When vibrations (in the form of longitudinal pressure waves) in the sonic or acoustic range reach the ear — whether transmitted through air, liquid, or solid — the person receiving those vibration hears sound. If the sound is unpleasant — because it lacks agreeable musical quality or is too loud — it becomes noise. Excessive noise can be mentally and physically dangerous for workers. For both mandated and marketing reasons, much work has been done on reducing noise in the workplace over the past decade. Sound-producing components, such as pumps, are tested for noise generation and their ratings are published as part of their basic specification.
Yet, if you walk through any large automotive plant, you'll probably pass some hydraulic power units that have sound-deadening enclosures built around them. The presence of a sound enclosure often indicates that the power unit did not operate at the sound rating shown in the manufacturer's catalog for the pump. In other words, the catalog sound rating for the pump is usually much lower than the sound rating for the completed hydraulic power unit. Something happens during assembly of the power unit to metamorphose a relatively quiet pump into an unacceptably noisy power unit.
It's generally understood that pump pressure and pump size have about an equal effect on the hydraulic noise that a pump will generate. In addition, pump speed has about 300% greater affect on noise than either pressure or size. (This is the reason some pump manufacturers recommend lower-speed electric motors.) And fixed-displacement pumps are usually quieter than variable-displacement models. But all of these factors are noted in the pump catalog. Something else — not mentioned in the catalog — is contributing to the increase in noise level.
The problem is that any slight noise generated by the pump is amplified throughout the structure of the hydraulic equipment. Lab tests show that pump noise levels are increased by 2 to 3 dBA just by adding 12 ft of outlet and return lines. These lines do not generate noise - instead they radiate noise when they respond to pulsations or vibrations generated by the pump. Long hydraulic lines, whether pipe, tube, or hose, also frequently provide the primary path for propagating pulsations from the pump to components — such as the large flat metal surfaces the typically are part of hydraulic reservoirs — that react to them and radiate even higher noise levels. These phenomena help to explain why many pumps have a low noise rating, but when they are installed on a power unit, the assembly's noise rating is much higher.
Taking control of noise
In any hydraulic system, the pump is the main source of pulsations and vibrations. While pump manufacturers have made noise reduction a design goal, every pump still produces some ripple - the pump manufacturers' term for pulsations. Ripple produces the line vibrations which cause additional noise. System designers cannot change how much ripple the pump produces, so they must find ways to control the propagation of that ripple out through the rest of the system.
One of the first areas that should be reviewed when attempting to reduce power-unit noise is the hydraulic conductors. Somewhat surprisingly, one factor that can contribute much to the noise level is improper use of hydraulic hose. Recent research at a large pump manufacturer showed that they could take an average of 5 dB(A) out of a standard power unit merely by changing the configuration of the hydraulic hose. Frequently, a 90° curved hose is used when a horizontal line has to be connected to a vertical line, and 180° hose curves also are quite common. Experiments show that both of these configurations actually increase system noise level. The solution: don't bend hydraulic hose; instead, substitute bent metal tubing. Only use hose in a relatively straight line.
It is well known that introducing a compressible medium such as nitrogen into the relatively incompressible medium of hydraulic fluid will help reduce pulsations. The challenge is to get the fluid to interact with the nitrogen so the nitrogen compresses and the fluid merely loses its pulsation.
Over the years, nitrogen-charged accumulators have been installed in many hydraulic circuits to absorb pulsations. At first, accumulators were used as appendage devices — teed off the hydraulic line. The designer hoped that the pulsations would wander into the accumulator. However, experience showed that the majority of the pulsations bypassed the line leading to the accumulator. Different designs then evolved in which the full flow was diverted into the accumulator. Correctly sizing this type of accumulator is complicated and the circuit that directs flow into the accumulator is very expensive. Also, pressure drop through these accumulators may be unacceptably high.
Another method of using compliant nitrogen to deal with noise-causing pulsations is to mount an in-line nitrogen-charged noise suppressor right at the outlet of the pump. (This suppressor is described in detail in the box at right.) This design is more efficient than a large conventional accumulator because the fluid flow-path to the bladder is short and unrestricted, and the fluid contacts a much greater bladder area.
To reduce noise in a power unit:
* select an appropriate pump with a low noise-level rating
* use resilient mounts when possible
* organize the fluid conductors to minimize pulsation transmission; avoid long, unsupported conductor runs
* stiffen any large, flat metal areas, and
* install suppressors with compliant nitrogen-charged bladders if the power unit still does not meet the noise specification.
In-line noise suppressor
The suppressor consists of three concentric, cylindrical metal noise baffles or diffusers inside a nitrogen-charged tubular rubber bladder. The inner baffle has ½-in. diameter holes cut into it; the second layer is a coil spring that helps support the thin outer baffle; and the outer baffle is perforated by more than 4,000 1/32-in. diameter holes. (These holes are small enough that the surrounding bladder cannot extrude into them.)
Pulsations enter the suppressor and then pass through the three baffles — a total radial distance of only ¼ in. — and strike the bladder,which typically is charged at 50 to 60% of the hydraulic-system operating pressure. The bladder deflects each time it is hit by a pulsation, and this slight deflection absorbs and reduces noise — and as a bonus, any shock waves. The bladder's large area, its ability to oscillate at high frequency, and short travel distance combine to absorb pulsations with frequencies above 600 Hz.
The size of the suppressor is determined simply by the size of the hydraulic line in which it will be installed. Models are available for pipe and tube sizes from 3/8 to 3 in., with NPT pipe, SAE tube, and split-flange port connections. Sizing older-style, accumulator-type hydraulic pulsation dampeners was a long and complicated process. With this design, the size of the line becomes the size of the suppressor.
The remaining decision concerns pressure rating. Models for 3000- or 5000-psi systems are available. (A stainless-steel option converts the 3000-psi unit for water use.)
One pump manufacturer built 60 double-pump power units for an automotive plant. The completed power units registered a noise level of 82 dBA, but the plant's noise specification was 80 dBA. The manufacturer decided that the least expensive solution was to install in-line noise suppressors directly at the outlet of the pumps. The suppressors brought the noise level down to 78 dBA), and the cost was considerably less than building noise enclosures around the power units.