1. The Field of the Invention
The invention relates to the structure of and methods of presetting bellows-type gas lift valves.
2. The Prior Art
A wide variety of bellows-type gas lift valves is known in the art, many of which include a fixed mechanical stop to limit the valve stem travel. Such a stop is desirable to prevent damage to the bellows in the event that an excessively high operating pressure is applied to the valve.
One example of a bellows-type gas lift valve having a mechanical stop is illustrated in U.S. Pat. No. 3,192,869, issued July 6, 1965, to McCarvell et al.
Other gas lift valves, such as the Camco type "BK" bellows-type valves, also have a mechanical stop for limiting the valve stem travel. In this type of valve, the stop is positioned prior to welding the bellows into the valve, and no provision is made for repositioning the stop after the bellows is affixed, i.e., during the bellows presetting operation.
It is known in the art to preset the bellows of a gas lift valve after assembly by overpressuring the valve. For example, a valve which is to have an operating pressure of 600-800 psi may be overpressured to, for example, 3500 psi. The purpose of presetting the bellows is to permanently deform the bellows so that the valve will open at the desired operating pressure. The purpose of presetting the bellows is to assure the operating pressure will not change if an excessive pressure is encountered in the well, say as high as 3500 psi.
Since it appears that the conventional presetting procedure described above does not cause sufficient compression of the bellows, the bellows will "stack up" at operating pressure, thereby limiting the stem travel before the mechanical stop is reached. Moreover, it has been found that stem travel is non-linear with applied pressure, and it would be desirable to make the relationship more nearly linear. Still further, the prior art valve assemblies and presetting methods do not provide entirely consistent operating pressure/stem travel characteristics from valve to valve. That is, some valves will fully open at a pressure differential which differs significantly from that required by other valves of the same construction and preset by the same process.