The issue of 'dew point of natural gas' is a complex one that is often misunderstood. Thorough discussion of this topic would take many pages so I must be overly brief.
The 'dew point' of the natural gas is the temperature at which a component of the gas mixture begins to condense. The condensing component could be either water or a heavy hydrocarbon.
Dew point is a complex function of both the precise gas mixture (C1 through C15+, water, etc.) and pressure.
Dew Point Temperature = F( [c1],...,[c15],[N2],[CO2],[H2O],etc.}
The Gas Research Institute, and others, have determined a relationship between the CONCENTRATION of water and the 'water dew point' down to a temperature of -40 for a single 'ideal' natural gas composition (the relationship below -40 can only be an educated guess). All 'water dew point' measuring instruments use this, or similar, data to create their 'dew point' calibration curves. Since 'dew point' is a function of the gas matrix (not simply concentration & pressure) a sensor calibrated on nitrogen suffer from addition error if used in natural gas and the magnitude of the error will change with gas composition.
So now what do you do?
Focus on the key issues: 1) too high a hydrocarbon dew point will clog fuel injectors, change the combustion profile (temperature, rpms, pollution), and possible cause damage to the turbine; 2) a high water dew point causes hydrate formation in the gas supply pipeline and absorbs BTUs.
Gas turbine operators will rarely experience a problem due to water dew point - this problem is a biggie for gas transmission companies. If you must know, the best choice would an instrument that measures concentration as concentration is independent of both composition and pressure.
The Hydrocarbon Dew Point should be the key question for all natural gas fired turbines as it is the direct cause of several operational problems. The incidence of 'hot' gas has increased drastically in the past two years as fewer gas processing facilities are removing the C3+ components of the natural gas due to the low market price for those components.
Your best choice is to go with a direct measurement of hydrocarbon dew point. I specifically say direct measurement to guide you away from a GC. GC's can estimate the HC dew point only if they accurately measure every component in the natural gas. Since most GC's cannot individually measure C5 through C15 their estimations of dew point can have huge errors. In addition to being able to predict an impending operational problem (if gas has high HC dew point expect a problem) you will also know whether or not you need to preheat the incoming gas and by how much. A simple control loop based upon the measurement can save lots of money in unneeded pre-heat energy.
