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# Pressure drop

Member Posts: 6
I'm studying the code books to take the mechanical exam in June. When I look at the Pipe sizing tables, when there is a greater pressure drop, the BTU levels are higher? Why would this be? It makes sense to me that we would have less heat with less pressure. Thanks

• Member Posts: 7,357
When you move more BTUs through the pipe, you are moving a higher volume of gas. This increased volume of gas encounters more friction which equates to a higher pressure drop.
There is probably a better way to explain this but that is how I visualize it.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 6
Thanks Zman, my first post here, and my first attempt at learning about this...Please excuse my temporary ignorance on this, if this doesn't make sense...The tables I am speaking about have the same inlet pressure, same specific gravity, same type of pipe, same size pipe, and in the Cubic feet of gas per hour section the number is much higher on the table with a greater pressure drop. (The pressure drop is listed at the top of the tables to define the circumstances.) (The only difference that I can see in the tables is the drop. (IFGC table 402.4 (10) and (11). The cubic feet on the table tells you what size pipe to use for any given appliance rating. For some reason I can get 5 times the cubic feet of gas through the same 1/2 inch pipe with the same inlet pressure when the top of the table tells me that a 17 in w.c. pressure drop is present instead of 1 inch. I guess I am not understanding what pressure drop is?
• Member Posts: 403
Pressure drop is the loss in pressure from the inlet pressure to the outlet pressure. So a 1 inch drop from an inlet pressure of 20 inches would give an outlet pressure of 19 inches, while a 17 inch drop would Lee suit in an outlet pressure of 3 inches. With a 17 inch drop you are using more pressure to move the gas through the same size pipe than when you have a 1 inch drop. Just like with a water pipe or garden hose when you use more pressure you get more flow, and therefore more btu's delivered.
• Member Posts: 6
Thanks Gary, I'm feeling pretty dense right now I'm still missing a factor that would make sense out of this. "With a 17 inch drop you are USING MORE pressure"... How is this possible without affecting the predetermined inlet pressure? And before that even, how does a 17 inch DROP in pressure allow you to use MORE pressure? "Just like with a water pipe or garden hose when you use more pressure you get more flow, and therefore more btu's delivered." ...Exactly! which is why my brain tells me that if you have MORE of a drop you have less flow and you have less BTUs delivered at the outlet.
Does natural gas condense and become more efficient in that way? It would have to condense in order for any of this to make sense to me. Thanks again for trying to get it through my head.
• Member Posts: 19,156
You have to start your thinking at the source -- in this instance, the place where the gas pipe comes in. At that point you have a certain pressure -- let's say, for the sake of discussion, 20 inches. Now to make gas flow through the pipe in question, you have to lose some of that pressure -- no pressure loss, no flow. This pressure loss is referred to as the drop. The more flow you have, the greater the pressure loss (or, to look at it another way, the greater the pressure loss permitted, the greater the flow). So -- if you have a very small drop, you have a very small flow. If you increase the drop, you have a greater flow. If you want to have more BTU output -- that is, more gas -- you have to use have a greater pressure drop. That can be thought of as using some of the available pressure at the inlet.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 4,517
The lower the allowable pressure loss listed on the tables allows a larger size pipe therefore less overall pressure loss per foot of pipe. The pressure loss are factors for each table so you can accurately determine correct pipe size based on pipe length and pipe size.
• Member Posts: 15,432
What Tim, Jamie and Gary said. And Zman nailed the reason for the drop- friction as the gas moves through the pipe. A larger pipe will offer less friction to a given amount of gas.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
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• Member Posts: 6
So my mistake in thinking, is that I am associating amount of flow purely on the inlet pressure and the diameter and length of pipe. I'm hopeless I need to go to school. Thank you all very much. Wait!.....I think I just got it! Pressure drop would be considered a drop in resistance to the flow? Causing less pressure and better flow? ...Nope...I still don't know what would determine the drop, between two tables containing same size pipe, same pressure, same everything, except pressure drop. Is it the gas itself that determines it? Never mind, I'm sure you all are thinking this guy is pathetic !
• Member Posts: 4,517
-Your forgetting the end result is the equipment which usually consists of a pressure regulator to reduce the pressure coming out of the gas valve (outlet pressure) and then the orifices on the burners which control flow. We strive to have the same pressure at the furthest appliance as we have coming out of the gas meter. The pipe size determined by the charts is how we do that.

If you are looking for training I own and operate the Gas Training Institute located in Rhode Island. We also have 34 manuals available on gas related subjects.
• Member Posts: 4,517
Think also about this the higher the gas pressure we can have the smaller the pipes we can use.
• Member Posts: 500
edited April 2017
Nube said:

So my mistake in thinking, is that I am associating amount of flow purely on the inlet pressure and the diameter and length of pipe. I'm hopeless I need to go to school. Thank you all very much. Wait!.....I think I just got it! Pressure drop would be considered a drop in resistance to the flow? Causing less pressure and better flow? ...Nope...I still don't know what would determine the drop, between two tables containing same size pipe, same pressure, same everything, except pressure drop. Is it the gas itself that determines it? Never mind, I'm sure you all are thinking this guy is pathetic !

Maybe this will help : Example : 20 inch wc differential across a closed ball valve ( or balancing device ) would be 0 and not 20 inches for this purpose. If the valve were open to atmosphere on the " load " side it would be 20 inches , but for the purpose of measuring the dynamic operation of the device the closed valve represents zero differential. Slowly open the valve , the differential will creep off of zero to 1 , 2 , etc. The closer we get to full open the more gas is passing through our valve. When we were barely open it didn't take a lot of gas to maintain our 20 inch input , but with the valve full open it takes a lot of gas to maintain it. You have to imagine these charts as fuel flowing and not static measurements. I hope that helps.

hvacfreak

Mechanical Enthusiast

Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

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• Member Posts: 12,157
Forget gas for now and relate the problem to something you can feel.

Lets say you have 50 PSI water pressure in your house and it feeds an outside sill cock which has a hose on it to water your garden.

Lets say you have 2 pressure gages. 1 is connected to the sill cock outlet (where the water hose connects) it reads 0 because the valve is shut.
The third gage is connected at the far end of the hose on a tee. This gage reads hose pressure at the far end but does not block the hose which is free to flow This also reads 0 when the valve is shut.

Now open the sill cock , just a little, you now have water flowing through the hose.

gage at the beginning of the hose reads 5psi. gage at the far end of the hose reads 1psi. You can here the sill cock valve screaming because it is restricting the water flow.

Now lets look at pressure drop. You have a pressure drop of 4psi through the hose (gage -gage) 5psi-1 psi.

Now open the sill cock all the way so the hose is screaming...you have more flow.

gage at the beginning of the hose reads 40psi, Gage at the end of the hose reads 25psi

So now you have more flow. More flow through the same size pipe (or hose)= bigger pressure drop. 40psi-25psi=15psi drop= more flow

In the first example with the valve just opened a little you had a 4psi drop through the hose low pressure drop =low flow
• Member Posts: 2,693
edited April 2017
This is not an endorsement for CSST nor is it against CSST .

Read the following sizing and installation methods . It's all about the math and this book explains it very well . In the rear of the book ( pg 90 - 93 ) there are also Iron Pipe pressure drop tables , they include the pressure drop through several sizes of iron pipe at ranges through 15,000,000 BTUh . It sounds as if your question may have to deal with allowable Pressure drop but I am just guessing.

Is what is confusing you the fact that the charts reference different delivery pressures ? 17" w.c pd cannot happen in a system whose inlet pressure is not 1 PSI or 27.684" w.c , whereas 1" w.c pd is common in a system that is 1/2 PSI ( common) inlet pressure .

For instance , a 1/2 " Iron pipe feeding a 100,000 BTUh appliance will have a pressure drop of .019" per foot but that same line feeding a 200,000 BTUh appliance has a .067" per foot pressure drop . If the appliance requires 5" at the gas valve and you started out with 7" at the meter or the tee and you are using allowable pressure drop you could run
7.00" - 5.00" = 2.00" APD
100,000 BTUh appliance 2.00" / .019" =105' of pipe
200,000 BTUh appliance 2.00" / .067" = 29.85' of pipe

The pressure drop per foot varies dependent on the load it serves

2 PSI = 55.368 " wc
1 PSI = 27.684 ' wc
1/2 PSI = 13.842" wc

Fell free to contact me if you'd like to discuss in detail , maybe I can assist you in better understanding this whole thing
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 6
Thanks!! I'm getting it now thanks to all of you. Rich and EBEBRATT Ed, got me real close here. Ok, in simplest words...Ed, your example got me visually thinking, except for the fact that inlet pressure on the tables are the same with different pressure drops (Both written in the legend), I guess one could call it a legend? Correct me if I am wrong Rich, but I think you educated me by making me think of the appliance on the end of the line. An appliance putting out more BTUs, (having a larger orifice), will create more flow and less pressure in the line,(greater drop),. I wasn't thinking about the appliance restricting flow causing less drop, or a larger appliance causing more flow and a greater drop.
This was key... DUE TO THE LOAD IT SERVES!! Thank you!
I got it right? I'm not missing something? Did I misinterpret?

I'm just learning the basics, and had this hurdle that got me, and I didn't even feel like picking the books back up until I could get that straight. I can't tell you all how much I appreciate taking the time for me, on something that I'm sure is a pretty simple aspect of the mechanical field. Thank you!
• Member Posts: 6
Was just looking back on the posts, now that I understand... Tim said what I was missing too, earlier on. And several other good posts too. Thanks Again.
• Member Posts: 12,157
Everyone learns differently. Some can learn from books, some learn better seeing something visually.

Me, I guess I'm somewhere in the middle. Don't give up. Read some of @Dan Holohan's books. He is a great teacher. Once you start to understand you will learn fast. Weather the pipe or ductwork moves air, gas, steam or water the basics pretty much stay the same
• Member Posts: 220
think of pressure differential. the greater the differential the more you get. like temperature. the great the temperature differential the greater the Btu's transferred between the two mediums