Ashrae Journal - December 2008 - 52

can be traced over this curve with respect to equipment weight. If the equipment or assembly is simple, then shock calculations can be done manually using the procedure mentioned in the BR 3021 standard.2 For complex assemblies, a finite element analysis using commercial software can be done. The results of these calculations and analysis are useful in predicting the failure of components parts, joints, etc. The weaker sections can be discovered and necessary modifications in the design are performed until design requirements are met for proper post-shock functioning of equipment. Reduce shock transmitted to equipment. It may not always be practical or economical to design all items of machinery and equipment to withstand the maximum imposed acceleration due to shock, it is necessary in many cases to protect these items by using suitable shock mounts. Using flexible bellows/rubber expansion joints, and multistage structures also reduces the shock transmitted. Position the mounts so the height of the center of gravity above the plane-of-fixing of shock mounts does not exceed half of the minimum span of mounting. Perform a physical shock test. For a number of similar plants and equipment, a prototype plant and/or equipment is shock tested by a noncontact underwater explosion in its proximity or by a physical shock test on a shock testing machine. The prototype plant and/or equipment are examined for failures, and the necessary design modifications are done until the prototype successfully passes a shock test without any major physical failure. Post-shock, performance of the prototype plant and/ or equipment is checked against acceptable conditions in the performance test. The tested prototype plant and/or equipment is then yellow-banded and never used onboard. Brittle materials like grey cast-iron or any material having an elongation capability of less than 10% should not be used for naval equipment, as they are more prone to failure under shock conditions. However, if the equipment passes the specified shock test and stresses are proved within acceptable limits the brittle material can be used. Structure-Borne Vibration Tips for Reducing Vibration • Design structure by using FEA analysis. • Provide stiffeners; avoid stress concentration (sometimes even by drilling holes) wherever necessary. • Isolate the moving parts, i.e., vibrating structure from non-vibrating structure. • Provide adequate support to vibrating equipment. • For vibrations isolators, do not use central bolt of longer distance than the height of mount to avoid direct vibration transmission through central bolt. • Tighten the hold-down bolt, and use spring washers at appropriate places. • Provide support for pipes cables using special clamps with internal rubber lining to avoid damage due to friction. • Use flexible coupling. • Keep required tension in the belts. • Do quality welding and brazing. • Do proper lubrication of moving parts. • Use rubber paints and standard painting procedure. • Follow a condition-based monitoring program. • Undertake active vibration isolation. Typically for naval applications, the structure-borne vibrations are measured as per MIL-STD-740-2.3 This standard provides scope, purpose, application, implementation, approach, measurement procedure, and acceptance criteria for different types of naval equipment mounted in different methods such as resiliently/solid, etc. In some cases, the mechanical vibration requirements for plants and equipment are to be in accordance with MIL-STD-167-1.4 Type I is associated with environmental vibration, and Type II is associated with internally excited vibration. All equipment must withstand, without any reduction in reliability and performance, the effects of environmental vibration as defined by MIL-STD-167, Type I. The dynamic stiffness of mounts must not exceed the levels specified by a navy, and above-mount, narrow-band vibrations levels must be below the one-third octave levels specified by a navy. The analysis of vibration data reveals the source of vibration, its transmission path, and probable occurrence of failure. The vibrations can be minimized from these results in following ways (also see sidebar “Tips for Reducing Vibration.”): Minimizing vibrations produced by source. In refrigeration plants and chiller packages, the only moving parts are the compressor and motor, pump and pump motors, fan, and air-cooling unit motor. The reduction in speed of rotation minimizes the vibrations. Use of semi-hermetic compressor, flexible coupling, and mono-block pump reduces the vibrations. Properly lubricating the moving parts and operating the plant close to design conditions reduces the vibrations. Minimizing vibrations in its transmission path. If equipment is rigidly mounted/connected, then vibrations pass easily through them. Using multistage structure, shock mounts, and flexible piping rubber expansion joints mitigates the ashrae.org December 2008 Vibration refers to mechanical oscillations about an equilibrium point. The medium in which sound (vibration) exists often is described by airborne, waterborne, and structure-borne. Vibrations are undesirably produced due to unbalanced forces in reciprocating and rotating machinery such as the compressor used in the refrigeration plant and chiller packages. The unbalanced forces are generated due to imperfect design, manufacturing, assembly, installation, operation and maintenance. The vibration causes failure of components, parts, or assemblies. The vibrations may also be detected by an enemy. A navy will specify the acceptable limits for structure-borne vibrations. The refrigeration plant and chiller package designed must conform to these requirements. For any vibrating structure, when vibrating frequency matches with its own natural frequency, resonance occurs. At resonance, the amplitude of vibration increases, and if it crosses the tolerable limits, the structure fails. 52 ASHRAE Journal

Ashrae Journal - December 2008

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