Understanding Hydraulic Shock and Water Hammer

Understanding Hydraulic Shock and Water Hammer

In building water distribution piping, we often heard of the terms “Water hammer” and “Hydraulic Shock”, however do we really understand the principles behind these occurrences. In this blog we will dive deeper into the reasons behind these occurrences in our water distribution piping systems. We will also explore its effects in our piping systems.

Most of us, even those who are not practicing the plumbing profession, may have likely witnessed these scenarios within the piping systems in our homes or offices. In my apartment when I was still boarding during college days, I often heard the exposed piping installation create a rattling sound. This occurs around 3-5 seconds. The noise it creates was really a nuisance and I was wondering then why it occurs.

Hydraulic Shock VS Water Hammer

Hydraulic shock is often erroneously referred to as water hammer, however, these two terminologies are not synonymous. A water hammer is one of the most common manifestations of the effects of hydraulic shock. A flowing water has an inertia proportional to its weight and velocity. A hydraulic shock occurs when there is a sudden change in velocity such as when we turn-off a quarter-turn ball valve and faucets quickly as well as sudden cut-off in flow in washing machine solenoid valves. This rapid stopping of fluid momentum converts the kinetic energy of water into a dynamic pressure wave, which could travel up to 5,000 km./hr which are more prevalent in longer pipe lengths and higher velocities.

Imagine a train running at high velocity with its heavy mass. When a train is suddenly blocked or derailed, we could see accidents wherein the cabins at the rear will have no where to go to release its inertia than to either compress the front cabin or offshoots and fly anywhere into the direction of travel. The same scenario is happening within our piping system except that water has nowhere to offshoot to but to bounce back not as a volumetric flow but as a pressure wave.

Why does it become a pressure wave and not just a pressure spike? This is because fluids and pipes are not perfectly rigid. Water is slightly compressible and steel and plastic pipes are slightly elastic. So instead of a single instant pressure spike, what you get is a traveling pressure disturbance — a pressure wave.

Wave Propagation (Water Hammer Effect)

Right after a valve closure, the fluid at the valve compresses which results in pressure within the pipe to rise sharply. This compression pushes the upstream fluid forming a pressure front. That front travels backward through the pipe at the wave speed. This pressure wave travels back and forth within the piping system until energy dissipates. When the piping is not properly secured or supported or when the pipe supports or fasteners are widely spaced, this rebounding wave causes the pipe to vibrate and hit against the surrounding structure like a concrete wall or metal structure or even in the brackets or hangers if these are not properly tightened and there is a room for the pipe to wiggle. This noise is called a water hammer.

Though water hammer causes user disturbance especially amplified during sleeping hours, this noise is not as dangerous as the hydraulic shock that the system is experiencing during these scenarios. The hydraulic shock can weaken joints resulting in leaks, vibrate hangers which would cause hanger nuts to tear loose, rupture diaphragm tanks or pressure tanks, rupture heaters, damage meters, gauges, and regulators which would eventually cause the early deterioration of the piping system. One of the most common break points in the point of use is at the flexible supply hoses as these hoses are just flexible tubing inside a metal clad and are not usually rated to resist extreme pressure caused by the pressure waves.

Other common causes of hydraulic shock are at the booster pumps during the starting and stopping of pumps, especially in single speed pump systems. Incorrect check valve specifications at booster pumps also trigger hydraulic shocks. Swing check valves installed immediately after the booster pump cause rapid closure when the pump stops which triggers pressure surges. A Spring-Check valve or a silent check valve is the most appropriate non-return valve to be used immediately after the pump. These valves are engineered to minimize system noise and hydraulic shock by controlling how quickly the valve closes during downstream pump shut-off.

Figure 1 - Example of a Silent Check Valve

Figure 2 - Brass Threaded Spring Check Valves

The Spring/Silent check valve starts closing as soon as forward velocity drops. It uses spring force to track deceleration which results in near-zero velocity at the instant seating. The swing check valve relies only on gravity and flow inertia. After the pump shut off, the disc in the swing check valve is still open due to inertia and it needs to wait until the reversal of flow, then the disc slams shut rapidly which generates high-frequency pressure waves.

Figure 3 - Different check valves designs with its slamming tendencies.

Hydraulic Shock Mitigation

One way to minimize hydraulic shock is to properly size the piping system. The size of the pipe directly affects the velocity of water within the pipe. This is the reason why one of the important engineering factors in pipe sizing is velocity limitation. When velocity within the piping system reaches 10 fps, the likelihood of hydraulic shock dramatically increases which results in the piping problems mentioned above.

When sizing the piping system, the following are the recommended maximum velocity for the common pipe materials:

SUGGESTED MAXIMUM PIPE VELOCITIES (fps)
Steel or Cast Iron	4-8
Copper (Hot Water)	5
Copper (Cold Water)	5-8
PVC	4-6
PPR (Hot Water)*	5
PPR (Cold Water)*	8

Source: ASPE 2024 Study Guide; * Added by author for Philippine setting.

Air Chamber VS Water Hammer Arrestors

Increasing the piping system's compressibility is another method for minimizing and countering hydraulic shock. This old but still a prevalent method in the Philippines is the use of the Air Chamber. An air chamber is basically a simple piping assembly which entraps air by providing a raised chamber above the water outlet. Unlike water, air is highly compressible and elastic. The air serves as a cushion and compresses during sudden pressure spikes. The problem with air chambers is that air dissolves into water over time, then it becomes waterlogged. When this happens, the piping loses its surge protection.

Figure 4 - Air Chambers: Plain Air Chamber (a,b); Standpipe Air Chamber (c) ; Rechargeable Air Chamber (c)

In contrast, Water Hammer Arrestors (WHAs) maintain stable performance over many years because the gas—typically nitrogen—is sealed and isolated by a piston. This pre-charged gas behaves consistently, providing reliable and repeatable damping characteristics. Furthermore, WHAs offer a superior damping mechanism due to the friction of the piston against the cylinder, which prevents the oscillatory behavior (pressure bouncing) that can occur with a simple air chamber containing regular air.

Figure 5 - Parts of a Water Hammer Arrestor

Figure 6 - Different designs and sizes of Water Hammer Arrestors

The use of air chambers to control water hammer is no longer considered an approved method. This is because updated codes, such as the latest International Plumbing Code (IPC), mandate the use of listed and performance-rated devices for hammer control. Since air chambers are neither listed nor performance-rated, they are effectively excluded under these new requirements.

To summarize, hydraulic shock presents a genuine risk, potentially causing damage and early failure in water piping systems. Consequently, registered master plumbers must prioritize mitigating this hazard. Key strategies include correct pipe sizing, incorporating suitable non-slam check valves upstream of booster pump installations, and utilizing a listed water hammer arrestor, particularly near quick-closing fixtures such as solenoid valves and fast-closing motorized valves.

Author:

RMP James Ray L. Magdadaro, ASPE

Managing Partner - MEP Global Co.

Principal Architect - JLM Architects