Surging In Compressors and It's Effects On Compressor

     Before understanding the surging process, you need to understand head and capacity concept of a compressor.

Maximum Head of Compressor:

     Maximum head of the compressor is the head achieved when the compressor performs maximum amount of work on per mass of gas. This is practically achieved when operating the compressor at it's minimum capacity. 

Minimum Capacity of Compressor:

     When the pressure at the discharge vessel is quite low than the maximum head of compressor, the compressor will work at  at maximum capacity and minimum work per mass of gas. When the discharge pressure starts increasing, the pump have to work more on per mass of gas and starts running on less capacity, until a minimum capacity and maximum head of compressor is achieved.

SURGING:

     Suppose a compressor is connected to a system that needs a large amount of gas or is at very low pressure. The suction of the compressor is connected to the system having good enough pressure. You want to transfer the gas from high pressure vessel to the low pressure vessel. You start the compressor that starts transferring the gas from high pressure vessel to the low pressure vessel. Since at this stage, there is a little resistance of pressure at the discharge side of the compressor, the compressor capacity is high at start.

     Transferring the gas continuously will increase the pressure continuously at the discharge vessel and hence the resistance to the discharge also increases. This happens if you are not using that gas at the same rate. The compressor capacity also starts decreasing continuously, since the compressor discharge pressure have to overcome that discharge vessel pressure in order to continue to transfer the gas. Hence compressor will have to use more power to work more on per mass of gas. 

     A stage will come when the pressure at the discharge vessel becomes higher than the maximum head of the compressor. The flow of gas stops i.e, the capacity of the compressor drops less than the minimum capacity and the gas will flow in reverse direction i.e, from discharge vessel towards the compressor. This suddenly decreases the pressure in the discharge vessel than the maximum head of the compressor. Therefore, the compressor again starts delivering the gas to the discharge vessel. 

     Gas pressure in the discharge vessel again becomes more than the maximum head of the compressor and the phenomenon of reverse flow and forward flow happens so on.

     This rapid flow of gas back and forth in the compressor is called Surging. In other words, one can say that the surging occurs when the compressor is operated below it's minimum capacity.

Effects Of Surging On Compressor:

     Surging sets up severe vibrations in compressor and associated piping which can cause damage to the compressor.