Blowcase, Blow Case principles & sizing, thread469-52806 1

[AMenJr]

Hi, I have searched Eng-tips for sample blowcase sizing information. I located the following: thread469-52806. I am familiar with blowcases however I can't seem to find some sample sizing calculations. Does anyone out there have samples that they can share with me? I'm sure I can develop it, however it would be very nice if someone has already gone through the steps. I would like to use this as reference to develop my own spreadsheet to use now and in the future.

 

[zdas04]

Funny you should ask right now. I spent the weekend trying to refine my blowcase sizing arithmetic.

Determining the capacity of a blowcase has to assume a discharge pressure. For a compressor blowcase, I use the scenario of discharging back into the gas flow line downstream of gas measurement (assuming that power gas comes from upstream of the cooler, so a 5 psi dP from power gas to line pressure).

The approach I was taking (in MathCad) is to:
1. Divide the pressurization period into 10 time increments (the 10 is arbitrary, but 3 resulted in too much error and 15 gave me the same answer as 10 within the other assumptions) and for each increment calculate the volume flow rate (using AGA) and pressure at the beginning of the increment, then using that rate calculating the mass that was added during the interval. Since PV = nRT that gives me the pressure at the start of the next increment to calculate the flow rate. At the end of the loop, compare your final pressure to the power gas pressure. When they match close enough, multiply the increment times 10 to get pressurization time.
2. Using the final flow rate from step 1 determine how long it takes to fill the blowcase with enough gas to displace the liquid between the dump-open and the dump-close heights.
3. Repeat #1 for the vent line dumping the pressure back to separator pressure.

Add the three steps together, add in a latency time for valve repositioning, and then add a drain period to refill the blowcase and you have the elapsed time per cycle. Turn that into cycles/day and calculate the volume dumped each cycle and you have a volume/day.

What I'm finding is that step 1 is pretty short, step 2 is 5-10 times as long, and step 3 is 100 times step 1 for the vent line sizes that I keep seeing (3/8 tubing is really common).

Right now my MathCad file has a bug that is causing an infinite loop, but you should be able to write your own from this.

David