Throttling issue of globe valves
Thread Content
Can the decrease in flow rate and the increase in pressure before the valve, which occur when using a stop valve to throttle the gas, be explained by PV=nRT?
No, that’s not possible, because both volume and temperature here are not variables. As for why the pressure increases, I think it’s due to a violation of the law of conservation of mass; in other words, the throttle valve restricts the flow, but the amount of material coming from upstream remains unchanged, which is what causes the P-value to rise – it’s simply due to an increase in density. If the container has no source and only outputs without any input, then it’s even less likely for the P-value to increase.
This post was last edited by ylb913 on 2016-5-22 07:06. The assumption is an ideal gas, but this applies to most gases as well; the answer is correct! The key issue here is that n is changing (increasing). PV=nRT, where V is a variable quantity and R is a constant. With the temperature T remaining constant and the feed rate unchanged, the discharge valve being closed leads to an increase in n within the constant volume V, thereby causing the pressure P to rise.
It makes more sense to use Bernoulli’s equation.