PVC Fitting Failures
on Golf Course Irrigation Systems
August 31, 1993
by Michael Harrington
HARCO Corporation
Lynchburg, VA
Automatic golf
course irrigation systems possess unique operating characteristics
that damage PVC pipe fittings that result in failures. Automatic
irrigation systems incorporate many valves that open and close
creating cyclic pressure surges. PVC fittings, because of their
geometry and material characteristics, are uniquely unqualified
to withstand the water hammer and cyclic pressure surges present
in these systems.
PVC fittings used in automatic golf course irrigation
systems were developed for and widely used in commercial irrigation,
plumbing, waterworks, water well, and industrial markets. The
quantity of PVC fittings
used on golf courses represents a small percent of the total.
These same fittings on golf courses account for the majority of
reported failures of PVC fittings and are a significant maintenance
problem for golf courses throughout the United States.
The majority of these failures have been incorrectly judged
to have been caused by faulty installation, defective manufacturing
or incompetent design. PVC pipe used on golf course irrigation
systems however, has experienced very few problems and has
provided cost effective and reliable service for many years.
Automatic valve-in-head golf
course irrigation systems are continuously pressurized and supply
water to hundreds of valve-in-head sprinklers throughout a course.
The opening and closing of these sprinklers create pressure surges
that telegraph through the piping system in the vicinity of the
sprinklers. A typical golf course system may experience from
40,000 to 100,000 cycles per year with magnitudes of 10 to 80 psi.
(1). The attached graphs illustrate
the frequency and magnitude of pressure surges on a golf course
irrigation system.
Pipe
fittings, defined as the intersection of two or more cylinders,
are more complex geometrically than pipe. Hoop stresses of
the intersecting cylinders combine at the points of intersection.
The inside corner of the cylinder intersection is subjected
to twice normal hoop stress. In tees, the stress at the corner
is further increased because of the unsupported area defined
by the overlap of the two cylinders when viewed in the horizontal
plane. A cutaway of the fitting across this unsupported plane
illustrates the oval shape geometry.
Manufacturers
have understood these additional stresses for years and have
added material to the outside of the fittings to compensate
for the additional stresses. While the additional material has increased
quick burst strength, it has not increased the fitting ability
to withstand pressure surges and cycles.
As
PVC fittings are pressurized, the fitting deflects internally,
attempting to “round up” the oval geometry of the
unsupported intersection area. This creates a hinge at the
internal intersection that bends in the vertical plane during
every pressure surge. At this same point in the horizontal
plane, additional bending is taking place created by the differential
expansion of the intersection and the cylinder portions of
the fittings near the bells. The extra stiffness of the intersection
prevents the intersection area of the fitting from expanding
to the same degree as the bell areas.
Every
pressure surge “flexes” the intersection, similar
to bending a hinge in two directions. The visco-elastic
properties of PVC are unsuited to withstand repeated flexing
and fittings fail at a number of cycles dramatically less
than pipe which does not have any internal “hinges”. The
hinge on a plastic tackle box that fails after a short period
of time is an example of a plastic “hinge failure”.
The
magnitude of this reduction of fatigue strength of fittings
is illustrated in FATIGUE TESTING OF PVC PIPE FITTINGS (copy
attached). 6” schedule 40 PVC tees withstood less than
15,000 cycles of 100 psi every
2 seconds. This compares to 6” PVC pipe which withstands
1.5 million cycles of the same test parameters.
The
effects of this “internal bending hinge” are
greatest on full size opening tees, where the branch diameter
is equal to the run diameter. 90 degree bends and first order
reducing tees are the next most susceptible with 45 degree
bends and service tees being susceptible to a lesser degree.
For
golf course irrigation systems subjected to cyclical pressure
surges, it is recommended that HARCO deep bell ductile iron
fittings be used for all 4” and larger fittings and all 2”,
2 ½, and 3” tees and 90 degree bends.
-
R.D. Bliesner, “Designing, Operating
and Maintaining Piping Systems Using PVC Fittings”, Keller-Bliesner Engineering,
Logan Utah, (1987).
- D.
B. Edwards, R. M. Cohen, B. Lehman, “Fatigue Testing of PVC
Pipe Fittings”, ANTEC (1992)
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