Mar-2013

Recips need dedicated vibration monitoring systems

Avoiding ‘crater maintenance’ and the ‘doom loop’ Reciprocating compressors have a reputation as bad actors among the rotating equipment fleet; showing the highest number of damages while being process critical at the same time.

Oliver Franz
PROGNOST Systems GbmH

Article Summary

Although this is a crucial combination, insufficient protection and condition monitoring systems can still be found on reciprocating machinery. Prognost recently asked a potential customer’s reliability engineer whether he employed time-based or condition-based maintenance strategies. He was completely caught by surprise and replied, “None of these. We operate on ‘crater maintenance.’ As long as there is no crater visible where once was a recip, no maintenance is needed on the machine.”

His reply was in jest, but it underscores the fact that “cheap” maintenance usually is not cost-effective at all.

Even after numerous catastrophic recip failures, some operators still use inadequate Machinery Protection Systems (MPS). A review into the history of applicable standards offers some insights.

From the first publication of the API670 in 1976 (“Non-contacting Vibration and Axial Position Monitoring System”- dedicated to proximity sensor-based machinery monitoring) until its currently valid 4th edition “Machinery Protection Systems,” the API670, largely focused on the technical requirements of centrifugal equipment (gas and steam turbines, centrifugal and axial compressors).

Besides other important factors, API670 requires monitoring of dynamically changing parameters, such as axial and radial shaft displacement, speed, surge along with bearing, and casing vibration. But none of these (except vibration) deliver effective and reliable machinery protection on a reciprocating compressor.

Until 2007-08, when the API670 5th edition task force was formed, reciprocating compressors have not been in focus within API670. This latest edition, which is expected to be released this year, hopefully will offer valuable information and guidance on how to effectively protect reciprocating compressors.

It would be desirable if the standards include crosshead acceleration as a safety protection parameter, based on practical lessons learned. That topic will be discussed below.

The IEC 61511 provides the official framework for operators of rotating equipment, potentially bearing risk to harm Health, Safety and Environment (HSE). It also offers guidance to the process equipment operator, defining the SIL (Safety Integrity Level) requirements which have to be fulfilled by the machinery protection system to be installed.

One way to obtain an overview which SIL is appropriate to mitigate the inherent remaining risk to an acceptable level is offered by the Risk Graph. Using the four different risk parameters (Occurrence Probability, Extent of Damage, Exposure Time and Hazard Avoidance [once damaging occurs]) the appropriate SIL1 to SIL4 will be indicated  (with SIL4 being the most stringent level).

Not all machinery protection applications may justify a SIL1, SIL2 or even SIL3 certified solution. One thing, however, is important when performing an evaluation likely driven by budget constraints: The likelihood of a safety protection system failing on demand can be 10 < 100 times higher if the operator choses a SIL 1 over a SIL2 solution. If a system meeting SIL3 requirements over a SIL1 system is chosen, the likelihood of the systems to fail on demand can be 100 < 1000 times higher.

Guidance in the SIL standards (IEC 61511) regarding machinery protection systems (API670) can help operators to reduce HSE and economic risks appropriately.

To understand the limitations of the standard approach of monitoring vibrations on reciprocating machinery, it is important to understand the difference between a uniform rotating movement of a turbine shaft in comparison to a oscillating operation mode. Due to the large effort put into the dynamic rotor balancing of centrifugal machinery, they show minimal shaft deflection, resulting in low casing vibration levels.

In contrast, a reciprocating compressor shows a very different behaviour: Massive pistons are driven back and forth by crosshead-type drive trains, involving reversal of piston rod-forces from tension into compression, making the entire frame with all its components shake and bend to a good degree. Additionally, multiple groups of suction and discharge valves create opening and closing impacts, within every single revolution leaving distinct vibration events – and we call this a “normal operating condition.”

When comparing the working principles of a recip with a centrifugal machine, it becomes apparent that a recip requires a more dedicated monitoring system designed to handle the specific challenges.

Due to its working principal, the crosshead and its crosshead pin clearly are the focal points of reciprocating machines. Here, the rotating movement of the crankshaft is transformed into a reciprocating (linear) movement of the piston rod. It is the central component where all main forces are transferred via the crosshead pin/wrist pin to the piston rod.

To facilitate movement of these forces in the right direction, a solid crosshead guide is an integral part of each recip. The crosshead guide offers the closest connection of the moving parts within the drive train and is the best position to effectively pick up abnormalities in the vibration signature by the means of acceleration sensors.

As illustrated in the lower left side of a five-month 3- waterfall trend plot in Figure 2, reciprocating machinery typically shows very smooth crosshead acceleration characteristics except two distinct impacts during each revolution around the two rod load reversal points (approximately 40°CA past TDC and BDC). Changes of the acceleration signature and amplitude indicate different mechanical machine behaviour.