EPRI - Isokinetic Sampling Nozzles (Probes)

Sampling gases and liquids for on-line and grab sample chemical analysis can be a major source of errors. Up to several orders of magnitude concentration changes of dissolved and suspended impurities have been observed when sample withdrawal, cooling, or transport are not properly performed. Multi-port Steam Sampling Nozzles (Probes) are rarely used because of their complexity, cost, non-isokinetic characteristics, and problems with use in large pipes and they are no longer a part of the ASTM D1066 standard.
 
A more appropriate and functional design was developed and the design and performance of these nozzles was verified in the Electric Power Research Institute (EPRI) research project RP2712-8. This design was then included in ASTM D1066, "Standard Practice for Sampling Steam". Additional information about the need for isokinetic sampling and the proper design and operation of a sampling system can be found in a technical article entiled "Sampling Savvy".  

Background
A photograph of a typical Weld-in Nozzle (probe) design is shown in Figure 1 and Flanged Nozzles are shown in Figure 2. The nozzles extract a representative isokinetic sample from a flow region removed from the pipe surface and at the average flow velocity of the sample fluid. This arrangement results in the withdrawal of the sample with representative concentrations of dissolved, suspended, and volatile constituents.

An appropriate nozzle (probe) is designed for the desired sample flow and typical conditions of the sample fluid. When the flow velocity through the pipe changes, sample flow should be adjusted to maintain isokinetic sampling.
 
Each Nozzle (probe) is designed with considerations of vortex shedding, resonance, vibration, erosion, and strength of the attachment to the pipe.