The heating value of natural gas has a significant impact on its monetary value. In general, the heating value of natural gas increases as the concentration of low volatility, high molecular weight components increases. Condensation of gas phase components, which reduce the proportion of high molecular weight components, therefore tends to decrease gas phase heating value, while vaporization of entrained liquid has the opposite effect.
In order for natural gas supply to keep up with demand over the next 10 to 20 years, it will be necessary to increase production from deep-water fields in the Gulf of Mexico. (Refer to Volume 1, Fall/Winter 1997 official newsletter of Colorado Engineering Experiment Station Inc.) Gas produced from said deep-water fields contains higher concentrations of low volatility components, such as water vapor and heavy hydrocarbons, and has a higher susceptibility to condensation than shelf and onshore production gas.
Additionally, some onshore produced gas, particularly in low ambient temperature regions, is susceptible to condensation of low volatility components. Condensation of low volatility components distorts the remaining vapor phase composition thereby changing its physical properties, heating value and monetary value.
The American Petroleum Institute (API) and the Gas Processors Association (GPA) are two leading industry organizations, having recommended standard practices for sampling and analysis of natural gas.
Both organizations require that the temperature of Natural gas samples be maintained above their hydrocarbon dew-point temperature. Most compositional analyzers require the sample gas pressure to be reduced substantially below the supply gas pressure. A gas pressure regulator is typically utilized for this purpose. In many cases, during pressure reduction, the J-T cooling effect cools the gas below its hydrocarbon dew-point temperature resulting in condensation of some low volatile components thereby altering its vapor phase composition and monetary value. This can be seen in the Phase diagram of FIG. 1.
Note that this particular gas composition, at the original source pressure, point “A” (2014.7 PSIA and temperature of 60° F.), all the components are in the gas phase. However after pressure reduction to 24.7 PSIA (point B) the J-T cooling effect has reduced the gas temperature to −27.9° F., which is well below its hydrocarbon dew-point temperature of −6.93° F. at that pressure. The sample gas at point B no longer conforms to the Industry standards and is no longer representative of the original source gas.
There are cases wherein Point B is in the gas phase, however the adiabatic or near adiabatic pressure drop line AB traverses the liquid phase envelope (2-Phase region) as seen in the Phase diagram of FIG. 2. When this occurs, the possibility exists for the transitional liquid formed to become separated from the gas phase, thereby causing compositional differences along a sample gas passageway.
It is possible, in some cases, to preheat the gas sufficiently to prevent it from traversing the 2-Phase region during pressure reduction. However it is not always safe or practical to preheat the gas sample to the required level.
An additional problem which occurs during pressure reduction and regulation of a high pressure gas is the instability of the secondary pressure or “set pressure”. A common trait of single stage pressure regulators is that the secondary pressure is affected by changes in the primary or supply pressure. In many analytical applications, changes in the sample gas pressure feeding an analyzer has a negative impact on its accuracy.
Therefore it is advantageous to reduce high pressure natural gas sample sources in a manner which prevents condensation of some vapor phase components and provides a secondary pressure essentially independent of source pressure changes.
Further, it can be beneficial to reduce and regulate natural gas sample pressure internal to the pressure source. In such case, a flowing natural gas source, such as in a pipeline, can be utilized to maintain the sample gas at a near isothermal condition during pressure reduction. Refer to Mayeaux U.S. Pat. Nos. 7,004,041; 6,904,816; and 6,701,794.
However, as previously mentioned, there are cases wherein, at the initial and reduced pressure, the gas is totally in the gas phase but has traversed through the 2-Phase region, thereby subjecting it to distortion from condensation.
Insertion-type “probe regulators” such as described in Mayeaux U.S. Pat. Nos. 7,004,041; 6,904,816; and 6,701,794 (the contents of which are incorporated hereto by reference), employ a single stage of regulation. As previously mentioned, there are cases where a single stage of pressure reduction, providing essentially an adiabatic pressure drop, can result in distortion of the sample gas composition by cooling the gas below its hydrocarbon dew-point. Refer to FIG. 1.
Prior art multi-stage pressure regulators are bulky and typically limited to two stages. Further, the output pressure setting for stages upstream of the final stage in prior art systems are typically preset internally or externally adjustable for specific circumstances. The emphasis in prior art multi-stage regulators is on regulated pressure stability. Little or no consideration is give to minimizing the J-T cooling effect. Therefore prior art multi-stage pressure regulators designs do not address the major issues involved in the modern day sampling of Natural gas for compositional analysis.
FIG. 4 illustrates a typical two stage pressure regulator wherein with the first stage output set at 500 PSIG (which is typical) the J-T cooling effect is not distributed evenly between the two stages. Additionally, the actual J-T cooling effect distribution between the two stages varies significantly, and by reducing the gas pressure from 2014.7 to 514.7 in one stage at essentially adiabatic condition utilizing conventional methods, the gas is cooled sufficiently to penetrate the 2-Phase region, resulting in distortion of the a gas sample composition derived therefrom.
Accordingly, it is believed that the prior art has failed to provide a system to reduce and regulate a hydrocarbon sample gas stream pressure in a manner so as to prevent condensation from occurring during and after its transition from high to low pressure due to J-T cooling.
In addition, it is further believed that the prior art has failed to provide a method to regulate and maintain a constant and stable secondary pressure independent of variations of the primary (upstream supply pressure).