B R AT I O N D ETECTION

V I B R AT I O N D ETECTION

Vibration can be detected by measuring displacement, velocity, or accelera- tion; therefore, vibration transducers can measure any of these factors. One of the most commonly used vibration transducers, the piezoelectric trans- ducer , is based on the piezoelectric accelerometer, which produces an

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S ECTION PLC Process Data Measurements C HAPTER 4 Applications

and Transducers 13

electrical output (voltage or current) proportional to the acceleration of the vibration. A piezoelectric transducer does this using a piezoelectric crystal, which is a crystalline substance that exhibits electric polarity under pressure. The transducer, which is spring loaded with a crystal of known mass, reacts to acceleration by creating a voltage across the crystal, generally in the millivolt range. It measures acceleration by detecting the force applied to the known mass, since force is equal to mass times acceleration (F = ma).

International standards for rotating machinery (the ISO 2378 and 3945) specify that vibration severity is directly related to vibratory velocity for machines running at and above 500 rpm. Vibration velocity can be found using a vibration transducer/transmitter that integrates the acceleration measurement taken from a piezoelectric-based accelerometer, resulting in a velocity measurement proportional to the vibration. The vibration trans- ducer/transmitter then sends a 4–20 mA signal to the PLC that is propor- tional to the velocity of vibration in inches or meters per second. Figure 13-

37 shows vibration measuring devices that provide a 4–20 mA output.

Remote Reset

6 Reset

9 Vibration Level

Shutdown Circuit

Common 11 4-20mA

(b) Equipment Power

13 Alarm Circuit External

Alarm

AC 1 Power

Grounding Wire

Shielded Cable

Figure 13-37. (a) Vibration measuring devices and (b) the connection diagram for the first device in part (a).

There are several guidelines for determining the level at which vibration becomes critical. Figure 13-38 illustrates a vibration warning level guide provided by PMC/BETA LP (Natick, MA), a vibration transducer manufac- turer. A PLC can monitor the level of vibration in a machine or equipment and provide the operator with a warning indication, according to the guide, before damage occurs. Figure 13-39 illustrates a severity chart for machines with vibration warning levels of 0.2–0.4 inches/sec over a frequency range of 100 to 100,000 rpm. This chart shows the variable peak-to-peak displacements for

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IO N

A p p P Vibration Protection Levels L C lic

In (unloaded) d

• Motor/generator sets

0.5 lT 0.5

(electric driven)

• Conveyors

x • Fans/blowers

e 0.5 • Hammermill

.in • Motor/generator sets

(engine driven)

u s (centrifugal) o m

tri • Compressors a p

• Gear boxes

a (reciprocating) lte n y

• Pumps:

x -gear t.c

Velocity (inches/sec)

-centrifugal

om

0.05 • Electrical motors 1 0.05 -8

0 Typical vibration trip levels are shown for different types of machinery.

0 • Machine tools

-7 These trigger limits are recommended starting points for setting vibration

(unloaded)

2 D These ranges are typical. Each machine will have its own personality a a -8 ta

5 • Turbines

warning levels, defined as levels at which abnormal wear occurs.

depending on how it is loaded, the particular installation, and the tolerances

of the machine itself.

ra a 0 n su re u sd m c

e n e rs ts

(Courtesy of PMC/BETA LP, Natick, MA)

Figure 13-38. Vibration warning level guide.

Displacement–Mils (Peak To Peak)

Velocity Limits

ro 0.2 tio c e s n ss

Very Rough

Danger of

In Very Bad d breakdown

oil film

u 0.1

s 0.09 tri 0.5

(shutdown)

Rough

a 0.07 0.4 Bad

lT 0.06 e wear

Causes major

t& 0.04 w

Fair Correct to

save wear

Fair

w id e Good .in o

d 0.02 C Good u

Minor faults

s o m 0.1 tri p

a Excellent a Normal for

Smooth

lte 0.07 n y

x 0.01 t.c new equipment

-7 5 D 0.02 a 2

100,000 e n e

Velocity–Inches/Sec (Peak)

(Courtesy of PMC/BETA LP, Natick, MA)

Figure 13-39. Vibration severity chart.

S ECTION PLC Process Data Measurements C HAPTER 4 Applications

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smooth to very rough severity and indicates possible consequences (bold blue lines indicating very bad, bad, fair, etc.). For machines with higher or lower warning levels, the limits shown on the vibration severity chart should

be increased or decreased, respectively. If the vibration velocity surpasses the maximum allowable limit in a

vibration monitoring system, the PLC can annunciate an alarm condition and initiate a shutdown of the system before a catastrophic failure occurs. Figure 13-40 illustrates a typical direct interface application of a vibration transducer (4–20 mA) to a PLC system where the PLC is responsible for a shutdown command to the machine, if necessary. A vibration transducer/

4–20 mA PLC

System

Vibration Transducer

Machine (a) (a) Digital signal to control a shutdown if vibration limit exceeds the max level. Digital signal controls a shutdown if the vibration limit exceeds the maximum level.

Transducer/Transmitter To PLC

Machine

Remote Reset

7 8 4–20 mA

6 Reset

9 To PLC Analog

4 Equipment Common Shut 11 Power

Machine Circuit

AC Power

Alarm

1 Power 14

Grounding Wire

Monitors Input Levels of Vibration

Alarm

Alarm Circuit

Power

(b) Digital signal to annunciate working levels of vibration. (b) Digital signal annunciates working levels of vibration.

Figure 13-40. Vibration transducers used in a PLC system to (a) control a shutdown

command and (b) monitor vibration levels.

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S ECTION PLC Process Data Measurements C HAPTER 4 Applications

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transmitter combination can also be interfaced with a programmable control- ler to monitor vibration. The internal contacts of the transmitter can then be used to shut down the machine or system if the vibration level surpasses the specified alarm limits.

1 3 -9 S U M M A RY

In this chapter, we introduced basic measurement concepts that explain how data and errors are interpreted and analyzed. This information, which is based on statistical analysis, is very helpful when implementing an intelligent or knowledge-based PLC system.

We also explained different techniques used to transform the physical measurement of a transducer sensor into a voltage or current signal. Trans- ducers, in general, are composed of several intermediate measurement and connection elements, which vary depending on the type of transducer— thermal, displacement, pressure, or flow. Process control systems, which we will discuss next, use these devices to monitor system variables.

K EY bridge circuit T ERMS Bourdon tube

displacement transducer flow transducer guarantee error linear variable differential transformer (LVDT) load cells mean median mode orifice plate potentiometer pressure transducer propagation error resistance temperature detector (RTD) standard deviation strain gauge thermal transducer thermistor thermocouple thermopile turbine flow meter Venturi tube vibration transducer

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