Myth Busters: You Cannot Avoid Compressor Reverse Rotation

We sometimes come across customers who have reverse rotation issues with their centrifugal compressors. This is usually the case when operators have inadequately designed piping systems and check valve locations, especially on machines with large suction/discharge vessels and side streams. We have even seen compressors that went up to and beyond their maximum allowable continuous speed in reverse during a trip event, which resulted in damage to the machine. Bi-directional seals and bearings can help and are often the quick fix, but are really not the right answer. The correct answer is always a properly designed compressor piping and surge control system.

Why Does Reverse Rotation Happen?

Reverse rotation can happen during a compressor trip (emergency or fast shutdown) when the driver is de-energized and thus does not provide torque to the compressor. Since there is a large volume of high-pressure process gas that is stored on the discharge side of the compressor, this discharge gas will now act against the rotation of the compressor. That is, with no driver there is no power to the compressor, and the compressor rapidly decelerates due to the high aerodynamic load.

Simultaneously, the loss of wheel speed rapidly reduces the compressor's ability to produce head. According to the fan laws, the pressure-rise capability of the compressor is the square of its speed. The loss of speed reduces the ability of the compressor to generate head. If the discharge pressure has not had time to drop (through a recycle valve), the flow reverses and the compressor surges. After it surges and decelerates to zero speed, if the pressure differential across the compressor remains sufficiently large, the compressor will start acting like a radial inflow turbine and begin rotating backwards.

Not only will the compressor spin in reverse, but all gears, drivers, and anything else physically connected to the drive train will reverse rotate. Also, while it is theoretically possible that the compressor does not cross the surge line during spin down, in most cases of reverse rotation the compressor also experiences a transient surge event. Thus, this can be a very destructive event for the machine that can lead to catastrophic failure.

What Makes Predicting Reverse Rotation So Difficult?

Predicting and quantifying the severity of reverse rotation is a very complex problem. Most conventional compressor surge transient analysis software will only accurately predict the onset of surge and the coast-down deflection points but does not properly predict the behavior thereafter. Classical transient surge analysis does not model what happens after the compressor reaches zero speed.

The further reverse rotation simulation also requires a transient analysis since both the compressor suction/discharge pressures as well as its speed still vary with time after crossing zero speed, but it is more complex since the compressor (now radial inflow turbine) behavior outside its standard head-flow map is unknown. From a fluid dynamic perspective, the reverse inflow into the compressor causes an impulse force on the backswept blades and a reaction from the inducer. If the compressor has a vaned diffuser the fluid dynamic problem becomes even more complex since the inducer will act like an inlet nozzle in flow reversal.

How Can Reverse Rotation Be Avoided or Its Impact Minimized?

The design of the suction and discharge piping (and placement of non-return flow check valves) as well as the proper functioning of the surge control recycle loop are the most critical factors in avoiding reverse rotation. Both the suction and discharge volumes should be limited by placing check valves as close as possible to the suction and discharge flanges (but still outside of the recycle loop). The recycle loop piping and valve should be sized sufficiently large (and actuate sufficiently fast) to get to pressure equilibrium before the compressor has decelerated to zero speed. If a sidestream exists, a check valve should be placed close to that compressor flange also.

A transient surge analysis should be performed to determine the proper sizing and control reaction time requirements of the anti-surge control system and valves. Most cases of reverse rotation can be traced back to an undersized surge control piping, slow surge valve actuation response time, or a valve system with inadequate flow capacity to lower the compressor discharge pressure quickly during an emergency shutdown.

What Hardware Measures Help When Reverse Rotation Cannot Be Avoided?

If the transient surge analysis determines that reverse rotation is unavoidable, then a further analysis should be performed to quantify and estimate the maximum speed the reverse rotation will reach. In general, reverse rotation speeds below the first critical speed (first bending mode) of the compressor can be handled if all the shaft and rotating components are properly designed for this scenario.

The more conservative approach is to utilize bi-directional components: all bearings (including compressor, driver, gear, and anything else that spins with the shaft) should be bi-directional with no offset on the pads for tilt-pad bearings. Dry gas seals and any other seals must also allow bi-directional operation. Gears and coupling vendors should be individually consulted and cannot have anti-rotation devices installed.

However, if the prediction (or field testing) shows that the compressor will spin in the reverse direction beyond its first critical speed then there may not be an acceptable mechanical solution that avoids damage during a trip event. As a minimum, a full rotordynamic analysis of the train including lateral and torsional must be performed, and as a consequence, other hardware changes to the shaft, seals, and bearings may be required.

What Should Operators Do After an Unexpected Reverse Rotation Event?

If an operator experiences an unexpected reverse rotation event on a compressor in the field during a trip, a careful inspection before restart is always recommended. This is especially the case if the reverse rotation speed went beyond the first critical speed, and always must be performed if it went above the maximum continuous speed (since damage is likely).

One should also note that other system and process components may be affected by compressor reverse rotation. For example, the lubricant rundown tank sizing may need to be increased to handle the additional time required for the equipment string to reach a full stop. Thus, a process system risk review should be performed to make sure all equipment that is potentially affected by a reverse rotation event is identified and properly designed for it.

A centrifugal compressor is not a radial inflow turbine and is not designed to operate like one. Reverse rotation should always be avoided and usually is not a problem if the compressor piping and surge control system is properly designed. If reverse rotation cannot be avoided then the manufacturer should be consulted and detailed analysis (and hardware changes) will likely be required.