The lesson is that general sizing rules must be tempered with good engineering. Before applying a generalisation that you read on the internet or given by a colleague, take the time to understand the derivation for that rule of thumb and ensure it applies to your situation. Pipe sizing isn’t an exact science and rules of thumb are perfectly acceptable when choosing a size, but this should be done with eyes open.

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Arrestors are available for every type of pipe. The exact tools you’ll need will depend on what type of pipe you’re working with, and where you’re installing the arrestor. You may need to cut into the pipe, or you may be able to install a threaded connection. They’re typically installed at the end of a water line near the plumbing fixture that’s causing the hammering. If that’s not enough to eliminate the water hammering, Shalyn King suggests installing them at any 90-degree turns.

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Arrestors have a piston or diaphragm separating the water line from an air-filled chamber. When the water’s momentum is high enough, the piston becomes the path of least resistance. The water pressure pushes the piston into the air on the other side, which compresses, absorbing and dispersing the water’s momentum.

Water hammer arrestors do wear out over time. Aaron Adams suggests occasionally checking arrestors for leaks at their connections to stay ahead of any problems caused by aging.

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The most obvious issue that arrestors solve is that annoying water hammer rattle. But that noisy rattle is the side effect of the real problem: water slamming at full speed into the pipe junction. Over time that force can shake pipes loose from hangers, making the noise even worse, or even shake fitting loose can cause leaks.

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The result can be a shocking level of rattling and banging, explains master plumber Aaron Adams. “Water hammering is very loud and causes pipes to shake violently. Other times homeowners may hear loud intermittent bangs when they turn water on and off in certain parts of the home.”

Aaron Adams is a Master Plumber and CEO of Aaron Services: Plumbing, Heating, Cooling, a family-owned company serving the Atlanta Metro area since 1978.

A water hammer arrestor provides an off-ramp for the water’s momentum, like the emergency off-ramps on mountain highways, where trucks can safely divert if they get too much speed on a steep downward slope.

Calling in a pro will typically run $250 – $500. Shaylin King explains that the price largely depends on the placement within your plumbing system, since an arrestor install sometimes requires opening walls to access the end of pipe runs. In cases like that, where the plumbing is difficult to access or will require drywall removal and repair, the cost can balloon up as high as $1,000.

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One approach is to perform a life-cycle cost analysis that balances operating and maintenance costs against the initial capital investment. This analysis usually results in a velocity around 2-3 m/s, giving that rule of thumb. More expensive piping materials and larger pipe sizes tend to favour sizing for higher velocities whereas common materials and smaller sizes indicate lower velocities.

Pharmaceutical engineers often cite “turbulence” as the driving criteria for sizing the piping for high purity water. The idea is that turbulent flow, defined as Reynolds number >4,000 (or >10,000, depending upon which engineer is speaking), greatly reduces the propensity for a biofilm to establish on a pipe wall. However, there is always a laminar layer at the pipe wall; it is many times deeper than a typical biofilm thickness even at a Reynolds number of 50,000. Studies show that microbes will tightly adhere to stainless steel pipe walls when the shear stress is less than 8-10 Pa. As seen on the charts, the Reynolds number needs to be well over 50,000 to achieve 10 Pa shear stress at the wall.

Another sizing approach is based upon existing infrastructure limitations. For example, your plant’s chilled water pressure or a pump’s capabilities might limit the maximum pressure drop. However, you might not be able to provide a target of, for example, 60 kPa/100 m. Instead, the pipe sizing might simply be based on the available pressure drop regardless of how that fits into the generality.

Depending on the volume, water hammering can be mildly annoying or highly alarming. But the noise is only the symptom. The real issue is that the forces involved can damage the pipes, potentially resulting in breaks and leaks.

Shaylin King, a licensed plumber with Mr. Rooter, compares water hammer arrestors to shock absorbers for your plumbing. “While not always necessary,” says King, “especially if your plumbing is properly sized and secured, they’re a smart preventative measure in many homes to protect against sudden pressure surges and potential pipe bursts.”

Where the pros disagreed was on how long they typically last. I heard estimates ranging from 5 to 25 years. Why so much variety? Because arrestor lifespan depends on a number of factors.

“Plumbing pipes are not designed to withstand shaking or movement,” explains Adams. And that shaking, over time, can cause connections to come apart, or even wear holes through the pipes.

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This depends on the specific arrestor. Most – but not all! – can be installed in any direction. It’s important to check your specific model’s instructions for the manufacturer’s guidance.

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How do you determine the pipe sizes for a system? The first step is establishing the process requirements: flow rates, fluid properties, and pipeline configuration. Many engineers then apply a rule of thumb that says to use a velocity of 1.5 to4 m/s. Higher velocities mean higher pumping cost and possibly damage to the piping due to erosion or water hammer. Lower velocities mean larger (more expensive) piping and possibly problems from fouling or solids depositions in the pipes due to insufficient turbulence.

Before applying a generalisation that you read on the internet or given by a colleague, take the time to understand the derivation for that rule of thumb and ensure it applies to your situation

RULES of thumb are sometimes misleading. Engineers often discuss rules of thumb for pipe sizing. Velocity (1–4 m/s) and Reynolds number (>10,000) are cited, as are pressure drop (20-80 kPa/100 m) and shear stress (>5 Pa).

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Also consider the nature of the fluid being pumped and the material that the piping is made from. Corrosive fluids such as acids, and abrasive fluids such as slurries, will tend to be more aggressive toward the piping at higher velocities. Higher velocities will also create greater water hammer if valves are suddenly closed. If the fluid carries large particles, such as sand, the velocity should exceed the settling rate of the particles (typically about 1 m/s).

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Before you install a water hammer arrestor, determine which faucets or valves in your house cause the noise. Washing machines and dishwashers are prime suspects because their automatic valves close fast.

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This current design is an upgrade over old-school “air chambers” found in older homes. Air chambers were essentially arrestors without the piston. Over time, however, they filled up with water, making them ineffective.

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Water moves through your pipes fast. Turn your kitchen faucet on full blast to get a feel for just how fast it moves. When it’s flowing and then is abruptly shut off, the water’s momentum slams into the end of the pipe, causing it to hammer against the hangers and wooden framing it touches.

The pipe size choice may be influenced by other factors: 1) Pipes are typically one size larger on the suction side of a pump than on the discharge; 2) a smaller-than-expected size might be selected to make installation in tight quarters easier; 3) when a very small line, such as ½” or ¾”, is acceptable, the engineer might upsize it to 1” or even 2” to provide strength, so someone who steps on the line won’t break it; and 4) short pipelines will be smaller diameter than long ones with equal pressure drops.

Of course, not every appliance needs a water hammer arrestor. If the appliance cuts off water gradually enough, the change in momentum is minimal. If you’re not suffering from water hammer, then you don’t need to worry about it.

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A water hammer arrestor costs from $15 to $50. They’re available online, at home centers, and in hardware stores. Most of the cost of a water hammer arrestor installation is in the labor.

A rule of thumb that incorporates pipe size is to choose liquid lines to handle a velocity of 1.5 +d/10 where “d” is the pipe diameter, inches. This gives 1.6 m/s for 1-inch and 2.5 m/s for 10-inch piping, and about 20 kPa/100 m pressure drop. Similarly, gas lines can be sized for 20d m/s and steam lines 35 m/s (maximum).

Destry Thompson has been in the plumbing industry for nearly 9 years, and is currently the General Manager of John Stevenson Plumbing.

Clearly, this is an area where a DIY installation can save a great deal of money. But it’s important to do things right. As Aaron Adams points out, “cutting corners to save dollars in plumbing can lead to nightmares for homeowners.”

Personally, when installing an arrestor on older washing machines I like to take a moment to evaluate the shut-off valve itself. If the valve needs to be replaced soon, I’ll upgrade to new valves with integrated arrestors.

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This is the fifth in a series that provides practical insights into on-the-job problems. To read more, visit the series hub at https://www.thechemicalengineer.com/tags/rules-of-thumb

Does your plumbing bang and clang like a truckload of scrap metal? If so, your pipes may be suffering from “water hammer.” The cure for this condition is a water hammer arrestor, an inexpensive device that can often be installed in a single afternoon.

Use the rules of thumb judiciously. Let them give you a first estimate, and use them again to reality check your final selections. Pipe sizing exercises are filled with calculations, but the formulas used in those calculations are themselves based upon empirical data and sometimes fuzzy data. These notions make the whole sizing process an engineering art form, which gives my creative side great joy.

“Generally, there is no ‘upside-down’ for a water hammer arrestor,” says Adams. “Arrestors are effective when installed vertically (in either direction), horizontally or at an angle.” But that’s not true of all models, so always check the manufacturer’s instructions.

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I made these to help me visualise the relationships between flow rate and pipe size. From these charts you can see that the “rules” are not consistent. For example, if you hold velocity constant at 2 m/s, the pressure drop varies from about 10 kPa/100 m for 12-inch pipe to >125 kPa/100 m for 1-inch.

You may want to coordinate a water hammer arrestor install with other work. Destry Thompson, of John Stevenson Plumbing, suggests installing them “in conjunction with a thermal expansion tank for your water heater to relieve excess pressure in the plumbing system.”

Another factor is how frequently the arrestors are used. If your arrestor is on a washing machine supply line, for example, there’s a dramatic difference in wear and tear if you wash a load of laundry every other day compared to once a week.

Given a flow rate and pipe size, these charts give a Reynolds number and velocity. The charts trace any one parameter along with pipe size and yield the remaining flow parameters.

King had one more tip: give some thought to what access you’ll need down the road and install arrestors in areas you’ll be able to access when it’s time for a replacement. And that brings us to our next topic: replacing older arrestors.

I spoke with a trio of licensed plumbers to learn what causes water hammering, why it’s a serious issue, and how water hammer arrestors can solve the problem.

Luckily, water hammer arrestors don’t need to be replaced all that often. “Their lifespan is fairly long considering the forces they encounter,” says Adams. That said, every plumber I spoke to agreed that water hammer arrestors have a limited lifespan.

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