Machinery Vibration and Your PLC

Note: Because of the length of this article, we recommend that you print it to read it. It is also a great article to keep on file.

 

This article explains how to monitor vibration on common plant machinery using accelerometers. It includes a review of the hardware available today and what challenges condition monitoring providers are faced with to meet the requirements of the future.

 

Monitoring the running condition of plant machinery is, without a doubt, important. Most of the efforts to move away from Preventive Maintenance (time-based maintenance) to Predictive Maintenance (also called condition-based maintenance) have come in the last 15 years, particularly in large industrial plants. We have learned that by monitoring plant machinery, we can plan outages and repairs and save a lot of money in doing so. Unplanned outages, resulting from machinery failures, are very costly. The outages are usually longer and the repair cost is often higher because damaged parts have to be replaced or repaired instead of just replacing worn parts.

 

Today's Monitoring Practices (Condition Monitoring)

Vibration monitoring is relatively easy to install and use on a large variety of plant equipment. Other aspects of a "good" equipment reliability program include temperature monitoring, analysis of lubricating oil, careful alignment of equipment following installation or repairs, and thermographic profiling using an infrared camera.

In a large industrial plant, such as a refinery or chemical plant, vibration monitoring is commonplace. Many operators of smaller processing plants are also using vibration monitoring because they have learned of the benefits in doing so. Vibration monitoring is either "continuous" or "periodic":

  • Continuous monitoring means a machine is equipped with vibration sensors and dedicated monitoring instruments that are permanently installed to monitor the machine on a continuous, around-the-clock basis.

  • Periodic monitoring is usually done with a portable data collector that records and stores data that is later downloaded to a computer running a database program. Here it is compared with previous readings. The data can be analyzed and important trend information is revealed by these computer programs. In smaller processing plants, periodic monitoring is also done with a portable "vibration meter". The vibration levels read from the meter might be logged manually in a log book instead of a computer program.

Critical Machinery

Here are the factors to help you decide when to use continuous monitoring and when to use periodic monitoring:

  • Criticalness of the machine to the process

  • Size of the machine, which generally is proportional to the cost of replacing it

  • Availability of spares (both machines and spare parts for them)

  • Safety or health factors 

Type of Machine (Examples)

Critical

BOP (Balance of Plant)

Steam Turbine Generator

X

 

Integral Gas Fired Reciprocating Compressor

X

 

Boiler Feed Pump

 

X

Large Fan or Blower

 

X

Cooling Tower Fan

 

X

Large Pump

 

X

 

Transducers and Measurements

Two of the most common vibration transducers used to measure machinery vibration today are the non-contacting Eddy Probe displacement measuring system and the industrial Accelerometer. These transducers are very different and their measurements are very different as well. It is important to understand their use.

 

Eddy Probes

The non-contacting eddy probe displacement measuring system, Figure 2, measures relative movement between fixed parts of the machine (bearings) and moving parts of the machine (shaft and rotor).

  • Shaft vibration is the measurement of relative motion between a shaft on a machine and the bearings supporting the shaft. It is measured in mils peak to peak (one mil is equal to 1/1000th of an inch).

  • The axial position of the rotating element is a measurement of the thrust load against a thrust bearing. It is measured in mils.

  • The displacement measurement system output for vibration is alternating (ac) while position is a stationary (dc) measurement. These signals have to be processed differently by the instrumentation systems they are connected to.

It is important to note that relative shaft vibration measurements, using non-contacting eddy probe displacement measuring systems, only apply to machinery with sleeve type bearings. This transducer should not be used with rolling element (anti-friction) bearings.

 

 

Accelerometers

Industrial Accelerometers, as applied to machinery, measure casing vibration and bearing vibration. This is a measurement of the effect of the forces that originate from within the machine. These forces are mechanically (and sometimes acoustically) transmitted to the surface of the machine or to the bearings (where the transducer is located).

 

Accelerometers are installed on the outside of the machine casing or on bearings which makes installing them relatively easy. While the signal generated within the accelerometer is acceleration, (measured in g) it is usually converted to velocity by the transducer itself or within the instrumentation systems. Velocity is the preferred measurement type (inches par second, ips) because it tends to have a "leveling" effect at both low frequencies and high frequencies. It is easy for engineers and technicians to relate to velocity levels, when judging severity, because they do not have to consider the frequency part of the equation. The relationship between acceleration, velocity, and displacement is illustrated in Figure 4. There is also a comparison table in Appendix A.

 

Transducer Placement

 

Generally, non-contacting eddy probe displacement transducers are installed inside machine cases (machines with sleeve bearings) and accelerometers are installed outside machine cases and on bearing assemblies as illustrated in Figures 5 and 6. Wiring should always be shielded and should be installed in conduit for maximum protection.

Non-contacting eddy probe drivers and signal conditioners have to be mounted near the machinery due to the limited length of the transducer systems coaxial cable. These should be mounted in stainless steel enclosures or explosion proof enclosures, depending on the area classification. Signal conditioners, or other electronics, can be mounted away from the transducers at aconvenient location or in control rooms.

 

 

Monitoring Methods

Critical machinery should always be monitored with a permanently installed continuous monitoring system to protect the machine, the process, and personnel. Often the monitoring systems for these machines are configured for safety shutdown. Typically two levels of alarms are provided; a first stage alarm providing a warning, and a second stage alarm signifying an operating condition that has progressed to a more serious level or triggering a shutdown. Contact closures are often taken to a PLC where these events are logged and announced to operating personnel.

 

A monitoring objective for less critical machinery (or Balance of Plant machinery) might be very different. For example, a remotely located water supply pump routinely has its supply restricted by clogged screens. This causes the pump to run rough and even cavitate. There is nothing wrong with the pump but if left to run this way for extended periods, damage would eventually occur. In this case, by continuously monitoring this pump, a vibration alarm would signify time to clear the screens of debris.

Another example might be a case where an air supply fan routinely becomes unbalanced due to product buildup on the fans blades. When this condition occurs the vibration level increases due to the amount of unbalance. If left to shake, damage is inevitable. In this case vibration would be used to notify maintenance when cleaning is required.

 

In both of these examples, a PLC is well suited as the monitoring instrument. The vibration is monitored continuously, the levels can be displayed and alarms can be used to indicate needed action.

 

Understanding Vibration Data

 

Overall Vibration

Overall vibration is a broadband measurement of the signal from the transducer. The vibration signal is not broken down into specific frequencies. It consists of energies generated at running speed, resonances, and multiples of running speed, for example. The largest frequency component of the overall vibration signal is usually the running speed signal. An example of a time based overall signal, where all of the energy is at one frequency, is illustrated in Figure 7. It is wrong to assume this to be the case all of the time. The typical vibration signal usually contains other frequencies.

 

The overall vibration measurement can be regarded as the first step in determining the running condition of a machine. All permanently installed continuous monitoring systems measure and display the overall vibration level.

 

Vibration Spectrum

The vibration spectrum is generated by doing a frequency analysis of the signal from the transducer. The process of generating a spectrum is called an FFT (Fast Fourier Transform) analysis. The vibration spectrum is a display that shows how much of the energy from the vibration signal occurs at specific frequencies. It clearly shows how much of the vibration level is at running speed and how much is at other frequencies, as seen in Figure 8.

 

The vibration spectrum is a powerful tool for understanding causes of vibration. It helps analysts diagnose machinery operating characteristics. Keeping in mind that vibration caused by unbalance always occurs at running speed, consider the case where an alarm on a monitored pump is tripped indicating high vibration. You would get ready to balance the pump. A portable data collector to analyze the signal first would show the high vibration to be at vane passing frequency (running speed times the number of vanes). This can result from eroding vanes. Trying to balance this pump to correct the high vibration would not be time well spent! The vibration analysis identifies the source of the vibration.

 

Today you can buy advanced monitoring systems that have built-in diagnostic software. Vibration spectrum analysis is used by these systems to automatically perform diagnostics and give the user a list of suggested causes when high vibration is identified. Similar software is also available for the database programs that are a part of the periodic monitoring programs.

 

Trending Vibration Data

Displays of vibration trends provide another method of determining the severity of a measured condition. A trend is an easy display to interpret-even by personnel that are not trained in vibration! For example, Figure 9 shows a trend that is increasing. If the display has alarm levels superimposed on the same display, it is easy to tell when the alarm levels will be reached, if the rate of change of the trend continues at the same pace. It is also easy to see accelerated rates of change on trend displays which mean the alarm levels will be reached sooner.

 

The trended data is usually the overall vibration level but some of the vibration analysis software programs provide for trending selected frequencies from vibration spectrum data. The latter can be useful when trying to understand unusual machinery behavior. An example might be trying to develop an understanding of the effects of process changes on machinery characteristics.

 

Using a PLC to Monitor Vibration

Why not? The PLC has demonstrated extreme reliability. Instrument and controls personnel understand programming PLCs to obtain suitable data displays and setting alarm set points. The vibration levels can be made clearly visible to operators in the control room, right along side all of the other important data that is crucial to continuous plant operation. Vibration data has proven to be valuable in determining the running condition of plant machinery and even extending running time.

 

Large Plants

In a large plant that has an equipment reliability group, there are trained vibration personnel who already have their hands full each day. If this plant moved some of the points that they are monitoring, using a periodic monitoring program, to a program where these points were monitored by a PLC, there would be a lot gained:

  • Agroup of machines would now be monitored continuously rather than periodically

  • The vibration expert would have more time to resolve other critical vibration problems in the plant. The plant monitoring effort would become a shared responsibility between the controls group and the vibration experts.

Small Plants

A small plant cannot afford to have a resident vibration expert. This work is typically contracted for when needed. A small plant will not have as much machinery as does a large plant but the machinery they do have has to be kept running. It makes sense to have a PLC monitor the bearings on the fans. This is especially true if the plant already has PLC's in service. The incremental cost for vibration transducers, signal conditioners, wiring, and an analog input module provide a significant benefit at much less than the cost of a permanently installed continuous monitoring system. The PLC can now notify operators when it is time to have routine maintenance work done on the fans. In the worst case, where there might be a more serious machine problem, an outside contractor can be called in.

 

PLC Interfaces

The most common PLC interface for vibration data is the 4-20 mA signal. This is an old reliable favorite and is easy to use. It is relatively immune to noise and ground loop problems by following simple installation guidelines. The 4-20 mA signals from vibration transducers are handled the same way 4-20 mA signals from other sensors or devices, already in widespread use, are handled.

Vibration transducers, with their companion signal conditioners, and 2-wire vibration transmitters provide conditioned signals that can be applied directly to a PLC using an analog input module. The signals from these devices are accurately measured, filtered or otherwise conditioned, and scaled to a proportional 4-20 mA output signal. Instrument and controls technicians already know how to program the PLC to display data, build trend files, and to set alarm points.

  • The 2-wire vibration transmitter is the easiest to install and to use. It is a loop powered transducer which means it will need an external 24 Vdc power supply to power the loop. One power supply can provide power for several transmitters. The transmitters output can be applied directly to an input channel on an analog input module set to accept a current signal. The signal can also be dropped across a resistor of a module set to accept 1-5 Vdc.

  • Using signal conditioners and separate vibration transducers requires some additional planning, although the 4-20 mA connection to the analog input module in the PLC is the same. As a point of caution, signal conditioners have two types of 4-20 mA outputs; active or loop powered. If it has an active output, the loop is powered from within and no external loop power supply in needed. Signal conditioners are used because they offer more options for tailoring measurements to specific applications. They offer filtering, choosing measurement types and setting special scales. They can also be configured with set point and relay drivers for special tasks.

The signal conditioners themselves need power which typically is a 24 Vdc. Again, one power supply can power several signal conditioners. The power supply and signal conditioners can be mounted in a convenient location in an equipment enclosure or in a separate steel enclosure. They usually need to be protected from RF sources. Signal conditioners for eddy probe displacement transducers have to be located near the machinery due to the limited length of the coaxial cable that is part of this transducer system.

 

Future Challenges

While the 4-20 mA method of connecting to a PLC is technically a sound approach, it has two shortcomings:

  • It requires wiring runs from the PLC to each transducer. As the number of points to be monitored increases, so does the cost of installing wiring.

  • The vibration spectrum information still has to be collected manually by a portable data collector on a periodic basis.

Keep in mind that vibration monitoring instrumentation systems, with the capability of automatically acquiring vibration spectrum data, are available today. These systems have software programs that process the data and provide the user with advanced warnings of pending machinery problems and can even offer suggested corrective action. This information can even be transmitted to remote sites where a specialist can receive the same information and help the plant make important decisions about machinery operating conditions. Systems with these capabilities are expensive and if they are networked, this is done with proprietary hardware and software which limits their effectiveness as information systems.

 

The challenges that vibration instrumentation vendors have today are to provide the industry with monitoring and analysis capabilities that the user can benefit from at reasonable costs. If monitoring systems can be installed at reasonable costs, more points can be monitored in a plant which expands the benefits even more.

 

Monitoring system wiring and hardware costs can be reduced by clustering the transducer signal conditioners. Figure 11 shows one technique for connecting groups of sensors. Wiring can be simplified by connecting the "multi-channel module" to a plant network. A network, such as DeviceNet, would provide a direct connection to a PLC. There are many advantages in using industry standard networks. Most importantly, they are controlled and they are understood by users. A multi-channel vibration module would become another node on the network. Such a module could handle both overall vibration and vibration spectrum data. However, there is a cost barrier to provide the latter. This is because acquiring and processing the data in order to obtain the vibration spectrum requires considerably more electronic parts.

 

A machinery vibration monitoring capability, using a PLC, that just measures overall vibration levels is a valid approach for many applications. Such a system would not carry the extra burden (cost) required to provide the vibration spectrum data.

 

For applications where the user can benefit from also having vibration spectrum data, more coordination is required between display hardware and software providers. These users want to be able to have all forms of the vibration data available to them on their displays. The response of the display systems must be fast and, in some cases, remote. A vibration analyst will want to switch from a display of bar graphs, showing vibration level and alarm limits, to a trend display, and to a spectrum display at a near instantaneous rate. The analyst will also want to compare one measurement point against another. This means rapid switching between measurement points is a requirement for these new systems. Better display drivers and better data exchange software will be required in order to achieve these display goals. Database software will need hierarchy levels to support facility, plant, machine, and point data.

 

Summary

Monitoring vibration to determine the running condition of plant machinery has proven to be a sound practice. Vibration sensors, monitoring channel electronics, and installation techniques have been refined to provide very reliable monitoring, and safety shut down capabilities, for important plant machinery. Future monitoring systems will take advantage of developing device level, control level, and information level networks.

 

Many plants can benefit significantly by adding vibration measurements to their PLCs. Loop powered vibration transmitters or vibration signal conditioners, connected to an analog input module, can be a relatively low cost addition to a PLC. Plant operators can have machinery running condition information available to help them make important decisions about keeping a process running or stopping it. Plant personnel know and understand the 4-20 mA signal very well. New hardware for communicating 4-20 mA signals, including wireless links, are being introduced on a continuing basis.

 

.

 

Appendix A

Acceleration, Velocity and Displacement Vibration Level Comparison Table
 

Freq. in Hz

Freq. in CPM

Acceleration

g peak

Velocity

ips peak

Displacement

mils p-p

10

600

0.08 g

0.5 ips

15.92 mils

20

1200

0.16 g

0.5 ips

7.96 mils

30

1800

0.24 g

0.5 ips

5.31 mils

40

2400

0.32 g

0.5 ips

3.98 mils

50

3000

0.41 g

0.5 ips

3.18 mils

60

3600

0.49 g

0.5 ips

2.65 mils

62

3720

0.50 g

0.5 ips

2.57 mils

70

4200

0.57 g

0.5 ips

2.27 mils

80

4800

0.65 g

0.5 ips

1.99 mils

90

5400

0.73 g

0.5 ips

1.77 mils

100

6000

0.81 g

0.5 ips

1.59 mils

120

7200

0.97 g

0.5 ips

1.33 mils

140

8400

1.13 g

0.5 ips

1.14 mils

160

9600

1.30 g

0.5 ips

0.99 mils

180

10800

1.46 g

0.5 ips

0.88 mils

200

12000

1.62 g

0.5 ips

0.80 mils

220

13200

1.78 g

0.5 ips

0.72 mils

240

14400

1.94 g

0.5 ips

0.66 mils

260

15600

2.11 g

0.5 ips

0.61 mils

280

16800

2.27 g

0.5 ips

0.57 mils

300

18000

2.43 g

0.5 ips

0.53 mils

400

24000

3.24 g

0.5 ips

0.40 mils

500

30000

4.05 g

0.5 ips

0.32 mils

600

36000

4.86 g

0.5 ips

0.27 mils

700

42000

5.67 g

0.5 ips

0.23 mils

800

48000

6.48 g

0.5 ips

0.20 mils

900

54000

7.29 g

0.5 ips

0.18 mils

1000

60000

8.10 g

0.5 ips

0.16 mils

2000

120000

16.2 g

0.5 ips

0.08 mils

5000

300000

40.5 g

0.5 ips

0.03 mils

 

This article is provided by Hardy Instruments. www.hardyinst.com.