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The Layman’s Guide To Acoustical Analysis – Pt2 Noise

Let’s start with a recap from my last post.

Acoustical analysis is the study, and subsequent control and abatement, of disruptive or destructive noise in a closed fluid system.

In the previous post, I discussed what Acoustical Analysis is and gave some of the why for the above statement.

In part 2, I want to dig into the big question that nearly everyone has:

Where does the noise come from, and why should I care?

I can remember sitting at my favorite coffee shop getting ready to start writing another blog post and then wondering why the guy next to me is yelling into his phone or why the person a few tables over is watching cat videos without headphones.  Both are equally annoying, but calling them destructive seems like an overreaction to a scenario that I can easily remedy by leaving or putting on my own headphones.

Another trip to Wikipedia will help to clarify this:

Noise = unwanted sound judged to be unpleasant, loud, or disruptive to hearing.

From a physics standpoint, noise is indistinguishable from sound.  Both are pressure waves traveling through a medium.  For most people, the medium they think of is the air around us.  These pressure waves (sounds) are traveling from a source, such as a loud speak, thru the medium then impacting on something that can detect them like our ears.  The medium could also be a solid or liquid flowing through a piping system.

Longitudinal wave direction

Again from a physics standpoint sound is neither good nor bad.  The transition from pressure waves being noticed simply as sounds to something defined as noise is context dependent.  One person’s Mozart is another person’s nails on a chalk board.

But we still have not gotten to where the sound comes from, or why it can be destructive in fluid systems.

Fluid Systems

The image above shows a very simplistic pump.  During the suction stroke fluid is pushed into the cylinder thru the inlet valve.  On the discharge stroke the same fluid is pushed through the outlet valve.  As the piston in the pump travels back and forth it acts like the cone in a loud speaker, creating waves within the fluid that travel in the direction of the motion.

WAVE DIRECTION IMAGE

One of the questions I’m frequently asked is if the piston, or “loudspeaker cone”, travels at the speed of sound to do this?  The answer is no.  The motion of the piston or cone will impart the energy into the medium sending it off at the speed of sound for that medium, but the driver is not required to travel that fast to create the wave.

Now that you understand where the sound comes from, let’s cover the destructive part.

As sound travels along fluid piping it will be bouncing around against different features that cause direction or size changes such as bends, elbows and tees along with all the other components that make up the full system.  Every time the sound wave hits or passes through a change, it will impose a force on the structure.

An easy way to illustrate this is to look at an elbow.  When the sound wave (U) hits the elbow and is reflected/deflected it will create a resultant force that can be broken into a horizontal and vertical component.

Sound Wave Elbow

The force applied is proportional to the pressure variation that the sound wave creates.  In the world of pulsation control, this is called the peak-to-peak (PK-to-PK) which represents the high and low of the signal.  The larger the PK-to-PK, the larger the force pushing against the pipe.

A small value of pressure variation would cause a small inconsequential force.  However, as the pressure variation increases, so does the force until it eventually becomes destructive.  This brings us back to the question of when does sound become destructive noise.  All systems have sound in them, but levels high enough to qualify as noise can impart destructive forces on the piping and effect other components causing them to no longer function properly.  An example of this is a relief valve that chatters due to hammering from the pulsation.  The valve rapidly opens and closes as the waves pass.

One source for help with defining the limits in a system is the American Petroleum Institute (API).  They have produced numerous standards for many different types of equipment to give guidance on acceptable pulsation levels and remediation suggestions.  PPC Engineering also has plenty of real world experience to define when a system is in danger and how to protect it.

Now that you understand what acoustical analysis is, where the sound comes from, and why it needs to be abated, I will go more in-depth with specific issues and ways to deal with them in subsequent postings.

James D. Barlow, P.E.
Principal Engineer

 

 

 

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