vibrationvibration isolators specification

ELCEN Vibro-matic Co. has been a designer and manufacturer of vibration control products as per ASHRAE Guidelines. The isolators illustrated in this catalogue include a wide range of spring and rubber mount products for the control of vibration, shock and noise in the HVAC, Marine and industrial fields. In addition to the vibration mounts presented in this publication, our factory engineers are always available to assist customers in selecting and designing isolation systems for special projects, including seismic applications.

PLANNED ISOLATION

The isolation of machinery to prevent the transmission of vibration and noise has become one of the important phases of modern building engineering. Light weight construction and locating mechanical equipment on upper floors, adjacent to quiet areas, increases the requirement for vibration control. The use of isolation is primarily for reducing the effect of the dynamic forces generated by moving parts in a machine into the surrounding structure.

Hence, a proper understanding of the reasons for production and then the propagation of vibration and shock, involves technical and theoretical knowledge, which is crucial for reducing the effect of the dynamic forces generated in the moving components of equipment’s. Complaints related to vibrations in building emanate from following;

i) Noise level experienced by the residents, within the building exceeds the acceptable levels of the local set standards. ii) Vibration from mechanical equipment’s movements transmitted to the building structure. iii) Effect of vibration on the function of sensitive equipment’s and instruments.

Based on these criteria ELCEN Vibro-Matic Company has a wide range of products, systems and solutions to mitigate vibration and sound propagated by the motion of mechanical equipment’s. A broad selection chart for various types of mounts relevant to the equipment location and area of operation is provided in Figure 1.

FIGURE -1 ISOLATION THEORY

Every machine by its very action of operation creates a vibration or shock of varying intensity or amplitude. The requirements for isolating this vibration depend upon the local conditions of installation. Three principle factors control the selection of an isolator for a particular machine. The first is the weight to be supported, the second is the disturbing frequency of the machine and the third is the rigidity of the structure supporting the machine.

Vibration is a force and establishing an opposed force can effectively reduce its transmission. This is accomplished by incorporating a truly resilient material, which when subjected to a static load, deflects and by so doing establishes the natural frequency of the isolation system. When the natural frequency of the isolation system is lower than the operating or disturbing frequency of the supported machine, each cycle of vibratory force finds the resilient material in the returning phase of its cycle. The effectiveness of the isolation then, is a function of the distance of return travel remaining at the time of impact. When the ratio of disturbing frequency to the natural frequency attains a ratio of about 3 to 1 then, theoretically 90% of vibration is eliminated.

This is best explained by visualizing each cycle as an individual blow. This blow drives the isolator into dynamic deflection. When the force of the blow is spent, the isolator starts its return at its own frequency. Since the frequency is slower than that of the blows, it is obvious the return will be only partial before next impact. Because the isolator possessed the energy with which to complete its return to equilibrium, the unaccomplished portion of travel represents the amount of opposed energy that will absorb the next impact. Therefore, the greater the ratio of disturbing to natural frequency the more efficient the isolation, subject to diminishing returns. It is evident any truly resilient material capable of the required static deflection, operating within its elastic limits will produce the required results. The essential factor of an isolator is it must be truly resilient. It must have the ability to return to its original height when loads or forces are removed. Such a material, when loaded within its elastic limits, will have a long effective life.

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VIBRATION ISOLATORS SPECIFICATION


In the case where vibrations are present due to a constant steady state oscillation of imbalance in a machine a precise formula may be applied with reasonable certainty of attaining desired results. in substance, this formula is based on the ratio of the operating frequency of the machine or other equipment to be isolated, to the natural frequency of the isolated system. The disturbing frequency f d of a machine can be readily determined either by measurement or by the known operating characteristics of the equipment. Generally the lowest R.P.M. in the system is used as the disturbing frequency.

The natural frequency f n of a machine set on resilient material is a function of the static deflection of the resilient material under the imposed load. For practical purposes the natural frequency f n is described by the formula which is applied for theoretical efficiency

Where d = static deflection in inches, in the resilient supports. The ratio (f d/n) establishes the efficiency of the isolation from the following formula:

E = percentage of vibration isolated. f d = Disturbing frequency of the isolated machine. f n =Natural frequency of the isolated machine.

The percentage of isolation efficiency attained as a measure of the amount of reduction in the amplitude of the transmitted mechanical vibration. Refer figure '2' to readily select the static deflection required to attain desired isolation efficiency.

FIGURE -2

To determine the percent of isolation efficiency from figure '2', read from the graph at the intersection of vibration (disturbing) frequency and static deflection.

SHOCK ISOLATION Shock can be described as a motion in which the velocity changes very suddenly causing a single or multiple impacts into the ground or surrounding structure. Consequently, the theory of isolation outlined above is not applicable as the forcing (disturbing) frequency is usually low (less than 100 cpm). However, best results are obtained with isolators providing low natural frequencies. The design and selection of an isolation system for impact machines such as drop hammers and punch presses, must take into account that the energy of each blow is dissipated before the next impact occurs. When necessary, use of inertia mass and damping devices may be employed to attain desired stability.



NOISE

An additional benefit of machinery isolation is the associated noise reduction. Introduction of a resilient medium between the equipment and the structure acts to break the path of structural borne noise. This results in noise reduction in areas outside the machinery room and to limited extent, in the area around the machine. This is achieved by reducing the sounding board effect normally associated with machinery mounted solidly to the structure. Use of isolation does not reduce air-borne noise if found to be above allowable levels. Air-borne noise must be treated acoustically with acoustic enclosures or other sound absorbing devices.

Vibration levels measured on equipment’s and equipment components by vibration transducers can be affected by unbalance, misalignment of components and resonance interaction between the equipment and structure. If excessive vibration levels exist even after proper balancing, then the equipment and its installation should be checked for possible resonant condition. Maximum allowable RMS velocity level for some equipment’s are shown in Table 1

TABLE 1

EQUIPMENT ALLOWABLE RMS VELOCITY, in/s Pumps 0.13

Centrifugal Compressors 0.13 Fans ( vent sets, centrifugal, axial) 0.09

LOCATION

In evaluating and selecting an effective isolation system, consideration must be given to identifying critical areas. In buildings where equipment is located directly over or adjacent to quiet areas, offices, libraries, studios, operating rooms, etc., a very critical condition exists, requiring extremely high isolation efficiency. Basement locations and areas with high ambient noise such as factories, warehouses, power plants, etc., are considered less critical and can tolerate larger transmitted forces. In addition, the geographic location of the building is also important. if located in a designated earthquake zone, the potential seismic forces produced will dictate the incorporation of restraints to protect the resilient isolated equipment.

MATERIALS Elastomers describe natural rubber and the various synthetic materials such as neoprene, EPDM etc. It possesses inherent damping and sound deadening characteristics and can be moulded into any shape or hardness and vulcanized to metal. The term rubber-in-shear is used to describe the deflection in shear rather than compression. Elastomers limit static deflection to a maximum of 1/2 inch and can be incorporated to provide damping in spring isolators.

Metal springs become preferable when the required static deflection exceeds 1/2 inch. They can be designed to provide large static deflection and depending on the application are available either free standing, housed or vertically restrained. Free standing springs are usually unrestrained devices which must be laterally stable meeting a minimum of 0.8 ratio of spring diameter to compressed height. Housed springs are used to counteract the effect of lateral forces, while vertically restrained springs are used for equipment whose weight varies with the addition or removal of large amounts of water and for rooftop equipment subject to wind loads. Since steel springs will transmit noise, introducing an elastomeric pad to break the direct structural borne noise path is necessary.

Definitions: In understanding vibration isolation theory, it is helpful to become familiar with some of the terms and definitions relating to vibration and shock.

Frequency is defined as the number of complete cycles of oscillation that occur per unit of time.

Natural frequency is the number of complete cycles of oscillation a mass will vibrate in a given unit of time if a force displaces it from its equilibrium position and allows it to vibrate freely. Disturbing frequency is the frequency of vibration produced by an unbalanced, rotating or reciprocating movement in mass.

Resonance is when the disturbing frequency equals the natural frequency of the isolation system resulting in an amplification of vibration producing an excessive violent movement. This is common as the system passes through resonance.

Static deflection is the distance an isolator will deflect under the static or dead weight of the equipment.

Spring rate is the weight required to induce a spring to deflect a given distance.

Damping is the ability of an isolation system to absorb and dissipate a significant amount of energy and convert it to heat. The importance of damping is to reduce the tendencies of the isolated equipment to "bounce." This occurs during acceleration or slowing of the equipment when the speed reaches resonance and the amplification of the unbalanced forces occurs. It will be noted, damping reduces the efficiency of the isolator. However, this reduction is so minor it can well be ignored or compensated, by a very small increase in the static deflection requirements for the isolation system.

Vibration isolator selection guide in Table 2 takes into consideration building span, equipment running speed, power requirement, damping, etc. instead of theoretical aspects, isolation deflection is taken into account. These isolator specifications in turn form the project mechanical specification. Recommended isolator type, base type and minimum static deflection are within safe limits of most HVAC equipment’s.



Table 2

Table 2 Selection Guide for Vibration Isolation

slab on grade Up to 20 ft floor span(6 mtr.)

20 to 30 ft floor span(6-9 mtr.)

30 to 40 ft floor span(9-12 mtr.)

Equipment Type Horse power

And other

RPM

Base Type

Isolator Type in.

Min. Def.

Base Type

Isolator Type in.

Min. Def.

Base Type

Isolator Type in.

Min. Def.

Base Type

Isolator Type

Min. Def. in.

Refrigeration Machines and Chillers Bare Compressors All All A 2 0.25 C 3 0.75 C 3 1.75 C 4 2.50

Reciprocating All All A 2 0.25 A 4 0.75 A 3 1.75 A 4 2.50 Centrifugal All All A 1 0.25 A 4 0.75 A 3 1.75 A 3 1.75

Open Centrifugal All All C 1 0.25 C 4 0.75 C 3 1.75 C 3 1.75 Absorption All All A 1 0.25 A 4 0.75 A 3 1.75 A 3 1.75

Air Compressors and Vacuum Pumps

Tank-Mounted

Up to 10

15 & Over

All A 3 0.75 A 3 0.75 A 3 1.75 A 3 1.75

All C 3 0.75 C 3 0.75 C 3 1.75 C 3 1.75

Base-mounted All All C 3 0.75 C 3 0.75 C 3 1.75 C 3 1.75 Large reciprocating All All C 3 0.75 C 3 0.75 C 3 1.75 C 3 1.75

Pumps

Closed coupled

Up to 7.5

All B 2 0.25 C 3 0.75 C 3 0.75 C 3 0.75

10 & Over All C 3 0.75 C 3 0.75 C 3 1.75 C 3 1.75

Large inline 5 to 25 All A 3 0.75 A 3 1.75 A 3 1.75 A 3 1.75

30 & Over

All A 3 1.75 A 3 1.75 A 3 1.75 A 3 2.50

End section & split case

Up to 40

All C 3 0.75 C 3 0.75 C 3 1.75 C 3 1.75

50 to 125

All C 3 0.75 C 3 0.75 C 3 1.75 C 3 2.50

150 & Over

All C 3 0.75 C 3 1.75 C 3 1.75 C 3 2.50

Cooling Towers

All

Up to 300

A 1 0.25 A 4 3.50 A 4 3.50 A 4 3.50

301 tO 500

A 1 0.25 A 4 2.50 A 4 2.50 A 4 2.50

500 & over

A 1 0.25 A 4 0.75 A 4 0.75 A 4 1.75

Boilers-Fire -Tube

All

All

A 1 0.25 B 4 0.75 B 4 1.75 B 4 2.50

Axial Fans, Fan Heads, Cabinet Fans, and Fan Sections

Up to 22” Dia. 24” Dia. & over

All All A 2 0.25 A 3 0.75 A 3 0.75 C 3 0.75

Up to 2” s.p. Up to 300

B 3 2.50 C 3 3.50 C 3 3.50 C 3 3.50

301 to 500

B 3 0.75 B 3 1.75 C 3 2.50 C 3 2.50

Up to 22” Dia. 24” Dia. And over

2”s.p.

501 & over

B 3 0.75 B 3 1.75 B 3 1.75 B 3 1.75

& over 2.1” s.p.

Up to 300

B 3 2.50 C 3 3.50 C 3 3.50 C 3 3.50

& over 2.1” s.p.

301 To 500

C 3 1.75 C 3 1.75 C 3 2.50 C 3 2.50

Up to 2.1” s.p. 501 & over

C 3 0.75 C 3 1.75 C 3 1.75 C 3 2.50

Centrifugal Fans Up to 22” Dia. All All B 2 0.25 B 3 0.75 B 3 0.75 C 3 1.75

24” Dia. & over

Up To 40 Up to 300

B 3 2.50 B 3 3.50 B 3 3.50 B 3 3.50

301 to 500

B 3 1.75 B 3 1.75 B 3 2.50 B 3 2.50

501 & over

B 3 0.75 B 3 0.75 B 3 0.75 B 3 1.75

50 &

over

Up to 300

C 3 2.50 C 3 3.50 C 3 3.50 C 3 3.50

301 To 500

C 3 1.75 C 3 1.75 C 3 2.50 C 3 2.50

501 & over

C 3 1.00 C 3 1.75 C 3 1.75 C 3 2.50

Propeller Fans Wall-Mounted All All A 1 0.25 A 1 0.25 A 1 0.25 A 1 0.25 Roof-Mounted All All A 1 0.25 A 1 0.25 B 4 1.75 D 4 1.75

Heat Pumps All All A 3 0.75 A 3 0.75 A 3 0.75 A/D 3 1.75 Condensing Units All All A 1 0.25 A 4 0.75 A 4 1.75 A/D 4 1.75 Packaged AH,AC,H & V Units

All

Up to 10

All

A 3 0.75 A 3 0.75 A 3 0.75 A 3 0.75

15 & over

Up to 300

A 3 0.75 A 3 3.50 A 3 3.50 C 3 3.50

Up to 4” s.p.

301 to 500

A 3 0.75 A 3 2.50 A 3 2.50 A 3 2.50

501 & over

A 3 0.75 A 3 1.75 A 3 1.75 A 3 1.75

15 & Over

Up to 300

B 3 0.75 C 3 3.50 C 3 3.50 C 3 3.50

4” s.p &

over

301 to 500

B 3 0.75 C 3 1.75 C 3 1.75 C 3 2.50

501 And over

B 3 0.75 C 3 1.75 C 3 1.75 C 3 2.50

Packaged rooftop Equipment

All

All

A/D 1 0.25 D 3 0.75 --------------------------------------------------------------- Isolator 15 times additional roof deflection over 0.25 in.

Ducted Rotating Equipment

Small fans , fan-powered boxes

Up to 600 cfm

All

A 3 0.50 A 3 0.50 A 3 0.50 A 3 0.50

601 Cfm & over

All

A 3 0.75 A 3 0.75 A 3 0.75 A 3 0.75

Engine-Driven Generators

All

All

A 3 0.75 C 3 1.75 C 3 2.50 C 3 3.50

Base Types: - Isolators Types:- A. No base, Isolators attached directly to equipment 1. Pad, rubber or glass fiber B. Structural steel rails or base 2. Rubber floor isolator or hanger C. Concrete inertia base 3. Spring floor isolators or hanger D. Curb –mounted base 4. Cased spring mount or restrained spring mount

5. Thrust restrained mount



Base Types: - A. No base, Isolators attached directly to equipment B. Structural steel rails or base C. Concrete inertia base D. Curb –Mounted Base



Direct Isolation (Type A )

Direct Isolation (Type A ) is used when equipment is unitary and rigid and does not require additional support. Direct isolation can be used with large chillies. Packaged air-handling unit, and air-cooled condensers. If there is any doubt that the equipment can be supported directly on isolators, use structural bases (Type B) or inertia bases (Types C ) or consult the equipment manufacturer.

STRUCURAL BASES (TYPE B)

Structural bases (Type B) are used where equipment cannot be supported at individual locations and/or where some means is necessary to maintain alignment of component parts in equipment. These bases can be used with spring or rubber isolators (Type 2 and Type 3) and should have enough rigidity to resist all starting and operating forces without supplemental hold-down devices. Bases are made in rectangular configurations using structural members with a depth equal to one-tenth of the longest span between isolators, with a minimum depth of 4” (100 MM). Maximum depth is limited to 12”(300 MM) except where structural or alignment considerations dictate otherwise.

STRUCTURAL RAILS (TYPE B)

Structural rails (Type B) are used to support equipment that does not require a unitary base or where the isolators are inside or outside the rails and the rails act as a cradle. Structural rails can be used with spring or rubber isolators and should be rigid enough to support the equipment without flexing. Usual industry practice is to use structural member with a depth one-tenth of longest span between isolators with a minimum depth of 4”(100 MM). Maximum depth is limited to 12”(300 MM), except where structural considerations dictate otherwise

CONCRETE BASES (TYPE C)

Concrete bases (Type C) consist of a steel pouring form usually with welded-in reinforcing bars, provision for equipment holt-down, and isolator brackets. Like structural bases, concrete bases should be rectangular or T-shaped and, for rigidity, have a depth equal to one-tenth of the longest span between isolators, with a minimum of 6”(150 MM). Base depth need not exceed 12” (300 MM). unless it is specifically required for mass, rigidity, or component alignment.

CURB ISOLATION (TYPE D)

Curb isolation system (Type D) are specifically designed for curb-supported rooftop equipment and have spring isolation with a watertight and airtight curb assembly. The roof curbs are narrow to accommodate the small diameter of the springs within the rails, with static deflection in the 1” to 3” (25 MM TO 75 MM) range to meet the design criteria described for Type 3.

Isolators Types:- 1. Pad, Rubber or Glass Fiber 2. Rubber Floor Isolator or Hanger 3. Spring Floor Isolators or Hanger 4. Cased Spring Mount or Restrained Spring Mount 5. Thrust Restrained Mount

RUBBER PAD TYPE 1 ELCEN MODEL CRMP

Rubber isolators are available in pad (Type 1) and mulded (Type 2) configurations. Pads are used in single or multiple layers. Molded isolators come in range of 30 to 70 durometer (a measure of stiffness) as per ASTM D 2240. Material in excess of 70 durometer is usually ineffective as an isolator. Isolators are designed for up to 0.5 inch (13 MM) deflection, but are used where 0.3 inch (8 MM) or less deflection is required. Solid rubber and composite fabric and rubber pad are also available. They provide high load capacities with small deflection and are used as noise barriers under columns and for pipe supports. These pad types work well only when they are properly loaded and the weight load is evenly distributed over the entire pad surface. Metal loading plates can be used for this purpose.

ACUSTIC RUBBER MOUNT TYPE 2

ELCEN MODEL NRFM

GLASS FIBER PAD TYPE 1

ELCEN MODEL EGFP

Recompressed glass fiber isolation pads (Type 1) constitute inorganic inert material and are available in various sizes in thicknesses of 1” to 4” ( 25 MM to 100 MM) and in capacities of up to 500 psi. Their manufacturing process assures long life and a constant natural frequency of 7 to 15 Hz over the entire recommended load range. Pads are covered with an elastomeric coating to increase damping and to protect the glass fiber. Glass fiber pads are most often used for the isolation of concrete foundations and floating floor construction.

ACUSTIC RUBBER HANGER TYPE 2

ELCEN MODEL ENAH SPRING HANGER TYPE 3

ELCEN MODEL ECSH COMBINATION VIBRATION HANGER

TYPE 3

ELCEN MODEL ARSH

Isolation hangers (Type 2 and Type 3) are used for suspended pipe and equipment and have rubber, spring, or a combination of spring rubber elements. Criteria should be the same as for open spring isolators. To avoid short circuiting, hangers should be designed for 20 to 30° angular hanger rod misalignment. Swivel or traveler arrangements may be necessary for connections to piping systems subject to large movements.

Free standing spring mount TYPE 3 ELCEN MODEL EOSM

Steel springs are the most popular and versatile isolators for HVAC applications because they are available for almost any deflection and have a virtually unlimited life. All spring isolators should have a rubber acoustical barrier to reduce transmission of high –frequency vibration and noise that can migrate down the steel spring coil. They should be corrosion –protected if installed outdoors or in a corrosive environment. The basic types include Open spring isolators (type 3) consist of a top and bottom load plate with an adjustment bolt for leveling. Springs should be designed with a horizontal stiffness at least 100% of the vertical stuffiness to assure stability, 50% travel beyond rated load and safe solid stresses.

RESTRAINED SPRING MOUNT TYPE 4

ELCEN MODEL ERSM

Restrained seismic spring isolator (Type 4) has hold-down bolts to limit vertical movement. They are used with (a) equipment with large variations in mass (boilers, refrigeration machines) to restrict movement and prevent strain on piping when water is removed, and (b) outdoor equipment, such as cooling towers, to prevent excessive movement because of wind load. Spring criteria should be the same as for open spring isolators, and restraints should have adequate clearance so that they are activated only when a temporary restraint is needed.

CLOSED SPRING MOUNT TYPE 4

ELCEN MODEL ECSM

Closed (Housed) spring isolators consist of two telescoping housing separated by a resilient material. Depending on design and installation, housed spring isolators can bind and short circuit their use should be avoided.

THRUST RESTRAINT TYPE 5

ELCEN MODEL ETRS

Thrust restraints (Type 5) are similar to spring hangers or isolators and are installed in pairs to resist the thrust caused by air pressure



vibrationvibration isolators specification

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