In most process manufacturing situations, particularly those with continuous processes, the measured variables tend to be fairly stable. Temperatures, flows and pressures generally run in a narrow band, so instrumentation is sized and ranged to reflect the normal parameters. But there are applications where variables can fluctuate widely, and these are the situations where we have to depend on the turndown ratio of a given instrument.
Wally Baker and his colleagues talk about just such a situation in the October 2017, issue of Flow Control in their article titled Proper Instrumentation is Key to Profitable Plunger Lift Applications. For those who don’t spend their days at natural gas well sites, they explain what a plunger lift system is:
The action is simple: A plunger, sized to be close to but less than the diameter of the well tube, drops to the bottom of the well, where it lands on a bumper spring. The well is shut off at the top, allowing pressure to build up. When pressure is high enough, the well is opened and the pressure can push the plunger up the tubing, lifting the liquid above it and freeing the gas flow so production can resume. The plunger is captured when it pops out and retained until the water column eventually reforms and the entire cycle repeats. On some wells, the clearing cycle may run several times per hour.
So, every time the plunger is activated, there is a cycle where above-ground gas pressure drops very low while the well is shut off, followed by a major spike when the plunger pops out. Pressure soon settles back to its normal production rate, then it begins to fall off as water accumulates, and everything repeats. The whole cycle may be as short as 15 minutes.
The problem is measuring differential pressure (DP), needed for the DP flowmeter used to measure gas flow. If the DP flowmeter it is sized and ranged for normal production, the operator can get into trouble:
Most wells that use plunger lift systems typically install DP flowmeters designed to operate with DP values below 250 inches of water column (wc) across the primary element during normal operation, which is up to 96 percent of the time. Consequently, users select DP transmitters with ranges that will be the most accurate during normal operation. However, a problem results when users choose transmitters with upper sensor limits close to 250-inch wc because any reading above the upper limit will register as a transmitter-high saturation. The huge burst of gas released when the plunger comes out of the pipe can send the DP reading across the primary element as high as 800-inch wc, pushing the flowmeter off the scale.
The result: the big burst of gas released with the piston is not measured correctly. The gas is still captured and sent down the pipeline, but it doesn’t get properly credited to the well, so production is understated. The operator may end up giving it away to the pipeline company. The obvious solution is to put a higher capacity flow meter in place, but it ends up running at the low end of its range much of the time. This brings us back to the question of turndown ratio. A flow meter expected to run at maybe 20 percent of its capacity all the time has to have an excellent turndown ratio.
Fortunately, the Rosemount 4088 Multivariable Transmitter not only makes readings more accurate across a wider operating range, but extends its measurement capabilities by gathering additional information, including temperature. For sophisticated flow measurements, such as at a gas wellhead, having a true process temperature reading can provide important information. The data collected by the line pressure, differential pressure and temperature sensors can be combined with the density value of the process fluid (liquid or gas) to measure directly, or calculate, a long list of variables:
This can drive major improvements in wellhead performance measurement as the article summarizes:
Pressure and flow instrumentation optimized for wellhead operations can help users solve some of their tough gas flow measurement problems. Flow meter measurement ranges should be wide enough to capture the entire plunger lift cycle while maintaining accuracy at nominal flow rates. By doing this, producers can capture full production flow data with high degrees of accuracy, avoid giving away product and maintain an accurate picture of well production.
You can find more information like this, and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Pressure Group and other specialty areas for suggestions and answers.