Radar level gauge system and method providing a signal indicative of process reliability

The present invention relates to a radar level gauge system, for level determination of a product contained in a tank, and to a method for achieving such a filling level determination.

Radar level gauge systems are in wide use for measuring process variables of a product contained in a tank, such as filling level, temperature, pressure etc. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged vertically from top to bottom of the tank. The electromagnetic signals are subsequently reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.

More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and receipt of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity along the probe of the electromagnetic signals.

However, the electromagnetic signal transmitted by the transceiver along the probe is typically not only reflected at the impedance transition constituted by the interface between atmosphere and surface, but at several other impedance transitions along the probe. Such impedance transitions may, for example, result from product residue that may have adhered to the probe as the filling level of the product changes inside the tank.

There is therefore a risk that the system attempts to determine the filling level based on an erroneous reflected signal.

Moreover, the reflected signal resulting from reflection at the surface of the product may under some conditions be intermittent, which may result in an unreliable filling level determination.

In order to improve the reliability of the filling level determination, US 2006/0052954 and DE 10 2004 052 110 disclose methods and systems for determining which received echo signals are surface echo signals resulting from reflection at the surface of the product in the tank, and which received echo signals are parasitic echo signals. According to each of the methods disclosed in these documents, the received echo signals are classified into surface echo candidates and parasitic echo candidates based on their respective movement over time—non-stationary echoes are classified as surface echoes, while stationary echoes are classified as parasitic echoes.

Neither of the systems and methods disclosed in the above documents provide any indication of the reliability of the level determination.

In view of the above-mentioned and other drawbacks of the prior art, a general object of the present invention is to provide an improved radar level gauge system and method, and in particular a radar level gauge system and method capable of providing an indication of the reliability of the level determination performed by the system.

According to a first aspect of the present invention, these and other objects are achieved through a method for determination of a filling level of a product contained in a tank, comprising the steps of generating and transmitting an electromagnetic signal; propagating the transmitted electromagnetic signal towards the product in the tank; receiving echo signals resulting from reflections at impedance transitions encountered by the transmitted electromagnetic signal; classifying one of the echo signals as a surface echo signal resulting from reflection at a surface of the product inside the tank; evaluating an additional echo signal; providing, if an amplitude of the additional echo signal is greater than a predetermined disturbance echo threshold, a signal indicative thereof and determining the filling level based on the surface echo signal.

It should be noted that none of the methods according to the various aspects of the present invention is limited to performing the steps thereof in any particular order.

The tank may be any container or vessel capable of containing a product, and may be metallic, or partly or completely non-metallic, open, semi-open, or closed.

The present invention is based on the realization that the reliability of filling level measurements can be improved by identifying and indicating sources of disturbance echoes at an early stage, preferably before a disturbance echo signal is in the same order of magnitude as the echo signal from the surface of the product.

Through the provision of a signal, based on which conclusions regarding the reliability of the process may be drawn, the operator of the radar level gauge system can be given an early warning and gets the opportunity to decide appropriate action for ensuring the continued reliability of the filling level determination.

The methods according to the various aspects of the present invention are especially suitable for a radar level gauge system of guided wave radar (GWR) type in which the transmitted electromagnetic signal and its reflections are guided by a probe that is connected to the transceiver and arranged to extend towards and into the product in the tank. It should, however, be understood that the present invention may also be useful in non-contact type radar level gauge systems in which a transmitted electromagnetic signal is radiated towards the product using an antenna, such as a horn antenna, an array antenna etc.

In the context of the present application, the “probe” is a waveguide designed for guiding electromagnetic signals. Several types of probes, for example single-line (Goubau-type), and twin-line probes may be used. The probes may be essentially rigid or flexible and they may be made from metal, such as stainless steel, plastic, such as PTFE, or a combination thereof.

In the case of a GWR-system, action taken by the operator in response to a signal provided by the system may, for example, include cleaning the probe and identifying a “lump” echo signal as such and removing it from the echo profile prior to determination of the filling level.

The early warning system of the present aspect of the invention will reduce the risk of erroneously classifying an echo signal resulting from reflection at probe contamination, such as product residue accumulated somewhere along the probe, as the surface echo signal.

For determining whether to provide a signal to indicate possible probe contamination, the additional echo signal that is evaluated may advantageously result from reflection at an impedance transition located above the position associated with the surface echo signal, that is, above what is presently believed to be the surface of the product in the tank.

Alternatively, all additional echo signals may be evaluated, but only the presence of such an echo signal both having an amplitude greater than the probe contamination threshold and resulting from reflection at an impedance transition located above the position associated with the surface echo signal lead to the provision of the above-mentioned signal indicating possible probe contamination.