Apr-2014

Calculating and mitigating the effects of water hammer in a liquid sampling system

In a liquid analytical sampling system, a common problem has the potential to destroy pressure gauges, flowmeters, pumps, and other sensitive components.

Dean Slejko
Swagelok Company

Article Summary

It’s known as water hammer, and it occurs when a valve abruptly closes and liquid flow is suddenly stopped. Water hammer strikes as a sudden pressure pulse that is in addition to the working pressure in a system. This pulse of additional pressure can be enough to damage every pressure gauge or flowmeter upstream of the closed valve. It can even burst lines or end connections if the pulse is too great.

Yet operators often don’t realise what caused the damage. They know the maximum process pressure won’t exceed the system’s design. So they have trouble understanding where the extra pressure originates.

Let’s take a closer look at why the phenomenon of water hammer occurs and how to estimate its effect in your sampling system. In addition, we’ll review a few methods for mitigating the water hammer effect, including using needle valves for shutoff, diverting flow instead of shutting it down, and using pressure snubbers to protect gauges.

Estimating Water Hammer
When the operation of a valve suddenly interrupts the flow of a liquid, something has to absorb the momentum of that liquid and bring it to a standstill. The liquid is heavy, and it’s travelling fast. Therefore, the force required to decelerate the liquid creates a rapid increase in pressure. This pressure surge originates in the compression of the liquid and from deformation of the tube walls. The resulting pressure wave travels upstream in the tube and, unfortunately, the pressure rises too quickly for a typical proportional relief valve to open and release it. Without a means of escape, the pressure spike can be severe enough to damage components anywhere in the line.

A variety of mitigation methods can help to avoid the sudden pressure spikes of water hammer. We’ll cover those methods in a moment. But first, let’s review how to determine if water hammer may impact your sampling system. You’ll need to know a few specific details, including the density (ρ) and velocity (u) of your sample fluid. These parameters can be entered into an equation known as Joukowski’s Law, which estimates the magnitude of the water hammer pressure pulse (∆P) experienced in a line when the liquid velocity (u) suddenly changes by ∆u. The equation for Joukowski’s Law reads:

ΔP = c · ρρ · Δu

The parameter c is the speed of sound in the liquid under actual operating conditions. For example, the speed of sound in water at 20°C is about 1400 m/s. Therefore, when the operation of a valve suddenly stops a water flow with a velocity of 1 m/s, the calculated pressure pulse is:

ΔP = 1400 m/s × 1000 kg/m3 × 1 m/s
ΔP = 1400 kPa

That’s 14 bar of additional pressure that you have likely not planned for in your sampling system. This extra pressure spike may not be tolerable for this particular system. And the pressure pulse will be even higher if the liquid sampling system flows at a higher velocity than 1 m/s.
In practice, the pulse pressure may not be as high as that calculated by the Joukowski’s Law equation. The full force of the pulse develops only when the change in velocity is sudden, specifically when ∆u occurs within a given time period (t), as determined by the following equation (in which L is the length of the liquid line):

t ≥    2L
          c

If the water line in the example above is 100 m long, then:

t ≥   200
       1400    s

t ≥ 142 ms

The time period (t) is the time the pressure wave takes to spread along the upstream line and reflect back to the valve. The full pressure pulse develops only when the reflected pressure wave returns to find a closed valve. The above result indicates that the pulse will be less than the calculated pressure if the valve takes longer than 142 ms to shut off. In other words, you can minimise the water hammer pulse by closing the valve in more than 142 ms.

Mitigating Water Hammer
In domestic water systems, it’s common to mitigate water hammer by installing vertical pipes containing trapped air in water lines near each faucet. The air in the capped pipes absorbs the water hammer pulses. This design could work in sampling systems, too, but it would create an unacceptable dead leg that could lead to sample contamination. Instead, you’ll need to use other remedies to avoid water hammer. First and foremost, don’t use ball valves to shut off the flow in a liquid sampling system. Ball valves shut off flow instantaneously, which can cause large pressure surges in the sample transport and conditioning systems.

Instead, you may opt to manage flow via one of two methods that help to mitigate water hammer issues, including:
1. Using multi-turn needle valves with tight shutoff seats to stop the flow, as shown in the fast-loop module in Figure 1. The needle valves isolate and bypass the fast-loop flow. They close the flow slowly, which dissipates the momentum over a longer duration and minimises the water hammer pulse. Rising plug valves may also be effective.
2. Diverting the flow instead of stopping it. In Figure 2, a common handle operates two bypass valves, so they can’t move separately. The three-way ball valves have special porting that opens a bypass flow path before closing the main flow path. The valves are never off, so water hammer never occurs.

As noted earlier, pressure spikes due to water hammer are too fast for a pressure relief valve to open and relieve the excess pressure. The rupture disc in the pressure relief valve might burst, but it will do so too late to avert severe damage to an upstream flowmeter or pressure sensor. Those components will receive the full force of the water hammer spike before the relief valve opens.

Snubbing Pressure Pulses
Even by following the precautions noted above, valve operation in a liquid sampling system is likely to cause water hammer pressure pulses that may be large enough to damage pressure gauges. Your best defence is to use pressure snubbers to protect all gauges potentially exposed to the pressure pulses. A pressure snubber slows the response of a pressure gauge so it doesn’t respond to the transient pressure pulse in the line.

Protecting Your System
In summary, suddenly stopping flow in a sampling system can create a pressure surge that’s large enough to damage multiple components in the line. Known as water hammer, this surge is a system performance and maintenance issue, and could even be a safety issue if toxic system media were to escape due to a burst line or component. To mitigate water hammer, use needle valves to shut off sample lines, or divert the flow rather than shutting it down. In addition, consider pressure snubbers to protect pressure gauges in your system. These practices will help to minimise the effects of water hammer, reducing maintenance costs and system downtime, as well as enhancing safety.