It begins with a bang, and that’s not a good thing. Where it ends could be with damaged pipes, a leaking irrigation system, countless headaches, and thousands of dollars worth of preventable repairs.

It’s commonly known as “water hammer”; the annoying noise that comes from the irrigation system whenever there’s a sudden change in water flow. Larry Workman, of Lasco Fittings, Brownville, Tennessee, defines it as “the wave that is transmitted at sonic speed throughout the pipeline as a result of a sudden change in the velocity of the fluids.”

Put another way, water hammer is the impact force caused by the sudden change in pressure due to variations in the velocity of water traveling through a pipe. Those changes are caused by things such as the opening and closing of valves, and the starting and stopping of pumps. Now, you could just hope the noise will go away and do nothing about it, but that’s probably not a good idea.

“What happens typically is that you’ve got a column of water flowing down the pipe at five feet per second; then all of a sudden it stops short,” says Workman. “There’s the momentum of that water being stopped by the valve that sends a shockwave through the length of the pipe. When it comes back down towards the valve, that’s when you hear a bang.”

It’s like a train heading towards a tunnel in a mountain, but there’s no tunnel.

The train hits the mountain, and all the cars slam into the engine. Instead of the train being a ¼-mile long, now it’s a block long. That’s your water hammer.

Because that surge wave travels down the pipe at sonic velocity, it can create a huge, yet short-lived spike in the system’s water pressure. A system that normally operates at 150psi can see a jump to as high as 500psi. That jump can occur in a matter of milliseconds and is often undetected by gauges on the system, creating nothing more than a flicker. But the damage it can do is enormous.

Photo courtesy: Lasco Fittings Inc.

“Your components, the pipe and the fittings, aren’t designed for those kinds of pressures,” says Workman. “You’ve got a system that has PVC pipe with a pressure rating of 315psi. It’s normally operating at 150psi, and all of a sudden you’re getting 400-500psi; you’ve got enough pressure to blow up that system.”

“The parts that typically go are the directional fittings -- the tees, 90s and 45s -- because the pipe itself just grows in diameter a little bit and then comes back. But when that shock wave tries to turn the corner, it bounces off the corner. That’s the noise you hear. And it’s like hitting the pipe with a sledgehammer, only from the inside.”

If you have water hammer or pressure surges 15 times a day, areas susceptible to fatigue will fail more quickly. A higher, but less frequent, surge of pressure will take longer to do damage. It isn’t a matter of if, just when.

“One of the most ideal pressure vessels is a cylinder,” says Workman. “As soon as you bend that pipe into a 90 degree elbow, that perfect vessel no longer exists. In effect, what the water pressure tries to do is take that 90 and bend it into a coupling. With a lot of water hammers, what you see is a fatigue failure in the crotch of tees and 90s.”

“What you’ll get is a little crack in the crotch of the fitting, and it will develop into a pinhole with a little stream of water. That little stream will churn up the sand and the soil. It looks like gophers have been gnawing at the fitting. But it’s a result of the surging in the system that wears out the fittings.”

It’s called cyclic fatigue. Even though you may not have the type of water hammer that busts the fittings, the irrigation system is experiencing fatigue, and that will cause critical components to eventually wear out.

Even if a system is operating properly, water hammer can still be a problem. That’s because one of the greatest contributing factors isn’t functionality, but design. And design flaws are only the beginning. Factors such as the velocity of fluid in the system will also affect whether or not a system falls victim to water hammer.

“Typically, the plastic piping industry does not recommend PVC being used with velocities higher than five feet per second. If you’re pushing water through the pipe at a speed faster than that, you’re just asking for water hammer,” says Workman. “Your frictional head-losses go way up too.”

Another problem occurs when there’s a sudden change over a short period of time in the velocity of the water going through those pipes. That usually happens when the system is shut off. Because the mechanism of a diaphragm valve can take up to a couple of seconds to completely shut off, the variation in flow during that critical valve close time can cause an enormous spike in velocity.

In a typical diaphragm valve, approximately 80% of the flow is shut off in the last 25% of the valve close time; defined as the “critical close time”.

The material of the piping system also enters into the equation. Metallic pipes are generally very rigid, and because the surge travels faster through them, it tends to get dissipated during the closing action of the valve. In other words, the surge pressure is not trapped within the system

The elastic or non-rigid nature of PVC will greatly reduce the pressure wave surge that travels through the piping system, but also requires a much longer valve closing time than the more rigid materials. That ability to swell or grow slightly helps to dissipate some of the energy created by the pressure wave. Consequently, unaddressed water hammer in a PVC system will create stress and fatigue in the pipes that will ultimately lead to failure.

The bad news is there’s no quick fix to stop water hammer; the good news is something can be done to minimize its effect.

“There are virtually no products on the market that can stop water hammer,” says Workman. “They have surge arresters out there, and air vent valves. Every one of these things can help a little bit, but they can’t stop water hammer because it happens so fast that all of these things have mechanical inertia of the components that cannot react quickly enough.”

Think of a bumper on a car that can withstand a five-mile-per-hour impact. At the slower speed it will protect your car; at 15 miles per hour you’re going to have damage. By keeping the water velocity below five feet per second, there’s very little chance the system will experience water hammer.

If you hold those velocities down within the system, you’ll have less chance of getting water hammer. The slower that water moves, the quicker the time the valve can close and not cause this rapid change in velocity.

The first thing that can be done is to cut down the volume of water being pumped through the system. That will slow down the velocity. Since every system will be a little bit different, that can be achieved in several ways.

On some systems, you might be able to increase the diameter of the pipe going out to the sprinkler heads. Another method involves creating a loop system. Instead of the pipe going out from the valve out to the last sprinkler head, install them on a circular loop.

“What you’re doing by sending part of the water down the right hand side of the circuit and the rest down the other side,” says Workman, “you’re lowering the velocity in that pipe.”

Another thing to look for when correcting water hammer in a preexisting system is to consider what’s causing the water to travel at such a high velocity in the first place. More than likely, you’ve got openings in the system large enough to allow such a flow. That could include too many sprinklers or nozzles that are bigger than needed, and are using too much water. A simple solution could be smaller nozzles and watering just a little bit longer.

“All of these are compromises. You have to balance it. It’s a give and take situation,” says Workman. “But the key thing is you’ve got to get that velocity in that pipe down by increasing the diameter of the pipe, or reduce the reason to have the demand in the first place by cutting down the size of the nozzles.”

Air in the system, also known as “air slugs,” can also cause problems. It will travel through an opening at five times the velocity that water will travel.

“Even in a perfectly designed system, with a velocity of five feet per second design, air can get in,” says Workman. “So when you turn on the system, because air can travel through the system faster than water, the water behind that air bubble will speed up. When that air bubble runs out of the system, you’ve got a 20-foot-per- second velocity change. You’ve got yourself a water hammer.”

That’s one of the reasons that some contractors install air bleed valves. Located on the systems’ high point where the air bubbles will collect, these valves allow trapped air to vent, preventing it from exiting the sprinkler heads and creating water hammer. These release valves open to release the air when the system is turned off, but close when water is running through the pipes.

“Pressure relief valves are one good way to relieve water hammer,” agrees Kevin Rost, of Dura Plastics, Beaumont, California. “Monitoring the system is also a good idea, especially if you’re coming off a well tank or a pressure pump where you’re inducing more air into the system every time that pump or tank turns on.”

Winterizing a system can also allow air into the system. After draining the fluids for the winter months, it’s important that when you fill the pipes for the fist time, they are filled slowly as to not trap air. That’s achieved by opening the valve to about ¼ of the way. And remember to remove the furthest sprinkler, allowing the air to bleed off while you’re filling the system.

“Anytime a system has been off for a long period of time,” says Rost, “you need to turn that system on very slowly. That will prevent air from binding in an elbow or a transition point.”

When you’re installing a new irrigation system, make sure you consider water velocity, and the effect valves and sprinkler heads will have on it. The same holds true for a pre-existing system. Unlike a new installation, you can’t do much about the design. And often, you are inheriting someone else’s mistakes. Still, something can be done. While there’s no magic bullet, by taking the proper steps you can manage the problem and avoid getting hammered.