Inductive analog sensor with three programmable switching outputs
July 2000
Analytical Instrumentation & Environmental Monitoring
Retron
The sealing of the spindle in the Arca Series 08C is achieved through different methods, depending on the required operating conditions. This is to ensure the lowest possible fugitive emission and minimise the friction. The stem and the packing box hole have a super-finished surface produced by roller burnishing. The surface is compressed and becomes harder and more corrosion resistant.
The standard maintenance-free PTFE-ring
packing with a redundant micro-sealing
element (Figure 1)
To guarantee the efficiency at lower pressures, a corrosion resistant spring applies a pre-calculated load. As internal pressure increases, the sealing lips are pressed against the spindle and the stuffing box hole. A set of V-rings, made of different PTFE compounds, guarantees sealing under changing operating conditions. The base ring of the V-ring-set performs as a wiper ring. The redundant micro-sealing element is a profile ring, made of special elastomer material. Its distance from the upper, outer wiper ring is equal to the valve stroke. This avoids any ingression which may occur around the outer spindle. This may get past the wiper ring but will not get as far as the micro-sealing element when the valve is closed.
Graphite stuffing box (Figure 2)
A safe and reliable packing assembly requires even compression for all upper and lower graphite rings transmitted by the stuffing box nut to the stuffing box packing. This may be compared with hydraulic pressure being identical throughout a liquid in a chamber. During assembly, the lower rings have to be compressed more than the upper ones, to achieve an even compression throughout the stuffing box packing. To achieve this, the patented Arca-Optipress pre-loading device, activated by the actuator, is recommended.
If this is not executed as recommended, careless assembling of the stuffing box rings by compressing the total packing set by fastening the stuffing box nut alone would not result in a satisfactory seal. In such a case, friction results in the upper rings being compressed more than the lower ones, which results in an uneven compression of the rings, with a likely early seal leakage. Bear in mind that the torque applied to the stuffing box nut has to be such that the sealing is effective, while at the same time the hysteresis is not made too high by the friction of the seal.
Bellows seal bonnet (Figure 3)
Hermetically sealing bellows are available for process fluids where very little fugitive emission is acceptable, as is the case with highly toxic fluids. The pressure-resistant, elastic, double-layer bellows is seal-welded together with the valve spindle and the upper adapter ring. For safety reasons the system has a redundant maintenance-free safety spindle sealing. There is also an optional socked screw connection available for between the bellows and safety spindle sealing. This could be either for leak detection or filling with an inert gas.
Wear and corrosion-resistant bushing
(Figure 4)
To prevent any galvanic corrosion (normally associated with stuffing box packing such as graphite and plain carbon steel material of the bonnet), the design offers a special treated stainless steel bushing for all bonnets of valves larger than DN 65 (2,5"). Valves equal and smaller than DN 65 all have bonnets of stainless steel. This bushing, with a hole straight through it, allows a super-finished hole and perfect sealing between the stuffing box packing as well as sealing rings and the bushing. This stainless steel bushing avoids or minimises any corrosion and degradation of the sealing surfaces.
Valve & Automation
(031) 579 2593
[email protected]
www.valve.co.za
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