Valves are fundamental devices in the industries to transport or regulate fluids (liquid, gas, steam), so they must have the ideal standards and materials to ensure the safety of personnel and proper functioning.
A recurring situation in equipment are leaks that can occur inside or outside. For exterior leaks it can occur in:
“Cap-body union in any of the designs in which the body has one or more union flanges. Such is the case of single seat globe valves, the various types of ball valves and some rotary valves. The stems, both in reciprocating-linear movement valves, and in the axes of rotary valves: butterflies, balls and eccentric rotary. For this sealing, packing or stuffing box and closing bellows are used”.
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The function of the control valve is “to regulate at an intermediate point in its travel, so it could be questionable to require a closure like the one that an isolation valve must guarantee, especially in a critical circuit. In reality, a reasonable leak could be admitted, since, under normal conditions, we are not even going to work close to the minimum adjustable flow that we define with its capacity, where its stability may jeopardize the controllability of the variable”. For this reason, on many occasions the user demands a certain requirement from the control valve due to its economic value and its isolation or “fail-closing” functions, among others.
The action of avoiding leaks in this equipment will be assigned various forms: “hermetic valve”, “zero leak”, “bubble proof”, “total closure”; however, as A. Campos López specifies, “there is no “absolutely zero” closure in its literality”, “they were ambiguous forms that actually sought the best but without defining how. When talking about sealing tightness, we must take into account not only the leakage between the shutter-seat, but also the possible leakage between the seat-body, and in the sealing rings in cage-guided shutters, in short, the total leakage”.
Faced with this situation, the manufacturers carried out tests, improved the tightness, and established standards and recommendations to improve operation. In the case of control globe valves, due to such complex applications, it is not possible to ensure the concept of “zero leakage” or “zero leaks” by valve design.
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