My initial impression of the collection of valves sitting atop a gas well was that the assemblage of hardware was unnecessarily complex. This turns out to be a false first impression. Both the casing and the tubing strings terminate at the tree. Figure 3-1 assumes that only the tubing string will be used to produce gas. This is called “single completion well”. The casing below the packer has- been perforated to communicate with a gas bearing sand formation. If the casing has also been perforated to draw gas from a shallower formation, then the well would be termed a “dual completion”.
The wellhead pressure is shown on a gauge atop the tree. This pressure does not bear a direct relationship to the critical bottom hole pressure (i.e. the pressure inside the tubing at the level of the perforations). There are a minimum of two valves available on the tree to shut-in the high pressure gas flow from the tubing; the master and the secondary (or wing) valve. The master valve is upstream of the secondary valve. The master valve is intended to last the life of the well, while the secondary valve is replaced when it starts to leak. Whenever a high pressure, flowing gas stream is blocked-in, the valve so used will be subject to the erosive force of rapidly moving sand. When two valves are located in series on a gas line, the valve closed first will erode. Alternately, when gas flow is to be restored, the valve opened last will experience the effect of high velocity, erosive sand. This concept applies to casing wellhead valves and liquid drains from high pressure separators, as well as production tubing isolation valves.
How does one know when a secondary isolation valve is leaking and requires replacement? Simply close the valve and see if it stops the gas flow to downstream equipment. If a secondary valve (which I like to call the “throwaway valve”) is not replaced in a timely fashion, the master valve (which I refer to as the, “permanent valve”) will also start leaking. I leave it to the reader to imagine the difficulty and expense of replacing a leaking master block valve on a 4000 psig gas well.