Gas Pipeline Blowdown Calculations

[denniskb]

Where would I find a comprehensive method for calculating the pressures and temperatures vs time resulting from blowdown and pressurising of pipelines and compressor stations?


Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[zdas04]

Good luck finding anything on blowdowns. It is something I've been looking for for many years without success. I've frequently had to calculate blowdown times for pipeline work. My approach was (this approach assumes that you have a really good handle on the total system volume):

1. Pick a pressure increment (I usually use 100 psi, smaller improves accuracy at the cost of complexity).
2. At each increment I check to see if the flow to atmosphere is choked (i.e., sonic velocity), if it is then I calculate sonic velocity, convert that to a mass flow rate using the blowdown valve I.D. and compute the time required to remove the mass involved in lowering pressure to the next increment.
3. Keep the mass flow rate constant until you've removed enough mass to lower the pressure to the next increment.
4. Itterate steps 2 and 3 until you no longer have choked flow.
5. From the onset of sub-sonic flow to zero I use flow-through-an-orifice arithmetic to calculate a volume flow rate at an upstream pressure, call that constant for a 1 psi increment, and calculate mass flow rate and elapsed time to the next step.
6. Sum all of the times to get blowdown time.

I've found that the dramatic pressure/temperature effects (in piping larger than 6-inch) are all localized to within inches of the blowdown valve. Typically the bulk flow is laminar if you're using a 2-inch or 3-inch blowdown valve and mostly you can ignore flow effects 20 pipe diameters from the blowdown.

I hope this helps, the process is so non-linear and non-steady-state that most of the published stuff just fails to handle the complexity. I've had 3-hour blowdowns that I was able to predict the elapsed time within 10 minutes using this technique.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[sailoday28]

As I stated in my response in the forum for thermodynamics. To solve transient fluid flow problems, one should use the method of characteristics.
This method converts the partial differential equations of motion, cons. of mass and energy into ordinary differential equations which are then more easily solved.

The simple water hammer equation is the result of converting partial differential equations into a simple intergratable differential equation. The results are well known.