IINDU


NDUSTRY FOC


OCUS


CHEMICALS & P


S & PHARMACEUT


UTIICAL S


Acoustic emission can identify valve gas losses and save costs


fy


platforms, acousti emssion technology i also fin infinding use in other process industries such as power generation, nuclear, chemicals and pharmaceuti als, says Ti ma


r p s in tries such


pow r gen ration, n clear, chemicals an pharmaceuticals, says Tim Bradshaw, general manager, MISTRAS Group, UK operations


STRAS Group, U


disruption, and by personnel with just one hour of training.


T


he detection of leaks by acoustic methods was used well before the


technology of acoustic emission (AE) was applied elsewhere. However, the equipment that was available was susceptible to noise fromsources other than leaks. As the AE industry grew in other areas, technology became available which meant that the problems of environmental noise could be largely overcome.


A trials programme carried out by BP in the early 1980s selected the AEmethod as most promising for development as a method of quantifying leakage throug h valves. The field development programme lasted six years and involved removing more than 800 valves fromservice that had been identified as leaking, and retesting these in the laboratory to build up a correlation between physical loss and acoustic signal level.


Following this, a ‘best fit’ correlation was developed so that the technology could be easily applied. Factors having a significant effect on the a coustic signa l level include valve type, size, and


differential pressure. This development was followed by the commissioning of a new instrument able to make accurate field measurements to make practical use of the database. The new instrument, the VPAC II from Mistras Grou intrinsically safe, portable


and simple to p, is


use by virtue of all the measurement functions being automatic. This instrument together with the ‘VPAC’ Valve Leak Technology Package as the quantification method is known, has become widely used in the oil and gas industry for the identification and estimation of through-valve gas losses, thus enabling cost effective operational s to be made plant


,


quickly and easily without and maintenance decision


12 12 DECEMBER/JANUAR 201 ANUARY 2017 | AU AUTOMA MAT ATION


The quantification of allows the identification


ifi


he quantificatation of through valve lossesthrough valve losses allows the identification of significant cost savingsf


o sign cant cost saving s through loss contthrough loss controlrol


DE EC ION PRINCIPL DETECTION PRINCIPLE


The source of the acoustic emissions should be considered initially. These are generated by a fluctuating pressure field that is associated with turbulent flow of the fluid at the leak site. The conditions for turbulent flow are met when the inertial effects of the fluid flow overcome the viscous drag; the ratio of the former to the latter is defined as the number. Turbulence has been commence when the value for the


found to Reynolds


Reynolds number is between 103 and 104. Acoustic emission therefore, is an


,


effective method for detection of through valve leaks where the velocity across the leak is sufficiently high with respect to the size of the leak orifice to produce a Reynolds number in this region.


In one petrochemical plant four 24-inc control valves were


,


In one petrochemical plant, four 24-inch control valves were tested using V


tested using VPAC. Two of these were shown tot be leaking more than 2,500 tonnes/year. One oil company identified


C. Tw Two


of these were shown o be leaking more than 2,500 tonnes/year. One oil company identified losses of $14 millionlosses of $14 million from four refineriesfrom four refineries


The calculation to deduce the flow rate in a cylindrical orifice would be simple. However, this is far fromthe real world situation where a leak is likely to be anything but a cylindrical orifice, and is also likely to be made up fro ma number of smaller leaks around the entire valve seat. As a result, an alternative method of correlating the AE received and the flow rate through the leak was required. This was achieved empirically by testing 800 valves in the field and repeating the tests with the valves removed to a flow rig. Valves in the size range one to 18 inches, and of a range of types, were used in this exercise and a database of results was compiled fromwhich a predictive .


equation was derived It is this predictive equation that allows the quantification of through valve leaks in the field.


TEST METH TEST METHOD The operation of the instrument is


simple. The sensor is held in contact with MISTRAS Group ww.mistra


a flat surface, using a suitable acoustic T: 01954 231612 T: 0 www.mistrasgroup.co.uk co.u /AUTOMATION AT /AUTOMATION 4 231612


the 300 memory loc stored with a single


Bradshaw genera l io


couplant such as grease, on the valve to be tested. The current value of the signal level (dB) is noted. This may also be


key-press in one of ations. If a leak is


indicated by a reading greater than normal background (12 -16dB) then readings are taken on the pipework upstream and downstream of the valve. As the signal level will be highest close to the leak and attenuate as the distance from the leak increases, these upstream and downstream figures will be lower if the valve is truly the source of the acoustic emission. The noted reading is then inserted into a PC spreadsheet along with the othe r relevan t


information: valve inlet size, differential pressure across the valve, and valve type. This information is used in the


spreadsheet by the predictive equation to calculate the loss rates. The spreadsheet is often modified to present the loss rate in convenient units such as tonnes/year, m3/day or even product value/period. The VPAC II systemhas now been licensed for use onmore than 200 sites and has proved cap able of quickl y surveying large numbers of valves and estimating losses fr omthe leaking valves .


FUR HER DEVELOPMEN FURTHER DEVELOPMENTS


The detection of through valve leakage using VPAC is not confined to gas systems. Where there is sufficient differential pressure to satisfy the


conditions for turbulent flow, then liquid leakage can also be readily detected. The database of results on liquids has now been built up allowing quantification of through valve liquid losses. At present, work is ongoing to expand this database, which will further improve the liquid leak correlation to larger valve sizes.


Originally developed for estimating leakage rates through valves to flare in oil refineries and offshore platforms, acoustic emission technology is also in


igin lly throu lo fo estim tin leakage rates valves to flare in oil refin ries and offsh re


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