Hi all,
I'm studying for the upcoming controls PE exam and am having problems with a sample question that I suspect has the wrong answer in the answer key:
Given a basic gas flow control valve loop where conditions are as follows:
Inlet P: 80psia
Outlet P: 30psia
Op. Temp: 140F
Mole weight of the gas: 40
If only the upstream gas (P inlet) increased by 10 psi, which of the following best describes how the mass flow would change?
A) F outlet = 1.062 * F inlet
B) F outlet = 1.095 * F inlet
C) F outlet = 1.125 * F inlet
D) F outlet = 1.162 * F inlet
I believe the answer should be A - due to the relationship between P inlet and Q (flow) - which would be the square root of the increase percentage - i.e. sq root of 1.125 = 1.0607
The answer key says C, which implies a linear relationship between the flow increase and the inlet pressure increase. Either it's a typo, or am I just missing something? The only thing I can figure is that there is something attributed to gas density change due to the pressure increase, but I cant put my finger on it.
Thanks for any help you can offer with this; I'm new to valving (background is in manufacturing - PLC's robotics, instrumentation - but NO valve experience) and didn't expect the controls exam to include so much emphasis on valve mechanics and theory.
I'm studying for the upcoming controls PE exam and am having problems with a sample question that I suspect has the wrong answer in the answer key:
Given a basic gas flow control valve loop where conditions are as follows:
Inlet P: 80psia
Outlet P: 30psia
Op. Temp: 140F
Mole weight of the gas: 40
If only the upstream gas (P inlet) increased by 10 psi, which of the following best describes how the mass flow would change?
A) F outlet = 1.062 * F inlet
B) F outlet = 1.095 * F inlet
C) F outlet = 1.125 * F inlet
D) F outlet = 1.162 * F inlet
I believe the answer should be A - due to the relationship between P inlet and Q (flow) - which would be the square root of the increase percentage - i.e. sq root of 1.125 = 1.0607
The answer key says C, which implies a linear relationship between the flow increase and the inlet pressure increase. Either it's a typo, or am I just missing something? The only thing I can figure is that there is something attributed to gas density change due to the pressure increase, but I cant put my finger on it.
Thanks for any help you can offer with this; I'm new to valving (background is in manufacturing - PLC's robotics, instrumentation - but NO valve experience) and didn't expect the controls exam to include so much emphasis on valve mechanics and theory.