07 Rod Drop(Compressed)

Rod Drop

Rod Drop

Reciprocating Compressors Condition Monitoring & Diagnostics

© 2007 General Electric Company. All righ ts reserved.

Rod Drop monitors are used to detect excessive wear of the piston’s consumable rider bands (also known as wear bands). With this information, we can plan to shut down the machine for a relatively inexpensive inexpensive routine rider band replacement job – avoiding the expensive repairs that would be required if metal-to-metal contact were allowed to occur between the piston and the t he cylinder liner. Traditional rod drop monitors simply simply triggered an alarm alarm when the vertical position of the piston rod exceeded a setpoint corresponding to significant rider band wear. Newer methods, such as Rod Position monitoring, measure the dynamic position of the rod throughout its entire stroke. Typically, these methods use an orthogonal (perpendicular) pair of eddy current proximity probes as well as high speed data acquisition and processing (up to 720 samples per crankshaft revolution). Some other variations on the Rod Position measurement are known as Rod Deflection, Rod Motion, and Rod Runout. Rod Position monitoring will be introduced in in the module that follows this one.

Rev 0. November, 2007

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Rod Drop

Presentation Contents • • • •

Fundamentals Instrument Settings Determining Trigger Angle Probe Calibration

2 © 2007 General Electric Company. All rights reserved.

can allow us to proactively Fundamentals – Effective monitoring of rider band wear can that appropriate maintenance actions in a timely manner. Monitor Settings – Rod drop monitoring can only be effective when instrumentation settings are appropriate.

important to establish an appropriate crank angle Determining Trigger Angle – It is important value for collection of instantaneous instantaneous rod position measurements, if this type type of signal processing will be used. Probe Calibration – For accurate measurements of piston rod displacement, it is important to calibrate the proximity transducer for fo r the actual conditions under which it will be applied.


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Rod Drop

Fundamentals • Over time, soft rider bands wear away – especially in non-lubricated cylinders. • If the excessive wear is not detected early enough, the piston can contact the cylinder liner, causing expensive damage.

Piston g n i R

Rider Band

Cylinder Liner

3 © 2007 General Electric Company. All righ ts reserved.

Both the rider bands and the piston rings contact the cylinder liner. However, the rider bands are designed to carry the full weight of the piston while the piston rings are designed only to provide sealing between the piston and bore. The piston rings and their mounting grooves are purposely designed with clearance to allow the rings to “float”, without carrying any of the vertical load of the piston’s weight. The rider bands are intentionally designed to seat fully within their mounting grooves, so that they can support the piston’s weight. Historically, rider bands were made from soft metals such as babbitt. In modern designs, most rider bands are fabricated from plastics or composite materials – especially for non-lubricated cylinder designs. Rider bands can be one-piece design (required by API-618) which are installed while segmented pistons are disassembled – or split design – with step-cut or butt-cut ends. Rider bands are designed to slowly wear away over time. In good operating conditions, the rate of wear can be very low – especially in lubricated-cylinder applications. However, adverse conditions such as improper cylinder lubrication, component misalignment, or excessive particulates or moisture in the process gas can increase the wear rate.


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Rod Drop

Piston Damage Fundamentals

Scuffing Scuffing Wear Wear


This photo shows what can happen to a piston when excessive rider band wear is not detected in time. The scuff marks on the piston often correspond to matching gouges in the cylinder liner. Due to material differences between the piston and the cylinder liner (example: aluminum piston and steel liner) it is quite likely that the cylinder will not be damaged as extensively as the piston. Note: Some compressor designs incorporate replaceable cylinder liners, while others use special treatments, such as “nitriding” to create cylinder surfaces that are extremely resistant to erosion wear. For visual reference – this piston has been removed from its cylinder, rotated so that it is “upside-down” and placed on a soft mat on the deck grating. The rigging chainfall and hook that were used for lifting the piston and rod assembly are still visible in the photo. The piston rings have been removed, but the one-piece rider bands are still in place, awaiting disassembly of the segmented piston. Observe that the rider bands appear to have been burnished down to the point where they are completely flush with the outer surface of the piston.


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Rod Drop

Contact Sensors (Historic) Fundamentals

Fuse Metal Plug (pressurized chamber)

Contact sensors are installed as close to the piston as possible (typically on the pressure packing gland). They are mounted below the piston rod and are activated ONLY when the rod contacts them with adequate force.

Roller Switch (electrical contact)


Historical methods used physical contact between the piston rod and a sensor placed beneath it to indicate that the rod had dropped to the operational limits of the rider bands. These sensors include roller-equipped limit switches and pressurized chambers sealed with fuse metal plugs that melt due to frictional contact, causing a measurable pressure drop in the detector. Contact sensors are simple and inexpensive, but they only provide a “one-shot” signal with no warning. They do not allow for any trending to be done over time. A more effective monitoring method uses a non-contacting eddy current proximity detector to take continuous measurements. This allows us to watch the rod drop values closely as the rider bands wear, and to better anticipate and plan for maintenance outages to replace the bands before metal-to-metal contact occurs. (see next page)


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Rod Drop

Eddy Current Sensors (Modern) Fundamentals

(above or below rod)

Eddy current sensors are also installed as close to the piston as possible (typically on the pressure packing gland). They can be mounted either above or below the piston rod, and can take measurements at ANY time.

Bently Nevada, LLC

Eddy Current Proximity Probe


Since eddy current displacement probes do not contact the surface being measured, they may be installed either above or below the piston rod, as appropriate for the specific compressor design and component layout.


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Rod Drop

Rod Drop Trend Example Fundamentals


This plot shows an actual rod drop trend that was collected for a hydrogen compressor in refinery service. The monitor in this example was programmed to collect instantaneous values at an appropriate value of crank angle (known as “trigger angle”) that was determined for this specific installation. Trending a measured parameter in this way enables long-term outage planning for a proactive maintenance program. In this cylinder, the rider bands eroded at a steady rate, losing approximately 50 mil (1270 microns) of material over a seven month period. Note: Trigger angle is discussed further in subsequent pages of this module.


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Rod Drop

Measurement Geometry Fundamentals

PISTON

PROBE POSITION

MIDPOINT

Per the “similar triangles” relationship, L1 B1

=

( L1 + L2) B 2 PROBE VIEW

B 2 =

CL

PLANE

We know L1 & L2, so B2 can be calculated from measured value of B1:

L1

L2

NEW

B1

ACTUAL RIDER B2

( L1 + L 2) B1

BAND WEAR

WORN

L1

PISTON ROD LENGTH


Due to the cramped conditions inside the cylinder, it is difficult to install a sensor to measure the piston-to-bore clearance directly. For this reason, we normally measure vertical rod position at the face of the pressure packing case, and indirectly infer the piston position. Simple geometry is used to calculate the base (B2) of the large right triangle with known dimensions L1 and L2, assuming that it is “similar” to the smaller right triangle with known dimensions, B1 and L1. The L1 and L2 dimensions need to be measured accurately, and entered into the monitoring system. The monitor will use these values along with the measured value of B1 to calculate B2 for each stroke of the piston. This example shows the probe located above the piston rod looking downwards, although depending on the specific installation, it may be more convenient to mount it below the rod looking upwards. Either location may be used. There are several factors that can add error to our rod drop monitoring. We depend on four basic assumptions (listed on the following slide) to perform rod drop measurement.


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Rod Drop

Assumptions Fundamentals

1. The distance measurement made at the pressure packing case changes in direct proportion to the change in rider band wear (assumes the similar triangles relationship is valid). 2. Gravity is the predominant vertical force acting on the piston and rod assembly. 3. Rod flex is negligible compared to the amount of rider band wear being measured. 4. Operating temperature of the piston is relatively constant.


1. Other factors besides rider band wear can cause the piston rod to drop. For instance, wear of the crosshead shoes or pin bushing can introduce additional clearance at the crosshead end of our assumed similar triangles. 2. The piston may not stay on the bottom of the cylinder during the entire stroke. This situation is more likely with relatively small diameter, pistons used in high-pressure cylinders that have many stages of pressure packing. 3. Rod flex may be significant, and it may occur in the horizontal direction as well as in the vertical direction. 4. The piston may expand and contract as operating temperature varies due to startup, load changes, or other factors – such as unexpected transients occurring to the cylinder cooling system, upstream gas process, or interstage coolers. Note: Rod Position monitoring is less susceptible to these errors than Rod Drop monitoring. It is introduced in the module that follows this one.


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Rod Drop

Ideal Piston Rod Behavior Fundamentals

• This animation illustrates an idealized rod drop monitoring situation. • The rod drop measurement varies as a gentle sine wave shape over an entire revolution of the crankshaft. Rod-Drop-Ideal.gif


This animation shows the simulated displacement signal from a proximity probe that is mounted vertically beneath the piston rod on a perfect machine. Depending on the specific compressor, it is just as valid (and sometimes preferable) to install the probe above the rod, looking vertically downward. Trigger angle is a simply a specified single value of crank angle at which the rod drop monitor can be programmed to collect an instantaneous measurement of rod displacement. This animation displays the crank angle numerically as it changes from 0 to 360 degrees through each stroke of the piston.


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Rod Drop

Actual Piston Rod Behavior Fundamentals

• The animation shows rod position test data collected from a highlyinstrumented compressor.

Probe pair at piston rod packing gland

3 probe pairs installed for testing

(animation)


The linked animation shows the motion of an actual piston rod by displaying data that was collected from a real machine as part of a research project. Proximity probe pairs were installed at the scraper packing case, the intermediate wiper packing case, and the pressure packing case, so that the lateral rod displacement could be measured in two dimensions at three different locations. Note: This much instrumentation is not typical! The scale on the left side of the animated displacement plot has a scale of 2 mils of displacement per division. The top pane of the animation is the view looking down on the compressor, and shows that there is significant side-to-side motion of the piston rod. The bottom pane is the view looking at the side of the machine, and shows the vertical movement of the rod. The animation shows significant side-to-side movement as well as vertical movement. Maximum displacement is roughly 80 mils peak to peak (40 divisions on the scale).


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Rod Drop

Instrument Settings • Installation dimensions (L1 & L2) • Once-per turn reference signal • Trigger angle (if collecting one instantaneous sample per crankshaft rotation) • Transducer scale factor

PISTON

PROBE POSITION

MIDPOINT

PROBE VIEW CL

PLANE

L1

L2

NEW

B1

ACTUAL RIDER B2

BAND WEAR WORN

PISTON ROD LENGTH

Compressor Monitoring Instruments


(1) The monitor needs accurate values for L1 and L2 to calculate B2 from measured values of B1. (2) The monitor also needs to know when each cycle of crankshaft rotation occurs, so that it can properly synchronize its data collection. This requires the use of a onceper-turn reference signal (shown on following slide). (3) For each stroke of the piston, the Rod Drop monitor can be programmed to provide either a calculated Average value, or an instantaneous value that is collected at a specified trigger angle. If an appropriate trigger angle can be determined, the instantaneous values often provide more useful trending data than the average values (described in subsequent slides). (4) Finally, it is important that the proximity probe is calibrated for the actual piston rod that it will be measuring. Standard scale factor is 200 mv/mil, but several factors may cause the as-installed scale factor to be different. Factors include composition of the piston rod alloy, surface treatments, coatings, or contamination, and the roundness of the cylindrical rod itself (described in subsequent slides).


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Rod Drop

Required Measurements Instrument Settings


This slide shows the values that the monitor needs to have to calculate accurate Rod Drop data. These values include the physical dimensions of connecting rod length, piston rod length, stroke length, and probe position. The monitor will also need to be programmed with the piston angle for this throw. Recall from the Crank Angle module that Piston Angle is simply the number of degrees of crankshaft rotation from the time the once-per-turn event is triggered to the time that the throw of interest reaches TDC. In the example shown here, the piston angle is approximately 40 degrees.


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Rod Drop

Signal Processing Instrument Settings

Average Value

Instantaneous Value

• Monitor processes the proximity • Monitor collects a single sample at transducer signal to create a single a specified value of crank angle average value over a complete (“trigger angle”) for each stroke. stroke (one full crankshaft rotation). • Requires once per turn crank angle • Does not require crank angle reference signal. reference signal. • May provide more accurate • May be adequate for general trending for a compressor with trending of rod drop values. changing load conditions.


Average Value – This measurement has existed since the early days of analog eddy current monitoring instruments. It requires only very simple processing to filter the voltage signal from the proximity transducer to produce an average value. One small advantage of this method is that it does not require a once per turn reference signal. However, as we have seen in the Cylinder Pressure and Cylinder Performance modules, it is vital to have an accurate crank angle measurement to evaluate the dynamic pressure signal with respect to displaced volume. The average rod drop value may be adequate for general trending, but significant errors can sometimes be introduced when a compressor operates at a variety of different loading conditions. Instantaneous Value – This measurement is collected only once during each stroke – at a specified value of crank angle, known as the “trigger angle.” It is not averaged or filtered. If the trigger angle is selected carefully, this measurement may provide more accurate trending for a compressor with changing operating conditions. The next four pages describe some of the details that should be considered when selecting an appropriate trigger angle.


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Rod Drop

Determining Trigger Angle Selection Guidelines • Select a value of crank angle where the piston rod is in tension. • Select a value of crank angle where the piston rod displacement is consistent at different compressor loads.

Note: The “default” monitor setting for Trigger Angle may not be appropriate for your application.


Selecting a trigger angle that occurs while the piston rod is under tension may reduce errors introduced by flexing of the rod. However, this might not always be true (Recall the research data that was shown with the animation on Page 11 – Actual Piston Rod Behavior). Selecting a trigger angle that corresponds to a relatively constant value of rod drop over a variety of load conditions will usually reduce the errors that may be introduced by changing compressor capacity. Rod drop monitoring instruments may include default trigger angle settings. For example, Bently Nevada� rod drop monitors have traditionally used 240 degrees as a default setting, based on research performed by a North American compressor manufacturer. These default values provide a starting point for trigger angle selection, but they should not be accepted as-is without validation.


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Rod Drop

Rod Load vs. Crank Angle Determining Trigger Angle

40°

120°

240°

40° 120°

240°

tension tension compression compression

Full Load Conditions

Part Load Conditions

The piston rod is under compression at the default trigger angle setting (240°). However, it is under tension for both full & part loads between 40° and 120°. 16 © 2007 General Electric Company. All righ ts reserved.

For the compressor in this example, pressure monitoring instrumentation and diagnostic software was already in place. This allows us to determine values of crank angle where the combined load is positive (tension) on the piston rod. For this particular machine, the default trigger angle setting is not the best place to start, as the piston rod is under compression at 240 degrees at both full load and part load conditions. For Full Load conditions, the piston rod is under tension between approximately 40 degrees and 196 degrees of crank angle. For Part Load conditions, the rod is under tension from 0 to about 120 degrees, and again from roughly 296 to 360 degrees. There is an overlapping crank angle range, between about 40 and 120 degrees, where the piston rod is in tension for both Full Load and Part Load conditions. This is a good place to start looking at actual rod displacement to select a trigger angle that will work satisfactorily at all compressor loads. Note: If the compressor being evaluated does not already have permanent pressure monitoring installed, it may be possible to install temporary sensors to perform a rod load evaluation. Otherwise, it may be necessary to use calculated data from the manufacturer, or operating experience from similar machines to select a crank angle range where the piston is most likely under tension.


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Rod Drop

Rod Displacement vs. Crank Angle Determining Trigger Angle

40° 120° 60°

40° 120° 60°

tension tension

tension tension



Piston rod displacement is ~46 mil for both Full & Part loads at 60° crank angle. 17 © 2007 General Electric Company. All righ ts reserved.

On the previous slide, we observed that the piston rod is under tension at both Full Load and Part Load conditions between a crank angle of roughly 40 to 120 degrees. Within this range, our goal is to find a crank angle value where the piston rod displacement is fairly consistent no matter what load the compressor is experiencing. This will allow us to establish setpoints for alarms that are not activated spuriously every time the compressor is running at part load conditions. For this particular machine, the piston rod displacement is about 46 mils at 60 degrees crank angle for both Full Load and Part Load conditions. So 60 degrees would probably be a more effective Trigger Angle than the default value. Note: In “real-life” it may be more typical that we would be using portable test instruments to perform such an evaluation as part of an initial installation. The following slide includes an example of the type of information that might be available in such a situation.


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Rod Drop

Rod Displacement (Oscilloscope) Determining Trigger Angle

360°

360°

360°


360°



The purpose of this page is simply to emphasize that, although the concepts involved in selecting a good trigger angle are simple, the process itself is often not as straightforward. Portable test instruments, such as this digital oscilloscope, are VERY flexible, and their settings must be understood very clearly in order to make meaningful comparisons between rod position waveforms collected at different times and operating conditions. These two oscilloscope traces each show piston rod displacement values, as read in voltage levels, from eddy current proximity transducers. In the timebase display mode that was selected, each trace shows data for just over two and a half complete revolutions of the crankshaft. Observe that the waveform pattern repeats fairly consistently at each of the blue vertical lines. When evaluating rod displacement values at different load conditions, it is especially important to ensure that the oscilloscope is selected to the DC-coupled mode, so that the voltage (and therefore displacement) readings can be evaluated on a consistent scale.


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Rod Drop

Probe Calibration • Verify probe calibration to the actual piston rod surface to reduce errors caused by the following factors: – Specific composition of the piston rod alloy – Surface treatments, coatings, or contamination – Roundness of the cylindrical piston rod surface


If the piston rod material is different from the material that was used for the factory calibration of the proximity transducer, the scale factor will be incorrect. This will result in inaccurate distance measurements. Even very thin wear-resistant plating and surface treatments can change the electrical properties of the piston rod enough to affect the scale factor of the displacement transducer.


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Rod Drop

Piston Rod Material Effects Probe Calibration 24 22

) C 20 D V ( 18 l a n 16 g i S 14 t u p 12 t u O 10 r e c 8 u d s 6 n a r T 4

4140 Steel Tungsten Aluminum Copper

2 0 0

10

20

30

40

50

60

70

80

90 1 00 110 120 130 140

Actual Probe Gap (mils)


This example shows how a probe calibration curve can be affected by different material properties of the “target” surface that is being viewed by a proximity transducer. The slope of this curve is called the Scale Factor. In this example, the Scale Factor for 4140 steel is approximately 200 mV per mil of displacement: Scale Factor = (output voltage range) / (displacement range) = 20 V / 100 mil = 0.200 V / mil, or 200 mV per mil.


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Rod Drop

Micrometer Kit Probe Calibration

• Micrometer kit accommodates creating a probe calibration curve for the actual piston rod.

Bently Nevada, LLC


The shaft micrometer kit has a V-shaped saddle machined into its base, so that it will remain secure while it is temporarily strapped to any cylindrical shaft. The micrometer moves the probe closer or farther away from the shaft surface, in a very precise manner. Plotting the voltage of the transducer output signal vs. the physical distance of the probe tip from the surface results in a probe curve, with Scale Factor indicated by the slope of the curve (as shown on the previous page). This scale factor value must be programmed into the Rod Drop monitor so that it can accurately convert the proximity transducer signal into a distance measurement. Note: The need for accurate probe calibration also holds true for the Rod Position monitor, as we will see in the module that follows this one.


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Rod Drop

Practice Exercises Rod Drop

• • • •

Fundamentals Instrument Settings Determining Trigger Angle Probe Calibration

22 © 2007 General Electric Company. All rights reserved.

Practice exercises for each module may be used for self-study or for instructor-led group review sessions.

For Further Reading: General Electric public reference document, GER-4274, “Is Rod Drop the Right Measurement for My Reciprocating Compressor?


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07 Rod Drop(Compressed)

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