Valves and valve actuation – Valve – Removable seat engaging element
Reexamination Certificate
1999-02-22
2004-09-21
Scherbel, David A. (Department: 3754)
Valves and valve actuation
Valve
Removable seat engaging element
C251S368000
Reexamination Certificate
active
06793198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to industrial valves. More specifically, this invention relates to valve plug heads for industrial valves. Still more specifically, this invention relates to valve plugs which employ dissimilar materials and devices for attaching valve plug heads to valve plug stems.
2. Description of Related Art
Valves, valve plugs and associated components are well known in the art. Valve plug heads are generally positioned within the valve in the middle of the flow stream to control the volume of flow that is allowed to pass through the valve. By varying the position of the plug head relative to the valve seat control of the flow volume is achieved. In sum, the valve plug head is used within a valve to divert and restrict flow. Plug heads are subjected to fluid forces, chemical attack, thermal stresses, impact from particulates and debris, as well as the forces used to attach it to the plug stem. Additionally, the plug head is subjected to seat loading forces should it contact the valve seat.
The typical valve head is attached to a plug stem, which in turn is connected to an actuating device. This actuating device, which is typically positioned outside the internal portion of the valve, is controlled to move the plug stem, thereby changing the position of the plug head and controlling the volume of flow passing through the valve. Thus the attachment between the plug stem and the plug head is inside the valve, exposed to the flow stream, while the actuator and the attachment between the actuator and the plug stem are outside the valve and are not exposed to the flow stream. Typically, between the internal portion of the valve stem, which is exposed to the flow stream, and the external portion of the valve stem, which is connected to the actuator, is a smooth cylindrical section that is used as a sealing surface. Valve packing is placed around and against this cylindrical section, permitting in and out movement of the plug stem, without flow stream leakage out of the valve. The plug stem is therefore subjected to axial forces as the actuator moves it, mounting forces relating to the actuator attachment, and the long cylindrical section is subjected to bending forces. The plug head and the plug stem perform distinctly different purposes and are subjected to very different forces. The plug head, sitting in the middle of the flow stream, diverts and/or restricts flow, and is subjected to fluid and seat loading forces and to forces related to attaching the plug head to the plug stem. While the plug stem, adapted to be moved by an actuating device provides a sealing surface and is subject to axial and bending forces. In industrial, high volume, high flow rate valves these forces on plug stems and plug heads are typically significant contributors to valve failure.
Traditionally, valve plug heads are either composed of one monolithic material or make use of more than one material. Plug heads employing more than one type of material have particular advantages, in particular better erosion and corrosion resistance, improved shock absorption, working life, and thermal expansion qualities. However, typically the use of a plurality of material types has been limited by the ability to effectively join the materials together economically and without creating stress points that limit the life of the plug head. The most common current methods of fixing dissimilar materials together in a valve plug are taper fitting or interference fitting, both of which employ a retaining ring that is fixed around the plug head.
Taper fittings have been shown to subject the plug head to undesirable stresses, contribute to thermal expansion problems and are not practical to repair. The typical taper fitting design requires a mating of two conical surfaces, one on the plug head, and the other on the retaining ring. Since neither the plug head nor the retaining ring can be manufactured to completely ideal cone shapes, the plug and seat may not mate perfectly, therefore loading between the two when mated may not be uniform. Moreover, the force of the retaining ring on the plug head, that holds the plug head in place, is located close to the edge of the plug head and is generally perpendicular to the angle of the conical surface. The location and angle of this force are undesirable because they introduce tensile forces into the portion of the plug head that bears the force. Often the desired plug head material may demonstrate weak tensile strength, therefore, introducing additional tensile forces may either limit the selection of plug head materials or, if desired plug head materials are used, may tend to break off the edge of the plug head, separating the plug head from the plug stem and causing valve failure. Also, as the retaining ring wears away, through corrosion and erosion, the shape of the contact area can change, typically moving closer to the edge of the plug head. This contact area change tends to concentrate forces on the edge of the plug head and increases the likelihood that the edge of the plug head will fracture, thereby also causing the plug head to separate from the plug stem. The stresses induced with the taper fit are difficult to quantify and, therefore, can detract from a valve plug's performance. The stresses are difficult to predict because the plug head is held in place by the retaining ring and the retaining ring is attached to the plug stem via welding. Variables in the welding process such as weld shrinkage, inter-pass temperature, amperage of weld, inert gas environment, number of passes between welding pauses, the amount of initial burn in, as well as other related welding factors can change the amount of stress in the plug head.
As noted above, typical prior taper fit designs attach the taper fit ring to the plug stem via welding. This approach results in the retaining ring and the plug stem essentially becoming permanently joined into one component. If the plug head wears away or breaks and the plug stem is still usable, the typical taper fit design does not lend itself to achieving the proper concentricity between the plug head and the plug stem after the plug head has been replaced. When taper fit valve plugs are originally manufactured, the plug stem is the last portion of the plug to be machined. This is done so that it can be machined concentric with the plug head. When a taper fit valve plug is repaired, the plug stem has already been machined, so it is not possible to make adjustments in the plug stem to ensure concentricity with the plug head. Also, if the plug head is misaligned, when the taper fit ring is welded in place, adjustments cannot be made for concentricity without cutting the taper fit ring off again. Since the welding of the taper fit ring involves difficult to predict shrinkage and distortion of the taper fit ring, it is not generally possible to assure that the plug head will be properly concentric with the plug stem after the taper fit ring is welded in place. For these reasons, it is the current industry practice to discard taper fit valve plugs when the plug head has broken or worn away, rather than to attempt to repair them.
Additionally, with many plug head material classes, the coefficient of thermal expansion is less than that of many taper fit ring material classes. Generally, in assembling the part, the taper fit ring is fit tightly around the ceramic and the taper fit ring is welded to the plug stem. At elevated operating temperatures, the taper fit ring increases in size more than the plug head, and the plug head becomes somewhat loose in the taper fit ring, thereby leading to early failure of the fit in operating conditions.
Interference fittings typically require a bulkier retaining ring, contributing to the load on the plug head; a complex difficult procedure to replace plug heads; and are generally limited in their service temperature ranges. An interference fit achieves more uniform loading of the plug head than does the taper fit. However, the typical interference fit uses a on
Chipman Stephen R.
Robison Jeffrey C.
Smith Craig C.
Caldera Engineering LLC
Keasel Eric
Sadler Lloyd W.
Scherbel David A.
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