Table of Contents
4.1 Control measures
On our car we know two different causes for maintenence measures. The exchange of wear parts like tires, brakes and the clutch normally takes place not before monitoring probes visual impression or the function show the need. Such an approach is called an „on condition“- repair, i.e. if necessary.
In contrast the producer can demand the exchange for other components under the threat of loss of the guarantee. Typical example is the prescribed exchange of the tooth belt for the valve train after ca. 100 000 service hours. Thereby it does not depend on whether the failures announce itself. A special gentle operation mode also does not extend the space of time. The kind of repair respectively maintenance can be named „life limited“.
The repair of worn parts takes today paticular occurres almost no more. Instead the exchange with new parts is usual. In contrast to this, the repair of gas turbines is of great importance. For example the life time limited rotor components are concerned. Other parts can be reused without rework after an intense check, or if necessary, can be repaired. In this the gas turbine distinguishes itself basically from a modern car.
"Illustration 4.1-0": (Lit. 4.1-6): Two concepts are established for the overhaul time intervals of a gas turbine:
Fixed interval maintenance/overhaul ( "Ill. 4.1-1") and on-condition maintenance/overhaul. The particular ‘philosophy’ must not be linked on a gas turbine type. Thereby an operator specific approach is possible. Both practices have their pro and contra.
Fixed interval maintenance is the traditional aproach of a gas turbines overhaul. It considers especially the design lifetime/guarantee lifetime of the hot parts. If the overhaul interval has to be extended, verifications are inevitable. For this the OEM gives assistance and recommendations. From the OEM in the manual determined overhaul interval refers normally to the operation hours and/or start/shut down cycles. At the on-condition maintenance the overhaul is carried out in terms of a directed exchange of a component/module that has reached its life limit. Naturally it is tried to combine favorable the lifetimes respectively remaining lifetimes of the components of a gas turbine. That counts as well for new as for used parts. This is not at least a demanding logistic task.
So that depends from the possibility to establish with sufficient certainty the condition (‘health’) of the component. For this different approaches that also can be combined, are possible (Chapter 5.1). Sensors monitor and record the lifetime affecting operation parameters. From those the life consumption respectively the remaining lifetime will be calculated by algorithms, established by the OEM. This approach is supported by modern „trend monitoring“ and electronic control systems. Borescope inspections ( "Ill. 4.1-6") have the ability to evaluate components with visual noticeable damage criteria in the installed condition. That can ensure the estimations of the trend monitoring (Chapter 5.1), based on sensor data and/or used alone as lifetime criterion. An example are damages like crack formation by thermal fatigue or thermal abrasion (oxidation) of the wall of a high pressure turbine vane ( "Ill. 3.3-9"). To change the affected parts the module design ( "Ill. 4.1-9") is helpful. This limits time and effort of a parts exchange.
Does an operator wish to proceed according to on-condition or changes to this practice a detailed preparation together with the OEM is inevitable.