A 0.70 SG natural gas mix expands through a 1/2 inch choke. Upstream pressure & temperature are 500 psig & 140 degF. Flow rate is closest to (Mscf/day): (A) 2,500; (B) 2,700; (C) 2,900; (D) 3,100.
8 FAC 2 has the required formula, including the Rawlins constant options where I need z for this gas mix. This takes time and has lots of room for error. So I just solve the problem quickly with z = 1, giving an answer 0 2,830, or (C).
A note in the Guidebook says this sort of estimate might be 10% low (i.e., using z = 1 if z is actually 0.8). So I quickly add 300 for 3,100.
Looking over the answer key, that's moving into (D) territory and thus too close to "estimate". So I run to 9 PVT 2 and lock down my true z using the chart & SG 0.7, Ppr=515/663= 0.78, & Ppr = 600/378 = 1.59. This charts out a z = 0.94, very close to my original guess. When I use this z I calculate q = 2,922, or (C), like my original calculation.
Were I in a bind for time on this problem I would have just ran with my original calculation with z = 1 and got the right answer. But this is risky and I would do this only if under time constraints, or if the answer key was so broad-spaced I knew could get away with.
Look, knowing what you can get away with, and taking calculated (heh) risks is a big deal on any exam.Get used to life not being perfect and learn to role with what you are given.
Also: on the CBT questions regarding equations in the SPE books not be in the provided reference may focus more "understanding" rather than plug-and-chug. In this example knowing about how much z accuracy matters is just the kind of question I might ask were I writing an CBT question to get everyone flipping through their resources in vain to look for an equation that is in the SPE text but not in the approved reference.. Look, if it's in an SPE book, the concept and understanding are fair game, even if you are not expected to memorize the equation. Personally I see no other way to study for this situation except to continue to do problems that use resources that won't be provided on the EBT. Focus on learning and understanding, not memorizing things, and never assume just because it's not in the provided resource the concept is off limits.
can you confirm that the gas Rawlins equation (or variation) is not given in the approved reference guide? If so, to your point... it could be a conceptual relative % question, or a gilbert's liquid question.
ReplyDeletePage 156-157 for equations are in the new Updated reference that will be provided on the EBT. If I were taking the exam, I would always work with the digital version like you will on the exam.
DeleteAlso, I would get used to searching fast. If you have been doing problems, you will have a mental clues needed to do a search.
So would Cd be given on the test or is there a place to find that for flow through a choke?
DeleteDo you mean in the Update reference?
Delete