Surface Equipment – High Pressure Separator
To prevent metering difficulties, and to reduce corrosion and pressure drop in downstream piping, liquids are removed from wellhead gas. Water plus natural gasoline condensate are drawn off as a mixed phase. Gas flows out of the separator, through the sales meter, and into the collection (i.e. lateral) piping. The two main problems associated with the operation of high pressure separators are:
• Liquid carry-over.
• Loss of gas through a leaking liquid dump valve.
Only rarely do high pressure separators carry-over due to excessive gas rates. The vessel, if properly sized to handle the initial well production, will be adequate to de-entrain liquids from their diminishing gas flow as the well ages. Usually, liquid carry over is due to high liquid levels. The liquid dump valve shown in figure 3-1 is actuated by “instrument gas” (i.e. natural gas) flow from a connection on the high pressure separator. Once the instrument gas bottle illustrated in this sketch fills with water, the dump valve may become inoperable due to water in the instrument gas. The resulting high liquid level in the separator will keep the instrument gas bottle liquid full and hence continue to prevent the dump valve from operating and draining the high pressure separator. Installing a larger instrument gas bottle and instructing field personnel to drain it daily is one answer. The ideal solution though, is to supply dry instrument gas from a nearby glycol dehydrator. Dump valve instrument gas tubing improperly aligned to resist freeze ups is also an . important factor in liquid carry over.
The most common cause of high liquid level carry over from high pressure separators is simply that the liquid dump valve becomes mechanically inoperable, or it is calibrated to hold too high a level. If one of the level gauge glass taps are plugged; or the glass has become opaque with dirt, field personnel will never realize there is a problem.
My first assignment in troubleshooting gas field operations was to survey the high pressure separators in a system encompassing four hundred wells for undersized vessels. The dehydration station servicing these wells was being menaced by an ever increasing brine content in the inlet gas. I discovered not a single undersized separator. What I did find was a hundred inoperable liquid dump valves. Almost without exception, the gauge glass isolation ball check valves had become stuck with age and disuse. As these valves could not be opened or closed, field operating personnel had discontinued blowing down the gauge glass to unplug the taps and clear the glass of fouling deposits. Without being able to visually locate the liquid level in the separator, they could not properly calibrate the liquid level control or know when the dump valve had become inoperable.
It is usually pretty easy to find a leaking liquid dump valve on a high pressure separator. Continuous or frequent venting from the low pressure, three phase separator is one tipoff. A cool line downstream of the dump valve, as well as lack of a liquid level in the separator’s gauge glass, are other indications of a leaking dump valve.
A grain of sand that has become lodged in the dump valve’s instrument gas bleed-off port will cause an “Air-to-Open” dump valve to stick open. Oft times, a stuck dump valve can be made operable by manually opening and closing it a few times. Not uncommonly, dump valve internals are damaged by erosive sand. To minimize this effect, the usual short stem plug inside the dump valve body should be replaced with a long stem carbide plug. On occasion, I have seen dump valves blowing through because a pebble had become lodged between the plug and the seat. The only tool required to disassemble a liquid dump valve to rectify such a problem is a large hammer.