PUMPS, VALVES & ACTUATORS FEATURE


LOCATION, LOCATION, LOCATION: Ensuring accurate pipeline sampling


Karim Mahraz, product manager, Analytical Instrumentation, Swagelok Company, describes the important points to consider to optimise nozzle placements and orientation, and enhance system accuracy in any sampling system


E


nsuring accurate and timely analytical measurements in any sampling


system depends on the proper location and orientation of the nozzle that supplies process samples to the system and online analysers. System designers must consider several variables when deciding where to place this interface to obtain consistent, representative samples. Nozzles are short and measure smaller


in diameter than the main process line. They often house a probe that protrudes into the process fluid for continuous analysis. Probes are useful because they reach into the centre of a process stream and allow greater flexibility for nozzle location. Nozzles without probes are also acceptable, but designers should account for the lack of a probe when determining nozzle location. Turbulence Considerations. You should


typically place sampling nozzles downstream from a point of induced turbulence, as the process fluid will be thoroughly mixed. A sample taken from this point will therefore best reflect real process conditions. In some applications, you can install an in-line mixer to better mix your sample fluid. Avoid placing the sampling nozzle


immediately after the point of turbulence, as you will likely experience pressure fluctuations or eddy currents, which will negatively affect analytical measurements. The U.S. Environmental Protection Agency (EPA) recommends placing the sampling tap at least two pipe diameters downstream of the last point of turbulence. In pipeline sampling, place the nozzle


by following that same rule. However, be more conservative if the process fluid is close to its bubbling point. To avoid bubbles in your samples, place the nozzle at least five pipe diameters upstream and two diameters downstream of turbulence (Figure 1). Vapour Sampling Considerations. In


certain applications, nozzle location requires greater consideration. When the process sample is a vapour, for instance, condensation occurs near flow disturbance points, impacting your sample. Here, you can locate the


Vertical stacks require much larger


horizontal or slightly inclined nozzles housing specialised probes. Base the nozzle location on the gas temperature, avoiding areas that are too hot (to keep material costs down) and choosing areas that are above the acid dew point temperature (so sulphuric acid does not condense in the probe). In Liquid Streams. For liquids, the best


location for your sampling nozzle is on a vertical pipeline flowing upward, where you can be sure the pipe is full (a downward-flowing pipe may not be full). Both horizontal and inclined nozzles are acceptable, but horizontal is preferred since it allows the use of a shorter probe, which is less susceptible to vibration. When sampling a liquid from a


Figure 1: The ideal nozzle placement in horizontal or vertical pipelines is generally at least five pipe diameters upstream and two pipe diameters downstream of any flow disturbances.


Image © 2013 “Industrial Sampling Systems”


sampling tap further downstream. The ISO 10715: 1997 standard for natural gas sampling states the tap should be located at least 20 pipe diameters downstream from the last point of turbulence. The API MPMS 14.1 standard requires just five pipe diameters. Meanwhile, if the pipe contains a supplemental probe, such as a thermowell, you should locate the sampling probe at least five thermowell diameters away from the thermowell. Nozzle orientation. Once you have


determined the proper nozzle location, it is time to consider its orientation. Proper nozzle and probe orientation will minimize the potential for sample contamination. Your ideal orientation varies depending on whether the sample is a gas or liquid and the requirements of the probe. In Gas Streams. When sampling a


process gas, use a vertical nozzle on a horizontal process pipe. Locate the nozzle on top of the line (Figure 2) so any contaminants fall back into the process pipe. If you orient the nozzle horizontally in a horizontal process pipe, solids may collect inside the nozzle, eventually resulting in a blockage. You may also consider an inclined nozzle; however, this orientation can still present blockage issues in dirty samples.


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Karim Mahraz is product manager, analytical instrumentation for Swagelok Company. karim.mahraz@ swagelok.com


Figure 2: The ideal location for sampling process gas is in a horizontal line with the nozzle mounted on top, allowing potential contaminants to fall back into the process stream. The nozzle may be vertical (left) or inclined (right)


horizontal line, locate your nozzle in an area where the process pipe turns upward after the horizontal run, so you can be sure the line is full. If the pipe turns downward, the horizontal section might contain a static layer of gas trapped above the flowing liquid, resulting in a two-phase sample. In addition, mount the nozzle on the


side of the horizontal line if your nozzle does not contain a probe (to minimise the risk of extracting vapour at the top of the pipe or sludge at the bottom) or on the top of the line if your nozzle has a probe (to allow heavy solids to fall back into the process pipe). Avoid locating the nozzle on the bottom of the pipe where contaminants may easily fall into it. Following the best practices outlined


here will help prepare any system for reliable operation. Sample accuracy depends on it. But do not forget about maintenance needs, too. Your final nozzle placement should leave enough physical space for installation and ongoing maintenance. REFERENCE - This article was adapted


from “Industrial Sampling Systems: Reliable Design and Maintenance for Process Analyzers” (2013), a process sampling textbook authored by Tony Waters and published by Swagelok Company (www.industrial-sampling- systems.com).


Image © 2013 “Industrial Sampling Systems”


Swagelok www.swagelok.com


PROCESS & CONTROL | FEBRUARY 2019 17


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