It’s no wonder inline tubing has become popular with landscape contractors. Easy installation, even coverage, lack of overspray and near invisibility are its real advantages over other types of sprinkler and drip systems. Unfortunately, many potentially great water-conserving in-line systems are underperforming in the field. The main culprit? Uneven coverage due to clogged emitters. Dirt and sediment aren’t getting properly flushed from the lines, leaving emitter flow paths vulnerable.
Here’s a look at ways to improve flushing with in-line tubing and restore your system to its optimum efficiency.
Automatic vs. manual flush valves
Automatic flush valves were designed to replace the manual flushing of laterals, making it easier to keep the lines clean and prevent emitters from clogging. They’ve had the potential of being real timesavers over manual valves, especially if you’re laying out in-line tubing in the typical drip system branching design. This kind of design has dozens of laterals and dead ends where these valves are needed.
Just so we’re on the same page, let’s go over how these valves operate. There are two types of automatic flush valves: normally closed and normally open. Normally closed valves open at the beginning of each watering cycle, flush and then close as pressure increases. Normally open valves are already open as the system starts its cycle, they flush and close after reaching a certain pressure, then open again when the system shuts off.
Sounds good in theory, but in the field there’s some doubt about the reliability of automatic flush valves. The concern is whether they stay open long enough for water to reach a high enough velocity for a thorough flush. Debris may be left in the system that could clog emitters.
“We recommend a flushing velocity of 1.5 to 2 feet per second,” says Dr. Charles Burt, chairman and professor of California Polytechnic State University’s Irrigation Training and Research Center in San Luis Obispo. “Automatic flush valves may reduce particle buildup in the ends of the lines, but they are not going to give you the recommended velocity or the sustained flush needed to really blow things out.”
Dean Dal Ponte, the marketing manager for Rain Bird‘s Landscape Drip Division, agrees.
“I’m positive that automatic flush valves seal off way before the flow gets anywhere near as fast as Cal Poly’s recommended velocity,” he says. “These types of flush valves are luring people into a false sense of security, and could potentially lead to system problems later on.”
Contractors are also finding that debris can easily get into these valves and get stuck in their mechanisms, causing them to malfunction.
“It doesn’t take much to clog an automatic flush valve,” says Eric Vilander, an irrigation technician for the City of Duarte, CA. “If debris is in the tubing and gets into the valve, the valve can get stuck open and as much as 10 GPM of water will run into the street and then I’ll get a call.”
The solution? Good old-fashioned manual flush valves – which do allow you to create a high-velocity, high-volume flush – and a grid design so that only one manual valve is necessary per zone.
Grid is great for manual flushing
Your typical drip system branching design has a supply manifold with multiple laterals branching off and dead-ending (see illustration on previous page). The problem with this design is it allows deposits to build up in the dead ends where the automatic flush valves are incapable of flushing them out completely, according to Dal Ponte. And to use multiple manual flush valves is impractical.
“The often-used branching lateral design is great for sprinkler systems but not for drip,” he says. “It can lead to problems with flushing, which can further lead to clogged emitters and uneven coverage. Instead, we recommend the grid, or looping design.
“There are lots of reasons the grid is so desirable,” continues Dal Ponte. “You get better uniformity because the pressure is better equalized across the entire system. Water comes from both sides so a pinch or break in the line is not as critical. But what it really allows you to do is collect all the ends of the laterals together and flush from one point in a collection manifold.”
A grid design consists of laterals that form a continuous loop so there are no dead ends. With no dead ends, only one manual flush valve is needed per zone and clogging is significantly reduced, according to Cal Poly’s Dr. Burt.
“It is very desirable to have a grid or looping configuration and a flushing manifold with a single flush valve,” he says.
Located on the exhaust manifold, a manual flush valve can be a simple tubing end closure, such as a figure eight, or a screw-off type that is manually removed at full-system pressure and velocity to thoroughly flush out the system.
The great news about a looping design is that, if you still want to automate your flushing, you can do something tricky: replace a manual flush valve with an electric control valve. The electric control valve is connected to an automatic controller on its own station so it can be programmed to flush when velocity is at its peak, and for a long enough time to flush thoroughly.
“What that does is give you a full open area for up to a couple minutes, which is enough time to get the water up to speed and allow the debris to travel through the system and out the valve,” says Dal Ponte. “And, automatic flushing can be programmed as frequently as needed.”
Burt says another common problem that leads to clogging is improperly sized components such as the manifold and flush valve.
“Often times the flushing valves and manifolds are sized too small when they should be as large or larger than the supply line,” he says. “The increased pressure loss means that you won’t get the pressure needed for proper flushing.”
With a grid or loop design, one manual flush valve (or electric control valve), one air relief valve and the proper filtration, your in-line tubing will go far in being the versatile, efficient, water-conserving low-volume product it was designed to be.
