2 replies to this topic

[Somnath Abhale]

Hello, I need to calculate the reverse flow through the 5 stage centrifugal compressor casing during trip.

 

During trip condition, the only check valve provided at the discharge of 5th Stage will fail (due to disc  dislocation or any other mechanical failure) and the high pressure system volume (considering the whole system downstream of 5th stage of compressor) will try to flow towards the low pressure system through Centrifugal Compressor.  The resistance offered through the interstage coolers and knock-out drums assumed to be negligible and the only controlling resistance for the reverse flow will be the compressor casing as it has very small clearance for flow.

 

Can anybody help me for calculating the same? or any reference where i can find?

 

your inputs regarding to the system behavior will be appreciable.

[chemsac2]

Abhale,

 

Two observations on your query:

1. Anti-surge valve would mean that gas need not flow through compressor to settle-out

2. In case gas flows through compressor, it can lead to reverse rotation of impeller, damage to motor, bearings and dry gas seal if not bidirectional

 

This is how I would estimate reverse flow through compressor:

1. Assume pressure at 5th stage suction.

2. Get impeller+motor MK2/GD2 values from vendor datasheet which represent inertia of impeller

3. From 5th stage differential, calculate torque available.

4. Ensure torque available due to pressure difference can overcome MK2/GD2 of impeller.

5. Iterate for different flows till 5th stage suction pressure ensures sufficient torque to overcome motor+impeller inertia

6. Continue procedure for different stages of compressor

 

This is not exact procedure, but treat this as general direction. I had done it once to simulate start-up of high energy pump till it attained final speed i.e. predict torque-speed curve.

 

Few transient analysis softwares do it for pump trip scenarios. You can consult standard water hammer handbooks as well.

 

Regards,

 

Sachin

[Somnath Abhale]

Thanks Sachin