Operation in deep choke, especially of axial compressors during unloaded operation, therefore, should be avoided.
In a centrifugal compressor, this usually occurs near the eye of the impeller. Centrifugal compressors are more commonly used due to their versatility: they can compress a wide range of flows to a high discharge pressure. During choke, the flow channels between blade rows may experience blockage effects.Choke can occur at any performance level, i.e., speed or position of the variable stator vanes (VSV) for a constant speed machine. Currently, no dedicated instruments are used for choke detection. This usually means a valve must be installed in the discharge that can be modulated to keep the compressor pressure ratio from dropping too much. The negative impact on the process and the potentially damaging effects of compressor surge are clearly observable. Investigation of several catastrophic failures using diagnostics tools point to choke (not surge) as the root cause of the failure due to fatigue of a rotating blade or fixed vane. Surge is a violent flow reversal that occurs when the process restricts the compressor flow below a certain minimum value.Choke occurs when the process does not create enough restriction to the compressor flow and the compressor operates at its maximum flow for a given performance level. All the flow through the fourth stage comes from the third stage and if the third stage flow is restricted (not using the flow for heating; e.g. Stonewall occurs in a centrifugal compressor when the velocity of the gas in at least one stage reaches the speed of sound (Mach 1) at the gas conditions in that part of the machine. This is most likely to occur when the MAB is operating at a peak high load that is far away from the surge and choke regions. Therefore, clamping must be avoided or used with special care with axial compressors.One further choke condition should be noted: In compressors operating in parallel, when one compressor trips, the natural response of a load sharing control system is to increase the performance of the running compressor to compensate for the loss. This leads to increased gas velocity in the compressor.The increase in gas velocity occurs until it reaches sonic velocity or resonance at the blade throat (Mach 1). This is in great part due to the natural shape of the performance curve that represents a significant change in flow from the limit of surge to the limit of choke (Figure 1).In axial compressors, however, this distance is significantly less (Figure 2). Figure 2 shows the performance curves of a 12-stage axial compressor. Further, the anti-surge valve may remain open.The MAB, therefore, is accumulating a significant number of additional choke cycles that will eventually lead to failure. Although the choke increases with the performance level, long-term operation even at low-performance level in a deep-choke condition can be damaging due to its low visibility and cumulative effect.Choke is difficult to detect by conventional vibration monitoring systems. There is also an increase in the compressor discharge temperature due to the increase in entropy across the region of sonic velocity.In general, the only way to prevent stonewall is to increase the pressure ratio across the compressor. The usual compressor control and protection schemes are not effective for preventing stonewall and adjustments to speed or recycle are usually ineffective in preventing stonewall. When a compressor transits back and forth from the stable area to the choke area, a minor change in noise frequency can sometimes be heard specially at higher performance levels. This is no longer the case. The stable operation of axial and centrifugal compressors is limited by surge and choke. Choking or stonewall The choking occurs in the compressor which operates at a low discharge pressure and maximum flow rate. Turbomachinery Magazine brings you the latest and most comprehensive news and perspective in the industry. Except for a few conditions (engines idle, taxi, partially during take-off), the pressure ratio of the fan is usually large enough to accelerate the flow to supersonic. Continue Shopping Fracture eventually occurs during operation at high load.
Stonewall occurs in a centrifugal compressor when the velocity of the gas in at least one stage reaches the speed of sound (Mach 1) at the gas conditions in that part of the machine. Compressor surge can be understood by the help of schematic of a compressor impeller in figure-3. during startup), the discharge pressure of the third stage will increase significantly and consequently the fourth stage pressure ratio will decrease significantly. In many cases, catastrophic failure will not take place for ten or even twenty years of apparently stable operation after many apparently successful startups.
Compressor manufacturers have found that prolonged operation of the compressor in stonewall can lead to fatigue failures of the impeller cover and blades. As the stone wall limit is approached, the control system modulates the discharge control valve to prevent the pressure ratio from dropping any further.There are certain ethylene plant designs that include a side draw from the discharge of the third stage of a four-stage propylene refrigeration compressor (PRC).
The cumulative effect of choke-induced fatigue gradually leads to the failure of blower components.