Mar-2013

Bridging the gap between safety protection and condition monitoring

In the process industry it is still common that users strictly separate safety protection equipment from condition monitoring. Machine operators frequently face a time-consuming troubleshooting based on minimum diagnostic data e.g. from the DCS system.

Jost Anderhub
PROGNOST Systems GmbH

Article Summary

Since many years, flight data-recorders are a common standard in the aviation industry to perform post-incident analyses of aircraft accidents. Such transient data recorders are also available for machinery protection systems. The captured data can be used to improve the traditional snapshot monitoring program and support cost efficient condition monitoring purposes without waiting for an analyst to come.

Maintenance strategies and condition monitoring of machines
When operating rotating equipment, different maintenance strategies are applied. The simplest method is to run a machine until it stops due to a failure - taking the risk that a comparably small damage can cause expensive consequential damages. When wear processes in the machine are well understood and the timely progress of wear is known, wear-parts should be replaced shortly before their lifetime is reached to maximise use of the wear potential and prevent component failures (Preventive Maintenance). However, the lifetimes of components of reciprocating compressors, such as valves and bearings, are not subject to linear wear what make it difficult to decide on the best time to replace a component mainly on operating hours. Maintenance decisions on components that are experiencing non-linear wear-out are challenging and can not be made time based. They require monitoring of certain parameters that allow judgement on the component’s condition and an estimation of the best time to replacement. This is named Predictive Maintenance.

Which maintenance strategy is applied depends mainly on two parameters:
a) the criticality of the machine
b) the availability of a spare compressor
The total population of machines can be grouped in four main categories.
Category I       Critical, unspared
Category II      Critical, spared
Category III     Uncritical, spared
Category IV     Uncritical, unspared

Generally: the more critical a machine - the less acceptable it is to run it to failure or to rely on preventive maintenance. An unexpected failure with a machine shutdown has a negative effect on the production or safety. The availability of a stand-by compressor will reduce the requirement for predictive maintenance, but has no effect on the level of safety that is required.

The gap between protection and condition monitoring
Especially uncritical and spared reciprocating compressors are still operated without or with a minimum of protection. Whereas most of the critical compressors are equipped with at least basic protection such as frame vibration, low lube oil pressure and discharge gas temperature. The number of primary damages is large and full coverage of all these would require an almost indefinite amount of monitoring parameters. Therefore a compromise between “limitation of consequential damage” and “amount of protection parameters” is widely practiced. In short: depending on the degree of the acceptable damage, the protection parameters are determined.

An overall guideline on the essential protection parameters can be obtained from the API618. However, this should be considered as the bare minimum and does not reflect all recent experiences of the industry.

Frame vibration is typically considered a parameter that provides basic information on the mechanical condition of the compressor. However, in the past years it has been widely accepted that frame vibration is not sufficient for mechanical safety protection. Monitoring vibration impacts on the crosshead slide has been recognized as a far better method for detecting all major mechanical problems of the motion work. Recent experience has also shown that piston rod failures can only be prevented by monitoring dynamic piston rod position (piston rod run out). Only if a piston rod failure is considered an acceptable damage, crosshead slide vibration is sufficient for mechanical protection.

Protection systems are typically based on simple, but reliable, RMS parameters e.g. RMS vibration. Some of the values are transferred to DCS systems, but most recommended protection parameters such as low frame lube pressure or low suction pressure are not useful to provide any diagnostic information for preventive or even predictive maintenance.

Piston rod position is a parameter that is directly “located” at the gap between protection and condition monitoring. The two different parameters:
a) vertical piston rod position to indicate rider ring wear
b) piston rod run-out indicating the mechanical health of the piston rod and its connections are coming from the same sensor and signal. It is economical to use both parameters, one for protection and one for condition monitoring. However, in most applications only one parameter is used, either for protection or for condition monitoring.

How can the gap between protection and condition monitoring be bridged?
Bridging the gap means using monitoring sensors and their continuous information stream for both purposes. This is more economic and increases the quality of both, protection and condition monitoring. One way to bridge the gap is to record high frequency signal data for detailed analysis in a device, attached to the protection system. The aviation industry has introduced flight data recorders since decades; however, it is only used in post mortem analysis. But why should recorded data only is used when a damage has occurred?

Using the protection sensors for condition monitoring purposes is one option; another option is increasing the frequency and signal quantity of monitoring sensors and continuously recording the data for on-demand diagnostic approach.
Reciprocating compressor diagnostic condition monitoring is typically based on:
a) p-V diagram analysis
b) Cylinder vibration
c) Vibration monitoring of crankshaft bearings