Fan Instalation, Operation & Maintenance - How to Avoid Problems with Your Fan.pdf - MiUE - E i TCH III rok - Shang90

F AN E NGINEERING

FE-500

Information and Recommendations for the Engineer

Fan Installation, Operation & Maintenance

How to Avoid Problems with Your Fan

Introduction

This document presents ways to avoid the most com-

mon fan problems caused by improper storage, installa-

tion, operation and maintenance. Installation, operation

and maintenance manuals give general instructions on

what and what not to do. This document will give more

detail as to why these steps are important.

Another good idea is to add grease to the outside of

the bearing seals as this will help seal out moisture.

It is not possible to add grease to some small fans

and motors that have “sealed for life” bearings. In this

case, rotate the shaft monthly.

Reduce the belt tension on belt driven fans. This

reduces the load on the bearings, minimizing the poten-

tial for problems.

Do not store the fan in a location where it will be

subjected to vibration. Vibration may cause internal sur-

faces to rub against each other, damaging the bearings.

Damage of this type usually does not cause a problem

right away; it may take a couple of months of operation

for it to develop.

Storage

Many fans do not have the chance to operate success-

fully simply due to their treatment and handling during

shipment and storage. Rough handling during shipment

and improper storage can cause damage that is not

noticeable until the fan is in operation. Fans are fre-

quently received on site well before they are put into

operation. This often happens on large projects where

the fan is set in place and then sits idle while the rest

of the project is completed. Sometimes several months

go by before the fan is started.

It is discouraging to buy a new fan, only to have

problems shortly after startup. This can be avoided with

proper storage techniques which drastically reduce the

likelihood of having problems.

Most problems associated with storage are due to

moisture getting into the bearings. The best way to

avoid moisture problems is to store the fan in a clean

and dry place, preferably indoors. Outdoor storage usu-

ally subjects the fan to variations in temperature and

humidity. As the temperature drops, moisture condenses

as dew. Condensation in the bearings can cause rusting

of internal bearing surfaces, known as puddle corro-

sion.

If fans cannot be stored in a controlled environment,

avoid puddle corrosion by packing the bearings full of

grease. This eliminates the air pockets where moisture

can condense. Many greases contain rust inhibitors.

Adding new grease every month adds more of these

inhibitors. Turn the shaft about ten revolutions while

adding the grease to make sure that all surfaces inside

the bearings are coated. Stop the shaft in a different

location than it was previously stopped at. This way if

any moisture does develop, it will not always be at the

same location. On fan startup the extra grease will purge

out of the bearings. This may make a mess, but it is

better to deal with a mess than with a bearing failure.

With split bearings, the caps can be removed prior to

startup to remove excess grease.

Fan Foundations

The structure that supports the fan must be strong

enough to support the loads produced by the fan. Many

“fan” problems are actually structural support problems.

The support must be designed to carry both the dead

weight of the fan and dynamic loads created while the

fan is operating.

A well-designed fan support is rigid enough to keep

vibration levels low. Before discussing the features of

good fan support design, we need to set up some

background information on vibration:

Vibration is the repetitive motion that results from

forces that vary in amplitude or direction over time. One

common cause of vibration is impeller imbalance.

Impeller imbalance is a result of the centrifugal forces

acting on an impeller whose center of gravity is offset

slightly from the center of rotation. Not all vibration is

bad. Only when the vibration levels exceed certain

amplitudes is it a problem. A well-balanced impeller has

its center of gravity close enough to the center of rota-

tion that the vibration levels are low.

Excessive vibration causes problems in many different

ways. It causes lubricant to break down, which allows

metal to metal contact of bearing surfaces, which then

results in premature bearing failure. It can also cause

fatigue cracks in the bearings, the bearing supports, or

other fan components. It can cause fasteners, such as

motor and bearing hold down bolts or the set screws

that hold the bearings and impeller to the shaft to work

themselves loose. Many precision processes, such as

the manufacturing of computer chips, cannot tolerate

high levels of vibration. In other installations, sound

caused by vibration can be annoying to the people who

must work nearby.

Vibration Spectrum Plots

Figure 1.

Figure 2.

Fan Engineering FE-500

Fan Vibration

Figure 1 shows the plot of a vibration spectrum, which

is a plot of vibration amplitude versus frequency. These

plots are used by vibration technicians to diagnose

vibration problems and the general condition of rotating

equipment. The amplitude relates to how “loud” the

vibration is, and the frequency relates to its “pitch.”

Amplitude can be expressed in terms of acceleration,

velocity, or displacement, all three of which are related

mathematically. When dealing with fans, it is convenient

to use cycles per minute for the frequency because it

is easier to identify the vibration levels at the fan and

motor speed. Common units for vibration amplitude and

frequency are shown in Table 1.

Figure 3. Concrete Pad Anchor

Leveling Nut, If Used, Should Be

Backed Off After Shimming For

Final Tightening of Hex Nuts

Fan Base Angle

or Structural Steel

Temporary Form For

Grout Pouring

Hex Nut, Split Ring

Lock Washer, and

Tapered or Flat Washer

1" to 1.5"

Grout Allowance

To Be Filled With

Nonshrinking

Machinery Grout

Full Width Stainless

Steel Shims

Shimming Surface To Be

Smooth, Level, Dressed

If Necessary

Pipe-Bolt Sleeve

Dia. 2 to 2 1 / 2 Times

Bolt Dia. For Correction

of Alignment Errors

Care Should Be Taken

That Anchor Bolt Sleeves

Are Filled With Grout

Table 1. Common Units For Vibration Analysis

J-Bolt Leg Should Be

Fastened To Foundation

Rebar

MeASuReMent

unItS

Peak-to-Peak Displacement

mils (1 mil = .001 in.)

Peak Velocity

inches / second (ips)

ing from an imbalance of the impeller ensures that the

vibration levels will be low. Also, because concrete pads

are so rigid, their natural frequencies are usually very

high, which avoids resonance problems.

Figure 3 shows the best way to anchor a fan to a

concrete pad. “T” or “J” bolts provide a strong, rigid

connection to the pad. The pipe sleeve allows for some

flexibility in case the bolt location does not exactly

match the hole in the fan base. Compression type

anchor bolts are sometimes used, but they can work

loose when subjected to loads caused by vibration. To

avoid this problem when using these types of anchors,

use as large a size as possible.

When the fan is anchored to the pad, level it using

shims. Use 1" to 1 1 ⁄ 2 " thick shims between the fan base

and the concrete pad. After leveling the fan, build dams

around the pad and fill the gap made by the shims with

grout. Grout is a masonry product, similar to the grout

used to set ceramic tile. There are many varieties of

grout, from mortar types to epoxy types. Epoxy grout,

while more expensive, is more durable and more resis-

tant to oil and moisture than cementitious grout. After

the grout has set, double check that all of the anchor

bolts are tight.

Often it is not practical to mount fans on concrete

pads and they are mounted on structural steel supports.

With steel supports it is essential that they be designed

for rotating equipment. It is not only important to con-

sider dead loads and natural frequencies, but the sup-

port must be rigid enough to keep drive belts or cou-

plings aligned. One way to make the design easier is to

locate the fan as close to walls or vertical columns as

possible. Roof mounted fans are a special case of

mounting fans on structural steel supports. The differ-

ence is that the steel structure is covered by the roof.

The same design criteria must be used.

When mounting the fan on a structural steel base

there may be gaps between the fan base and the struc-

tural steel base. This occurs because structural steel is

not perfectly flat, and neither are the bases of fans. Fill

any gaps with shims before tightening the fan to the

steel base. Since most fan designs have relatively close

clearances between the impeller and fan housing, tight-

ening the fan to the base without shims can distort the

fan so that the impeller rubs against the housing.

Acceleration

g (1 g = 32.2 ft/sec2)

Frequency

Cycles per second (Hz)

Frequency

Cycles per minute (cpm)

The spectrum in Figure 1 is for a fan operating at

1250 revolutions per minute (rpm = cpm), driven by a

motor operating at 1750 rpm. If we were to increase the

fan speed, the spike corresponding to the fan speed

would move to the right. If we were to slow it down,

the spike would move to the left.

Spikes will also be present at the natural frequencies

of the structure. Just as bells or tuning forks have dis-

tinct natural frequencies they “ring” at, structures have

natural frequencies. The fan support in Figure 1 has a

natural frequency at 2200 cpm.

Resonance

When the fan speed corresponds to the structure’s

natural frequency, the fan and structure are in reso-

nance. At resonance, small forces can produce high

levels of vibration. Even a well-balanced fan can produce

high vibration levels at resonance with a structure’s

natural frequency. Figure 2 shows what happens when

the fan speed from Figure 1 is increased to 2200 rpm.

As you can see, the vibration level at this frequency

increases dramatically at resonance. Sometimes the

vibration can be lowered by balancing the fan to an

even finer balance, but the fan and structure will be very

sensitive. A small amount of dust buildup on the impel-

ler, for example, will cause the vibration level to increase

again.

In order to avoid problems with resonance, the sup-

port structure for a fan should be designed so that the

natural frequency of the structure is at least 20% high-

er than the fan speed. When mounting a fan on an

existing structure, verify that the natural frequency is

high enough by having a vibration technician perform a

“bump” test. A bump test is simply striking the structure

and measuring the frequencies at which it rings. If there

is a natural frequency too close to the fan speed,

stiffen the structure so that the natural frequency

increases to the point where it will not be a problem.

The best foundation for mounting a fan is a flat, rigid

concrete pad that has a plan area of at least twice the

plan area of the fan and is thick enough that the weight

of the pad is at least three times the weight of the fan.

To keep the edges of the pad from breaking away, they

should be kept at least six inches from the fan. The

large weight of the pad compared to any forces result-

Vibration Isolation

This is a topic that is somewhat controversial in the fan

industry. There are those who advocate a “total system”

concept of evaluating a fan and its support system. This

Fan Engineering FE-500

type of analysis looks at the support of a fan impeller

component by component all the way down to the foot-

ings and foundation of the building. Looking at the total

system, the structure is designed to avoid resonance

without the need for vibration isolators. From a technical

point of view, this is the correct way to design fan sup-

port structures. This concept has been used success-

fully on vibration sensitive fan applications such as the

manufacturing of computer chips.

The other approach is to use vibration isolators

between the fan and the supporting structure. These

isolators, when properly selected, reduce vibration forces

transmitted to the structure by approximately 95%. This

much reduction reduces the likelihood of having a reso-

nance in the support structure. Depending on the fan

speed, isolators are selected to have a specified amount

of deflection when put under load. For example, a spring

selected to deflect 1" under load on a fan operating at

1200 rpm will reduce the forces transmitted to the sup-

port by 97%. Various isolator designs, such as metal

springs and rubber-in-shear, are available to accommo-

date different loads and speeds.

Advocates of “total system” design point out that

selecting isolators based only on the vertical load is an

oversimplification. Fans mounted on isolators not only

move up and down, but rock back and forth and move

side to side. These additional motions end up increasing

the loads transmitted to the structure, resulting in less

“isolation,” and can cause problems. On the other hand,

vibration isolators work successfully in the majority of

cases.

Figure 5. Fan mounted on an inertia base

Figure 4 is a photograph of a fan and motor mount-

ed on a structural steel base supported by spring vibra-

tion isolators. Like other fan support structures, the base

must be rigid and designed without natural frequencies

near the fan or motor speeds. Notice the routing of the

electrical conduit to the motor. It is flexible and takes

into account the movement of the motor.

Figure 5 shows an inertia base type of vibration base.

It is similar to the base in Figure 4, except that the base

is filled with concrete. The weight added by the concrete

creates inertia, reducing the amount of vibration. The

concrete also makes the base very stiff, making it

easier to design to avoid resonance. The disadvantage

of this type of base is that the isolators and the struc-

ture supporting the fan and base must be designed to

carry the extra weight of the concrete base.

Figure 4. Fan mounted on a structural steel base with

spring vibration isolators

Figure 6. Well-designed duct configuration

Fan Engineering FE-500

Figure 7. Poorly-designed duct configuration

creates an obstruction at the fan inlet and results in a

system effect on fan performance. On some fan designs,

this obstruction also causes an increase in the sound

levels produced by the fan. Increases as high as 20db

in the blade pass frequency have been observed.

In some cases fans rigidly mounted to their supports

need flexible duct connections. Fans handling high tem-

perature air need to have flexible connections in order

to absorb the thermal expansion of the ductwork. The

ends of large plenums can deflect due to pressure load-

ing. Ductwork connecting plenums to fans needs to have

flexible connections to prevent the transmission of these

deflections to the fan. In both of these cases, flexible

connections allow room for duct movement without

damaging the fan.

Be careful when using a fan to support ductwork, or

when using ductwork to support a fan. Most fans are

not designed to carry these external loads, and adding

them to the fan may cause the impeller to rub or cause

other misalignments which could damage the fan. Check

with the fan manufacturer before mounting the fan or

ductwork this way to make sure the fan design can

handle the loads.

Duct Connections

Any time ductwork is connected to a fan, it is important

to consider any effects the duct may have on fan per-

formance. Catalog fan performance is based on uniform

flow entering the fan and a straight run of duct on the

discharge. Many duct configurations do not provide

these flow conditions and as a result, the fans will not

perform at catalog levels. This loss in performance is

known as “system effect.” Figure 6 shows a well-

designed duct configuration that will not have any sys-

tem effects, while Figure 7 shows a poorly-designed

duct configuration that will lose performance due to

system effects. For more information on system effects

and methods for estimating their effect on performance,

see AMCA Publication 201.

Fans mounted on vibration isolators need to have

flexible connections between the fan and the ductwork.

Without flexible connections, the ductwork would prevent

the movement of the fan on its isolators, reducing the

effectiveness of the isolators. In addition, rigid connec-

tions transmit fan vibrations to the duct, opening up the

possibility of exciting resonant frequencies in the duct-

work.

It is important to mount flexible inlet duct connections

with the correct amount of slack. Figure 8A shows a

cross section of a properly mounted flexible connection.

There is just enough slack in the connection to allow

movement between the fan and ductwork. Figure 8B

shows an improperly mounted connection, with an

excessive amount of slack. Because of the negative

pressure at the inlet, the extra material is sucked in. This

Fan Startup

Figure 9 is a typical pre-startup checklist. Before starting

a fan go through the checklist to make sure the fan is

ready to run. Pay particular attention to safety. Be sure

to lock off electrical power before working on any fan.

Do not assume that because the factory tightened the

fasteners and aligned the belt drives or couplings at the

Figure 9. Pre-Startup Checklist

Verify that proper safety precautions have been fol-

lowed:

• Electrical power must be locked off

Check fan mechanism components:

• Nuts, bolts, and setscrews are tight.

• System connections are properly made and tight-

ened.

• Bearings are properly lubricated.

• Wheel, drives, and fan surfaces are clean and free

of debris.

• Rotating assembly turns freely and does not rub.

• Drives are on correct shafts, properly aligned, and

properly tensioned.

Check fan electrical components:

• Motor is wired for proper supply voltage.

• Motor was properly sized for power and rotational

inertia of the rotating assembly.

• Motor is properly grounded.

• All leads are properly insulated.

trial “bump”

• Turn on power just long enough to start assembly

rotating.

• Check rotation for agreement with the rotation

arrow. Does the assembly make any unusual

noise?

Correct any problems which may have been found.

(Follow safety guidelines. Make sure electrical

power to the fan is locked off.) Perform checklist

again until the fan is operating properly .

Run up to speed:

• Are bearing temperatures acceptable (<200°F)

after one to two hours of operation?

• Check for excess levels of vibration.

After one week of operation:

• Check all nuts, bolts, and setscrews and tighten

if necessary.

Figure 8A.

Correct Installation

Figure 8B.

Incorrect Installation

Fan Engineering FE-500

Fan Instalation, Operation & Maintenance - How to Avoid Problems with Your Fan.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: