Pressure gauges are an essential, if delicate, measuring tool for oil and gas production. Flow lines, separators, and even atmospheric vessels like stock tanks are all under some amount of pressure. Gauges allow you to monitor pressure levels throughout the operation, from the wellhead to the tank battery. Monitoring pressure downhole is also important for extending the production life of the well for as long as possible. If you plan on working in oil and gas production, it’s a good idea to be comfortable using, maintaining, and calibrating pressure gauges.
Pressure Gauge Basics
Pressure gauges are used all over a pumping lease, with a range of sizes, costs, and accuracy levels. Some gauges can take more abuse, but generally are less accurate. Others are more precise, but have to be treated more carefully.
Figure 1. A few different example gauges. (courtesy of Helicoid Instruments)
Less expensive gauges can be used for tasks where an approximation is enough, such as monitoring the pressure in a flow line. It’s more important to know the exact pressure at the wellhead and downhole, however, and so a more accurate gauge should be chosen for those uses. An appropriate gauge should be chosen for each task. The life of a gauge that’s measuring pressure down the flow from a triplex pump, or in another situation where pressure fluctuates rapidly, can be extended by using an adjustable vibration dampener to lessen the shock.
One type of gauge uses a spring to measure pressure and move the hand on the gauge’s face. A second, more common type uses something called a Bourdon tube. That’s a thin, flat tube that is curved into a c-shape. As pressure builds in the Bourdon tube, it attempts to straighten from its curved shape. This moves the hand on the gauge’s face, indicating the pressure.
Figure 2. The interior of a Bourdon tube gauge. (courtesy of Helicoid Instruments)
Types Of Gauges
In some contexts, it is wise to use a gauge with some built in safety measures, such as a stronger glass face. Some will have a rubber plug in the back of the gauge that can blow out if the pressure grows too great. Otherwise, safety glasses should be worn whenever opening a gauge or when reading a valve on a high pressure line.
Most gauges are gas filled, but some may be filled with liquid instead. Gas filled gauges may become scratched or otherwise difficult to read over time. Liquid filled gauges will remain readable much longer, but may have a bigger problem with corrosion.
Well Testing Gauges and Calibrating
Gauges used for testing well pressure work the same as any other gauge, they’re just generally more accurate and can be calibrated.
Figure 3. An example of a gauge and dead weight tester.
A test gauge will have an adjustment screw with which any error from the indicating hand can be corrected. Using a dead weight tester, the gauge can be checked for accuracy. Well testing gauges are generally fairly expensive and should be treated gently. A cam and roller geared gauge is a good option, as they are long lasting and accurate. It’s also usually best to select gauges that have the hand pointing directly up when indicating the middle of the pressure range. Gauges of this sort are more reliable. Occasionally it may be necessary to have a gauge calibrated at a laboratory, or by the factory, in situations when precision is important.
Calibrating a test gauge requires a dead weight tester. To calibrate a gauge using the dead weight tester in Figure 3, the black gauge in the back left (hidden by part of the tester) should be closed. The black valve in the front left should then be opened and the handle cranked up. Hydraulic fluid will be pulled under the crank’s plunger from the center reservoir. Now the two valves’ states are reversed, with the front valve being closed and the back opened. Cranking the handle downward puts pressure on the fluid so that it is pushed to the center pedestal stem. You can fit the gauge onto the tester, and then place the test amount of weight on the pedestal in the center. The left hand screw raises the weights to the correct height, and you can then check the gauge to see if it’s reading accurately. The gauge’s hand can be adjusted with a screw driver.
Measuring Pressure Without A Gauge
A dead weight tester can also be used to measure pressure directly when you need a very precise reading. Using a small diameter, high pressure hose, the tester can be connected directly to the wellhead. The tester can then be used to measure pressure.
Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles: How To Test Wells In Oil & Gas Production, Common Tests For Oil & Gas Production and, Special Tests for Flowing Wells in Oil and Gas Production – they’ll be sure to pump you up!!!