Proximity measurement to detect a fractured piston rod
Successful avoidance of major consequential damages using proximity measurements to detect a partially fractured piston rod.
One day, the online monitoring system at the Yara Brunsbüttel (Germany) urea plant site avoided compressor damage by detecting a cracked piston rod at the first stage. Prior to a complete rod failure, the machine automatically stopped, avoiding costly consequential damage.
Application
The urea plant operates, besides others, two Thomassen reciprocating compressors in carbon dioxide (CO2) service. Both compressors are equipped with a PROGNOST-NT online condition monitoring system. In recent years, several machinery failures have been detected by the system, such as rider ring wear, loosened components and valve problems. The machines are four-throw, double-acting, four-stage horizontal compressors and have been in operation since 1997.
Our monitoring system continuously monitors all safety and performance-relevant parameters on the compressors. Vibration and displacement sensors, as well as pressure probes, produce the signals needed for gapless and meaningful safety protection and condition monitoring.
Additionally, all relevant process data from the distributed control system (DCS) are integrated, monitored and trended: Yara decided to have an automatic shutdown in case a dangerous machine status occurred. Based on crank angle-related rod-position analyses, PROGNOST shuts down the machine. The system calculates the peak-to-peak value of the segmented piston-rod position (runout) signal, providing additional information regarding the condition of motion works components.
Sequence of events
After six years’ experience with this machine monitoring system, we were confident that the compressor would be safely tripped in time if the overloading caused bigger mechanical problems.
Yara Maintenance Manager Dr. Uwe Ladwig.
The compressor was intentionally operated beyond design limits. Yara simulated for a short time a capacity increase to identity potential bottlenecks in the downstream urea plant in advance of the installation of an additional compressor.
Yara increased the suction pressure to increase the volume of compressed gas per cycle; discharge pressure as well as running speed remained. The screenshots and data are extracted from the ring buffer data that the PROGNOST system automatically saved. This allowed a subsequent root cause analysis to be performed by the author.
Findings
“When the machine stopped, we found a partially cracked piston rod on the first stage,” Ladwig said after the stress lest. ”Thanks to the – as expected – timely trip, we were able to repair and restart within 24 hours.”
Today it is possible to shut down reciprocating compressors before a piston rod fails completely. The dynamic piston-rod position is the determining parameter to detect loose connections within the drive of the piston or cracks within piston rods at an early stage. Automated online machine monitoring, including safety shutdown functions, lead to lower costs of production and higher plant efficiency.
By Dr. Uwe Ladwig, maintenance manager, urea/rotating equipment, at Yara Brunsbüttel GmbH, Germany.
About Yara
Yara converts energy, natural minerals and nitrogen from the air into essential products for farmers and industrial customers. The main application is fertilizers, while industrial uses and environmental solutions are also important growth segments. The Yara Brunsbüttel site has two production units, an ammonia plant and a urea plant.
