Management

Setting Vibration Action Trigger values

An Action Trigger value is set which when passed will lead to some further action. This document demonstrates how to determine this value Intially i.e in installation and subsequently once an understanding has been built about the typical vibration levels experience in service.


Vibration Action Trigger Values are subject to periodic review.


Vibration Analysis – Initial Limits

                Initial Action  trigger levels  should be set as per the following

Setting of Alarm Limits through Experience


Determining validity of  readings for setting Alarm Limits

Before new levels can be set it is necessary to ensure that the data being used are suitable.


There should exists 10 or more  readings taken when the unit was operated in a consistent way

There are  5 rules that must be satisfied

No single  reading occurs  outside of (Xm+3.Dv) or (Xm-3.Dv)

No two out of three readings occur outside of (Xm+2.Dv) or (Xm-2.Dv)

No four out of five readings occur outside  of (Xm+1.Dv) or (Xm-1.Dv)

No eight successive readings occur on one side of the mean value

For   Safety Critical Equipment as identified in FMECA,  or Equipment which has been classed as Critical, there should be no consistent trend increase  or decrease point by point over more than 5 consecutive points. For over equipment this should not be for 7  consecutive points


Additionally for best results the Variation  value  Cv should be less than 0.33



Calculating Mean Value Xm


X =  A Measured Value in a group

Xm =  The average value of the Group

n = number of measured values




Calculating Deviation Value Dv


Dv = Deviation Value

X =  A Measured Value in a group

Xm =  The average value of the Group ( Xa=X/n)

n = number of measured values



Calculating Variation Coefficient Vc


Vc = Variation Coefficient

Dv = Deviation value

Xa =  The average value of the Group ( Xa=X/n)





Setting Upper Limit  (Action Trigger Value)


UL  =  Upper Limit

Dv = Deviation Value

Xa =  The average value of the Group




Setting Lower Limit


LL  =  Upper Limit

Dv = Deviation value

Xa =  The average value of the Group




Action Trigger Value

This is the value at which some other further action is taken. This  may or may not include equipment overhaul




Worked Example


Applying the rules to check that the readings are suitable to generate a new  Action Trigger Value


   






Rule

Result

OK/NOK

10 or More values

10

OK

No single  reading occurs  outside of (Xm+3.Dv) or (Xm-3.Dv)

UL and LL  are 5.59 and 1.1. Maximum value is 4.5 and minimum value is 2

OK

No two out of three readings occur outside of (Xm+2.Dv) or (Xm-2.Dv)

(Xm+2.Dv) = 4.84 ,(Xm-2.Dv) = 1.85. Neither of these are exceeded

OK

No four out of five readings occur outside  of (Xm+1.Dv) or (Xm-1.Dv)

Xm+1.Dv) = 4.14 ,(Xm-1.Dv) = 2.6. Neither of these are exceeded. In each case only a single isolated value exceeds these

OK

No eight successive readings occur on one side of the mean value

No more than two appear  successively either side of the mean

OK

For   Safety Critical Equipment as identified in FMECA,  or Equipment which has been classed as Critical, there should be no consistent trend increase  or decrease point by point over more than 5 consecutive points. For over equipment this should not be for 7  consecutive points

There are 3  only readings which decrease consecutively

OK

Variation Coefficient less than 0.33

Coefficient is 0.223

OK


The new Action Trigger value is the Ul value which is 5.59