Axial compressors have been known to fail catastrophically at high flows due to blade vibration and high temperatures generated when operated beyond the stonewall line.
Critical Speeds
A critical speed is defined as any operating rotational speed that coincide with one of the rotor systems damped, natural frequencies. Operation at the critical speed for extended periods of time can result in internal damage due to excessive lateral vibration levels. It is important for operators to know their compressor critical speeds so they can be avoided. The original equipment manufacturer usually provides a list of critical speeds for users to avoid. Force response plots such as the one shown in Figure 1.6 can be used to locate critical speeds once the compressor has been installed in the field. The reader can see by studying the forced response plots in Figure 1.4 that rotor vibration increases significantly at about 1350 rpms and 2385 rpms due to the existence of rotor critical speeds.
Figure 1.6 Here is a typical predicted forced response plot. The upper plot is the 1x phase versus the rotor speed and the lower plot is lateral vibration versus rotor speed. This analysis was run with mid-span imbalance. Notice: 1) there seems to be a rotor critical at about 1350 rpm where both the X and Y probes will see vibration peaks, which appears to be the rotor first critical, 2) there seems to be a second rotor critical at about 2375 rpm, 3) both critical speeds have major phase shifts associated with them.
Horsepower Limits
The horsepower load on a centrifugal compressor is governed by the flow, gas density, the rotor geometry, and the rotational speed. For a given set of impeller diameters and gas conditions, the fan affinity laws define how flow, pressure, and the required horsepower vary as the compressor’s rotational speed changes as follows:
1 The flow rate varies directly with the speed
2 The differential pressure varies with the square of the speed
3 The power use varies with the cube of the speed
Small operating changes can have significant effects on how a compressor performs, which is why operators should carefully track the compressor’s overall operating characteristics, including the horsepower load, and look for trends indicating process changes. An overload condition may be telling you something is wrong. Overloading may be manifested by high amp loads for compressor with electric motor drivers or a loss of speed for compressors with gas or steam turbine drivers. If amp loads are seen to be rising or dropping, try to determine the cause of the change. Rising amp loads may be caused by changing process conditions or machine degradation, depending on the shape of the compressor performance curves. Falling amp loads may be caused by a decrease in flow due to a flow restriction or a decrease in the suction pressure.
Temperatures
As most process gases get compressed, they heat up in a predictable way. For a given suction temperature, we should expect a predictable temperature rise due to compression at the discharge nozzle. However, deviations in predicted temperature values can occur, which is why operators need to keep an eye on compressor gas operating temperatures. Temperature levels provide vital clues to the condition of a compressor. High compressor discharge temperature or interstage temperatures can be a sign of compressor degradation or a higher than normal compression ratio, due to system fouling or a restriction.
Takeaway: Gas temperature trending is a simple way of assessing the overall aerodynamic health of a centrifugal compressor.
Bearing temperatures can also provide vital clues to the condition of a centrifugal compressor. An upward trending bearing temperature is a sign that something is changing, such as the bearing’s condition or the oil supply temperature. The most critical bearing temperatures are the thrust bearing pad temperatures. A failure of the thrust bearing pad will result in a catastrophic rotor failure as excessive axial movement occurs, which is why the axial position of the rotor should always be closely monitored and trended.
For an extra level of thrust bearing failure detection, a dual voting (2 out of 2, i.e., 2oo2) arrangement composed or two proximity probes are normally installed on critical compressors. (Note: The proximity probes must be installed axially so that they observe the thrust collar or other integral portion of the shaft surface. Probe targets, such as shaft sleeves, that are not integral to the shaft are not recommended.) Each machine train thrust monitor should be configured in a dual voting configuration, 2 out of 2 (2oo2), which is considered best practice by API 670 to ensure a high level of in machinery protection. In some highly critical applications, an OEM or end user may even request a triple modular redundant (TMR) configuration, 2 out of 3 voting (2oo3), to achieve the required reliability level.
A typical set of safeguards for a centrifugal compressor might include:
A well-written set of operation and start-up procedures to prevent the compressor from operating at any unsafe flow or speeds
Reliable flow and speed control
A reliable surge control system
A reliable vibration and thrust monitoring system
Periodic lubrication monitoring
Periodic field inspections of the compressor and driver
Compressor performance monitoring
The purpose of safeguards is to prevent bad things from occurring. In cases where the consequences of an initial failure can escalate and grow in severity due to a progression of secondary damage, multiple safeguards can be used to mitigate the consequences of a failure by breaking the sequence of unwanted events. Figure 1.7 shows a hypothetical installation with a group of safeguards in place to prevent a failure or mitigate the effects of a failure. Without safeguards in place, a failure could progress from internal machinery damage to, in extreme cases, a catastrophic release with the potential to impact the plant or the surrounding community. In general, the greater the consequences of a failure, the more safeguards are required. We must always strive to achieve a balance between cost of safeguards and the risks involved.
Now let’s use what we’ve learned and consider the risk matrix shown in Table 1.3. The far-left column shows event probability levels and across the top are event consequence levels. For example, the intersection of an “extremely high consequence event” and an event probability of “one event per year” represents an extremely high risk, as highlighted by the dashed oval. The idea of risk control is to design machinery systems so that all risks levels are at acceptable, low levels. If you find a particular risk level to be high during the course of your analysis, you can either reduce its probability of occurrence through machine redesign or by incorporating operating procedures or reduce the consequence level by adding additional protection, such as automatic shutoff valves, barriers, fire protection, etc. Moving the risk level to a cell that is either to the right or down from your starting point is desirable. An optimum outcome is moving your machine’s risk in the general direction of the arrow in Table 1.3, which points down and to the
