Continuous Rod Drop Monitoring using 3300/81 Six-channel Rod Drop Monitors

Continuous Rod Drop Monitoring using 3300/81 Six-channel Rod Drop Monitors, Proximity Transducers and a Keyphasor® transducer

Horizontal reciprocating compressors usually have rider bands to minimize wear of the piston rings and to eliminate cylinder liner damage due to contact between the piston and cylinder. The primary rider band problem is determining how much rider band wear has occurred while the machine is operating so the machine can be shutdown prior to contact between the piston and cylinder. In the past, the only way to check rider band thickness was to shut down the process, remove a valve and measure the available rider band thickness with a feeler gauge.

Bently Nevada does not recommend that customers use this parameter to shut down their machines. It is primarily a machinery management tool to help customers determine when to replace the rider bands.

Benefits
The Bently Nevada Six Channel Rod Drop Monitoring System continuously monitors the rider band condition in each cylinder in horizontal compressors. It is most effective for compressors with non-lubricated or lightly lubricated cylinders or for process compressors that are not compressing a sweet, dry gas. The rod drop monitor has the following features:

  • The display shows a continuous online indication of rider band wear, enabling you to schedule maintenance to replace them only when required. This also allows you to maximize the lifespan of your rider bands.
  • Alert and Danger alarm level indications warn you of impending compressor damage.
  • The automatic shutdown relay increases machinery and personal safety. The shutdown relay allows the monitor to shut down the compressor through the emergency shutdown system when the maximum wear of the rider bands is reached.

System Description
The 3300/81 Six Channel Rod Drop Monitor calculates and displays the rider band wear based on the position of the piston rod relative to the cylinder packing box. The system consists of a monitor, a Keyphasor® transducer and a proximity transducer system for each cylinder.

Installation and Setup of the Rod Drop Monitoring System

A good installation is critical for an accurate and repeatable rod drop measurement. There are a number of factors that impact the measurement, including vibration, thermal growth, rod materials and coatings.

Probe selection
One of the first steps in applying a rod drop system is selection of the proper proximity probe. Select a probe of sufficient linear range based on some basic parameters. The most obvious is the thickness of the rider band. Less obvious is the effect of piston thermal growth on the measurement. Bently Nevada Proximitor® Sensors for rod drop measurements include 8 mm, 11 mm, and 14 mm transducers. The 8 mm probe has a range of 2.0 mm (80 mils) starting at 0.25 mm (10 mils) from the probe face, the 11 mm has a range of 4.1 mm (160 mils) starting at 1.0 mm (40 mils) from the probe face, and the 14 mm has a range of 4.1 mm (160 mils) starting at 0.5 mm (20 mils) from the probe face. Typically for cylinders with a rider band thickness of 2.6 mm (100 mils) or less, an 8 mm probe is sufficient. Use an 11 mm or 14 mm probe where possible to guarantee sufficient range. It is especially important to use an 11 mm or 14 mm probes for large diameter pistons (500 mm, 20 inches, or greater) or for aluminum pistons. In large diameter and aluminum pistons, the thermal growth of the piston can be significant, and may grow out of the linear range of an 8 mm probe.

Probe Mounting
Ensure a solid probe mounting. To minimize measurement error, install the probe as close to the piston as possible and ensure that the probe mounting is free of vibration or thermal growth. We do not recommend using external probe housings. Thermal growth of the distance piece and high machine casing vibration make this unacceptable. The preferred installation is to mount the probe with a mounting block to the face of the packing box. This places the probe close to the cylinder and makes the measurement less susceptible to thermal growth and vibration effects.

Mounting of Rod Drop Transducers

It is best to install an additional probe in the same radial plane as the rod drop probe but oriented horizontally to the piston rod for diagnostic purposes. The ideal configuration is to input the Rod Drop signal into a 3300/81 monitor and into a 3300/16 monitor, and input the horizontal probe signal into the same 3300/16 monitor. If that is not feasible, taking periodic measurements of both signals to establish an trend of the piston rod movement using ADRE® is recommended.

Rod Materials and Coatings
Rod materials and coatings can have a significant effect on the probe curve. Reciprocating compressor rods are typically coated with tungsten carbide. Some OEM’s use chrome or other materials. In any event, run a calibration curve on the actual rod using the Bently Nevada Shaft Micrometer. The values for the calibration curve will be slightly modified if using intrinsic safety barriers. To ensure the readings are accurate, take the readings from the buffered outputs on the front of the 3300/81 monitor.

After determining the linear range of the transducer system, input the lower end and upper end values of the probe curve, including the physical gap and voltage, into the Rod Drop Monitor. The 3300/81 Monitor calculates an appropriate scale factor based on the upper and lower gap distances and voltages. If the rod is a material other than standard steel, you should use transducer systems specially calibrated for that material to maximize the linear range of the transducer system.

Calculating Rod Drop
The proximity probe detects the movement of the piston rod and is mounted to the rod packing. As the rider bands wear, the piston drops within the cylinder. This causes the distance between the probe tip and piston rod surface to change, causing a change in the gap voltage setting.

The rod drop distance observed by the proximity probe is proportional to the actual wear of the rider bands. It is necessary to program specific machine parameters into the monitor to calculate rider band wear. The connecting rod length, piston rod length, stroke length and probe location must be accurate. These parameters determine the correction factor, which relates the change in gap voltage measured by the probe to the actual rider band wear. Errors in these measurements will effect the accuracy of the correction factor, resulting in an error in the indicated rod drop.

Another important measurement to input into the monitor is the piston angle. The piston angle is a reference to associate each piston rod’s position with the Keyphasor® signal. It is the angle (in degrees) measured on the crankshaft from where the Keyphasor® probe first sees the notch on the shaft or flywheel to where each piston rod is at Top Dead Center (TDC), or fully extended. This can be different for each rod, or it may be the same for some rods on the same machine. The piston angle is used both for diagnostic purposes and to calculate rider band wear when the Rod Drop Monitor is setup to operate in the “instantaneous” mode.

Machinery Measurements Needed for Rod Drop Measurement

The monitor uses the principle of similar triangles and the geometry of the machine to compute the correction factor at various points in the stroke. The correction factor is multiplied by the probe gap to allow the monitor to display the actual rider band wear.

Principle of Similar Triangles

Each channel of the 3300/81 Six Channel Rod Drop Monitor has two modes in which data can be displayed: “average” and “instantaneous” rod drop. For average rod drop, analog filtering is used to measure the average gap voltage over a complete cycle.

Instantaneous Rod Drop
One important feature of the 3300/81 Rod Drop Monitor is the ability to set a trigger angle so that the measurement is made at the same point in the stroke each revolution. This eliminates the effect of rod flex and movement of the crosshead between the guides.

The trigger angle is the angle (in degrees) measured on the crankshaft from when the piston is at Top Dead Center to the location in the stroke where you want to sample the signal. The 3300/81 uses the Keyphasor® signal as a reference to determine when to “trigger” the monitor to capture the signal. Although the physical location of the Keyphasor® probe is fixed, the time at which the Monitoring System takes a rod drop reading can vary from 1 to 360 degrees, as related to the rotation of the crankshaft.

A trigger angle of 240° past top dead center (TDC) is the best trigger angle in most cases, and this is the default trigger angle for the monitor, as set at the factory. The trigger angle can be set for any point in the stroke if necessary. The monitor automatically recalculates the correction factor whenever the trigger angle changes.

Using a 240° trigger angle may not always provide the best readings. The trigger angle should not be at an unstable point on the waveform (a point on the waveform where the slope is high). An example of stable and unstable trigger angles are shown below:

Rod drop trigger angle selection

You may experience changes in your rod drop reading due to load or process changes. Examples of load and process changes in reciprocating compressors include a change in the molecular weight in the compressed gas, a variation in the compressor speed, varying the compressor load with clearance pockets or valve unloaders, or changes in suction temperatures or pressures. In instances where you expect the load or process to change, you should evaluate your trigger angle to minimize changes in the rod drop reading due to these process or load changes.

In instances where you need to vary the trigger angle, use the buffered outputs in front of the monitor to send the trigger pulse and the timebase signal into an oscilloscope. For each cylinder, compare the dc coupled timebase signal from the rod drop proximity probe with signals at various load conditions.

There are some critical factors to consider when determining where to set the rod drop trigger angle.

  1. Ensure that the trigger angle is at a relatively stable point on the waveform, i.e., the slope of the waveform at the trigger point is small or flat.
  2. Ensure that the trigger angle is at a point along the piston stroke where the piston is most likely riding at the bottom of the cylinder. For a probe mounted over the piston rod, that point should be near the bottom of the waveform; for a probe mounted under the rod, that point should be near the top of the waveform.
  3. The static rod drop reading should have very little change when the compressor changes load. In other words, ensure that the dc gap voltage at the trigger point on the transducer waveform does not significantly vary at each load step. To test this, change the load in the same fashion that you would normally vary the load under normal operations (i.e., using suction-line throttling, clearance pockets, suction valve unloaders, or speed control.)

Rod Drop Readings at various loads

  1. For increased accuracy, the trigger angle should be at a point where the piston is near the probe (closer to bottom dead center [BDC] than top dead center [TDC]). This should be between 90° and 270°.
  2. The trigger angle should not be located where inertial forces are at excessively large. Inertial forces are at their maximum at TDC and BDC (0° and 180° trigger angle). Near these points on the stroke, the piston rod may flex, causing the rod drop signal to have a “spike” which varies in amplitude. Therefore, avoid trigger angles between 340° and 20°, and between 160° and 200°.

Combining factors 4 and 5 above, the best locations for the trigger angle should be between 90° and 160°, or between 200° and 270°.

Re-zeroing the Rod Drop Monitor
Once the machine is at its normal operating conditions for a period of time, the position of the piston rod rises due to the thermal growth of the piston within the cylinder. It is very important to re-zero the rod drop monitor after approximately four hours under process loads to account for piston rise within the cylinder.

Additional Diagnostics
Having an additional proximity probe oriented 90° from the rod drop probe can provide additional diagnostic information. This is particularly important on small diameter (less than 200 mm [8 in]), high pressure cylinders. In certain instances, pistons in these applications appear to float in the cylinder bore. This occurs particularly if the process compresses a dense gas, such as CO2 or propane. In such a case, XY monitoring of the piston rod can help determine what is occurring within the cylinder.

Trending Systems
The effectiveness of the rod drop measurement is significantly enhanced when the reading is trended, either in a distributed control system (DCS) or in a machinery management system such as Bently Nevada’s Data Manager® 2000 for Windows NT. If all six channels are to be trended through a Bently Nevada communications processor, a third rack position to the right of the 3300/81 Monitor is required for a communications board.

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