Interface measurement through thick and thin

21 May 2019

Denny Nelson explains how the latest guided wave radar technology can improve interface level measurement accuracy by halving the minimum detectable thickness of the upper product.

Interface level in tanks or vessels is a vital measurement across many process industry applications. If inaccurate, it can lead to the cross contamination of products, which can prove costly. Although guided wave radar (GWR) is a widely-applied solution for interface level measurement, it requires the top product layer to be of a certain minimum thickness to be detectable. Advancing technology now enables GWR devices has enabled minimum detectable thicknesses to be halved.

The need to measure interface level arises when a tank or vessel contains two immiscible liquids, and in some applications, it is necessary to know the thickness of the upper product layer. A user may, for example, want to pour off only the top fluid and an indication of when to stop is required. 

Interface measurement is also essential for efficient separation processes, to control the flow of both fluids out of the vessel into independent channels with minimum cross contamination. Typical examples include oil over water, oil over acid, low dielectric organic solvents over water, and low dielectric organic solvents over acid. Low dielectric organic solvents include toluene, benzene, cyclohexane, hexane, turpentine and xylene.

In most cases, the two liquids will separate naturally because of their differing densities, with the lower density liquid settling on top of the higher density liquid. For example, when oil and water occupy the same vessel, the oil floats on top of the water. The interface in this example would be the upper level of the water and the lower level of the oil, and measuring it is important. In the oil production process, lack of visibility into separator upsets can result in oil being sent to the water tank or water being sent to the oil tank, both of which are undesirable. Unaccounted for or unauthorised hauling of produced oil in the water tank from a multiple well pad facility could cost nearly 900,000 euros every year in lost revenue, while excess water in the oil tank could cause an unexpected capacity loss, resulting in a spill or well shut-in from a high-level alarm.

The accuracy of interface measurement depends on process conditions, such as the dielectric constant of the products, and the presence of a distinct interface between the fluids. Sometimes the fluids do not separate entirely, and instead an emulsion or rag layer (a mix of the two products) forms between them. Typically, the thicker the emulsion layer, the more challenging it becomes to accurately measure the interface level.

Technology selection
The most basic way to measure an interface level is via a sight glass on the side of a tank. However, this has some obvious disadvantages. It is time-consuming and labour-intensive. Sight glasses also need regular maintenance, and in applications where condensation can occur, the operator may not be able to make an accurate measurement. Other alternatives include floats and displacers, capacitance transmitters, ultrasonic transmitters, differential pressure meters, and magnetostrictive sensors. However, all these technologies have various limitations in terms of their maintenance requirements and accuracy and reliability in certain process conditions.

GWR technology can provide accurate and reliable measurements in vessels with tight geometry, in chambers, and in tanks of all sizes. No compensation is necessary for changes in the density, dielectric, or conductivity of the fluid, while changes in pressure, temperature, and most vapour space conditions have no impact on its measurement accuracy. Further, the devices need minimal maintenance because they have no moving parts, they are easy to install, and can replace older technologies, even while there is liquid in the tank.

In a GWR installation, the transmitter is usually top-mounted, with the probe extending to the full depth of the vessel. A low-energy pulse of microwaves, travelling at the speed of light, is guided down the probe. At the point of the liquid level, a significant proportion of the microwave energy is reflected up the probe to the transmitter, which measures the time delay between the transmitted and received echo signal. An onboard microprocessor then calculates the distance to the liquid surface. As a proportion of the pulse will continue down the probe through low dielectric fluids, a second echo can be detected from an interface between two liquids at a point below the initial liquid level. Should the product lying on top have a higher dielectric than the one below, this prevents a top-down measurement using GWR. In this circumstance the mounting position can be inverted, so that it is installed on the tank bottom.

Thinner interface measurement
The reason why the upper product must be of a certain minimum thickness when using GWR in interface applications is to enable the device to distinguish the echoes of the two liquids. The minimum detectable thickness can vary between 50mm and 200mm depending on the transmitter model and probe style being used. However, Emerson’s latest Rosemount GWR transmitter enhancement provides functionality that enables the minimum detectable thickness of the upper product layer to be halved, to just 25mm.

This has been made possible by a new software algorithm which allows the transmitter to detect signal peaks that are closer together without having to decrease its signal bandwidth, reducing its high sensitivity and its ability to handle disturbances. This offers improved insight into the separation process.

The ability to detect a thinner upper product layer is a particular beneficial in cases where there should be no second product in the vessel, and where the presence of a hydrocarbon on top of methanol, for example, is an indication that there is something seriously wrong with the process. It can also be very beneficial in separators and scrapers, where the operation of a vessel can be optimised by reducing safety margins.

Denny Nelson is a marketing engineer at Emerson Automation Solutions.

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