Over the years, a number of technologies have been used to tackle the level measurement challenges associated with the various components of the natural gas industry. Some have fallen by the wayside due to performance or maintenance problems. Others have evolved along with the application challenges and proven to be tried and true. Applications range from fairly simple level measurement to those that involve harsh operating conditions and include high temperatures and pressures.

The level measurement industry has responded with devices that require no maintenance, offer high performance, and are able to withstand the harsh operating environments. Because of the variety of applications and operational differences of the processes, there is no single technology that will be appropriate for all applications.

The goal here is to present a basic user’s guide to the most appropriate level measurement technology for the various applications found within the natural gas industry. Its purpose is to briefly explain the technology choices, and recommend the best available and most practical solutions for the many of the applications that are typically found in the various phases of the natural gas industry, including production, processing, transmission, compression, and LNG storage.

Point level versus continuous

Regardless of the application, there are two major classifications of level measurement instruments:

• Point level (on/off) measurement refers to a measurement device that indicates the presence or absence of material at a given point or location in a process vessel, storage tank, or phase change (oil/water) in a separator. Point level can be used for either alarm or control of a process.

• Continuous level (proportional) measurement refers to a measurement device that measures the level over the full span range of a process. Continuous level can be used for indication of inventory or control of a process.

Technology choices

Choosing the appropriate technologies to measure high level, low level, total level, and phase separation (interface) depend on the process environment. Below is a list of the more common, non-mechanical, technologies used in oil and gas measurement:

• RF admittance employs a radio frequency signal to a sensor. A change in RF Admittance indicates either the presence or absence of the material, or how much material is in contact with the sensor. This makes RF a popular, versatile and robust technology for a wide range of application conditions and process materials for both point-level and continuous-level measurements. Additionally, RF Admittance has the inherent ability to ignore coating deposits that may develop on the sensor from the process material build up.

• Radar utilizes the frequency modulated continuous wave (FMCW) or pulsed time of flight (PTOF) through-air transmission that allows for an accurate non-contact reading of the reflected electromagnetic signals under some adverse measurement environments.

• Ultrasonic (continuous level) measurement uses a piezoelectric crystal transducer to generate a high-frequency ultrasonic pulse, and measures the transit time for the pulse to be reflected from the material surface and returned to the transducer to determine the level or distance.

• Ultrasonic (point level) measurement electronically resonates a crystal at a fixed frequency to generate sound waves that travel across an air gap to a second crystal. As the gap between the two crystals fill with a liquid media, the second crystal begins to resonate with the first. Due to the construction these are frequently referred to as “gap” switches.

• Tuning forks use piezoelectric crystals that vibrate the fork at a specific frequency. When the fork is covered by product, the system detects a change in frequency, which causes a change in the output state of the switch.

• Magnetostrictive technology uses a float with embedded magnets that rides on a rigid or flexible tube containing a magnetostrictive wire. The wire is pulsed with a low voltage signal that detects the position of the magnetic field within the float. This results in a transit time measurement that often exceeds the accuracy of radar. A magnetostrictive system may also contain temperature sensors providing many important measurements in one vessel penetration, including total level, interface level, and several temperature points.

• Time domain reflectivity (TDR), aka guided wave radar, takes a highly focused electromagnetic wave, guided by a metallic rod or flexible cable, to the surface of the process material and reflects it back along the wave-guide to determine the level.

Some typical level measurement applications are described below.

Gas processing

One example of a typical level measurement applications is gas plant/gas fractionation. In this process, liquid natural gas entering a fractionation plant, then goes through several steps to have the liquids (water and oils) removed from the gas. The separation process removes the liquids (also called condensate), sulfur (typically using amines), and other impurities. Gases are separated into their components, typically butane, propane, and ethane. At the end of the process, additives such as mercaptans are added to the gas to give it a noticeable odor, for safety reasons.

Natural gas processing starts at a wellhead where the gas and entrained liquids are removed for primary separation. Primary separation consists of a screen that the natural gas liquids pass through to remove any particulates in the liquid. This usually runs at pressures between 100 – 170 psig.

The second phase consists of a Demister membrane that removes the gas from the liquid. The gas is then sent on at pressures between 50 – 300 psig to a pipeline while the remaining liquids are fractionated to further remove propane, butane, ethane, and light hydrocarbon liquids for either storage, or sent via pipeline to marketing terminals.

Traditionally, displacers are widely used in the separation process. One of the more common problems is accumulation of iron oxides on the displacer mechanics. This causes the measurement to indicate a lower level than is actually present. This can cause an overfill situation and result in a fire.

In contrast, RF admittance (continuous level) has no moving parts and is immune to product build-up. The technology has been used to successfully remedy this problem for both total level and interface measurements. Alternatively, TDR technologies can be used if product build-up is not excessive. In addition, RF admittance (point level) may be used for either high-level or low-level alarms or control, and provides the best ability to ignore heavy coating deposits on the sensor.

Compressor stations

Gas compression is another application where level measurement technology can be effectively applied. As the natural gas is transmitted through a pipeline at high pressure, some trace impurities (wastes) remain in the gas. As the pipeline distance increases, the pressure decreases and the heavier wastes condense. Compressor stations need to be located at periodic distances to re-compress the pipeline and remove the wastes, which are collected at these remote stations along the route of the pipeline. These can be anything from water condensation to the condensation of the heavier hydrocarbons from the natural gas itself. The wastes are collected in one or several tanks depending on the size of the remote station. Typically, as a waste tank fills, tank trucks are scheduled to empty it. Since these wastes can be hazardous (flammable) materials, they are classified as Class 1, Div. 1 areas. Compliance regulation issues may come into play since one of the uses for the level measurement indication is to prevent the overfilling of any of these hydrocarbon waste tanks.

With regard to compressor station level measurements, a number of factors should be kept in mind. For total level and interface applications, magnetostrictive technology has the best ability to make measurements for both total level and interface levels between the condensed hydrocarbons and condensed water. The condensed hydrocarbons may still have some relatively low densities (specific gravities), and the correct float on the magnetostrictive system will correctly measure the levels. But, operators need to be careful in their choice of technology. Due to the possibility of low electromagnetic reflectivity characteristics of the hydrocarbons, combined with heavy condensation inside of the tanks, radar and ultrasonic technologies should only be used with caution.

LNG storage

The storage of liquefied natural gas (LNG) is typically under very low temperatures at ambient pressures, or ambient temperatures at elevated pressures, in order to keep the gas in a liquid state. Among the characteristics of LNG are its low and variable dielectric constant and low electromagnetic reflectivity.

With regard to LNG storage level measurement, operators should keep level/control indications in mind. These measurements are either in a relatively small vessel (less than 20 ft), or large spheres (30 to 50-ft diameter) where TDR technology is the preferred choice. TDR has the advantage in this case since it is unaffected by the variation in electrical characteristic.