Stalling of Compressors? 2

[Arshad Ahamed]

Greetings,

I tried researching about compressor stalling (and consequently came across surging as well) for more information. Sadly, the explanations found were too complicated. Could you explain (such that a fresher with 9 months of experience in the refining field) what compressor stalling and surging is - how it occurs, the effects on the system and what can be done to prevent the occurrence.


Thank You,
Arshad Ahamed.

 

[rotw]

When fluid flows through passages (vanes) such as impeller and diffuser, the geometry is diverging (diffusion effect) from inlet to outlet, this creates an adverse gradient pressure that push towards separation of boundary layer. When the angle of incidence of flow (which at constant inlet pre-rotation angle, typically relates to change of machine inlet flow) changes and reaches the so called critical angle (it is possible to identify the stall line using industry standard criteria such as Kobayashi-Senoo), the boundary layer separates locally, typically the weak spot is impeller exit / diffuser entrance. Separation means less pressure / head generation capability. In general, it is observed that the phenomenon rotates from one vane to the next in a direction opposite to the shaft direction of rotation. The consequences are typically high noise emissions than normal behavior and a vibration signature. The problem is that, that same phenomenon could transition the machine into surge, which means that the stall has generalized to all vanes and compressor has no longer the aerodynamic capability to keep pushing the flow forward as it has been overcome by downstream system; the system goes then through surge cycles, definitely a condition mandatory to avoid. Rotating stall is favorized by certain design, such as vaned diffusers and is prominent in high pressure machines as these are characterized by very low flow coefficient impellers, havong therefore thin flow passages. Low solidity vanes are often preferred to vaned diffuser for that specific reason too. So I suppose that the inherent danger of stall is when it can trigger transition machine into a much more severe condition that is surge.

 

[LittleInch]

That's a great explanation by rotw.

In a bit more simplistic terms though.

How does it occur? Usually by to low a flow through the compressor caused by downstream isolation valve closure or control valve closing took much.

The effects can be very violent and destructive with gas flow surging back and forth within the compressor.

To prevent it mostcompressors have anti surge valves or ASV which open very fast and connect the discharge to the inlet to allow sufficient flow to circulate through the compressor to prevent surge.

Most such compressors have the ASV wide pen on compressor start

Compressor performers charts clearly shoe the surge line so you know what not to design the flow to be.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Arshad Ahamed]

Thank you rotw and LittleInch for the insight. I now understand the phenomenon a bit better (this is proving to be quite a tough nut to crack).