Installing PVC Intake Lines
When installing thousands of feet of mainline every day the workers can fall into a mindless repetitive work pattern. Sometimes they stop using primer and cleaner, they stop cleaning the ends of the pipe, stop checking the ends for burrs and defects that occur when the pipe is dragged along on the ground at the supply house or on the job, and they stop checking for rocks and debris in the line. And because they lay so many thousands of feet of the pipe this way, and because they do not hear about many problems later when the system is started, those workers think they are doing good work, everything must be Ok. This is not good and when we start talking about installing intake lines those bad habits can be devastating.
Small leaks in a mainline may not show up for months or years, but a small leak on a pump intake line may prevent the pump from running properly or even at all if the pump loses prime. Small rocks may go through valves and clog up smaller zone lines, and they think those rocks came from careless work on zone lines instead of careless work on the mainlines. But small rocks get stuck in pump impellers and yet no one may notice that they have less pressure than they should unless a lot of rocks clogs up the impeller and then they are looking for the problem only to find the impeller full of rocks.
Technicians should treat pump intake lines with special care, with great respect and a healthy fear. Fear that if something goes wrong they will have a very difficult time finding the problem. Finding microscopic vacuum or air leaks in pump intake lines is often not possible and the problem can only be fixed by replacing the pipe, either all at once, or in sections. A few of these nightmares and perhaps they will be careful. Better to just be extra careful and avoid any problems on the pump intake lines.
The slightest error (by the pipe or fitting manufacturer, pipe delivery person, or the installing technician) can cause the loss of days of labor attempting to locate and repair a minor problem that causes major headaches such as loss of prime. Intake problems are hard to diagnose and repair, and perhaps many of these intake problems are some of the most difficult problems in all of irrigation repair and diagnostics. How many of us have worked long and hard attempting to prime a pump while wondering if we were only wasting our time because there might be an air leak, or is there just an air lock? Use extra care and caution when installing intake lines and follow the guidelines below. These guidelines are the result of many years of suffering, (I have made every mistake known to mankind in this area, in fact I have invented many mistakes and even hold patents on them).
Be alert for the following:
1. Examine every piece of pipe and every fitting before cleaning and assembling. Look for cracks, mold defects, scratches from dragging on the ground, be careful the pipe has no diesel fuel on it (pipe delivery truck exhaust stacks dump smoky soot and fuel on the pipe unless they cover that area of the load), and also check for size defects, pipe can be undersized if the molding machine runs too hot. The fit of the pipe to the fitting must be snug without cement in the joint.
2. Use a primer before applying the cement per ASTM Standard D 2855-93, which states that a primer should be used on any pressurized system before cementing the joint. Using primer on a line with negative pressure (less than atmospheric pressure, also called a vacuum) is even more important than on a pressurized line because air can leak into a pipe easier than water can leak out. In the irrigation industry this primer is commonly called “purple primer”.
3. If you use a PVC cleaner you must still use the “purple primer”. This author has used cleaners that are useless either through deterioration over time or they were made improperly. Quality PVC cleaner has three characteristics: the cleaner easily takes the printing off the outside of the pipe, the cleaner softens the pipe and makes the surface tacky, and the cleaner will dull the shiny surface of the pipe. If your cleaner cannot do those things then switch brands or suppliers.
4. Use only fresh newly opened cement on intake lines just to be safe. Do not use the cement if the cement seems “gooey”, has the consistency of jelly, has been contaminated with dirt, or water. Remember, PVC cement is “hygroscopic”, which means that the cement will absorb moisture out of the air. PVC cement, like brake fluid, will go bad quickly if left open and exposed to the air. A good policy for all contractors is to keep new un-opened quart cans of cement and primer on each truck. Whenever an intake line is being installed, always open a new can of cement and primer. This will do two things. First, the employees will see how serious you are about intake lines. Second, there will be no arguments over how to tell if cement is good or not, or how “gooey” something is before it is considered bad.
5. Use heavy bodied cement only. Heavy bodied cements contain more solids to fill gaps, and they also contain a different mixture of solvents that give a superior bond. Do not use fast drying cements. Typically, the heavy bodied cements are gray in color, and the fast dry types are blue in color. There is however no standard color code indicating the cement’s properties.
6. All pump intake lines must be graded up to the pump from the water source, or the line should be perfectly level. A pipe with a “bow” in it develops an air lock in the high portion of the pipe. This air lock can be very difficult to eliminate. Intake lines with air locks cost hours of gut wrenching work and worry and concern that there may be an air leak. Remember, even the slightest fall in elevation as the pipe runs up to the pump will cause an air lock, even a fall as small as 1/2”!
7. Avoid long intake lines by every means possible. The longer the line, the more likely a problem will develop, and the more likely an air lock will occur. A long intake line is a liability to both you and the property owner because any problem in a long intake line can cause thousands of dollars to fix and often the problem is only corrected by completely replacement of the line because it is just too hard to find a small pinhole leak.
8. Do not use PVC male or female adapters, instead use schedule 80 PVC nipples, flanges, or roll grooved fittings specially rated for high vacuum service. Male and female PVC adapters are the weakest of all PVC fittings and are susceptible to installation error (too tight or too loose), using them increases the chances of a problem now and in the years to come.
9. Keep it simple and install as few fittings as possible in the intake line. Aim the pump intake connection directly at the water source. Turn the discharge line into a pretzel if you want, but keep the intake line straight and clean. Avoid installing hose bibs, gate valves, and if possible avoid using any backflow devices that cause high pressure loss. If you must use a backflow device, do the NPSHR vs. NPSHA calculation. And yes, I know some people just have to have a hose bib, so call me, I make “dummy” hose bibs that you can install, one of my many patented devices.
10. Cut the pipes straight and then clean off the burrs.
