The current OSHA regulations are clear: Ambient sound level in plants must be kept at or below 85 DBA at one meter from the equipment generating the sound. The strategies for implementing the law are however, not as clear.
During the past fifteen years, Croll Reynolds engineers have worked with their counterparts in the field to identify those issues critical to the generation of sound in steam ejectors. In so doing, Croll Reynolds has succeeded in designing a new generation of vacuum systems suitable for those installations where noise is a critical specification.
First, Some Basics: The noise level generated by an ejector is not constant. Its highest level is at start up when suction pressure is closest to atmosphere. At its design operating pressure, an ejector typically generates its lowest sound levels.
Noise Reduction Strategies: There are two sources of noise in an ejector: internal noise from the body wall and noise from the high velocity discharge gases. They must be treated independently.
Sound propagated through the body of an ejector may be dampened with acoustic or, in some cases, thermal insulation. Insulation of the body of an ejector is accomplished either by:
Wrapping insulation around the unit. A typical installation utilizes low-density open-weave fiberglass, with a flexible barrier and aluminum facing. The insulation is applied from rolls, and thickness is added until the desired noise reduction is reached. The final thickness is generally 1″ or 1.5″. Croll Reynolds has tested several alternative materials and is prepared to advise on the most efficient installation.
Designing an acoustic jacket to serve as an integral part of the ejector. The jacket will typically encase the entire unit and should be filled with a sound-insulating material such as mineral wool or sand.
There are several options for addressing sound emanating at the discharge of an ejector:
The installation may be designed to discharge directly into a receiver, usually a vacuum condenser. Thermocompressors, for example, often discharge into shell-and-tube heat exchangers; standard ejector systems may be designed to discharge into inter or after- condensers. Each arrangement typically results in efficient sound reduction.
Alternatively, a separate silencer may be installed at the discharge of an ejector. This is standard operating procedure for hogging or priming ejectors, and may achieve up to a 30 DBA reduction in noise level.
Silencers are characterized as absorptive (packed) or reactive (chamber), or a combination of the two. Absorptive silencers are filled with either fiberglass or stainless steel packing. The use of stainless steel increases capital costs but results in longer life expectancy and better temperature resistance for the equipment.
Absorptive silencers work best at the upper end of the octave band frequencies.
Reactive silencers are designed with a number of consecutive chambers which operate to reduce the velocity of the entering gas or vapor. This results in higher pressure drop than that which is created by an absorptive silencer. This must be taken into consideration at the design phase of the project because ejector systems utilizing reactive silencers will characteristically require a higher motive steam pressure.
Speak with a Croll Reynolds engineer to develop your strategy for noise reduction. We are prepared to assist you with a thorough review and retrofit of your current system including a noise and pressure drop analysis or the design and installation of a new system.