What Do 1/4 NPT and 3/8 Orifice Size Mean for Solenoid Valves?

[franznietzsche]

I'm currently part of the propulsions team for a hybrid rocket project here on campus, working on the liquid fuel injector system. Specifically I'm trying to model which injector designs give us the best burn rate over the entire burn. However, I'm a physics major, not an engineering major, so I'm not familiar with the conventions used in the specifications of the various parts we are using. In particular, is this solenoid valve. I'm trying to determine the interior diameter in the valve through which the fuel will flow, since this will affect the mass flow rate. I see two numbers: 1/4" NPT, and 3/8" orifice size. Can anyone explain to me exactly what these numbers mean? I know the NPT number has to do with the threads on the valve, but I'm not clear on exactly what the two numbers are measuring. I've attached the specifications, these two numbers are in a table on the top of the second page.

 

[Q_Goest]

Hi franz'. The 1/4" pipe thread is the diameter of a tapered thread which can be cut onto a 1/4" pipe. It's tapered to aid sealing such that as you screw the thread into the body it gets tighter and tighter. Use Teflon tape on the male half of the part to help create a seal. Note that 1/4" pipe is actually 0.540" OD, there is nothing 1/4" about it. That's true of most pipe, the size is a nominal diameter, not a specific one.

Regarding the orifice, the 3/8" isn't sufficient to use for calculations. Use the Cv rating for the valve which is listed at 1.1 . If you're not familiar with using flow coefficients to determine flow rates and pressure drop, let me know.

 

[FredGarvin]

Just so you have it:

Q = C_v \sqrt{\frac{\Delta P}{SG}}

where:
Q = Flow rate in gallons per minute (GPM)
C_v = Flow coefficient of valve (unitless)
\Delta P = Pressure drop across the valve in \frac{Lb_f}{in^2}
SG = Fluid's specific gravity (dimensionless)

This is a standard equation. Make sure you calculate with these units. This will be the main thing to calculate what kind of delta p across the valve you'll need to supply to get the flow you'll need.

 

[Q_Goest]

Thanks Fred!

 

[FredGarvin]

I figured I had to throw in at least something to this thread...You beat me to the rest of it.

 

[franznietzsche]

Just so you have it:
Q = C_v \sqrt{\frac{\Delta P}{SG}}
where:
Q = Flow rate in gallons per minute (GPM)
C_v = Flow coefficient of valve (unitless)
\Delta P = Pressure drop across the valve in \frac{Lb_f}{in^2}
SG = Fluid's specific gravity (dimensionless)
This is a standard equation. Make sure you calculate with these units. This will be the main thing to calculate what kind of delta p across the valve you'll need to supply to get the flow you'll need.

Here's the thing, I can't calculate \Delta P across the valve. I know the pressure drop from the fuel tank to the combustion chamber, at least nominally, but the pressure drop across anyone part of the injector assembly is up in the air. It will depend on our injector design (smaller ports will increase the pressure gradient further down, decreasing it further up). Its because of this that I opted for directly solving the Navier-Stokes equations and modelling the whole assembly.

Of course, i know what mass flow rate we need, so this allows me to calculate the correct particular pressure differential across the valve. Ok, I figured out what I need to do. Thanks for your help.