How to Assess Turbomachinery Reliability and Strength

In certain operational and environmental scenarios, turbomachinery could be under dynamic loads such as earthquakes, impact/dynamic loadings, or blast loads (blast pressures). Although these might not be normal conditions and may only happen a few times during the life of equipment, they should be considered for overall safety and reliability. Machinery protection requires proper engineering safety and hazard assessment.

Earthquakes, water-hammer, dynamic forces due to the release of fluids, and similar transient cases could happen any time just as error in operation or malfunction could lead to such accidental loading cases. Blasts and explosions, too, must be considered, as well as accidents such as an item dropped from a crane during maintenance or a truck hitting machinery during an overhaul.

How machinery behaves under extreme loads depends on the quality of the design and the engineering/fabrication criteria met. This means going beyond code prescriptions and accurately predicting how equipment will respond during extreme events. Since experimental investigations are invariably limited, alternative approaches should be sought to evaluate the real behaviour and strength under dynamic/impact loads. This requires the use of realistic models or methods that, although may not be entirely accurate, provide some realistic estimates of load-carrying capacity.

COMPLEX RESPONSE

The complexity of the response of machinery subjected to large dynamic loads involves many phenomena. These include elastic and plastic wave propagation, material properties, elastic and plastic buckling, fracture, and rupture. Involvements of different kinds of jointing techniques (welding, bolting, etc.) make simulation or assessment challenging. Another difficulty is the time dependency of dynamic loadings and responses.

Machinery protection requires proper engineering safety and hazard assessment.

It might be assumed that dynamic responses resemble corresponding static responses. This is not the case in many applications. For example, the dynamic elastic-plastic buckling of mechanical parts has been different from static cases and can lead to counterintuitive behavior.

Sometimes a simple energy balance using the rigid-plastic method provides guidance on the overall response of machinery and is helpful in selecting dimensions. For instance, a large explosion can be approximated as a uniformly distributed loading.

Piping loads are particularly vulnerable to different dynamic, impact, and blast loads, resulting in serious damage or leaks. Assessments, simulations, and estimates are required for the resistance of pressurized piping to all possible dynamic/impact loadings to ensure their safety and reliability. In many cases, proper protections and barriers should be in place to reduce risk to acceptable levels. Supports and their structures play a major role in the dynamic responses of piping systems.

There could also be different types and modes of failure when studying machinery under dynamic/impact loadings. As examples, three types of defined failures are mentioned here.

The first type is defined as permanent ductile deformation of machinery without any major incident such as leakage, fracture, or rupture. For instance, the thin shell of a machinery casing might be deformed or wrinkle due to an earthquake. This should be regarded as a failure and should be repaired even if there is no leakage or rupture.

A second type of failure is when rupture, tear, or disconnection happens. In other words, this is when the maximum strain reaches the rupture/tear value for the material, usually in a localized area of the machinery. An example is permanent leakage of a flange joint due to large, exerted loads.

The third type of failure is transverse shear failure. This more readily occurs at boundaries and other hard points (or supporting points) of dynamically loaded machinery. For example, anchor bolt(s) of a machinery might be cut due to dynamic loadings. ■

Amin Almasi is a Chartered Professional Engineer in Australia and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.