VIBRATION MEASURING SYSTEMS FOR DYNAMIC MACHINERY
Contents
1. by demand even planned or only in alarm condition information to a database in order to establish the monitored machinery condition In the first case the number of relevant information is limited and the user can only compare the measured Total Value with a preset alarm limit In the second case the information provided by the independent monitoring system is helpful and relevant to accomplish a complex analysis and diagnose of machinery NOTE In a combined Machinery Protection MPS and Monitoring System CMS condition monitoring functionality is fed by machinery protection circuit not vice versa Communications from MPS to CMS utilize completely separate busses and networks Thus the MPS is not compromised or impeded in any way by the CMS Documentation Feedback Any suggestions and comments for improving this Application Notes should be e mailed at mainttech live se MaintTech Sweden uses feedback for continuous improvement of our documentation and for future MaintTech products We request comments be specific and include the product name and version We cannot provide personal responses to every message received but please be assured that all feedback will be given careful consideration for future improvements to the MaintTech documentation or software Technical Support Contact Details For any problem regarding this application feel free to contact our support team at mainttech live se To know more about us2. SC200 converters accept direct standard accelerometer connections The SC200 module has the following functions e transforms the accelerometer signal into velocity signal by applying additionally HP and LP filters e transforms the velocity signal into a 4 20 mA dc signal for the external remote indicators or for the acquisition systems SCADA e transforms the velocity signal into a 0 5 V dc signal to be directly connected to the CX RLY C module e ensures a buffered dynamic signal to a BNC connector or to be used for an analyzer temporary connection or to connect the dynamic signal to an external continuous monitoring system 0 to 5 Ydc To field accelerometer C200 CX RLY C 5 b eee seer oreo 4 20 mA dc Dynamic signal Figure 5 For a detailed wiring diagram download the SC200 User Manual from ctconline com NOTE _ CX RLY C can also be connected to a temperature sensor with 4 20 mA output Using more CX RLY C modules a combined vibration temperature protection system can be designed in the same way 5 MONITORING SYSTEMS The purpose of machinery monitoring systems is to indicate to register the measured parameters The main differences between monitoring and protection systems are Monitoring systems allow a longer response time including the measuring of parameters by channel multiplexing Monitoring systems can be used for data processing displaying and storing including persona
3. VIBRATION MEASURING SYSTEMS FOR DYNAMIC MACHINERY 1 INTRODUCTION The present document refers to the protection and monitoring of common dynamic machinery pumps fans centrifuges etc excepting turbo machinery turbines compressors etc The document focuses not only on the vibration measurement but also on other relevant parameters for the correct assessing of dynamic machinery condition 2 CLASSIFICATION OF MEASURING SYSTEMS Functionally speaking the measuring systems can be classified as follows a Protection systems b Monitoring systems c Combined systems protection and monitoring The monitoring systems can also be classified as gt Continuous monitoring systems that continuously measures the monitored parameters with the fixed mounted sensors on the machinery case gt Discontinuous monitoring systems these are usually portable instruments with a single transducer that can be used to perform a planed data collection of the important parameters weekly a monthly etc Between protection systems and monitoring systems there are essential differences that will be next detailed 3 PROTECTION SYSTEMS The main purpose of a protection system is to stop the machine anytime when the parameter values exceed a certain set limit when the power supply fails or when the system itself fails The protection systems may also have secondary functions such as gt the alert function which offers an optical and or ac
4. ence See below in Figure 3 the connection diagram of a measuring channel to an accelerometer placed into a hazardous area rey Sz Co ae aL mn j 6 gt J S Ck Ze Sim N Q g L il o Y Le Le Le LZ y N AA zi Q g as RA ARO la Figure 3 If the equipment is located into a safe area then protection barriers are useless To measure temperature into the combined protection systems a RTD thermal resistance is usually located closer to the bearing The RTD thermal resistance can be connected with two wires for small distances between transducer and electronic protection system or with three wires to compensate the ohm resistance of the connection cable Other facilities of protection systems are e Data transmission to a monitoring system analog or digital e The possibility of measuring the signal from the transducer using an external system such as a portable Spectra Analyzer In this case the output signal will have to be protected so that any short circuit 5 or any abnormal use of this signal does not affect the protection function In conclusion all the machinery protection systems must meet a number of special conditions e High reliability system e Short response time less than 1 second e The system must protect the machine by stopping it bot
5. failure e Ifthe failure is not detected by the protection system then the machinery will work unprotected To protect a machine the vibration parameters should be gt The Total Value which normally means the vibration speed in mm sec according with ISO 10816 Mechanical vibration Evaluation of machine vibration by measurements on non rotating parts gt Bearing condition A value that express the working state of bearings gSE SPM Envelope BC etc gt The amplitude of fundamental x1 RPM or harmonics of it gt The Total Value in a well determined frequency range The range recommended by the old ISO 7919 is 10 1000 Hz The most used transducers are the piezoelectric accelerometers placed onto the machinery case Because the protection systems are expensive people usually use only one transducer located on each bearing Sometimes a protection system with a single transducer is relevant and quite enough for example one transducer onto a fan bearing Pay a special attention to the machinery located in potentially hazardous areas In this case the transducer that was located in a dangerous place must be separated from the protection system located in the safe area using some safety barriers If the connection cable between transducer and electronic protection system is too long then you should carefully select it This means the cable should be screened and properly grounded to avoid any electromagnetic interfer
6. h from an accidental loss of voltage but also from any self failure or any cable connection failure e Alarm system must be latching type this means it must memorize an alarm condition until it is manually reset e Protection system must have the facility of multiplication of Danger Value eventually of Warning Value during machinery startup e The relay which produces machinery shutdown must be normal energized e Ifthe protection system is communicating either analogue or digital with a monitoring system then no monitoring system malfunction should affect the functionality of the protection system e An external device Spectra Analyzer connected to the protection system should not interfere or affect anyhow its proper functionality e The use of common computers PC type in the protection system scheme to switch the alarm relays is not accepted They can only be used in the monitoring system which is outside of the protection system The main conclusion is that the protection system must operate properly unaffected by the monitoring system if exists This way will be avoided any accidental stop which is not the consequence of exceeding the alarm limits 4 LOW COST PROTECTION SYSTEM EXAMPLE A low cost protection system can be designed using CX RLY C Module The system uses loop powered CTC LP250 Series accelerometers having the 4 20 mA dc output The output current represents the sensor zero level 4 mA and its full
7. h it is just an annunciator that doesn t lead to a radical decision to stop the machine A Danger Level which leads to the triggering of a relay that consequently stops the machine NOTE The two alarm levels have different names and they are presented by API670 as follows v Warning Alarm or Alert v Danger Shutdown or Danger Most of the protection systems have the retaining function they memorize the alarms Thus even when the alarm condition disappears the relay remains in the alarm position To return the relay to the normal state the system should be manually reset The alarm relays can be gt Normal energized if the measured value remains under the alarm limit the relay coil will be power supplied gt Normal de energized if the measured value remains under the alarm limit the relay coil will not be power supplied The API670 standard recommends that Warning Relays to be normal de energized and Danger Relays to be normal energized because if the power supply is interrupted the Danger Relays will become de energized and they will determine a stop condition Therefore the machinery would remain unprotected if an accidental power loss occurs which is undesirable 3 As we already mention any protection system should detect a transducer or a cable failure Detection means e The triggering of an additional fail relay eventually a light indicator e The direct triggering of the Danger Relay in case of
8. l computer data Monitoring systems cannot be used to stop a machine in alarm condition but they can provide relevant data about the alarm condition of measured parameters People often make confusion between these two systems just because there are many instruments available on the market that meet both functions protection and display register Protection system Monitoring system Figure 7 Monitoring system The response time of a monitoring system can be up to 20 30 seconds Usually the monitoring systems are multichannel devices The parameters can also be collected by channel multiplexing The vibration monitoring systems were used almost exclusively in the past for critical or very expensive machinery Nowadays the monitoring systems are 10 used increasingly more often for common equipments as well A relevant example in this sense is the monitoring of machinery difficult to be accessed or placed in hazardous areas where manual measurement involves a risk factor Now there are two different trends of using the vibration monitoring to add vibration measurements Total Value on the list of monitored parameters of the plant SCADA Systems DCS etc by using the well known loop powered 4 20 mA sensors or to use independent monitoring systems able to display more parameters than only Total Value like vibration spectra envelope time waveform orbits or other relevant graphs These systems can transfer
9. oustical signal when a set limit lower than the shutdown limit was exceeded gt local display functions for the measured parameter gt a function of long distance transmission analog or digital of information about the protection system condition and the measured parameters as well The Figure 1 shown below presents the block diagram of a protection system RTDs Sensor Thermocouples Accelerometers Transducer Velocity system Sensor leads Extension cables Accelerometer cables Machinery protection Signal processing systems a aaa Alarm Shutdown Integrity logic processing Monitor J p a i y Vibration system awer SapEW Urs Temperature Protective relays Figure 1 A protection system may consist of a single transducer mono channel protection system or many transducers multi channel power system The simplest protection systems are the vibration switches and they may be mechanic or electronic Figure 2 Mechanical vibration switches provide basic economical vibration protection by means of a simple and highly reliable over center snap action mass and spring mechanism In the presence of sufficient vibration along the switch s sensitive axis the switch snaps from a stable un tripped state to a stable tripped state allowing automatic machinery shutdown or alarming when excessive vibration Figure 2 occurs Once tripped the switch must be returned
10. scale output 20 mA Only two wires are required to send the current signal and to ensure the power supply of the sensor A loop supply voltage is used to power the remote sensor The remote sensor regulates the loop current so that the loop current represents the value of the parameter being measured by the sensor A series resistor RL at the loop power supply converts this current to a voltage that can be used by the process monitor controller to record the parameter being measured The range type and units of the measurements are fixed and must be established when ordering the sensor Example LP200 Series 4 20 mA Velocity Sensors Loop Power Sensor Velocity 4 20 mA Output for 0 10 mm sec rms A single CX RLY unit can be used to receive the vibration measurements from 2 4 field transducers See the typical wiring diagram in Figure 4 In the wiring diagram shown below the 24 V power supply is considered to be external To transform the transducers signal into voltage two 250 0 1 resistors are used The output signal of 4 to 20 mA will be transformed in a 1 5 V voltage range suitable for CX RLY C Module Because the input 3 and 4 are unused the I 06 and I 07 are connected to 24 V A single relay is used so the RLY1 must be correctly mapped to both CH1 and CH2 inputs Remember that the normal position of the relay contact can be NO or NC depending of the settings To fulfill protection system requirement the relay m
11. to its un tripped state via its local reset plunger or via voltage applied to its remote reset coil A single set of SPDT relay contacts are provided as standard for connecting the switch to a machine control or annunciation circuit Any protection system must have a response time less than 1 second The alarm response time should not be confused with the alarm delay time The alarm response time is not programmable but the alarm delay time is programmable The alarm response time is the maximum time that can elapse in the system before detecting that a channel s value has violated a set point Once a set point violation occurs the violation must be sustained for the alarm delay time before an alarm will be annunciated This means that set point violations shorter than the alarm delay time will be detected but not annunciated output relay will not act A special feature of the protection systems named Set Point Multiplier Factor is the capacity of temporary increasing the set alarm value for a short limited time at the machinery startup The Set Point Multiplier Factor is usually two or three times the set alarm value and the multiplication time is usually 10 to 20 seconds At the machinery startup a higher vibration limit may be temporary accepted so only for vibration measurements is necessary to multiply the alarm limit For each measured parameter a protection system must provide two alarm levels A Warning Level whic
12. ust be set to be CLOSE A key switch can be mounted between the I 02 terminal and 24 V When the switch is closed the relay will be placed onto the close position non alarm If the I 01 terminal is temporary connected to 24 V exactly with the startup time the alarm level will be doubled for 10 seconds Only a pulse is required a transition from the unconnected status to the 24 V status of the I 01 input That s how the machine will be still protected during startup to avoid excessive vibrations that may damage it A free relay contact from the electrical sub station is required an auxiliary contact of the machine start relay For more details see CX RLY C User Manual VIBRATION SENSOR VIBRATION SENSOR External External Power Power COM Machine trip replay Figure 4 The above diagram has two major disadvantages e Due to transducers type dynamic signal direct vibration for analysis is not available e Aremote indicator or the connection with an external SCADA system is not recommended If the 4 20 mA will be also connected to a SCADA input the efficiency of the protection will be lost any failure in the connection wire to the SCADA system will generate a false signal to CX RLY C module A better protection system requires a supplementary device a vibration converter In the scheme below Figure 5 is shown a protection system with two CTC SC200 series of vibration converter The
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