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Aug-2011

Early failure detection and shutdown of critical compressor due to a crosshead fracture

Over the last few years it has become more and more apparent that operators of piston compressors use specific monitoring systems particular for their reciprocating machines that implement an automatic machine shutdown function to prevent catastrophic machine failures.

Tobias Ahlert
PROGNOST Systems GmbH
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Article Summary
The current trend is moving away from simply placing a velocity vibration sensor on the frame of critical machines. In fact it is much more effective to have safety shutdown criteria on the basis of measured acceleration values directly at the crosshead slide and to measure the dynamic piston 
rod motion continuously. Specifically this 
kind of safety monitoring is possible, since modern and fast data acquisition systems are available.

Furthermore the method of automatic data interpretation plays an essential role. It is simply not enough to develop such an analysis to calculate an average value over a specific period and compare it with the appropriate alarm limits. With piston compressors it is important to recognize and analyse the individual events over one revolution. This however carries the responsibility to manage more system limit values. In doing so, the experience on which actual limit-values one adjusts with this modern monitoring system plays a very large and important role.

Usually these limit values are based on actual measured values. This is because each piston compressor has specific vibration responses and signatures according to its design and operating conditions. It is challenging to commission a machine with correct limit values without specific knowledge of the machinery vibration response or what we refer to as baseline data.

This paper describes that with only the default factory safety limits installed, a machine failure has been detected and an automatic shutdown successfully occurred. With this successful automatic shutdown, further damage was prevented.

Monitoring system equipment
A Prognost customer in Germany operates four 4-cylinder reciprocating compressors in natural gas service within a H2S environment. Since December 2006 all machines are equipped with a Prognost-NT Monitoring System. The system works with acceleration sensors on the crosshead slide (CHS) and on the cylinder. In addition proximity probes are mounted on the packing flange to measure displacement of the piston rod motion. Figure 1 show the arrangement, which is mounted on all cylinders.
The monitoring system itself is configured with the following software modules:
• Safety
• Early Failure
• Wear
• Process Data Analysis to give the operator a high grade on performance.

In January 17th 2010 the Prognost customer support specialist received a hotline call. This call was made by the customer to ask for verification of data based on a current automatic machine shutdown by the Prognost monitoring system. One of the 4-throw reciprocating machines was tripped on “CHS vibration RMS 36 Segment / cylinder 2”, but the operator could not see anything from the outside of the machine. Note: This call was based on the customer’s service agreement procedure.

The discussed machine has the following technical data:
Machine type:    4-throw, double acting
Service:    Natural gas transportation
Stages:     1
Piston stroke:     175 mm
Cylinder bore:     245 mm
Rotation speed:  380-740 rpm
Power:                1700 kW
Model year:         2005
Volume regulation:     Stepless speed & valve
                                   unloader(25-50-75-100%)
Suction pressure:       30 bar
Discharge pressure:    80 bar
Suction temperature:  15°C
Discharge temp.:        150°C

Safety limit configuration
The proven strategy for vibration analysis is the so called segmented vibration analysis based on 36 segments. Each 10° crank angle (CA) represents one segment per revolution on which the RMS vibration value is calculated. For each segment there is an individual limit which can be set independently from each other to avoid false alarms given by higher values caused for example by, high discharge valve impacts or higher vibrations levels at the rod load reversal points. As well the system checks the peak-to-peak value of the piston rod position signal based on 8 segments per revolution (Table 2).

In addition the system counts the quantity of segments during a specific number of rotations and in which and how many segments a limit is violated.

Figure 2 shows the safety limit adjustment of the crosshead vibration sensor from crank 2 with the actual measured values. In this case the limit is equal over all segments with a value of 40 m/s² instead of set the limits independently from each other which are displayed in Figure 3. The advantage to doing the settings independently is because of the different vibration behaviour within one revolution. The piston rod position safety limit adjustment is similar.
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