Natural Gas Piping - VP < 0.5 inWC

[Potcarb]

Hi,

I'm sizing a natural gas line that feeds a combustion blower. There is an Eclipse Engineering Guide that I found online that shows some tables for pressure drop in the line at varying flows. It also states:

Air, gas and mixture piping systems should be sized to deliver flow at a uniform pressure distribution and without excessive pressure losses in transit. Two factors cause air pressure loss and consequent pressure variations:
1) Friction in piping and bends, and
2) Velocity pressure losses due to changes in direction
In combustion work, piping runs are usually short (under 50 ft.), but often have many bends. By assuming that all velocity pressure is lost or dissipated at each change of direction and by using a pipe size to give a very low velocity pressure, other losses can be disregarded. In general, a velocity pressure of 0.3 to 0.5″ w.c. satisfies this need.

This sort of makes sense to me. I guess the goal is just to keep uniform flow and pressure? I'd like to understand this better so I can size pipes in the cases where I may need to go above 0.5" inWC.

Thanks

 

[Latexman]

I'm sizing a natural gas line that feeds a combustion blower.

Is this "combustion blower" a "combustion air blower" or what? If it is, in my experience I've never seen fuel feeding a combustion air blower. I've seen the fuel feed into the fired equipment (boiler, heater, etc.).

Good Luck,
Latexman

 

[Snickster]

I got a copy of the manual and read over the entire page 15 and the Example. I really cannot figure out what they are doing in their calculation method. They appear to be using some made up voodoo black box method for sizing pipe that makes no sense, in order to determine pressure drop in a line. Like they are trying to present a simplified method for non-technical people to size a line. I would just stick to normal methods for line sizing.

 

[Snickster]

Here is page 15 for reference.

 

[1503-44]

Typical nomograph for very low pressure line sizing.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Snickster]

I think I understand what they are doing now. I believe what they mean is that the static pressure loss in an 90 deg elbow equivalent to one velocity head or in other words LOSS = 1.0 V[sup]2[/sup]/2g. In the example there was 2 inches w.c available and four elbows so if the velocity head was 0.5 then you would have 4 x 0.5 loss or 2 inches w.c of static pressure loss. So they sized the pipe such that the velocity of 2800 FT/min gave a velocity head of 0.5. Therefore they appear to be ignoring the pressure loss in the straight pipe and K=1.0 seems very high for an elbow - they must be using very short radius screwed elbow. I don't think I agree with their calculation method as you need to consider the pipe friction losses and you can use larger radius elbow to get lower losses in elbows. Also there are other fitting in the line that contribute to pressure loss besides elbows.

 

[georgeverghese]

No one feeds natural gas to a combustion air blower.

 

[Snickster]

I think he means this system see attached.

 

[1503-44]

Nice mind read. That looks familiar.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Is it only me that's never heard the term "velocity pressure" used in this way before?

Seems to me that this is only useful when those exact situations occur, i.e. multiple sharp bends / elbows in a shortish distance.

So Potcarb - if you want to do anything outside this quite limited scenario, you need to look at pipe sizing charts based on equivalent length.

Many low pressure gas calculations ignore the effect of pressure and density change as there isn't much to start with, but always be careful to know when one chart or calculation applies and more importantly when it doesn't.

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

 

[1503-44]

See the note limiting loss to to 0.3" of water.

Fitting loss K factors

https://www.pipeflow.com/pipe-pressure-drop-calculations/pipe-fitting-loss-calculation

 

[PEDARRIN2]

On low pressure systems (< 1 psig), the gas companies typically "require" a maximum pressure drop of 0.3 - 0.5 in w.c.

 

[1503-44]

Yes, that's built into the table because of that reason.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[PEDARRIN2]

I have never liked the use of tables for sizing natural gas piping.

The 2003 and 2006 IFGC give differing guidance which differs from your table and leads to confusion.

 

[PEDARRIN2]

Seems it didn't load the 2003 version.

 

[1503-44]

I don't mind. I find tables can be pretty helpful in that they can often prevent errors typical of bad copy paste operations, or at least serve as a quick check of your own, or someone else's calculations that are not entirely transparent, or if I just need a quick look for commenting in this forum and I don't want to waste time turning on the desktop.

BTW it looks to me like all the tables agree pretty well, but
you do have to look at the same length and same diameter.
4"∅ x 60ft long all show 6800 ft3/h

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[PEDARRIN2]

I agree tables can be a quick check on things and I use them as such.

Your table seems to be a bit more conservative than the code tables, which is not bad. The reason I don't like the tables in the IFGC is the more recent versions got rid of the first table from 2003. The table from 2006 (and on) uses inlet pressure of less than 2 psig for this application. However, the formulas used for sizing that are also part of the code are different if inlet pressure is less than 1.5 psig than if it is greater than 1.5 psig. Also, the tables have a hidden "safety factor" which is not readily recognizable. Your table has an even greater safety factor than the code tables at smaller pipe diameters.

I prefer equations because then I can apply the safety factor I want to use.

 

[1503-44]

Of course you are free to use whatever you like, as pipe flow safety factors don't really exist in the world of codes. It only has to do with how well the maximum flow for any given pipe has been defined.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."