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Reciprocating Compressors are utilized in all manufacturing industries.
Because these machines are capable of providing high pressure along with
variable loading, they are favored for many gas process applications. The
total quantity of positive displacement reciprocating engines, pumps and
compressors far exceed the number of centrifugal units.
Past studies within the Hydrocarbon Processing
Industry (HPI) indicate that the maintenance costs for reciprocating
equipment are approximately 3.5 times that of centrifugal equipment.
Substantial savings in maintenance costs and an increase in run time may be
achieved through basic monitoring of some if not all of the following
Reciprocating Machine parameters.
Frame Vibration
The most
important vibration parameter of a successful monitoring program is frame
vibration. When properly applied, monitoring Frame Vibration will help
prevent catastrophic failures. For the greatest benefit, a Frame Vibration
Monitoring system should be wired to an automatic machine trip. To decrease
the possibility of a false trip, two (2) case mounted accelerometers should
be mounted on the frame relatively close to each other. The outputs from the
two transducers are signal conditioned, and their trip circuits are "AND"
voted. In other words, before a trip is initiated, both of the transducers
with their monitors must sense a high vibration level.
The
two accelerometers should be perpendicular to the shaft, and oriented in the
direction of the Piston travel, and mounted on a surface that allows direct
transmission of the Frame's Vibration.
A complete monitoring system would include:
Rod Drop
The vast
majority of Reciprocating Compressors are designed with horizontal Cylinders
and Pistons. This is primarily due to foundation requirements and the
popularity of opposed-balanced machine designs. A downside of
horizontal cylinders is the force of gravity causes the Piston to
"RIDE" more in the bottom of the Cylinder than in the top. This causes the
Piston to wear more in the "DOWN" direction. Machine manufactures provide
wear or rider rings to provide a replaceable wearing surface. The wear or
rider rings are rotated or replaced before damage to the cylinder lining
occurs.
Currently,
one popular safety device for detecting Rod Drop is a unit mounted under the
rod at a gap determined by the allowable wear of the wear ring. When the rod
contacts the safety unit white metal is worn through allowing instrument air
to escape. This in turn causes a pneumatic flag on the control panel to
change status.
There are
several disadvantages to the above-mentioned methods of Rod Drop detection:
-
A real
trend of ring wear cannot be established with a short amount of operating
time.
-
Since the
machine must be shut down, halting production, periodic inspections for
ring wear are expensive.
-
A change
in processed gas, load changes, and foreign matter can cause an extreme
change in ring wear rate.
For several
years, Eddy Probe systems have been utilized to measure Rod Drop. This
method of Rod Drop measurement has been gaining positive recognition with
Reciprocating Machine users. To measure Rod Drop with an Eddy Probe system,
the probe is installed in the vertical direction viewing the rod with a
probe housing. The CMCP801 proximity
probe housing provides an external adjustment (through the distance piece)
of the probe gap between the probe tip an rod. The proximity probe system is
interfaced to a CMCP545 Position
Transmitter to measure the probes DC output (Probe Gap).
The CMCP545 will provide a 4-20 mA
output that is proportional to the DC Gap Voltage. If a
CMCP545A Monitor is used, two levels
of alarms with corresponding Alert and Danger relays are provided. By
trending the DC Gap voltages from the eddy probe, it is possible to measure
the average horizontal running position of the piston rod.
A complete monitoring system would include:
Rod Run Out
Whereas Rod
Drop is a measurement of rod position, Rod Run Out is a measurement of the
rod's actual dynamic motion as it travels back and forth on its stroke.
Another term for this measurement is Rod Deflection.
One method
to make this measurement is to mount a dial indicator in the distance piece
riding on the piston rod. The machine is then barred through a complete
cycle. Indicator readings are taken in both the vertical and horizontal
directions during the machine's cycle.
The amount
of Rod Run Out is highly dependent on the cylinder alignment with the
Crosshead. Due to inherent looseness in the Crosshead and thermal growth of
the machine, higher readings of Rod Run Out are allowed in the vertical
direction. The horizontal direction allowances are much less and high
readings are attributed to misalignment. Typical Rod Run Out allowances are
3.5 to 6.0 mils Pk-Pk in the vertical direction and 1.5 to 2.0 mils Pk-Pk in
the horizontal direction.
An
alternative to dial indicators to make this measurement is again an
proximity probe system. Since dial indicators can only be used while the
machine is being barred, they do not provide an accurate measurement of Rod
Run Out. On the other hand, Eddy Probes can make this measurement while the
machine is operating. This provides a highly accurate measurement of the
actual dynamic motion of the rod under full load conditions.
One eddy
probe is mounted in the vertical (x) axis and one is mounted in the
horizontal (Y) axis in relation to the Piston Rod. Each Eddy Probe is
interfaced to a CMCP540A Vibration
Displacement Monitor for signal conditioning, alarming and interface to a
PLC or DCS.
The
vertical Eddy Probe can also be used as for Rod Drop measurements.
Therefore, the installation of X and Y Eddy Probes can be used for both Rod
Run Out and Rod Drop measurements.
A complete monitoring system would include:
Crosshead
Vibration
The
Crosshead of a Reciprocating Machine is made up of several major components:
Crosshead Bed, Crosshead, Slippers and Crosshead Pin. The purpose of the
Crosshead is to transform the circular motion of the crankshaft into linear
motion for the rod and piston. The Crosshead slides on a lubricated
babbitted surface much like a standard journal bearing. However, the
Crosshead slides back and forth instead of in a circular motion like a
shaft. Clearance between the Crosshead and the babbitt surface may be in the
range of 10 to 25 mils. As crankshaft rotates, the Crosshead is driven to
slide on either the upper or lower babbitt surface. As the clearance between
the Crosshead and babbitt surface increases, the Crosshead vibration
increases.
In a
compromise to measuring both vertical and in-line with the cylinder,
experience has shown that a CMCP1100
Industrial Accelerometer, mounted on a 45°
angle block, located in-line with the piston travel, will adequately measure
Cross Head vibration. The
accelerometer would be connected to a CMCP530(A) Velocity Transmitter
(Monitor).
By
processing the vibration signal in Peak Acceleration instead of RMS
detection, we can measure high amplitude short duration “peals” or “events”
that appear periodically. The Cross Head mounting location in-line with
Piston travel will see the mechanical transfer of energy caused by impacts
resulting from mechanical looseness on the compressor cylinders, such as
loose rod nuts and loose bolts. Liquid in the process can also be detected.
Competitive offerings may refer to this measurement as Rod Impact
monitoring.
Crosshead
Pin lubrication can also be diagnosed with this measurement. The Crosshead
Pin connects the connecting rod from the crankshaft to the crosshead. This
pin is force lubricated through "reversal" which allows oil between the pin
and its bushing. With each stroke, the oil is forced out. If reversal does
not occur, the pin and its bushing will fail rapidly.
A complete monitoring system would include:
- One accelerometer -
CMCP1100
- One 45° mounting block
- One vibration velocity transmitter or
monitor - CMCP530 or
CMCP530A
Main Bearing Vibration
Main
Bearing Vibration has not been proven to be a popular approach for
monitoring Reciprocating Compressors. Several end users have had problems
with broken crankshafts, which they thought were caused by unusual bending
of the crankshaft. In one documented case, machinists had over tightened the
drive belts powering the cooling fan on a reciprocating engine. This caused
unnecessary bending of the crankshaft.
Currently,
several end user’s have installed X and Y Eddy Probe systems on the main
bearings of large 12,000 HP Reciprocating Compressors. This installation
very nearly resembles that of a standard centrifugal compressor. However,
both probes view the crankshaft from the bottom bearing cap-mounted 90o
apart. As the lubricating oil cools the main bearing, no unusual measures
needed to be taken on this installation.
The Eddy Probes are
connected to a CMCP540 Vibration (Displacement) Monitor to measure radial
vibration. The full-scale range of the monitor is based on the bearing
clearance. As in all Reciprocating Compressor applications, the entire
bearing clearances are used.
A cost effective
compromise in lieu of installing X, Y Eddy Probes is to mount a CMSS2100
Industrial Accelerometer in-line with the crankshaft centerline and
interface it with a CMCP530(A) Velocity Transmitter (Monitor).
A complete monitoring system would include:
or
Valve Temperature
According
to industry studies, valve failures account for 41% of the problems
associated with reciprocating machinery.
In a Reciprocating Compressor, the valves are a pressure actuated "Poppet"
variety. Every machine manufacturer has favorite types of valves for
different applications. These valves operate utilizing a delta or
differential pressure technique. The opening and closing of a valve occurs
when the delta pressure is less than the force of their return springs.
When a
valve begins to fail, it usually begins to leak the process gas. This causes
the process gas to be re-compressed, further heating the gas. This higher
temperature process gas can be detected using a temperature transducer. This
transducer can be mounted through the valve cover plate measuring the gas
temperature near the valve. In some installations, the transducer is simply
imbedded in the valve cover plate, or within a valve cover plate bolt. This
mounting method is preferred when the process gas is explosive, which is
usually the case.
The
measured temperature of the process gas is then compared to the measured
temperature of the process gas at the same type valve, suction or discharge,
and the same stage of compression. Measured temperature differences of 4 to
20o F can indicate a problem with a valve.
A complete monitoring system would include:
- One RTD per valve
- One temperature transmitter or monitor per
RTD - CMCP560 or
CMCP560A
For additional reading STI offers several
application notes with a more in depth discussion of at:
STI Application Notes |
