PVC Control
What is PVC Control?
PVC Control is the Irrigation Craft control method for supplying water to PVC pipe systems. Irrigation Craft developed PVC Control specifically to deliver water into a PVC pipe system without damaging the pipe and fittings.
PVC stands for "polyvinylchloride", a type of plastic. PVC has A limited ability to make repetitive "folds" or "stretches". Whenever pressure changes in a PVC pipe the pipe either stretches or relaxes. Whenever pressure changes in a PVC fitting however, the fitting stretches or relaxes just like the pipe, but the fitting also experiences something else. In a fitting such as an ell or tee where the water changes directions, the plastic is folded in the crotch of the fitting. This folding occurs where the two hoop stresses meet each other at a 90 degree angle in the joint of the ell or tee.
Each pressure drop and rise is called a cycle. Each PVC fitting is able to withstand a finite number of cycles. When the number of cycles a fitting can withstand is exceeded, the fitting fails.
The following simple demonstration is an accurate representation of the principles involved. Fold a thin piece of any rigid plastic. Then fold the plastic back in the opposite direction on the same fold line. Repeat this procedure over and over again until the plastic fails at the "hinge point". This is exactly what happens with PVC pipe fittings. As the two opposing hoop stresses expand and contract, the plastic in the "crotch" folds just like a hinge.
How Important is PVC Control?
The advantages of PVC Control may not become evident in the first year or two in the life of a new PVC pipe system. In severe cases however the PVC pipe and fittings may actually begin to fail in less than two years. Systems have actually been studied and analyzed where a poorly designed pump station began causing dramatic PVC pipe and fitting failures within weeks of installation on a totally new irrigation system.
Pump stations can and often do abuse PVC mainlines when they are new but no one knows simply because the pipe and fittings do not fail. Property owners often know nothing until the invoices and complaints come in about frequent mainline blow outs. Sadly, when the problem becomes obvious, the owner is then exposed to confusing and contradictory information. Depending on who is speaking, accusations are thrown against the pipe or fitting manufacturers, the irrigation contractor, the irrigation designer, even on sprinkler equipment manufacturers.
But for a fact, the blame should often be placed on the pump station manufacturer. People find this hard to believe, almost too hard. After all, that company has been in business for years, they have a "good" reputation, they have done prestigious projects, they have been selling that equipment for years with no problems.
How can a pump station cause such a dramatic failure of a pipe system? By accelerated aging of the pipe and fittings. Plastic pipe systems can last for centuries and yet they can also be destroyed in weeks or months. Simply put, plastic pipe and fittings have poor tolerance for abuse.
Select the link below to see actual pressure recordings from real projects. These recordings show how a poorly designed pump station aged a mainline 10 years every month, and also how a correctly designed pump station protects the mainline.
