Ways to reduce pump noise in hydraulic circuits
09 October 2017
With the increase in the need for high pressure, high stiffness and quick response systems, the demand for hydraulic equipment started increasing, globally. The hydraulic system usage in aerospace and automobile industries is observed to increase by a CAGR of 5 percent in 2020.
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The ability of the hydraulic system in lifting and actuating heavy loads with an efficiency of 80-90 percent has made it superior to other systems available. The drawbacks of using a hydraulic system include leakage, fire prone, etc. One of the main problem with the hydraulic system, to be discussed, here is noise. Not only does it reduce the performance of a system but prolonged exposure of human ears to the occupational noise exceeding 85dB can lead to hearing impairments.
Sudden rattle and vibration (and noise) caused by pump cavitation/aeration, motor, improper valve installation, clogs in a suction strainer, etc. in the unusual range (>100dB) implies serious malfunctioning of a component in the hydraulic system. In order to attenuate this kind of inconvenience, an optimised system design shall be used to reduce the noise from the system.
The noise in hydraulic systems can be classified as follows:
1. Air-borne noise - the noise is radiated directly to the surrounding from the source
2. Fluid-borne noise - the noise is manipulated due to the flow ripples caused by the pressure fluctuations
3. Structure-borne noise - the noise is due to the transmission of vibration from one component to the other
The major source of noise in a hydraulic circuit is identified to be the power unit, pump, and motor. When the pump attempts to deliver flow exceeding the suction line flow, the pressure in the pump decrease below the vapour pressure of the hydraulic fluid, resulting in cavitation. Cavitation is the formation and breakdown of low-pressure bubbles in the pump flow. When the pressure increases as the low pressure bubbles come out of the pump, these bubbles are compressed and they break, radiating small pressure pulsation. The cavitation in the pump results in damage to the pump interiors, walls of the tubing and in turn, a noisy environment. Aeration is another major problem always confused with the cavitation. Aeration refers to the entrapped or dissolved air in the hydraulic fluid. Aeration and/or cavitation not only produce noise but also makes the operation spongy and slow.
Despite the issues associated with pumps, the global hydraulic market eyeballed a growth rate of 4.57 percent during the period of 2017-2020. The market reports that the pump industry made $9.3 billion in 2016 which is $0.4 billion greater than the sales in 2015. The cost of $1,000 associated with the pump repairs and replacement can be reduced by the Original Equipment Manufacturers (OEM) by proper design and installation of the power unit, valves, and other accessories. In some industries, sound-deadening enclosures are used to isolate the harmful noise range.
The other simple techniques to reduce noise from the pump include:
1. The length of the connectors (tubes) should not be long and sharp bends should be avoided.
2. Proper sizing of the pump must be carried out and larger size pumps can be replaced with the required number of small pumps as it will reduce the noise radiation.
3. Net Positive Suction Head Available (NPSHA) should be increased above Net Suction Positive Head Required (NPSHR) to avoid cavitation.
4. The power unit can be mounted on a rebound surface to reduce vibration and noise.
5. The metal tubes can be connected using small hoses as it does not radiate the noise.
6. More than 50 percent of the pump repair and replacement can be reduced on proper installation and maintenance of the suction strainer at the inlet of the pump.
7. Air entrapped must be removed to avoid noise due to cavitation and aeration.
8. Appropriate measures should be taken to reduce pressure pulsation and flow ripples.
Apart from these methods, scheduled maintenance of the hydraulic system itself will reduce the risk of noise and vibration. Even after all these techniques, if noise exists then the system can be isolated and insulated to dampen the noise.
About the author:
Aruna Rengasamy is a Hydraulics Simulation Engineer in India. Born in Tamil Nadu, India in 1993, Aruna has an M.E., in Aeronautical Engineering from Park College of Engineering and Technology, Coimbatore, India. She is currently working as Project Scientist at National Aerospace Laboratories-CSIR, Bangalore, India for Mechanical Systems Design Group under Structural Technologies Division. Her research areas include Aircraft Hydraulic Systems.