7.28 Checking for Choked Flow Conditions in a Control ValveΒΆ

Given the data from example 7.27, with a FL of 0.9. Confirm that choke will not occur in the control valve.

[1]:
from fluids.units import *
Psat = 4.75*u.psi
Pc = 3198*u.psi
P1 = 80.6*u.psi
P2 = 70.8*u.psi

P2_choke = control_valve_choke_P_l(Psat=Psat, Pc=Pc, FL=0.9, P1=P1)

ratio = (P2/P2_choke).to_base_units()
assert ratio.magnitude > 1

P2_choke.to(u.psi), P2
[1]:
(18.966081253330014 <Unit('pound_force_per_square_inch')>,
 70.8 <Unit('pound_force_per_square_inch')>)

We can calculate the choke pressure directly thanks to the nice ISA equations. With control_valve_choke_P_l, we calculate the downstream pressure required for choke flow to form. So long as P2 is larger than the choke pressure, no choke will occur.

Crane gives two solutions, one complicated and not necessary, and one that works on the same equation as control_valve_choke_P_l. Crane calculates FLP to use with that equation but only FL is supposed to be used, so they get a choke pressure of 20.8 psi instead of 19 psi here.