U.S. Nuclear Regulatory Commission
Particulate Sampling Line Bend Radii
Title: Particulate Sampling Line Bend Radii
See the memorandum from L. B. Higginbotham to W. L. Fisher
dated March 8, 1977, and the incoming request from W. L.
Fisher dated January 24, 1977. Stack and vent sampling
lines should have a bend radius equal to or greater than
five times the diameter of the sampling line.
During a preoperational inspection of Unit 1 at Davis
Besse, several right angle bends were observed in an
airborne sample line that lead to a particulate monitor.
In response to the deviation for failure to comply with
FSAR (Section 126.96.36.199) requirements for representative
sampling, the licensee stated that the right angle bends
had been replaced with bends of radii equal to five times
the line diameters. The licensee further stated that the
new line configuration was in conformance with ANSI
ANSI N13.1-1969 states: "Elbows in sampling lines should be
avoided if at all possible, but when they are required, the
bend radius of the elbow should be as long as practicable
..." (Section B5). Although the phrase, "as long as
practicable" does not appear to be defined further in the
narrative portion of ANSI N13.1-1969, Section A3.4 and
Figures A2 and A5 appear to give some credence to the
selection of R equal to or greater than 5D for sampling
probes, where R is the bend radius of the sampling line and
D is the diameter of the sampling line. Section A3.4 does,
however, contain the caveat that in "some probe
configurations ... deposition may be significant ...."
In examining the installation of stack and vent sampling
systems, a bend radius equal to or greater than five times
the diameter of the sampling line should be accepted.
However, some sort of an evaluation must be preformed by
the licensee to actually demonstrate that representative
samples are being collected. Such an evaluation can
sometimes be done by collecting special samples at the
location of the sample probe and correlating the results
with those obtained at the "remote" sample collector.
Regulatory references: ANSI N13.1-1969, Final Safety
Subject codes: 6.9, 7.3, 9.1