Jul-2014

Improving a compressor protection regime

Analysis of the build-up to the fracture of a compressor crosshead led a refiner to revise its approach to performance monitoring

BEN AUSTIN
Prognost Systems Inc.

Article Summary

Compressor crosshead fractures are costly, leading to unplanned outage of the system and loss of production. Prognost Systems claims that the development of fractures in the motion work, especially on the crosshead, can be detected by using segmented peak-to-peak piston rod position analysis. The company also recommends this as a mandatory parameter and for safety interlock. This claim was proven to be true by a complete fracture of a crosshead which could have been avoided if the automatic interlock had been activated by the operator. The experience of this case also motivated the operator to improve their site shutdown philosophy.

The installation
At a refinery in the US, a Nuovo Pignone reciprocating compressor was put into operation in 2004. Since the initial start-up, a Prognost system had been used to detect common faults such as valves, riders, rings and others. The compressor’s overall reliability had been very good. No failures in the motion works components occurred; no Prognost safety alert or alarm limit of CHS vibration (active) or rod position (inactive) had been reached.

Failure history
In 2011, the compressor had its first major overhaul in seven years due to a knock. Several parts such as a first stage connecting rod, bearings, crosshead pin and the main bearing liners were replaced. The crosshead failure described here occurred 11 months after these service works had been carried out by a non-OEM contractor.

Development of the crosshead fracture
At the time the event started, the compressor was running steadily at a normal load of 75%:
• Twenty-nine hours prior to the break, the Prognost-NT system issued the first diagnostic message. It reported the following damage class: ‘Crosshead/Piston Rod/Piston Stage 1 with a correlation of 100%’
• During the following minutes, there was a steady increase from the first diagnostic message (pre-alarms) to alarm (shutdown). In addition, the vibration safety limits of the crosshead slide were violated in several segments
• Sixty seconds before the crosshead failed, the first stage piston rod position reached trip levels
• After the crosshead parted and the vibration alert conditions on both crossheads were reached, automatic shutdown was initiated on the crosshead slide vibration
• On previous trips, the operators had seen the low oil pressure annunciator on and assumed that was the cause of the shutdown, when in reality this is one of the alarms that comes in when the machine is tripped offline and is coasting down. They checked the oil system, found it to be good, and restarted the compressor 32 minutes after the safety shutdown.

A log of the 30 minutes leading up to the fracture is shown in Figure 1.

Data analyses showed that the crosshead slide vibration and the rod position were both giving a normal online signature seven days prior to the failure (see Figure 2). The crosshead vibration safety limits were set between 2.0g and 2.3g.

Sixty minutes before the event, the crosshead vibration and the rod position were well below the safety limits. Two minutes prior to the fracture, development of the crack was visible in the CHS vibration RMS analysis in four segments (see Figure 3). For the safety shutdown, violations in six segments were needed.

Compared to the crosshead, the rod position increased 10 minutes earlier, averaging 51.23 mils 60 seconds prior to the trip. Some 15 seconds before the trip, the rod position reached 71.2 mils, exceeding 90 mils during the last three revolutions (see Figures 4-6). The sequence of events is shown in Table 1.

Taken together, all of the gathered data fully illustrated the reaction of the system shortly before the separation of the crosshead as well as what happened when the operator team overruled the shutdown initiated by the Prognost system. The shutdown settings from the site acceptance test (SAT) protocol showed the detailed safety limits and criteria for the activation of pre-alarm and main alarm. It also proved the disabling of analyses by resetting the safety alarm if the machine stops.

A new safety shutdown philosophy
After the complete fracture of the crosshead, the data analyses led to several conclusions on the operator’s side. First of all, Prognost monitoring showed that the developments of this type of crosshead fracture cannot be detected at an early enough stage by using the crosshead vibration alone. The use of piston rod position measurements has proven to be a reliable factor for safety protection.

As a result, the monitoring of piston rod measurements is now part of the site shutdown philosophy on machines equipped with Prognost at the refinery. In addition, operators are no longer permitted to restart a compressor tripped by a Prognost system. Thus, the Prognost-NT system now helps to avoid costly damages to rotating equipment, lowering outage and securing productivity at the refinery.

Ben Austin works in Customer Support with Prognost Systems Inc., Houston, Texas. He is an ISO Certified Category III Vibration Analyst with the Vibration Institute with over 17 years’ experience with rotating equipment.