Vibration isolation

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Vibration isolation and seismic restraint for mechanical equipment in Chile

Conference Paper · September 2016

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Structural Acoustics and Vibration (others): Paper ICA2016-78

Vibration isolation and seismic restraint for mechanical equipment in Chile

Nicolás A. Bastián-Monarca(a), Ian Azrak(b)

(a) Silentium, Chile, nbastian@silentium.cl

(b) Mason Industries, United States, iazrak@mason-ind.com

Abstract

Vibration control of mechanical equipment in seismic countries is a subject that should be addressed with some caution. One of the most utilized elements in controlling vibration is a spring isolator, one that has high deflection (relative to neoprene isolators). The problem is that the resonant frequency can coincide with the disturbing frequency of an earthquake, which means that in an earthquake the spring will begin to ‘jump’, causing the displacement of the mechanical equipment. The displacement of the equipment, beyond generating high costs (damage, loss of use, flooding, etc.), also creates the risk of personal injury and even death. Due to the aforementioned risks, it is necessary to consider the seismic ‘variable’ when doing a proper vibration control design in a seismic country or region. Criteria, designs and recommendations are presented in this report in order to perform proper vibration isolation in conjunction with seismic restraint for mechanical equipment and systems in Chile. In addition, you will be shown some failures from the M8.8 earthquake on February 27, 2010 in Chile.

Keywords: Vibration isolation, seismic restraint

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1. Introduction

The control of vibration transmission has received much attention in recent years due to its effect on the functionality of systems involved [1] and health. Vibration transmission can cause instability or even failure, as in the case in buildings subject to earthquakes [2]. Thus, vibration isolation is a vital requirement throughout much of engineering [3], particularly when there is a strong source of vibration such as a motor [4]. In this sense, technical or mechanical floors in any building project are a strong source of vibration due to the high density of equipment. In these scenarios, the direct radiation of a vibrating machine is usually not as serious as the radiation from the floor structure as a result of the structure-borne sound transmission from the machine to the floor structure [5]. In the past decade the use of active or semi-active vibration isolators had been a new trend for the designs of anti-vibration systems [6-11]. However, the advantage of the passive anti-vibration method are that they are stable and easy to set up [12]. Moreover, passive vibration isolators are the most simple, inexpensive and reliable means of protecting sensitive equipment from environmental shock and vibration [13]. Therefore, vibration isolation using passive isolators is very cost effective and hence is widely used in the engineering industry [14]. Thus, the passive isolation and seismic restraint of a structure from mechanical equipment is studied in this paper.

The use of vibration isolators (simply isolators from this point forward) in order to isolate mechanical equipment is a typical solution that engineers or acousticians always recommend. In the large range of isolators, neoprene and spring isolators are the most utilized, the reason being the static deflections that they offer. Spring isolators normally are offered between one inch and five inches of static deflection, which neoprene isolators are normally offered in a range between 0.15 inches and 0.55 inches [15]. Based on that, it is clear that when high deflection is required, springs be offered as the best solution. The problem with spring isolators is that they have a natural frequency that can potentially coincide with the frequency of an earthquake, which means that during an earthquake, the spring isolator can enter into resonance and being to jump violently. If the spring isolator does not have a seismic housing, the excitation of the spring will cause the equipment that it is supporting to being to move and fall over. If the machine is a fan or other airside type of HVAC equipment the danger is that it could fall over and pull down the ductwork, but if the equipment is wetside HVAC equipment it could fall over and bring down the piping with it, causing flooding within the building. Furthermore, if the equipment is located outdoors or on a roof, the possibility exists that the equipment could fall off the building.

In this work, the general concepts of the control of vibrations are presented along with a practical method of controlling vibrations of mechanical equipment and the design requirements per ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) [16] in order to achieve an effective vibration control system for particular pieces of equipment. Furthermore, the Chilean Seismic Building Code, NCh-3357/2015 [17] will be described in detail with regard to non-structural components, which applies to all types of mechanical equipment in buildings. After that, some real life applications in certain projects will be shown and the conclusions drawn.

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1. Vibration isolation

1. Overview

Vibration isolation is commonly adopted by engineers to reduce the vibratory effect caused by machines in buildings [18]. The mechanical vibration process can be subdivided into four main stages, generation-transmission-propagation-radiation. The transmission stage is often the best compromise for noise and vibration control activities in view of cost and practicability [19]. Under that premise, the most effective way to eliminate vibrations between the equipment and supporting structure is a vibration isolator [20].

1. Practical approach

The design of a complete vibration isolation system can be divided up into [21]:

1. The selection of the required transmissibility or static deflection for the vibration isolators.
2. Selection of the appropriate form of mounting.
3. Selection of the location for the vibration isolators and determination of the loads of each one.
4. The selection of suitable isolators to correspond with the previous three points.
5. The treatment of the various service connections to ensure that these do not cancel out the effect of the vibration isolators.

The static deflection needed to isolate a particular piece of equipment can be obtained by referring to ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) [16]. In general a higher deflection is required to isolate lower equipment motor frequencies [22].

There are normally three possible ways of mounting equipment. The first is to attach vibration isolators directly to the existing mounting feet of the equipment. The second method is to mount the equipment on a steel frame and to attach the vibration isolators to this, while the third possibility is to mount the equipment on a concrete ‘inertia’ base [21]. In figure 1, the three cases are shown.[pic 13]

Figure 1: Left: Chiller mounted directly on isolators. Center: Fan mounted on a metal base and isolators. Right: Pump mounted on concrete inertia base with isolators.

The manufacturer of the equipment generally gives the location of the isolators since the equipment usually has mounting hole locations (typically in the corners) where the equipment can be anchored [20].

For equipment that is installed on steel bases or concrete inertia bases where the base is too long with respect to the width, one ought to install intermediate isolators so that that base itself does not have excessive bending or collapse upon its length. This means that upon selecting isolators, the intermediate supports will typically have to support a greater load than that of the corner isolators. In Figure 2, an example of this case is shown.[pic 14]

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