Sample System Best Practices for Water and Hydrocarbon DP Applications

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Abstract
Sampling techniques for BTU and moisture measurements are well understood but applications beyond the routine requires additional attention to detail. Two such applications are the subject of this paper. The first is the measurement of low level moisture (1 – 1,000 ppbv) for cryogenic processes. Measuring moisture below 1 ppm is an entirely different world from the traditional pipeline application because of the highly polar nature of the water molecule, as it tenaciously adheres to all surfaces in the sample system. Without consideration of these surface effects, sample transport and subsequent analyzer response is compromised.

The second is measurement of the hydrocarbon dew point using a continuous device, a physical property rather than a quantitative method. The challenge is to sample the gas at pressure (or alternatively at a fixed inlet pressure), while separating oil, grease and liquid hydrocarbon without affecting the integrity of the sample gas.

Introduction
Mole sieve dryers at the inlet to the cryogenic recovery plant must dry the feed gas to a moisture concentration of typically less than 100 parts per billion (ppb, or 0.01 ppm). This concentration of moisture is only 1/1000th of the moisture limit that is commonly tolerated in pipeline quality natural gas. Too much moisture will lead to a loss of recovery efficiency and, if undetected, damage to the process equipment. Moisture dew points, needed for process control at these temperatures, test the ability of the analyzers to measure them. A well thought out, designed and executed sample system is essential if successful analyses are to be made.

Automated hydrocarbon dew point detection for applications where natural gas is processed, transported, stored or used as a feedstock (for natural gas fired gas turbines) is critical to determining the quality and the characteristics of the gas stream to be monitored. Sampling systems must be designed to ensure that the stream to be measured is representative of the process conditions. A system should be designed to ensure that it is suitable for the analysis method (technology) tasked to perform the measurement, while at the same time ensuring that none of the key constituents in the sample are removed or compromised.

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