Field
The present invention generally relates to gamma-transparent non-metallic windows used in the densitometry of pressurized fluids, and to gamma densitometers containing such windows. It is particularly concerned with a ceramic window that has been pre-loaded with compressive stress via a shrink-fitting fabrication method to better resist tensile stresses generated under high-pressure conditions.
Description of the Related Art
Gamma densitometry is a method used to characterize and measure properties of fluids or mixtures of fluids in many applications, including the oil and gas industry. Gamma densitometers are used in such methods. Typical applications of gamma densitometry and apparatus using the gamma densitometry method include level gauges for measuring fluid levels in tanks and separator vessels, densitometers for measuring the density of liquids in a pipe, and densitometers for measuring gas fraction in a multi-phase fluid flow. A special type of gamma densitometer is the dual-energy gamma densitometer. This apparatus uses gamma radiation of different wavelengths to simultaneously measure several properties of a fluid or mixture of fluids. It is typically used to measure the gas fraction and water cut simultaneously in a multi-phase well effluent. Such measurements are often performed on fluids under harsh conditions, such as high pressure and high temperature, for example, in a flow pipe, in a separator vessel, in a venturi, or some other pressure-containing body or vessel.
When a gamma densitometer is to be used to measure properties of fluids under pressure it is necessary to create an aperture in the pipe or pressure vessel that is transparent or nearly transparent to gamma radiation, but which still can function as a pressure barrier and withstand the high stresses experienced in such environment. Such an aperture is commonly known as a “gamma window.” If the gamma radiation is of a short wavelength, and hence high energy, such a window may be made from metal, or indeed be just a section of the pipe or the pressure vessel wall. Gamma radiation of lower energy, in particular energies lower than a few hundred KeV, is easily stopped by a few millimeters of metals. A notable exception is the metal beryllium, which is highly transparent to gamma radiation even at very low energies. However, this metal is highly prone to corrosion, and thus is not an ideal candidate for use for gamma windows in many applications.
Consequently, such “gamma windows” are commonly manufactured from ceramic materials. There are several ceramic materials that may be used to manufacture gamma windows since they combine high strength, in particular in compression, and low attenuation of gamma radiation. Unfortunately, such ceramic materials are brittle, i.e., they can support high compressive loads, but much smaller tensile and shear loads. This property makes gamma windows manufactured from ceramic materials prone to failure in high-pressure applications where the pressure applied by the fluid to the face of the window generates tensile and shear stresses that the non-metallic material forming the window cannot withstand.
It is therefore desirable to provide an improved pressure and temperature resistant window for gamma densitometry and other applications, and preferably a window that is transparent or nearly transparent to gamma radiation.