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A survey of a number of cooling tower fan operators and some vibration specialists revealed that the gear reducers are the most common mechanical problem. Most electrical problems were next, followed by motor drive shaft-gear reducer misalignment. Drive shaft unbalance, poor fan blade adjustment, bearing problems and occasionally fan unbalance and structural resonance complete the list. All of these problems produce vibration that can be used to detect the problem and what the problem is. An unbalance problem of fan is very rare. These few were out of balance because blade tip drain holes were plugged. The below table summarizes the problems, and the symptomatic vibration frequency and amplitude of each.

Vibration amplitudes are affected considerably by support rigidity which makes it extremely difficult to establish vibration standards. Be sure to coincide support rigidity when using any of available limits. Most of them were established for fairly stiff supports. The normal to maximum amplitude presented may be used as guidelines when manufacturers limits are not available.

Which parameters should be used, displacement, velocity or acceleration? The motion of an oscillating part can be described by its amplitude in terms of displacement (mils or microns, peak to peak), velocity (in/second or mm/second peak) or acceleration (g) and its frequency. In general, it has been used to refer to displacement as peak to peak (i.e. double amplitude) while velocity and acceleration are peak (i.e. single amplitude).

Displacement, velocity and acceleration are related to each other by frequency.

X = A Cos q
V = dX/dt = d(A Cos q)/dt = A w Sin q, Vmax occurs where Sin q = 1

Therefore,
V_pk = A w = A x (2pf) = (Disp./2) x 2 pf = pf Disp. (Double Amplitude)
= 3.1416 (CPM/60)(1 inch/1000)
= 52.36 x 10 ^-6 mils.CPM

Disp.= V_pk / pf = (1/p)(V_pk/f) = 0.3183 (V_pk/f)

a = dV/dt = d(A w Sin q)/dt = d(A 2pf Sin q)/dt = 2pf V ( - Cos q), a_pk = 2 pf V ( -1)
[The (-) means that g is 180 deg. out of peak with disp.]

G = a / 386.1 = 2pfV / 386.1 = 0.0162 fV (in/second²)

(1) Vibration in Cooling Tower: A cooling tower contains essentially 3 pieces of rotating machinery - the motor, the gear reducer and the fan. The primary forces which would cause an increase in vibration level include;

• ●  The vibration of the fan and fan shaft is caused by the fan unbalance (1 x rpm), blade pass frequency (number of blades x rpm), fan shaft misalignment (2 x shaft rpm), fan bearings (more than 10 x shaft rpm)
• ●  The motor and drive shaft i.e. motor or drive shaft unbalance (1 x drive shaft rpm) or misalignment (2 x drive shaft rpm)
• ●  The gear reducer itself - bearings (more than 10 x shaft rpm), gear mesh (number of teeth x rpm)

Of course, if there is a blade resonance or structural resonance which corresponds to one of the frequencies noted above, then a relatively small force can create a high vibration. In this case, since there is almost always some unbalance and misalignment present, the problem would be the presence of a resonance rather than too much unbalance, etc. In either case, of course, the electronic switch would trip.

(2) Location and Number of Vibration Switches: Most often only one switch is used. However, some customers use two, one on the motor and one on the gear reducer. When only one switch is used, the preferred location is on the side of the gear reducer. Since Vibration Switches measure the vibrations perpendicular to its base, the advantage of this location is that we will be sensing the radial vibration from both the fan shaft and the coupling shaft.

The vibration switch will then sense the vibration resulting from unbalance and misalignment of fan shaft and blade pass frequency, plus unbalance and misalignment of the coupling shaft. (Since these potential faults generates vibrations at different frequencies, it is essential that the switch trip on vibration velocity.) Further, protection is also afforded in the gear reducer itself.

In the past, some cooling tower manufacturers have mounted the switches on the motor rather than the gear reducer. This will protect for motor and coupling shaft unbalance. On wooden structures which are relatively soft, a severe fan unbalance may be sensed at this location (i.e. severe damage may result before a trip occurs) and affords no protections for the gear reducer. However, with a concrete structure, it is unlikely that fan vibrations would be sensed at the motor location.

The motor location was chosen for convenience when only mechanical type vibration switches were available. These required reset to be accomplished with a reset button on the vibration switch itself and it was inconvenient to get to the gear reducer to accomplish reset. Since the electronic type of vibration switches provide the capability for latching the trip and remote reset, when a trip occurs, it is not necessary to get to the gear reducer in order to reset the switch. A reset button could be located at the motor platform or other convenient location.

(3) Benefits of Electronic Switches over Mechanical Switches: Historically, users have specified mechanical vibration switches (like Murphy VS-2EX series vibration switches) because there have not been alternative and the limitations of mechanical switches have not been widely disseminated. The electronic switch has been in use for several years but is still not widely known. The mechanical switch is relatively cheap but is relatively ineffective and undependable. There are several fundamental reasons for this resulting from the inherent limitations of mechanical switch. Yet strangely, acceleration switches are used on many cooling tower fans. They may be effective with severe gear problems but not for drive shaft misalignment and unbalance or fan unbalance. A velocity sensitive device is appropriate for gear reducers. A careful look at how each type device operates and performs may help you to understand its capabilities and make the proper choice for his equipment protection needs.

Many large diameter cooling tower fans have inadequate vibration cutout devices. Devices designed for other applications, such as mechanical inertia spring or magnetic release - switches are often in use. Cooling tower fans up to 30 feet in diameter should have a system that can respond in a linear manner to preset vibration severity levels over a frequency range from 120 to 30,000 cycles per minute. Provisions for a time delay to avoid start-up trip out are also important.

These requirements are beyond the capability of most mechanical devices. Such devices set to trip on one frequency range may not respond to low frequency vibrations. It is also common to find that in order to prevent them from tripping out on startup, mechanical devices are set so high or are so remotely located as to be ineffective. Mechanical devices are also of no use in establishing monitoring systems.

Mechanical Vibration Switch, by its nature is acceleration sensitive. In principle a magnet exerts a pulling force on a weighted arm. This force is opposed by an adjustable spring by which the operator sets the trigger level-the greater the spring force, the less the acceleration force required to overcome the spring. When the acceleration force exceeds the magnetic force, the latching magnet pulls in the arm and mechanical relay is thus closed, proving the alarm or shutdown.

Electronic vibration devices that monitor vibration levels based on velocity measurements are available to meet requirements of large diameter cooling tower fans. However, a careful examination of their frequency response is required to assure protection at the large fan's lower operating speeds. Fans larger than 30 ft operate at speeds that are below the useful range of most velocity-limiting systems. These larger fans should have a system that reacts to displacement measurements to monitor fan conditions. Systems based on proximity sensors are best suited for this application.

Electronic Vibration Switch consists of a solid state crystal which produces electrons when it is deformed by the acceleration force. This electrical output is electrically integrated, producing a signal proportional to velocity. This signal is then compared with a preset limit and triggles a triac (in effect a solid state relay) if the level has been exceeded. This relay closure can be used by the customer to alarm, shutdown, etc. Thus it is a completely solid state, self contained unit. The customer need only bring 110 V power leads to it and bring leads to the relay.

Inputs from major users are that the mechanical switch frequently triggers when it shouldn't during its youth, and may not trigger at all after it has been installed for some time. (One year after installation.) Very frequently the user has jumpered them out. In short, as a protection device, they're very questionable indeed! Why?

• ●  They are mechanical and subject to corrosion, dirt, etc. (The electronic unit is all electronic - no moving parts.)
• ●  The mechanical construction is a very undesirable approach from a vibration viewpoint - the cantilevered beam, mass, and springs are subject to resonance being excited by higher frequency machinery vibration (which may be one reason for spurious triggering.)
• ●  The mechanical switch triggers proportional to acceleration. Damage potential is proportional to velocity in the speed ranges of most rotating machinery. The damage potential to your auto is very high at 60 MPH and zero acceleration. Likewise one could have a very high acceleration from 0 to 5 MPH but the damage potential would be relatively low at this low velocity.
  Further, acceleration increases linearly with frequency (RPM) as compared to velocity. Thus, the amount of protection depends on frequency and the device gives too much protection at some frequencies and not enough at others.
• ●  The accuracy of the triggering point is very poor with the mechanical switch - +/- 5% of full scale. Full scale for the mechanical switch is 4.5g which means accuracy is +/- 0.22g. This is a Catch 22, because typically desired limit levels are around 0.2g. So in this case triggering theoretically could occur anywhere from zero to 0.4g - very poor by anyone's standards. (The electronic switch is +/- 10% of set point, i.e. set at 0.2 in/sec velocity, triggering will occur at 0.2 +/-0.02.>

The solid state of electronic vibration switch has many advantages over the older mechanical design as follows;

• ●  Electronic switch uses a crystal to generate and electrical output to sense vibration level. No moving parts to wear or resonate. By contrast the mechanical switch uses magnets and springs to sense the vibration.
• ●  Built-in field adjustable time delay to avoid trip on high start-up vibrations or from transient vibrations during normal running. Mechanical switches do not have time delay. Consequently, trip points are frequently set so high that no protection is provided.
• ●  Trips on velocity rather than acceleration. The mechanical switch can trip only on acceleration. All authorities in the field agree that damage potential is related to vibration velocity. Acceleration trip provides over protection to fault which generate high frequency vibrations (i.e. false trip) and under protection to low frequency problem (i.e. they do not trip when a low frequency fault occurs.).
• ●  Calibrated setpoint dial. Permits setting trip level to known value in engineering units of in/sec. The user never knows where the mechanical switch has been set, or if it has been re adjusted by an unauthorized person.
• ●  Many options are available with the electronic switches that are not possible with the mechanical type switch including: separate transducer, dual trip settings and 4-20 mA analog output.

(4) Layout of Vibration Switch Arrangement

(A) Direct Mounting or With Remote Readout Option

(B) Remote Transducer Option or With Remote Readout Option or Remote Control