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The Sound Beneath the Floors

Studies have shown that the complete absence of sound can drive a person insane, causing them to experience hallucinations. Likewise, loud and overwhelming sound can have the same effect. This especially holds true in manufacturing and plant environments where loud noises are the norm.

This article originally appeared in IMPO's May issue

Studies have shown that the complete absence of sound can drive a person insane, causing them to experience hallucinations. Likewise, loud and overwhelming sound can have the same effect. This especially holds true in manufacturing and plant environments where loud noises are the norm.

A NJ-based food processing plant began experiencing this issue firsthand after building a new manufacturing space. Some of the plant’s equipment is powered by a series of hydraulic systems in a central pump room that contains a number of dual-stage hydraulic pumps. These 3,000-5,000 psi pumps push fluid through steel pipes that run under a conference room and up the side of a wall next to offices before moving out into the plant.

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Unfortunately, out of sight out of mind isn’t always the case with hydraulic machinery. The pipes connected to the pump were hard-clamped to the building structure to prevent them from moving around. This setup created a severe noise issue as the hydraulic pump went on stroke and pushed hydraulic fluid through the pipes, rendering the office space and a conference room almost unusable.

The unbearable and disorienting noise wasn’t coming from the pump itself. The pulsations, produced by the pump, created vibrations that radiated noise through the pipes and off of machine surfaces.

Norm Dotti, Consulting Acoustical Engineer, Russell Acoustics, diagnosed the source of the noise problem.

“The pipes were singing as fluid was pumped through them,” Dotti said. “They would shake the steel walls and cause them to radiate sound. The entire building structure around the office space was essentially acting as a loudspeaker to convey that sound.”

A simple house fan can be used to provide a layman’s view of the pump sound effect. If you take the number of blades on a fan (or the number of pistons in a pump) times its rotational speed that fan (or pump) is going to put out a tone at the rotational speed called blade passage frequency. A helicopter does the same thing. Take the number of blades times the speed at which the rotor turns and you get a component sound or vibration at that frequency.   

In our food processing plant scenario, even detaching the piping from the structure wouldn’t completely resolve the issue. The force and sound was coming from pulsations in the hydraulic fluid running down the pipeline. These pressure pulsations flowing through the lines are difficult to absorb since hydraulic fluid is hard to compress. Dotti determined that the best, most efficient solution would be to utilize a muffler.

The muffler needed for this application is very different from the commonly known car muffler used in exhaust systems. Rather than stifling the force of airborne sound, the muffler needed to reduce the pressure pulsations in the hydraulic fluid. Sound can travel through air, water or even hydraulic fluid, but properties such as viscosity and speed at which sound travels varies for each medium.

The major difference is that air is a compressible medium, while hydraulic fluid isn’t nearly as compressible. That makes it a challenge to absorb the pressure pulsations flowing through it. A specialized hydraulic muffler is needed to reduce the effects of hydraulic force. 

“A fundamental thing in engineering is energy conservation,” Dotti said. “If you don’t do something with the released energy, it’s just going to go someplace else. Hydraulic mufflers have cavities that hydraulic fluid flows in and out of to get viscous flow. That action sucks up energy without impeding flow. It takes the energy that’s in the form of pressure pulsations out of the system.”

A Parker Hannifin Inline Pulse-Tone hydraulic shock suppressor was installed on the discharge/outlet side of the food processing plant’s hydraulic pump to stop pulsations and resulting noise before it traveled through the piping and radiated off of other structural components. The conference room experienced a significant 20 dBA noise reduction, lowering the noise “loudness” by four times.  

As shown in figure 1, hydraulic fluid flowing into the Pulse-Tone goes through three different baffles or diffusers. These metal baffles are designed to convert 1/2” diameter holes to 1/32” diameter holes. The total radial distance through these baffles is only 1/4”.

After passing through the holes, pulsations strike a nitrogen-charged rubber bladder. The bladder deflects each time its hit by a pulsation. This slight bladder deflection reduces the shock and noise by absorbing the pressure pulse.

Overall, the large area of the bladder, with its ability to oscillate at a high frequency, and the short distance each pulsation has to travel once it enters the unit, are key performance features allowing the Pulse-Tone to reduce sound and shock in factory environments.

This sound phenomenon helps explain why many pump manufacturers list a very low dB rating, but when the pump is installed on a power unit, the resulting sound level is much higher. In fact, it’s almost impossible to forecast how much additional sound the hydraulic lines and surrounding structure will radiate.

A hydraulic shock suppressor reduces pump pulsations and shock. Decreasing these pulsations and vibrations not only reduces noise levels, but helps to prevent primary causes of component wear and leakage. This allows power units with an Inline Pulse-Tone to operate at higher RPMs with smaller, less expensive components and, most importantly for the food processing plant, less noise.

For more information, contact Parker Hannifin’s Accumulator & Cooler Division at 815-636-4100.