VALVE POSITION MONITORING

The Linear Variable Differential Transformer (LVDT) isideally suited for valve position monitoring.In this type ofapplication the LVDT is used to provide positional feedback to the governor control system to effect aclosed loop system.The electrical properties of theLVDT therefore play a key role in determining the systemresponse.Linearity is obviously key as well as a robustconstruction with flexible mounting options.AC type devices (as opposed to DC) are exclusively used for thistype of application,which permit long cable runs and offset adjustment with gain control.The picture belowshows a heavy industrial LVDT utilised in a valve positionapplication. The Aegis system offers a high integrity low cost multi- channel monitoring solution in a compact 3U,19” rack format.The system utilises a common display to view positional information as well as to set up the dual level alarm trip facilities.Each module excites an LVDT with a sinusoidal 3.0kHzwaveform of fixed magnitude.The construction of the LVDT provides a feedback signal to the module via inductance in to the secondary windings from the primary dependant on stroke position.The centre of the stroke range is normally the null point.The module performs rectification of the secondary induced signal converting to relative displacement.The module is usually configured to show a percentage of the total stroke length required.Calibration is achieved once the LVDT transducer isinstalled and the zero displacement point can be determined;this is then set on the module.The LVDT is then stroked to its maximum travel and the unit adjusted to display 100% displacement.It is also possible to display actual distance through the addition of a scaling factor.The module also has a calibrate function which can beenabled remotely.Once in calibration mode the transducer signal is disconnected and replaced with the calibration signal which is common to all units within the rack.The calibration signal is normally set to be 50% of the stroke length,therefore the standard analogue outputs available (4-20mA,0-5V) can be confirmed as mid range.The rack also has a common reset facility to clear alllatched alarms,which can be operated remotely in conjunction with the calibration function for alarm trip testing.As well as two sets of relay contacts per channel for the dual level alarms,(configurable for both positive and negative going alarms) each module also has a transducer integrity relay.This will indicate a fault if either the primary or secondary windings exhibit a continuity problem or short to earth.
Heavy industrial LVDT monitoring valve position Aegis valve position industrial monitoring system The usual LVDT’s for this application have universal joints at each fixing point.This feature allows for any lateral movement as well as being a positive fixing.Other options include rod end types,which tend to bespring-loaded but not suitable for this type of application since spring loaded LVDT’s are subject to bouncing when the valve is “hunting” or oscillating.Mounting of the LVDT is also critical to the applicationcustom bracketry may be necessary to position the transducer,away from locations where high temperature steam leaks or vents occur.Although the Sensonics range of devices operate up to 180ºC the life of the LVDT can be extended by minimising these temperature extremes.The LVDT drive electronics would normally be mountedlocally (up to 1 00 metres) either in a weatherproofhousing or in a modular rack assembly.The use of external electronics has a number of advantages over internal electronics. 1 .LVDT operating temperature increased to 220ºC 2.Setting up is made easier as zero and gain controls are accessible.3.Local 4-20mA output can be made available. The LVDT is usually driven by a 3kHz oscillator whichenables frequencies up to 300Hz to be monitored, permitting a fast closed loop response within the control system.Drive electronics can also be combined with signalconditioning functions to provide various forms ofpositional information in addition to alarm triggering and process outputs.A twelve channel Sensonics Aegissystem is illustrated below.

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