If there is, then I'm probably guilty of misuse.
I usually refer to a fast loop when I intend the understanding to be that the flow rate is sufficiently high and/or the flow path to the analyser is short enough that the measurement response time is very fast.
This is the case where the analyser is located adjacent to the control point for the reaction or process.
Some analysers are located in an analyser house remote from the process. Here one encounters the term fast loop as meaning the primary sample flow into the analyser house which is relatively fast flowing and designed to bring the sample as quickly as possible to the analysers. Some analysers then have a separate loop from the fast loop and may have relatively low flows through them, if not intermitent flow (e.g. grab sample analysers which do not use a continuous sample flow).
Thanks, JMW. One of the configuration I've seen is to use larger impluse line bring in more samples and branch out smaller tube to the analyzer and the rest return to the lower pressure process line. I think this can avoid any dead leg or sample lag.
Generally a "Fast Loop" is used when the analyser is located some way away from the sample point, the idea of the fast loop is to reduced the DV lag (Distance Velocity)that is introduced by the long sample pipline. The main problem with them is that they tend to work on the principle of large differential pressure to produce the hiher velocity, this then produces large quantities of "effluent" which needs to be disposed of, either to atmospere/drains or by piping into a lower pressurised part of the process.
"fast loop","speed loop", "sample bypass loop", they are all pretty much the same. the purpose is to pull a representative process sample and deliver it to the analyser pick-off, returning most of it to the process,
most analyzers require such small flows that the sample delay would be unacceptably large otherwise.
Standard design, no, it generally requires a custom engineer design, that is specific to your process.
Issues related to the fast loop include maintaining a representative sample. For gas samples the pressure can be reduced at the sample tap. This reduces the actual volume of gas transported. The risks include changing the composition or properties. Refrigeration occurs with the pressure reduction. Vaporizing regulators attempt to minimize this.
Larger impluse line increase the volume of the sample. This is a bad thing for transport time and generally avoided. It may be a good thing for viscous or plugging problems.
There are sample taps that use the head from the center of pipe to extract the sample and a return at the pipewall for a localized fast-loop. Analyzer packagers use lots of sample handling tricks to provide a representative sample to the analyzer, especially if used for process control.
