a feeling for what might fail

3
Feature 30 30 www.worldpumps.com WORLD PUMPS April 2013 A s a strategy, condition-based predictive maintenance (CBPM) provides the ideal compromises between failure-based maintenance (a.k.a. “run-to-fail”) and the potentially equally expensive practice of conducting maintenance on a periodic basis irrespective of machine use. Frequently, a rise, or in some cases a drop, in vibration levels provides the earliest indication that a bearing, gearbox, motor or other electro-mechanical component is in need of maintenance. Moreover, analysis of the vibration signa- ture can reveal the nature of the fault; for example, a broken tooth on a gear or a worn bearing. During one inspection it was noticed that the vibration levels at the impellor end of one fan were higher than normal. The data was analysed and it was found that the overall RMS vibration amplitude had more than quadrupled in the space of the few weeks since the previous survey. Mechanical looseness The fan was displaying symptoms of mechanical looseness; most likely some- where along the shaft or a shaft-mounted component judging by the vibration signature. Plant staff agreed to take action at the earliest possible opportunity. However, that evening, the fan threw an impellor blade which broke through the fan housing and embedded itself into a nearby control panel. Had they their time again, those involved in maintenance would not act in any other way. Indeed, based on the informa- tion to hand, the best course of action had been taken; as the data captured during the survey would not have condemned the fan and led to an imme- diate shutdown, impacting on production. In essence, whilst the vibration signature provided a strong, and as it transpired accurate, indication of what was failing it was not possible to estimate the rate of deterioration. Had vibration levels started rising the day after the previous survey or only a few hours before the Vibration monitoring is of particular use when it comes to protecting plant and machinery that is not only expensive in its own right but is also heavily depended on to keep operations going. One ques- tion though is: how often should vibra- tion levels be measured as part of a CBPM strategy? For instance, a steel mill in the United Kingdom, recognising the usefulness of vibration monitoring, used to conduct surveys on a monthly basis. These surveys included the use of portable measuring instruments to check the vibration levels of, amongst other machinery, fans supplying the furnace’s air/fuel mix and also used to expel waste gases. Condition-based predictive maintenance (CBPM) is increasingly popular as a practical and cost-effective means of protecting pumps, motors, gearboxes and other machinery. But of all parameters that might be monitored as part of a CBPM strategy, vibration could be the most useful – advises Monitran's Andy Anthony. A feeling for what might fail Operating 0262 1762/13 © 2013 Elsevier Ltd. All rights reserved Figure 1. An MTN/1185 vibration sensor on a pump at the Adom desalination plant.

Upload: andy

Post on 05-Jan-2017

218 views

Category:

Documents


4 download

TRANSCRIPT

Page 1: A feeling for what might fail

Feature3030

www.worldpumps.com

WORLD PUMPS April 2013

As a strategy, condition-based

predictive maintenance (CBPM)

provides the ideal compromises

between failure-based maintenance

(a.k.a. “run-to-fail”) and the potentially

equally expensive practice of conducting

maintenance on a periodic basis

irrespective of machine use.

Frequently, a rise, or in some cases a

drop, in vibration levels provides the

earliest indication that a bearing, gearbox,

motor or other electro-mechanical

component is in need of maintenance.

Moreover, analysis of the vibration signa-

ture can reveal the nature of the fault; for

example, a broken tooth on a gear or a

worn bearing.

During one inspection it was noticed that

the vibration levels at the impellor end of

one fan were higher than normal. The

data was analysed and it was found that

the overall RMS vibration amplitude had

more than quadrupled in the space of the

few weeks since the previous survey.

Mechanical looseness

The fan was displaying symptoms of

mechanical looseness; most likely some-

where along the shaft or a shaft-mounted

component judging by the vibration

signature. Plant staff agreed to take action

at the earliest possible opportunity.

However, that evening, the fan threw an

impellor blade which broke through the

fan housing and embedded itself into a

nearby control panel.

Had they their time again, those involved

in maintenance would not act in any

other way. Indeed, based on the informa-

tion to hand, the best course of action

had been taken; as the data captured

during the survey would not have

condemned the fan and led to an imme-

diate shutdown, impacting on

production.

In essence, whilst the vibration signature

provided a strong, and as it transpired

accurate, indication of what was failing

it was not possible to estimate the rate

of deterioration. Had vibration levels

started rising the day after the previous

survey or only a few hours before the

Vibration monitoring is of particular use

when it comes to protecting plant and

machinery that is not only expensive in

its own right but is also heavily depended

on to keep operations going. One ques-

tion though is: how often should vibra-

tion levels be measured as part of a

CBPM strategy?

For instance, a steel mill in the United

Kingdom, recognising the usefulness of

vibration monitoring, used to conduct

surveys on a monthly basis. These surveys

included the use of portable measuring

instruments to check the vibration levels

of, amongst other machinery, fans

supplying the furnace’s air/fuel mix and

also used to expel waste gases.

Condition-based predictive maintenance (CBPM) is increasingly

popular as a practical and cost-effective means of protecting pumps,

motors, gearboxes and other machinery. But of all parameters that

might be monitored as part of a CBPM strategy, vibration could be

the most useful – advises Monitran's Andy Anthony.

A feeling for what might fail

Operating

0262 1762/13 © 2013 Elsevier Ltd. All rights reserved

Figure 1. An MTN/1185 vibration sensor on a pump at the Adom desalination plant.

WOPU0413_Feat_Monitran 30 28-03-13 09:54:22

Page 2: A feeling for what might fail

Feature 31

www.worldpumps.com

WORLD PUMPS April 2013

one conducted that day? Just where

were they on the failure curve?

Had the survey taken place a day later,

the impellor blade would of course

already have been thrown; and from a

seemingly healthy machine going by the

previous survey’s readings.

There is therefore much merit in

performing condition monitoring all of

the time and vibration sensors are increas-

ingly being employed as permanent

fi xtures to pumps, motors, gearboxes and

other assemblies to supply monitoring

and control systems with vibration data.

Accordingly, alarms can be raised the

moment vibration levels warrant investi-

gation and, worst case, machinery can be

automatically shut down if vibration levels

rise so quickly there is no time to

investigate.

There is also another reason for

monitoring vibration levels. Vibration is

the movement of something, and that

requires energy. Hence, if a powered

electromechanical assembly is vibrating

more than it should be then it will

undoubtedly also be less effi cient (in

terms of power in vs. work achieved) than

intended.

For example, Monitran’s vibration

sensors were recently used as part

of a valve monitoring system for a

compressor. Specifi cally, valve wear can

result in a high pressure gas leak into

the low pressure side of the system

– i.e. it does not physically escape –

so the compressor just has to work

harder to produce a given volume

of compressed gas.

The pressure is ON

One sector making very good use of

vibration monitoring is the water industry,

particularly seawater desalination plants

using a process called reverse osmosis

(RO) to make water for human consump-

tion and agricultural use. While salt can

be removed from seawater in a number

of ways – including distillation, freezing,

dehumidifi cation – RO is used extensively

in many countries for producing drinking

water.

The RO desalination process requires

passing seawater at very high pressure

(circa 1,000 psi) through a semi-perme-

able membrane. This requires a great deal

of energy and is why most RO desalina-

tion plants are located close to power

stations.

One country particularly reliant on

seawater desalination is Israel. It has three

fully operational plants and a further two

scheduled to come online shortly. The

ones in operation are: Adom, near

Ashkelon, which came online in 2005; Via

Maris near Palmachim, which came online

in 2007; and Omis-Water, near Hadera,

which came online in 2009.

Between them, the three plants have

several hundred high pressure pumps,

each of which is fi tted with at least one

vibration sensor. The majority of these

sensors are Monitran MTN/1185 general

purpose, top-entry transducers (Figure 1

shows a sensor in situ on a pump in the

Adom desalination plant).

These sensors output a direct current

(DC) in the range 4-20mA, proportional

to velocity. The outputs then feed into

PLCs which control the pumps, and in

which two pre-determined alarm

levels have been set for each sensor.

If vibration levels reach the fi rst level,

an alarm is raised. If vibration levels reach

the second and higher level, the PLC

automatically shuts down the relevant

pump.

Advice

Above, we have cited the use of general

purpose sensors with DC outputs. Sensors

have a number of characteristics which

should be considered during the selection

process. In addition to top-entry (as per

Figure 1), sensors are available where the

cable enters from the side. Choosing

between top- and side-entry will be

governed by how you plan to run the

cables. Also, the connection to the sensor

can be integral (as per Figure 1) or via a

connector.

Should you wish to go down the route of

monitoring vibration levels at set intervals

(i.e. a temporary installation) then sensors

are available with magnetic bases. It is

essential to use the same sensor in

exactly the same position for each

reading and you may wish to consider

using paired ID tags/labels.

Next, the presence or risk of water, high

humidity levels or hazardous atmospheres

will steer you toward certain sensor

sealing characteristics (IP67, 68...) or

intrinsically safe (ATEX, IECEx) devices.

Temperature needs to be considered too,

as output drift (with temperature) may be

an issue; but only if you intend to operate

Vibration sensors are typically electronic

devices, employing either piezoelectric or

piezoresistive technology. Of these two

technologies, the former is more

prevalent in monitoring applications.

In the diagram, the mass is bonded to a

piezoelectric crystal which, when placed

under tension or compression, generates

an electric charge proportional to the

acceleration of the mass; hence why

vibration sensors are also referred to as

accelerometers.

The electric charge is conditioned by the

circuitry (PCB) to produce a more useful

output; typically a direct current (DC) in

the range 4-20 mA for ease or integrating

the sensor into a monitoring system or a

Programmable Logic Controller (PLC).

However, sensors with AC outputs are

also popular.

The output (proportional to acceleration

in mm/s/s) may also be conditioned to

represent velocity (mm/s) or displacement

(mm), and sensors are also available with

multiple outputs (e.g. acceleration and

velocity).

With no moving parts, a piezoelectric

sensor off ers long-term reliability and

stability. Typical frequency responses

range from about 1 Hz to 10 kHz or

more. As for the operating temperature

range, even a general purpose sensor will

cover -25 to 90°C.

The vibration sensor

WOPU0413_Feat_Monitran 31 28-03-13 09:54:22

Page 3: A feeling for what might fail

Feature3232

www.worldpumps.com

WORLD PUMPS April 2013

the sensor close to its declared operating

limits. See box ‘Taking Measure.’

Summary

CBPM helps plant/facility owners walk

that fi ne line between “run-to-fail” and

over servicing machinery, and there is

much to be said for monitoring 24/7.

Also, of all parameters that could be moni-

tored, vibration is perhaps the most useful,

as the analysis of vibration signatures can

provide an indication of which components

are failing or have already failed. With such

knowledge you can better schedule mainte-

nance routines and make sure replacement

parts/components are delivered in time.

There are several diff erent types of vibration

sensors available on the market, but selec-

tion need not be a daunting process if you

follow the above advice regarding charac-

teristics and measure normal operating

vibration levels to determine the sensor’s

sensitivity; thus enabling you to determine

the limits at which to raise an alarm or

perform an emergency shutdown.

www.monitran.com

For any monitoring application, it

is important to make optimum

use of the vibration sensor’s

output range, without sacrifi cing

sensitivity. For those interested in

the maths, it is worth noting that

many sensors are guaranteed to

operate across the range ±80 g,

though they can typically

experience far greater forces

without fear of damage.

Assuming a sensor’s sensitivity is

100 mV/g the output will swing

between -8 and 8 V across its

declared operating range.

However, if the sensor is likely to

be exposed to no more than ±8 g

then it would be wiser to select a

sensor with a sensitivity of 1 V/g

as the output voltage swing will

provide better resolution (10x in

fact) of the vibration levels.

Accordingly, if you are considering

mounting vibration sensors onto a pump

or motor as part of your CBPM strategy it

is always worth measuring the vibration

levels fi rst. For this purpose a hand-held

instrument like the MTN/VM220 will

certainly fi t the bill.

Conforming to ISO10816-3, the MTN/

VM220 can measure and store readings

for vibration (as acceleration, velocity or

displacement) and temperature against a

real-time clock. It is supplied with a probe

into which is integrated a dual-output,

general purpose sensor, the MTN/2200T

(with default sensitivities of 100 mV/g and

10 mV/°C) and to which can be attached

a magnetic base or spike.

Such a hand-held instrument is therefore

very useful for establishing the normal

operating conditions of machinery before

installing vibration sensors on a

permanent basis or for maintenance

engineers taking readings on a scheduled

basis, as part of their CBPM processes.

Taking measure

Contact

Andy Anthony

Monitran Ltd.

Monitor House Hazlemere Road

High Wycombe, Buckinghamshire HP10 8AD

United Kingdom

WOPU0413_Feat_Monitran 32 28-03-13 09:54:22