How to Calculate the Velocity of Gas Inside a Pipeline

Simple. Divide the volumetric flowrate by the cross-sectional area of the pipe.

Of what relevance is this with regard to welding a fitting into it?

Depends whether he wants the standard velocity or the actual. For actual he also needs to know the temperature.

Well, it doesn't say so in the original post.

That's the reason for my comment.

But do you really think it's relevant after 12+ years? I'm sure the OP has realised he won't get anywhere near the 0.4 m/s, (std or actual) and has done something different or given up the idea.

Are you going to try to weld the fitting while the gas is flowing?

yes we are going to weld the fitting while the gas is flowing as per standard min flowrate should be 0.4 m/s . .

Why you want us to prepare Excel format for you, you can prepare yourself. You have to calculate actual volume flow, for which you need to know the gas temp, pipe ID and absolute pressure.

I thought I'd posted a reply to this, but it seems to have gone missing.

He only needs gas temperature and pressure if he wants actual velocity. If he wants standard velocity the data is already there.

what formulae should i use once a get the gas temperature and pressure?

I'm kind of curious what equation you're talking about too. Why can't he just use volumetric flow rate divided by area?

What gas?

Ron

I am making a few assumptions based on some of the answers after the original post:

First is that you have measured the volumetric flow at some chosen point that is relevant to what you want to do.

Second, you have measured the pipe OD at the same location. The dimensions you listed are consistent with 8" pipe. You can find tables of standard pipe sizes and from that you can either look up or calculate the cross sectional area of flow.

Now do the division to get velocity at that specified point. SInce you have measured data, your calculation will be accurate.

If you are regularly involved in pipeline flows, I highly recommend you get a copy of Crane Technical Paper 410. It's just about the bible on pipeline flows and has all the formulas and data you need.

Now for my more pointed question: why are you welding on a fitting while the gas is flowing? You didn't say what gas it is, so are you sure it's safe to weld at high temperature? Will the gas combust or form dangerous compounds at welding temperatures (about 6000 deg F for oxy-acetylene welding)? Will the gas be contaminated by weld slag? Is it safe for anyone to be around it while welding?

Even if the gas is not combustible and is personnel-safe, you should be concerned if the gas will affect the weld quality by causing inclusions in the weld. If the weld needs to be leak-tight, you need to do good weld prep and that means cutting a hole and grinding the edges to have good weld surfaces (another reason to be concerned about personnel exposure). Otherwise you will have a weak weld that leaks.

Here it is:

V=Q_s/ID²·P_abs·2826

V - Gas velocity (m/s)

Q_s - Gas flow (sCUM/h)

ID - inside diameter (m)

P_abs - Gas absolute pressure (bar)

>>> your V=4.86 m/s

The above is based on: Q(actual)=V*A (pipe's internal cross section) and

Q(standard)=P(absolute)*Q(actual)

Also replying to cingold #11.

That only works if the gas temperature is already standard temperature, usually T_s = 15°C, 60°F or 70°F depending on the convention you're using. If not you need to correct for temperature as well as pressure. That's assuming the constant in your formula is right, I haven't checked.

Standard velocity V_s = Q_s/A where Q_s = flow at standard conditions, A = pipe inside area. Using consistent units, needless to say.

Actual velocity V_a = V_s*(P_s/P_a)*(T_a + 273)/(T_s + 273) where P_s = standard pressure, always taken as 1 atm (even if it's called 760mm Hg or 14.7psi etc), P_a = actual absolute pressure, same units as P_s. T_a = actual temperature.

Vstd = Qstd/(pi x(D(inside)/2)^2)

D(inside) = 219.08mm-2x(3.76mm), 211.56mm, 0.21156m, 8.33"

Qstd = 6172 std m3/hr, pi=3.14159..., thus

Vstd = 6172/(3.14159 x (0.21156/2)^2) , 175578 m/hr, 48.77 m/s

If I have my math right. The gas volume flow was already reported in std units.

Dear James,

There is no V std... only V actual, so 1st you have to convert to Q actual.

My equation considers all of it.

Regards

Then why did the original question state standard minimum velocity?

V std may not mean anything physically, but it's easy enough to define, as in several posts. And it's a useful concept - eg if you want to find mass velocity kg/(m².s), this is just V std*ρ_std, where ρ_std = density at standard conditions.

Also in my post#16 - I should have added that T_a and T_s are in °C.

OK, now I see my first answer was indeed wrong for the actual velocity by not factoring in the pressure. Taking my result and dividing by pressure: result is now 5.42 m/s, and this assumes there is no conversion factor for temperature (i.e. the actual conditions are 0 degree C, as long as IUPAC standard conditions apply).

Depending on what the actual standard units are, you may wish to apply the conversion from bar to atm (101.35 Kpa), and apply some temperature correction.

Anecdotally, I will add that a relative of mine was welding on a natural gas pipeline, and while the line was blocked in at both ends at first, and had been purged, someone apparently opened a valve on him, and there was a fireball explosion that blew him off the pipe and seared his lungs enough to put him in the hospital. Amateurs definitely do not want to "try this at home". If you are "merely" welding a hot-tap fitting onto the outside wall of the pipe, if the velocity is too low for the given pressure, there will be a plastic failure of the pipe during the welding procedure and someone is going to be hurt.

You didn't factor in the temperature either, that was the point I was making in #16.

0°C is usually defined as normal temperature in this context (commonly used in continental Europe). Standard temperature can have various values, depending on the convention adopted, see #16. It doesn't matter as long as everybody on a particular job uses the same.

I've never seen standard pressure defined as anything but 1 atm, but again it's OK to use something else as long as it's spelled out. But my formula has inputs for T_s and P_s so any values can be used, but obviously Q_s must be on same basis.

Nowadays it might be more logical to use 1 bar for standard pressure instead of 1 atm (at least outside USA!). I tend to use molar volume 22.7 m³/kmole at 1 bar, 0°C rather than 22.4 m³/kmole at 1 atm, 0°C.