Does a Restriction Orifice (RO) Actually Limit Flow??? 1

[Sirius P.Eng.]

I'm not clear on this issue...share your thoughts to enable me clear up this issue..

My thought is this...from the laws of conservation of mass and energy...when a restriction is created on the flow path of a fluid, pressure reduces, velocity increases and density reduces as well so that both mass and energy are conserved. (i.e for compressible flow a simple relation is m1 = m2 ==> rho1.A1.V1 = rho2.A2.V2. Obviously, for a gas at flowing conditions, the Volume or volumetric flow increases, but mass flow rate is conserved.
How then is it popularly said that restriction orifices (ROs) limit flow.?
Does this use of the term "flow" actually mean volumetric flow rate?
In a relief header how does an RO downstream of a BDV for instance limit the peak blowdown rate in order not the exceed the capacity of the vent handling system?

Please share your thoughts/experiences to help me clear my head.

 

[LittleInch]

For something like an RO on a blowdown line you often hit critical velocity, i.e. the gas is at sonic velocity.

Even if not critical, ROs result in pressure drops for certain flowrates and inlet / outlet pressures.

Increasing pressure upstream or lowering pressure downstream doesn't / can't change the volumetric flow rate through the orifice, but increasing upstream pressure does increase the mass flow as the density increases.

Yes mass is conserved.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[don1980]

Showboy, this RO example is a very good illustration of the law of conservation of energy, but it doesn't illustrate anything interesting about the law of conservation of mass. The RO causes the fluid velocity to increase (downstream side). That is, the fluid's kinetic energy is increasing, which means the potential energy (static pressure) must decrease such that the total amount of energy is conserved. This is easily observed, and it's a great illustration of conservation of energy.

The presence of an RO will absolutely result in a reduction in mass flow, as compared to a line that doesn't have an RO. The fact that the mass flowrate on the upstream side of the RO is the same as the mass flow on the downstream side is a obvious and sensible observation, which doesn't illustrate anything of interest relative to the law of conservation of mass. Conservation of mass is illustrated by things like chemical reaction, or combustion.

 

[RVAmeche]

A RO represents a pressure loss; the sum of the total pressure losses seen in the system will determine the mass flow rate of the system based on the pump curve or whatever is driving flow.

As don explained quite while, a RO will definitely effect the overall mass flow rate of a system.

 

[Latexman]

Showboy,

Read this thread798-51260 and this, for a somewhat thorough understanding of compressible flow through a thin plate orifice.

Good luck,
Latexman

Engineers helping Engineers

 

[MortenA]

@op you forgot the temperature of the fluid. For both a gas and a liquid the fluid is heated when pasning an orifice. For the gas the expansion Will also cool the gas. The constant enthalpy process such as a orifice will cool less than an expander where the enthrophy is (almost) constant. For a liquid it’s always warmer.