Why only negative 24 vdc supply give in vibration measuring

The principle is that analyzer supplies a negative voltage to transducer,who converts it into a frequency & sends it across the probe.Now as the probe comes into the area of rotor it creates a magnetic field & with it eddy current is produced on the rotor side.Which changes the amplitude of the frequency.It induces the voltage & remaining frequency comes to the transducer through the return way.Transducer converts it into the voltage .Now coming back to the question The gap voltage has a negative relation with the voltage induced i.e. as the gap increases the induced voltage decreases.So to make it to into a linear relation ship, negative voltage is supplied.i.e

Gap Gap voltage0 mm 0 vtg1 mm -9 voltage2 mm 18 voltage& when the probe is in air it will have no induced voltage so all -24 voltage without induced voltage will return back

Eddy Current (Proximity Probe) Transducer Installation

Part-1 Radial Vibration

Eddy Current Transducers (Proximity Probes) are the vibration transducer of

choice when installing vibration monitoring on Journal Bearing equipped rotating machinery. Eddy Current Transducers are the only transducers that provide Shaft Relative (shaft relative to the bearing) vibration measurement.

Several methods are usually available for the installation of Eddy Current Transducers, including internal, internal/external, and external mounting.

Before selecting the appropriate method of mounting Eddy Current Transducers, special consideration needs to be given to several important installation considerations that will determine the success of your monitoring program.

Theory of Operation

Eddy Current Transducers work on the proximity theory of operation. A Eddy Current System consists of a matched component system: a Probe, an Extension Cable and an Oscillator /Demodulator. A high frequency RF signal @2 mHZ is generated by the Oscillator/Demodulator, sent through

The gauge of the selected wire depends on the length of the instrument wire run, and should be as follows to prevent loss of high frequency signal:Up to 200 feet 22 AWGUp to 1000 feet 20 AWGUp to 4000 feet 18 AWG

The following wiring connection convention should be followed:

Red -24 VDCBlack CommonWhite Signal

Common Point GroundingTo prevent Ground Loops from creating system noise, system common, ground and instrument wire shield must be connected to ground at one location only. In most cases, the recommendation is to connect commons, grounds and shields at the Monitor location. This means that all commons, grounds and shields must be floated or not connected at the machine.

Occasionally due to installation methods instrument wire shields are connected to ground at the machine case and not at the monitor. In this case, all of the instrument wire shields must be floated or not connected at the monitor.

Conduit

the extension cable and radiated from the Probe tip. Eddy currents are generated in the surface of the shaft. The Oscillator /Demodulator demodulates the signal and provides a modulated DC Voltage where the DC portion is directly proportional to gap (distance) and the AC portion is directly proportional to vibration. In this way, a Eddy Current Transducer can be used for both Radial Vibration and distance measurements such as Thrust Position and Shaft Position.

Special Considerations

Number of TransducersAll vibration transducers measure motion in their mounted plane. In other words, shaft motion either directly away from or towards the mounted Eddy Current Probe will be measured as radial vibration.

On smaller less critical machines, one (1) Eddy Current Transducer system per bearing may be adequate for machine protection.

The single Eddy Current Probe will then measure the shaft's vibration in that given plane. Therefore, the Eddy Current Probe should be mounted in the plane where the greatest vibration is expected.

On larger more critical machines, two (2) Eddy Current Transducer systems are normally recommended per bearing. The Probes for this type of installation should be mounted 900 apart from each other. Since the Probes will measure the vibration in their respective planes, the shaft's total vibration within the journal

Dedicated conduit should be provided in all installations for both mechanical and noise protection. Flexible metal conduit should be used from the Eddy Probe to the Oscillator /Demodulator junction box, and rigid bonded metal conduit from the junction box to the monitor.

CalibrationAll Eddy Current Systems (Probe, Cable and Oscillator Demodulator) should be calibrated prior to being installed. This can be done by using a SKF-CM CMSS601 Static Calibrator, -24 VDC Power Supply and a Digital Volt Meter. The Probe is installed in the tester with the target set against the Probe tip. The micrometer with target attached is then rotated away from the Probe in 0.005" or 5 mil increments. The voltage reading is recorded and graphed at each increment. The CMSS601 Calibrator will produce a voltage change of 1.0 VDC +-0.05 VDC for each 5 mils of gap change while the target is within the Systems linear range.

GapWhen installed,Eddy Current Probes must be gapped properly. In most Radial Vibration applications, gapping the transducer to the center of the linear range is adequate. For the Model CMSS65 and 68 gap should be set for -12.0 VDC using a Digital Volt Meter (DVM), this corresponds to an approximate mechanical gap of 0.060" or 60 mils. The voltage method of gapping the Probe is recommended over mechanical gapping. In all cases, final Probe gap voltage should be

bearing is measured. An "Orbit" or cartesian product of

the two vibration signals may be viewed when both Eddy Current Transducers are connected to an SKF-CM Information System or an Oscilloscope.

Linear RangeSeveral versions of Eddy Current Transducers are available with a variety of Linear Ranges and body styles. In most cases, SKF-CM's CMSS68 with a linear range of 90 mils (0.090") is more than adequate for Radial Vibration measurements...

Model Range Output Size

CMSS65 90 mils200 mV/mil

1/4"x28 UNF 1" to 5" Length

CMSS68 90 mils200 mV/mil

3/8"x24 UNF 1" to 9" Length

CMSS62240 mils

50 mV/mil

1" x 12 UNF 1" to 5" Length

Target Material/Target Area

Target MaterialEddy Current Transducers are calibrated at the factory for 4140 Steel unless specified otherwise. As Eddy Currents are sensitive to the permeability and resistivity of the shaft material any shaft material other than 4000 series steels must be specified at the time of order. In cases of exotic shaft material a sample may need to be supplied to the factory.

Mechanical RunoutEddy Current Transducers are also sensitive to the shaft smoothness for Radial Vibration. A smooth (64

documented and kept in a safe place.

Internal Mounting

Internal Mounting is accomplished with the Eddy Current Probes mounted internally to the machine or bearing housing with a SKF-CM CMSS903 Bracket or with a custom designed and manufactured bracket. The Transducer system must be installed and gapped properly prior to the bearing cover being reinstalled. Provisions must be made for the transducer's cable exiting the bearing housing. This can be accomplished by using an existing plug or fitting, or by drilling and tapping a hole above the oil line. The Transducer's cables must also be tied down within the bearing housing to prevent cable failure from "windage".

For added safety and reliability, all fasteners inside the bearing housing should be safety wired, or otherwise prevented from working loose inside the machine.

Advantages of Internal Mounting

Most economical installation. Less machining required. True bearing relative

measurement. Usually good viewing surface

for Eddy Probe.

Disadvantages of Internal Mounting

No access to Probe while

micro-inch) area approximately 3 times the diameter of the Probe must be provided for a viewing area. The prepared journal area on most shafts are wider than the bearing itself allowing for Probe installation immediately adjacent to the bearing.

Electrical RunoutSince Eddy Current Transducers are sensitive to the permeability and resistivity of the target material and the field of the transducer extends into the surface area of the shaft by approximately 15 mils (0.015"), care must be taken to avoid non homogeneous viewing area materials such as Chrome.

Another form of electrical runout can be caused by small magnetic fields such as those left by Magna-fluxing without proper degaussing.

Perpendicular to shaft centerlineCare must be exercised in all installations to insure that the Eddy Current probes are mounted perpendicular to the shaft center-line. Deviation by more than 1-2 degrees will effect the output sensitivity of the system.

Orientation of Transducer(s)As most machine casings are horizontally split, transducers are commonly found mounted at 450 both sides of vertical 900 apart.

If possible transducer orientation should be consistent along the length of the machine train for easier machine diagnostics. In all cases orientation should be well

machine is running. Cables must be tied down due

to "windage". Transducer cable exits must

be provided. Care must be taken to avoid

oil leakage.

External/Internal Mounting

External/Internal mounting is accomplished when the Eddy Probes are mounted with a Mounting Adapter (SKF-CM CMSS911 or 904). These adopters allow external access to the Probe yet allows the Probe tip to be internal to the machine or bearing housing. Care must be taken in drilling and tapping the bearing housing or cover to insure that the Eddy Probes will be perpendicular to the shaft center line.

In some cases due to space limitations External/Internal mounting is accomplished by drilling or making use of existing holes in the bearing itself, usually penetrating at a oil return groove.

Advantages of External/Internal Mounting

Eddy Probe replacement while machine is running.

Usually good viewing area for Eddy Probe.

Gap may be changed while machine is running.

documented.

Transducer (Probe) side clearancesThe RF Field

emitted from the Probe tip of a Eddy Current Transducer in approximately a 450 coned shape. Clearance must be provided on all sides of the Probe tip to prevent interference with the RF Field. As an example, if a bearing is drilled to permit installation, the hole must be counter bored to prevent side clearance interference. Care must also be taken to avoid collars or shoulders on the shaft that may thermally "grow" under the Probe tip as the shaft grows from heat.

Eddy Current Probe tip to tip clearancesAlthough Probe tip to tip clearances are not normally an issue on most machines, it should be noted that Eddy Current Probes radiate an RF Field larger than the Probe tip itself. As an example, Model CMSS65 and 68 probe should never be installed with less than one (1) inch of Probe tip to tip clearance. Larger Probes require more clearance. Failure to follow this rule will allow the Oscillator/Demodulator to create a "beat" frequency which will be the sum and difference of the two Oscillator/Demodulator RF frequencies.

System Cable Length and Junction BoxesEddy Current Transducer Systems are a "tuned" length, and several system lengths are available. Length is measured from the Probe tip to the

Disadvantages of External/Internal Mounting

May not be true bearing relative measurement.

More machining required. Long Probe/Stinger length

(Resonance).

External MountingPure external Eddy Probe mounting is usually a last resort installation. The only valid reason for using this method is inadequate space available within the bearing housing for internal mounting. Special care must be given to the Eddy Probe viewing area and mechanical protection of the transducer and cable.

Advantages of External Mounting

Most Inexpensive Installation.

Disadvantages of External Mounting

May be subject to "Glitch" or Electrical/Mechanical runout.

Requires mechanical protection.

Installation Checklist

1. Mounting Type, Internal External/Internal External

2. Number of Transducers, X or X&Y

3. Target Material, 4140 Other 4. Smooth Target Area 5. Size of Target Area

Oscillator/Demodulator, and is measured electrically which can slightly vary the physical length. For example, the Model CMSS65 and 68 are available in 5 and 10 meter system lengths. Care must be taken to insure that the proper system length is ordered to reach the required Junction Box.

Grounding and NoiseElectrical noise is a very serious consideration when installing any vibration transducer, and special care needs to be taken to prevent unnecessary amounts of noise. As most plant electrical noise is 60 HZ, and many machines running speed is also 60 HZ, it is difficult to separate noise from actual vibration signal. Therefore, noise must be kept to an absolute minimum.

Instrument WireA 3-wire twisted shielded instrument wire (ie; Belden #8770) is used to connect each Oscillator/Demodulator to the Signal Conditioner in the Monitor. Where possible, a single run of wire from the Oscillator/Demodulator (Junction Box) to the Monitor location should be used. Splices should be avoided.

6. Junction Box Location(s) 7. Metal Conduit (Junction Box

to Monitor) 8. Flexible Conduit (Junction

Box to Probe) 9. Correct Instrument Wire 10. Shielding Convention,

Monitor or Machine 11. Calibration

12. Gap Set

Compressor Surge Control: Design And Modeling For Performance VerificationBy Marybeth Nored, Augusto Garcia-Hernandez, Klaus Brun and Jeff Moore, Southwest Research Institute, San Antonio, TX | September 2009 Vol. 236 No. 9

Figure 3: Recycle Valve Tested During ESD Event At SwRI MRF.

Buyer's Guide Compressor station pkgs. Compressor valves Compressors, gas

When a compressor reaches its surge condition, it loses the ability to maintain peak head and the entire system becomes unstable.

Surge is most accurately described as a system phenomenon - not a localized instability. Under normal conditions, the compressor operates to the right of the surge line. Compressor surge is sometimes viewed as a common occurrence but design of a proper surge control system should be regarded as both a necessary design practice and an effective risk-mitigation measure.

The surge-control system is an important element in the compressor system because it protects the compressor from surge over the range of compressor operations. Protection of the compressor through the surge-control system will help to avoid considerably more costly repairs or overhauls due to damaging surge conditions. Control systems may be implemented using a variety of methods and philosophies. However, the primary objective of any surge-control system should be to predict and prevent the occurrence of surge so as to reduce possible damage to the compressor and ensure a safe working environment for all station personnel. In order to ensure the system is designed properly for its various and often competing requirements, operators will choose to verify

component selection, response time and behavior through a dynamic compressor surge model.

The principle of a centrifugal compressor surge-control system is based on ensuring that the flow through the compressor is not reduced below a minimum flow limit at a specific head. The majority of surge-control techniques restrict the operation of the compressor to flow rates above a defined surge-control line based on the surge margin for a particular compressor. Restriction of the operating window of the compressor in order to avoid surge because of mistakes in the surge control system design should be avoided. A properly designed surge-control system can allow the operational range of the compressor to be extended based on the response of the surge-control system.

At a minimum, the control system should actively measure the compressor head and flow through the compressor system controls and determine the resulting operating point. The recycle valve should be opened in a specified time to a valve set point determined by the control system. This signal to the valve is based on the compressor operation, its proximity and its movement (rate) relevant to the surge-control line. Opening of the recycle valve in the surge-control system effectively avoids surge by providing more flow and reducing compressor head, to move the compressor away from its surge point. In Figure 1, the compressor rundown behavior is plotted over a head vs. flow map. The compressor flow begins to drop from 900 cfm to 500 cfm. Shortly before reaching the measured surge line, the recycle valve opens and the flow through the compressor increases, to effectively avoid reducing the flow further to the left of the surge line.

A surge-control system should be capable of monitoring the operation of the compressor continuously. The function of the surge-control system is to detect the approach to surge and provide more flow to the compressor through opening the recycle valve to avoid the potentially damaging flow reversal period and surge cycling. The surge-control system should be designed for the three surge environments (which may have competing demands) and the compressor operating parameters as well as manufacturer specifications.

Various design philosophies are also provided through the use of surge-control system design criteria, which allow the performance of the surge-control system to be evaluated. The actual choice of design philosophy rests with the operating company and compressor manufacturer - and may be based on experience with a particular compressor or station. Common surge-system design philosophies may include:

1. Design To Avoid Surge: The philosophy requires control system design criterion based on a calculated allowable discharge system volume. The allowable discharge piping volume should be determined by simple or more complex transient models of the compressor system.

2. Design To Permit Surge Under Specified Conditions: The design philosophy acknowledges that due to operational changes to the compressor station or cost-based decisions, the compressor may not be fully protected by the existing surge-control system.

3. Design Based On Risk Evaluation: The surge-control system is evaluated against a set of risk factors developed for a particular compressor and dynamic simulation is not necessarily required because of previous modeling efforts or experience.

The design of the surge-control system is more difficult than other station-control systems because of the high speed of disturbances and dynamic nature of surge. In addition, a variety of control system responses is required, depending on whether the compressor is starting up, operating in its normal operation at a low flow period, or undergoing a sudden shutdown.

Start-up Environment: The challenge to the surge-control system in the start-up environment is to quickly bring the compressor up to design speed without overheating the discharge gas. For steam turbine or single-shaft gas turbines, the start-up period will be lengthened and cooling of the recycle gas may need to be considered. In a typical start-up mode, gas is continually recycled to bring the compressor online. Operating in continuous recycle will cause the process gas temperature to increase until new gas can be supplied from upstream. With the recycle valve fully open and the downstream check valve closed, all compression horsepower will serve as heat input to the recycled gas.

Normal Process Control: The operation of the surge-control system under normal process operation is distinctly different. The surge-control system should not limit the operational range of the compressor. A relatively flat surge line equates to higher surge sensitivity to changes in compressor head. A steeper line indicates that the compressor is more sensitive to flow-rate changes or uncertainties near the surge line. In either case, the surge-control system must provide for smooth operation of the compressor. The challenge for the surge-control system in process control is to match the transition into surge (across the surge margin), which is typically gradual during normal process control, with a gradual increase in flow through the recycle valve. This requires precision control of the valve motion. The control signal and response of the recycle valve for normal process control will differ from the shutdown environment. During normal process control operation, lower gain signals should be used for adjusting the flow by opening or closing the valve in a controlled manner.Emergency Shutdown (ESD): In an ESD event, the compressor is suddenly shut down and driver power is removed. This operation requires distinctly different functionality from the surge-control system. Delayed shutdown or slowly decreasing speed is not possible as ESDs are intended to provide immediate shutdown of the unit due to safety considerations. The surge-control system must function quickly to open the recycle valve fully because the coastdown path is not being controlled by the station operator - only by the deceleration of the compressor based on the power train inertia and any residual power in the system. The emergency shutdown requires more demanding control-system response and may alter the surge-control system design because a single valve may not provide sufficient flow quickly enough. Additional evaluation of the system may need to be performed.

The worst-case emergency shutdown occurs when the compressor is operating at maximum head at the lowest allowable surge margin. This operating point should govern

the design of the surge control system - as the maximum possible differential which must be overcome by the recycle valve flow.

One of the most critical components in the surge system design is the recycle valve type and design (especially in high horsepower compressor installations, > 100 MMscf/d). Examples of the dramatic effects on the compressor rundown path are shown in Figure 2, for various valve capacities and delays in opening time. Note these lines are actual test results from the Southwest Research Institute closed-loop test facility on a Solar C-160 compressor. The 6-inch diameter recycle valve for the facility is shown in Figure 3. Test 7 on the performance map in Figure 2 corresponds to the normal operation of this valve.

Anti-Surge Valve DesignComplicated actuation systems on large recycle valves will introduce new dynamics into the valve actuation system that may cause a departure from linearity in the response. Response time or stroking time of the valve should not be used as the only criterion in selection of the recycle valve because this will impair the controllability of the valve and robustness of the design, needed for the process control environment.

Testing of the recycle valve for frequency response, amplitude step response in both directions (opening and closing) is recommended for valves configured with custom pneumatic systems or particularly large recycle valves (>12 inch diameter). In addition, noise attenuation characteristics and the turbulence through the valve can become major design issues for a station. High turbulence levels through a recycle valve have been shown to cause low-frequency turbulent excitation which can excite the recycle loop piping and structural support system. Recommended maximum noise level for the recycle valve is 85 dBA or less during normal operations at 1 m distance. Noise requirements for the valve during partial or full recycle may be different than those during a compressor emergency shutdown event.

Valve actuation system requirements primarily stem from the process control environment - not the shutdown environment. The key parameter to the recycle valve in the process control environment is precise control of the valve position. It is necessary to specify the speed for which the valve is allowed to change position and the amount of overshoot permitted.

In the startup environment, the key criteria is to match the valve characteristic to the compressor performance map to ensure that the compressor can come up to full speed as the recycle valve is closed. The key requirements for the recycle valve in an ESD are response time and flow-rate capacity for the given discharge system volume. For an emergency shutdown, the valve opening time is critical. Typically for the first one-second period after shutdown, the compressor deceleration will reduce the speed by approximately 30% and the head by 50 %. This deceleration period can take as long as five seconds, depending on the machine. To maintain flow through the compressor, the recycle valve must begin to open within the first second after the downstream check valve is closed.

The competing requirements for the recycle valve make use of multiple valve systems appealing. This approach to surge-control system design is not often the most cost-effective solution. In addition, the use of multiple valves for each surge-control function will require additional testing to ensure that the transition into and out of each operation is smooth.

Surge Control System ModelingA surge model can be used to effectively design the surge-control system based on the particular compressor system. Once the system-design criteria have been established, the surge model should be used to verify the design criteria have been met. The modeling process and interpretation of results should consider the high uncertainty associated with any numerical transient model and the assumptions governing the compressor surge model. If the transient model predicts the compressor operation to narrowly avoid the actual surge condition, the high uncertainty of the model results should indicate that surge is possible. In this case, the modeling effort would suggest that the surge-control system or downstream piping volume should be redesigned.

Modeling the surge-control system in the startup, process control and shutdown environment is recommended. The shutdown case will produce the most stringent requirements for limiting piping volume and maintaining fast recycle valve response. A basic fixed volume model can be implemented to determine surge-control system dynamic response in the shutdown environment - as shown in Figure 4.

A more complex dynamic model may also be needed to model more complicated systems with multiple recycle loops or more than one compressor unit. Several basic empirical rules are sometimes used to determine if the surge-control system adequately protects the

compressor. These empirical rules are not necessarily physics-based and only apply to a limited number of compressors and types of systems. A basic fixed-volume model can be used in a spreadsheet-based application to determine a more accurate system-specific discharge volume.

For more sophisticated systems and complicated piping geometries, transient (dynamic) models may be warranted. Dynamic models of the compressor surge system are distinctly different from a steady state model. The dynamic model is a purely transient study. The model will predict pressure and flow rate based on the mass accumulation in the system because these effects are transient processes. Dynamic models will predict intermediate process conditions when the flow through the compressor is changed. These are useful in the design of the compressor anti-surge control for all three working environments.

If properly modeled, the dynamic model should provide a resource for the operating company and manufacturer in protecting the compressor prior to the installation of the surge control system. An example of the amount of detail and component specifications to these models is shown in the basic 1-D Stoner model layout for the SwRI closed loop natural gas facility in Figure 5. In performing the recent testing of the Solar C-160 compressor, the authors determined the sensitivity of the modeling software to changes in valve geometry, piping selection, friction factor, and compressor deceleration curves. In addition, several non-dimensional parameters were developed to compare different compressor piping systems in terms of deceleration, system energy and bleed volume. 1 This work was published recently at the ASME Turbo Expo by J. Moore et al.

The dynamic model can also be a valuable tool in the design of a new compressor system installation. Results from the transient analysis should be used to evaluate the system piping design, placement of the downstream check valve and anti-surge valve, and the valve selection. The study should confirm the safe design of the surge-control system to

adequately protect the compressor from surge. A transient simulation of the surge-control system can be used to save time and expense in changing the system after installation. The transient modeling process should be employed in the design stage of a compressor installation and adapted to suit the range of application needed. Many references in industry are available to provide examples on the use of dynamic modeling to optimize or predict the behavior of a surge-control system.

AcknowledgementThe authors would like to thank the Gas Machinery Research Council for the funding and support given to the compressor surge control research program