Definitions

When the natural (critical) frequency of a machine coincides with the rotational frequency of rotating machines, resonance occurs. Every machine has one or more natural (critical) frequencies.

When a system function has been developed with multiple means for the same function. Consider two vent valves that serve the same function. When one of these valves fails, the other will still perform the function. Redundancy is often related to Hardware Fault Tolerance (HFT). However, HFT is not the same as redundancy.

The probability that an Independent Protection Layer (IPL) fails when called upon. PFD is used to determine the reliability of the protection layers (IPL) that are supposed to reduce the risk of failures to a tolerable level.

An Independent Protection Layer (IPL), or protection layer, that serves to prevent the occurrence of dangerous situations or events.

In a parallel alignment error, the axes are not aligned straight.

An oil-whip occurs when the oil-whirl frequency is equal to the critical (natural) frequency.

An oil-whirl occurs when the shaft in a bearing vibrates at a frequency of about half of a rotational speed. The amplitude of the movement at the center of the shaft increases as a result.

An Independent Protection Layer (IPL), or protection layer, that is used to mitigate the consequences of a dangerous situation or event.

The initiating event is directly related to the enabling condition. It is the cause of a dangerous situation or event. For example: corrosion in a pipeline (initiating event) which leads to a gas leak (enabling condition), which in turn leads to a gas explosion (dangerous event).

The Safety Instrumented Functions (SIF) of a Safety Instrumented System (SIS) fulfill the protection layers, or the Independent Protection Layers (IPL). These protection layers should be independent of each other, so that if one layer fails, it does not affect the other layers. Each of these independent protection mechanisms reduces the risk of machine failures …