fan vibration specifications

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Fan vibration specifications

by Michael DiGiovanni, Thomas R. Spearman

Engineers specifying allowable fan vibration levels are challenged to balance the need for lower initial

equipment cost with the need for higher equipment reliability. This is especially important in critical

applications. This article is a case study of a multiyear process that addressed the balancing of these

competing needs as they related to critical pharmaceutical cleanroom applications. The knowledge obtained

is transferable to other critical applications.

This case study shares the problems encountered with air-handling units (AHUs) in an existing parenteral

(injectable) pharmaceutical facility. The ensuing repairs required the efforts of cross-functional teams

comprised of skilled vibration specialists, craftspeople, technicians, and engineers both internal and external

to the firm. The result was more reliable equipment and increased understanding of fan, AHU, and vibration

specification requirements. The specification requirements were incorporated into master specifications,

improving the overall performance and reliability of air-handling equipment critical to the operation of the

facility.

HVAC and Cleanrooms

Cleanrooms are used in pharmaceutical parenteral product facilities to provide a controlled environmentsuitable for manufacturing parenteral medicines. The control of viable and non-viable particles is critical to

preventing contamination. The HVAC system is a key item in obtaining the controlled environment by

diluting and purging particles from the cleanroom. The HVAC system also provides a pressure cascade

between the cleanroom and adjacent spaces, preventing contaminants from entering the cleanroom. Fans in

the HVAC system provide the motive force for dilution, purging and pressurization. Failure of the fan will

cause an immediate interruption in manufacturing, as well as costly product loss. Prior to the resumption of

manufacturing, multiple time-consuming cleanroom sanitizations are required. Fan failure usually will

disrupt manufacturing operations for 24 to 48 hours, depending on the duration of the downtime and

whether the event is controlled or catastrophic.

HistoryThe parenteral facility experienced several catastrophic bearing failures on two existing air-handling systems

in 2003. These events resulted in the destruction of the fan shaft and required an evacuation of the facility in

response to smoke and fire alarms.

Follow-up investigations revealed critical design factors that directly affected the performance of the units,

including the use of roller bearings, as opposed to spherical ball bearings, the installation of locking collars

at both ends of the shafts or at one end, and lubrication selection based on the temperature of the unit.

The facility uses belt-driven fans with motors mounted on an integral base. Prior to the 2003 events, each

fan bearing and motor bearing had one vibration monitoring sensor in the radial direction. Our follow-up

investigation recommended a number of additional monitoring points be added including:

* A second radial vibration monitoring point, perpendicular to existing radial vibration monitoring point;

* Axial vibration monitoring; and

* Temperature monitoring.

These additional monitoring points provided insight into the performance of the existing units and a basis for

developing a technical understanding of the data being generated. As a result, potential failures could be

detected more easily and better root causes could be identified more effectively.

Two new adjustable speed drive (ASD) air-handling units were purchased in the fall of 2003 for installation

in early 2004 as part of a major facility renovation. Each AHU included supply and return fans. The existing

AHUs were more than 25 years old and were nearing the end of the equipments' useful life. With knowledge

gained from previous experiences, plant engineering worked with corporate engineering to modify thecorporate master specifications for this equipment type. The primary changes included adding vibration

monitoring along two axes for each bearing, and the ability to record these values at multiple speeds, since

operating speeds for the ASD fans could not be finalized until testing and balancing were completed. The
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tolerances and acceptance criteria for vibration levels were also tightened. This information was included in

the specification sent for bid.

During factory acceptance testing (FAT) in December 2003, some testing indicated problems with

equipment resonance. One fan on each AHU was unable to meet the vibration criteria, and the root cause

was undetermined. Due to scheduling constraints, the units were allowed to ship with the intent to test and

repair them on site.

Over the next few months in collaboration with the air handler manufacturer and the fan manufacturer, the

source of the vibration problems with these fans was intensely investigated. After installation, each AHUwill support between 20% and 25% of the parenteral facilities' total production area. Their performance will

be critical to the operation of the manufacturing facility.

The investigation revealed that at various speeds, the fans were exciting resonance frequencies in the fan

shafts, thus causing higher vibration loads at the fan bearings. To shift the resonance frequency away from

potential operating speeds, the fan shafts and wheels were replaced with stronger shafts and lighter wheels.

With this issue resolved and construction complete, the project moved into a start-up and return-to-service

phase. Early in the start-up phase, the fans began to fail due to problems with belt slippage. Over a number

of weeks, this problem occurred repeatedly on both units.

Another investigation was performed to resolve the belt problems. The investigation identified a number ofcritical points related to belt-driven systems. They included strict adherence to tensioning specifications

identified by the manufacturer using proper tensioning tools, allowing for runtime and re-tensioning after

installation, proper belt quantities based on load, sheave wear specifications and acceptance criteria, and the

implementation of an appropriate monitoring program to determine if slippage was occurring.

A major learning point was the criticality of the fan base design to the fan operation. Using a modal analysis

of the fan base, it was determined that the fans were again exciting resonance frequencies, this time

in the fan base. Frame resonance caused the fan and motor shafts to deflect in opposite directions during

operation, causing the belts to repeatedly go into tension and then relax. This caused tremendous belt wear

in a short period of time, ultimately resulting in premature belt failure.

Due to critical production needs, the systems were put into service even with this known issue. Belts were

over-tensioned to compensate for the problem, critical speeds were locked out of the ASD to reduce the

level of excitation, and monitoring programs were increased to predict failures early and plan for

replacements. While the belt over-tensioning helped the equipment limp through this time period, it also

overloaded the bearings and resulted in premature bearing failure 18 months later.

Over the next few months, additional failures were experienced. Many other problems were detected before

they occurred, and were effectively addressed without seriously impacting production. The process of

designing and fabricating new bases for these fans was also started. The new integral fan/motor bases were

designed to ensure resonance frequencies were outside the operating range of the fans. The new bases were

installed in spring 2005 and have resulted in a significant improvement in overall performance.

Over the last five years, nine new AHUs have been installed in this parenteral facility. Most units serve

production areas. Others serve warehouse and office space. The new units and the existing systems they

replaced have been tremendous resources for information and learning. Much of this learning has been

incorporated into corporate master specifications and maintenance practices, resulting in major

improvements in the start-up performance of new units and improved reliability of existing ones.

Specifications

Many ways exist to organize specification requirements. This case study chose to organize the corporate

master specifications into the following groups AHU specifications; fan specifications; and vibration

specifications.

Requirements in fan and vibration master specifications were divided into two categories: business criticaland business non-critical. These terms were selected to reduce confusion with other definitions of critical. In

pharmaceutical manufacturing, the term "critical" identifies system components that have a direct impact on


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product safety, identity, strength, purity, or quality. Cleanrooms, data centers, and vivariums are examples of

business critical applications within the pharmaceutical industry.

AHU Specifications

The three AHU master specifications used by the firm in this case study are packaged, semi-custom, and

custom AHUs.

The three AHU specifications require that the fans are furnished in accordance with the fan specifications.

The packaged AHU specification includes an editing comment that it is not recommended for applicationswith ASDs. The semi-custom AHU and custom AHU specifications require that the fan manufacturer

furnish the fan to the air handler manufacturer as a complete assembly, including the fan, motor, base, drive,

and bearings.

Fan Specifications

The two fan master specifications are centrifugal HVAC fans and axial HVAC fans.

For business critical and adjustable-speed applications, the principal flexural natural frequency of the fan

rotors must be a minimum of 2.15 times the maximum rotational speed of the rotors. For business non-

critical and constant-speed applications, the principal flexural natural frequency must be a minimum of 1.3

times the maximum rotational speed of the rotors. The higher limit for business critical applications is used

to ensure that the second harmonic of the natural frequency is avoided.

To avoid base resonance, the natural flexural frequency of the isolator base must be a minimum of 10%

greater than the maximum rotational speed of the motor or fan, whichever is greater. This does not include

the natural frequency associated with the rigid body motion of the isolator base due to deformation of the

isolator springs. The natural frequency specified is at-rest values, which can be determined by an in-situ

impact test.

Fan and motor combinations must meet the vibration limits in accordance with the vibration specification

prior to shipment from the AHU manufacturer. Fans with ASDs shall be vibration tested throughout the

rotational speed ranges indicated on the Fan Data Sheets. The default range is 40 Hz to 60 Hz of the ASD

output, with 60 Hz coinciding with the fan maximum speed. Reducing the speed range may reduce fan cost

since it reduces the probability of a resonant condition existing within the operating range.

ASDs are usually used to compensate for filter loading. The specifying engineer should determine the

minimum and maximum design filter pressure drops and use the fan laws to establish the tested rotational

speed range. A typical pharmaceutical AHU contains a MERV 7 prefilter, MERV 13 intermediate filter, and

MERV 17 final HEPA filter. (1) If all the filters are rated for 500 fpm (2.54 m/s), Table 1 lists typical clean

and dirty pressure drops for each filter type. Using the fan law (2) shown by Equation 1 with [RPM.sub.2]

equal to 60 Hz, the minimum rotational speed [RPM.sub.1] is calculated as 40 Hz.

[RPM.sub.1] = ([RPM.sub.2]) x ([square root of ([SP.sub.1]/[SP.sub.2])]) (1)

where

[RPM.sub.1] = fan minimum rotational speed in revolutions per minute or Hz

[RPM.sub.2] = fan maximum rotational speed in revolutions per minute or Hz

[SP.sub.1] = initial (clean) static pressure in in. w.c. or (Pa)

[SP.sub.2] = is the final (dirty) static pressure in in. w.c. or (Pa)

For business critical applications, machine surfaces of components that mate with machined components

such as bearing supports or motor slide base must be machined flat to 0.002 in./ft (0.17 mm/m) after

fabrication.

For business critical applications with brake horsepower greater than 20 hp, ball or roller bearings shall have

uniform or concentric inner race attachment to the shaft. Ball or roller bearings with inner race locking

setscrews are unacceptable.For FATs, mechanical run tests shall consist of operating the fans at the design rotational speed for a

minimum of 15 minutes. Vibration readings shall be obtained and documented during the mechanical run

test in accordance with the vibration specification. The principal flexural natural frequency of the fan rotor


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and natural frequency of the isolator base shall be obtained and verified to be within the design and

performance requirements given here. Fans with ASDs shall be tested throughout the rotational speed

ranges. Vibration readings shall be obtained and documented in accordance with the vibration specification.

Vibration Specification

The vibration master specification is vibration limits for rotating equipment.

Vibration data for equipment operation shall be obtained during FAT at the AHU manufacturer's facility

prior to shipment. FAT results shall be approved by the owner or owner's representative prior to release forshipment. If it is impossible to test the equipment at the manufacturer's facility, the owner or owner's

representative will arrange for site acceptance testing after installation.

Equipment testing will be performed by a technically qualified person trained and experienced in vibration

data collection and analysis. "Technically qualified" means the person has attended at least two vibration

data collection and analysis courses, and "experienced" means the person has more than three years of

vibration analysis experience.

Vibration measurements are obtained with the equipment at stable operating speed, load, and temperature.

Data is taken with a spectral resolution of 1.25 Hz or greater to separate the rotating excitation frequencies.

The bandwidth covers at least 65 times the maximum turning speed of measured component shaft.

Axial readings are taken parallel to the shaft axis of the equipment as close to the shaft as possible.Horizontal or radial readings are taken at a right angle relative to the shaft at the 3 o'clock or 9 o'clock

position when facing the shaft. Vertical or radial readings are taken at a right angle from the horizontal or

radial readings.

For belt-driven fans, the measurement locations include:

* Motor outboard bearing horizontal;

* Motor outboard bearing vertical;

* Motor outboard bearing axial;

* Motor inboard bearing horizontal;

* Motor inboard bearing vertical;

* Fan inboard bearing horizontal;

* Fan inboard bearing vertical;

* Fan outboard bearing horizontal;

* Fan outboard bearing vertical; and

* Fan outboard bearing axial.

For fan assemblies, the allowable maximum overall filter-out vibration levels are given in Table 2. Most

fans use an isolator base with springs which is categorized as a flexible foundation. For all applications, the

frequencies related to bearing faults and harmonics, generally in the range of 50 Hz to 500 Hz, the allowable

vibration limit is 0.01 in./second-peak (0.254 mm/ second-peak). The bearing fault frequencies or harmonics

of interest are the fundamental train frequency (FTF), ball spin frequency (BSF), bearing pass frequency

outer race (BPFO), and bearing pass frequency inner race (BPFI).

For equipment using ASDs, start at the minimum operating speed of the equipment. Begin taking vibration

measurements. Slowly increase the ASD in one Hz increments up to the maximum speed. If the equipment

exceeds the acceptable filter-out vibration limits as listed in Table 2, then complete a full spectrum analysis

at that speed for each required measurement location. Complete a full spectrum analysis at the maximum

speed for the design operating range at each required measurement location.

Conclusion

Acceptable vibration levels are difficult to balance due to the competing needs of low cost and high

reliability. The engineer should consider the business criticality of the equipment to determine this balance.

Fans with ASDs are often challenging due to the changing speeds operating near a component resonant

frequency. Resolution of these issues often requires the efforts of cross-functional teams; maintenance

groups are often an overlooked valuable resource.
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Acknowledgments

This article is a summary of a multiyear process to improve the overall performance and reliability of the

air-handling equipment critical to the operation of this parenteral facility. This effort required leadership,

support, and commitment from multiple groups and people during various times in the process. The

problems addressed were complicated, and the investigation and resolution process was long and intense.

We were fortunate to have support from skilled technical vibration specialists both internal and external to

the company, as well as skilled, conscientious, dedicated craftsmen, technicians, and engineers from various

parts of the company whose joint efforts enabled us to overcome these obstacles. Their abilities and desire toshare this learning have contributed to the improvement of the overall capital delivery process for this

company.

The contributions of the following people to the resolution of these issues and the development of this article

are greatly appreciated:

Jeffrey L. Bradley, Donald R. Bush, Daniel C. Carroll, Beau P. D'Arcy, L. Douglas Elam, P.E., Douglas E.

Ebert, P.E., Donald R. Moore, P.E., Duane A. Mowrey, P.E., Associate Member ASHRAE, R. Scott

Ronczka, P.E., Phillip M. Sergi, Justin L. Stahl, David Stoner, Jerry L. Van Blaricum, Sr. at Eli Lilly and

Company; Beverly K. Flick, CREW Technical Services; Dave Franks, Air Applications; Tim Kuski,

Member ASHRAE, Greenheck Fan Corporation; Robert J. Sayer, P.E., Sayer and Associates; Dick

Williamson, Twin City Fan Companies (retired).

References

(1.) ISPE. 1999. ISPE Baseline[R] Pharmaceutical Engineering Guide, Volume 3: Sterile Manufacturing Facilities,

p.152. Tampa, Fla.: International Society of Pharmaceutical Engineers.

(2.) 2004 ASHRAE Handbook-HVAC Systems and Equipment, Chap. 18.

(3.) ANSI/ASHRAE Standard 52.2-1999, Method of Testing General Ventilation Air-Cleaning Devices for

Removal Efficiency by Particle Size.

About the Authors

Michael DiGiovanni is a team leader and Thomas R. Spearman, P.E., is an associate engineering consultant at Eli

Lilly and Company in Indianapolis.

By Michael DiGiovanni, and Thomas R. Spearman, P.E., Member ASHRAE

Table 1: Typical filter pressure drops.

Filter Clean Pressure Dirty Pressure

Type (3) Drop in w.c. (Pa)Drop in. w.c.

(Pa)

MERV 7 0.3 (75) 1.0 (250)

MERV 13 0.5 (125) 1.2 (300)

MERV 17 1.4 (350) 2.8 (700)

Total 2.2 (550) 5.0 (1250)

Table 2: Allowable vibration levels.

Criticality Equipment Size Rigid Foundation Flexible Foundation

Business Critical5 hp to < 20 hp

0.12 in./second-peak 0.20 in./second-peak

(3.1 mm/second-peak) (5.1 mm/second-peak)

Business Critical20 hp and above



Business Non-Critical5 hp to < 20 hp



Business Non-Critical20 hp and above



Business Non-Critical

Packaged AHUsAll




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COPYRIGHT 2008 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

COPYRIGHT 2008 Gale, Cengage Learning

Bibliography for: "Fan vibration specifications"

Michael DiGiovanni "Fan vibration specifications". ASHRAE Journal. FindArticles.com. 13 Jul, 2012.

COPYRIGHT 2008 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

COPYRIGHT 2008 Gale, Cengage Learning
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