Natural Gas Sampling Probes
In order to analyse natural gas in a pipeline, the gas must be extracted from the pipeline and transported to the analyser in question. This process has a number of inherent difficulties amongst which are that the natural gas usually flows at a considerable rate through the pipeline and so careful design of the probe is essential and response times must be considered which depend on (amongst others) the internal volume of the sample system and the physical properties of the gas itself.
Inserting a probe into a fast flowing gas stream may not be as simple as it may first seem. The problem is that, in the same way that a car aerial can oscillate at certain car speeds and tall narrow chimney stacks can oscillate at certain wind speeds, oscillations can occur in the sampling probe at certain gas flow rates which can result in the eventual catastrophic failure of the probe due to sheering under stress. This is the result of a phenomenon known as Kármán vortex street formation, often referred to as vortex shedding. Under certain conditions eddies are formed downstream of the probe which shed in the wake of the probe, the local changes in pressure around the probe due to this alternate shedding effect is what causes the probe to oscillate.
Orbital have a solution to the vortex shedding problem. Our design of probe, known as the VE probe, represents the cutting edge in gas sampling technology. A triple helical fin is machined onto the outside profile of the probe: this fin dramatically changes the flow dynamics of the gas around the probe profile thereby virtually eliminating vortex shedding effects and eliminating the potential risk of the probe breaking away and falling into the pipeline.
Another key design feature of Orbital’s natural gas sampling probe is a very small internal volume. Probes designs with large internal volumes generally have slow sample transfer times because a larger volume of gas sample must be transported along the probe; attempts to improve transport time by increasing “fast loops” has the effect of increasing gas wastage. Our design of probe has a minimal internal volume which, in conjunction with low volume impulse pipework to the analyser, results considerably faster sample transport time to the analyser.
We have already installed in excess of 40 fixed and retractable VE probes on the high pressure transmission system where fast gas quality analysis is a necessity. VE probes are also being installed as standard with all of our fiscal measurement systems.