laboratory gas pipe sizing

[nicolai]

Hi there
what would be suggested guideline to follow for a gas reticulation in a laboratory environment to distribute gas consisting of Nitrogen Hydrogen, Argon and instrument air to the various lab equipment
the total instantaneous flow of Nitrogen is expected at 350 ml/min. Is max flow velocity the governing factor?
what formulas is recommended to calculate flow velocity and pressure loss? what is safe maximum flow velocities for gasses applicable.

thks for any assistance

 

[PEDARRIN2]

For lab gases, the governing factor is typically pressure drop, not velocity because the gases are typically pure.

I have used the Darcy equation for compressible fluids which seems to work fairly well. Maximum pressure drop is typically 10% so density does not become an issue, but I usually use 5%.

 

[nicolai]

Thks PEDARRIN2

I have noted your suggestion on Darcy formula

what is the typical maximum velocity limitation for gas in piping?

 

[PEDARRIN2]

If your gas is of high purity (typical of lab use), velocity is typically not a significant concern for the pipe. The lack of particulates which, at higher velocities, could erode and cause leaks is the reasoning behind this.

However, if you are still concerned, there is a formula V max = 100√(ZRT/29rP)

Z is compressibility factor
R is gas constant
T is temperature in Rankine
r is the specific gravity
P is absolute pressure psia

Erosion can be an issue for constrictions, like partially open pressure regulators, orifices, etc. where the velocity can approach sonic. You would want to refer to the valve manufacturer's literature for their guidance on these situations and avoid exceeding the orifice maximum pressure drop ratio.

I have used Crane's TP 410 for this. It is an invaluable tool which I highly recommend.

 

[LittleInch]

The key issue is going to be pressure of the gas in the pipe.

The higher the pressure, the lower the pipe size needs to be for the same standard volume of gas.

Where you place pressure letdown or pressure/flow regulators makes a big difference, i.e. at the start point then distribute a low pressure (bigger pipes) or at the point of use (smaller pipes)

If it's all low pressure then pressure drop is your issue. Many online calculators and formulae for low pressure gas are only valid if the pressure drop from one end to the other is <10%.

Engineering toolbox site has a wide variety of charts and calculators.

E.g

https://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html

Valid for pressures up to 1 1/2 psig

Velocity in low pressure gas I wouldn't go for much more than 10-15 m/sec, but you'll probably run out of pressure drop before you hit a velocity limit.

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

 

[racookpe1978]

Your requirement is for a lab - which guarranttees you several things.
No long-term production at known useage rates.
No 24x7 production or demand, up until the experiment/test run requires a steady 24x7 flow rate at some unknown mass/sec rate that NOBODY is goingt o be able to predict before that particular problem/test occurs.
Many, many hours of non-use, followed by a few hours or minutes of excessive but very critical use.

I would recommend buying a much larger pipe delivery system and control and regulator system than you think you will ever need. That reserve capacity will pay for itself many times over - you just won't know when the extra charges for the larger-than-minimal-size pipes and valves will pay for itself.

for example, Boeing paid for a high-speed wind tunnel complex capable of large airplane models and near-Mach 1 testing BEFORE WWII, but it was available in 1946-47 for testing the high-speed, swept-wing, podded multi-engined jet and turb0-jet engine designs for the Boeing B-47, B-52, and 707-C135 designs of the early 1950's!