of resilient seals should be monitored
during welding process to ensure maximum temperature
of seals are not exceeded. Refer to General Arrangement
drawings for details.
All welding should be performed in accordance with
qualified procedures.
(f) Position Switches
Where a valve is fitted with a position switch, the switch
must be wired in accordance with the switch datasheet.
Ensure that the switch is not exposed to excessive voltage.
Care should be taken not to hit the switch housing as this
may damage the threads and cause leakage.
4.0 OPERATION
4.1 OPENING
Reduced pressure, generated by increased velocity in
the minimal flow area results in additional force to assist
the Disc to open and allows for extra spring loading that
facilitates a faster closing time.
This spring force is balanced in the fully open position of
the disc for a given velocity of flow. This velocity depends
upon the flow imposed by the service conditions and the
Specific Gravity of the fluid.
The Noz-Chek II ® geometry is established by considering
the design velocity required to ensure that the disc is
stabilised open against its stop even if moderate flow
oscillation occurs.
4.2 CLOSING
When the flow through the valve reduces below the
critical velocity (velocity when the valve is just 100%
open) of the valve, the disc reacts immediately, limiting
backflow and valve slamming.
The spring load, disc inertia and short displacement
ensures a rapid self-dampening response.
4.3 POSITION SWITCHES
With reference to valves fitted with a position switch, the
purpose of the switch is to determine if the disc is in the
opened or closed position.
The switch is activated by the presence (close proximity)
of a magnetic field, the magnetic field is emitted from a
magnet located in the sealing disc, and is sensed by the
switch located through the valve wall.