How to Guide

51
Entrapped Air
Source
There are many potential sources for air in pipelines. Air may
be introduced at the point where fluid enters the system or
during initial filling of the system.
Problem
Air in a piping system tends to accumulate at high points in the
system. As the flowrate increases, the entrapped air is forced
along the pipeline by the moving water. These pockets of air
cause flow restrictions reducing the efficiency and performance
of the system. Water is about 5 times more dense than air at
100 psi, so when a pocket of air reaches an outlet, it escapes
rapidly and water rushes to replace the void. Such pressure
surges can easily exceed the strength of a piping system and
it’s components.
UV Exposure
PVC, CPVC and ABS pipe can suffer surface discoloration
when exposed to ultraviolet (UV) radiation from sunlight. UV
radiation affects PVC, CPVC and ABS when energy from the
sun causes excitation of the molecular bonds in the plastic.
The resulting reaction occurs only on the exposed surface of
the pipe and to the extremely shallow depths of .001 to .003
inches. The effect does not continue when exposure to sunlight
is terminated.
A two-year study was undertaken to quantify the effects of
UV radiation on the properties of PVC pipe (See Uni-Bell’s
UNI-TR-5). The study found that exposure to UV radiation
results in a change in the pipe’s surface color and a reduction
in impact strength. Other properties such as tensile strength
(pressure rating) and modulus of elasticity (pipe stiffness) are
not adversely affected.
The presence of an opaque shield between the sun and the pipe
prevents UV degradation. UV radiation will not penetrate thin
shields such as paint coatings or wrappings. Burial of PVC,
CPVC and ABS pipe provides complete protection against
UV attack.
WEATHERING
The most common method used to protect above ground PVC,
CPVC and ABS pipe from the sun is painting with a latex
(water base) paint. Preparation of the surface to be painted is
very important. The pipe should be cleaned to remove moisture,
dirt, and oil and wiped with a clean, dry cloth. NOTICE:
Petroleum-based paints should not be used, since the presence
of petroleum will prevent proper bonding of paint to pipe.
Reference: Uni-Bell PVC Pipe Association 2001.
Heat Build-Up
In addition to considering ambient air and operating tem-
peratures in a piping system, piping designers must consider
the radiant effect of sunlight when selecting piping material.
Testing to the ASTM D 4803 Standard Test Method for Pre-
dicting Heat Build-up in PVC Building Products indicates
that radiant heat from the sun can increase pipe surface tem-
peratures by 50ºF or more, possibly causing a piping system
to exceed maximum working temperature or de-rated pres-
sure carrying capability. Painting dark colored pipe with a
light pigmented water based paint may reduce, but will not
eliminate heat build-up.
Solution
Designers should be concerned about entrapped air, but the
issue of entrapped air is very complex. The behavior of air
in a piping system is not easy to analyze, but the effects can
be devastating. Obviously, the best way to reduce problems
would be to prevent air from entering the system. Systems
should be filled slowly and air vented from the high points
before the system is pressurized. Additionally, air relief valves
should be installed at high points in the system to vent air that
accumulates during service.
DESIGN & ENGINEERING DATA
Entrapped Air
• Pressure surges associated with entrapped air may
result in serious personal injury, system failure, and
property damage.
• Install air relief valves at the high points in a system to
vent air that accumulates during service.
• Failure to bleed trapped air may give faulty test results
and may result in an explosion.