In an analytical sampling system, a nozzle is used at the tap location to supply the sample to the analyzer. Proper placement of this nozzle is critical. It must be located in the right position and oriented correctly to ensure timely, accurate analytical measurements. Poor nozzle placement may lead to analysis delays, sample contamination and inaccurate results.
Ideally, a sampling system engineer will dictate piping layouts and even process vessel design to make certain the nozzle is properly situated. However, it’s more likely the engineer must work with existing schematics. And it’s possible the prescribed nozzle location isn’t in the right place or doesn’t have the correct orientation to guarantee a timely, uncompromised sample.
So, here, we’ll review some important considerations for locating and constructing nozzles for both gas and liquid stream analysis. Follow these guidelines for your facility. In addition, involve qualified analyzer engineers, process engineers and chemists, and component suppliers to ensure you consider every variable.
CHOOSING NOZZLE LOCATION
Nozzles typically are short and have a smaller diameter than the main process line from which they branch off. They often house a probe, which in its simplest form is a metal, glass or ceramic proboscis that pokes into the process fluid and withdraws a continuous flow for analysis.
When determining where to position the nozzle in a pipeline or vessel, select a location where the process fluid has thoroughly mixed so your sample accurately reflects process conditions. If possible, install an inline static mixer in the process line. If that isn’t an option, locate the tap downstream from a point of induced turbulence such as a pump discharge, flow orifice or piping elbow. The turbulence will help mix the process fluid before sampling.
Placing a sampling tap immediately after a source of turbulence isn’t good practice. The turbulence will cause pressure fluctuations and eddy currents — either of which may affect the analytical measurements.
You are better served by placing the tap at least two pipe diameters downstream of the last flow disturbance. The U.S. Environmental Protection Agency (EPA) recommends this practice. It permits two locations for manual stack gas sampling: 1) at least eight stack or duct diameters downstream and two diameters upstream of any flow disturbance; or 2) at least two stack or duct diameters downstream and a half diameter upstream from any flow disturbance. The EPA considers the first location to be ideal. If you can’t meet the first rule, EPA guidelines require additional sampling points to guard against the possibility of stratification.
For pipeline sampling, locate your tap at least two pipe diameters downstream of the last flow disturbance, anywhere it doesn’t interfere with a flow-metering element. However, if the process fluid is a liquid that’s close to its bubble point, it’s wise to be more conservative. To avoid getting bubbles in your sample, locate the tap where there’re at least five pipe diameters of clear straight pipe upstream and two diameters downstream (Figure 1).
When the stream being sampled is a vapor at or near its dew point temperature, the tap location becomes more critical. Some condensation may occur at pressure points near flow disturbances; you don’t want your gas sample to contain liquid condensate. To minimize this potential, one European standard (ISO 10715 1997, 13) for measuring natural gas requires the sampling tap to be at least 20 pipe diameters downstream of the last flow disturbance; the relevant American standard (API MPMS 14.1 2006, 15) requires at least five pipe diameters. If the pipe contains another probe, such as a thermowell, the sampling probe should be located at least five thermowell diameters away from the thermowell.
Even greater separation is necessary for isokinetic sampling, which calls for the velocity in the probe to match that in the process line. For example, for saturated steam, the relevant American standard (ASTM D1066) recommends the sampling tap be at least 35 pipe diameters downstream and four pipe diameters upstream of a flow disturbance. Because this separation may be difficult to achieve, ASTM suggests that noncompliant locations maintain a 9:1 ratio of upstream to downstream distances.