Gas Load Pressure Drop Question

niklaw1

I asked a question here a year or two ago trying to understand pressure drop on gas systems, I'm hoping someone could help me with a little more thorough understanding on which table to use in the IFGC when do load calcs for pre-existing residences. These would be when we would need to submit a gas load for the addition of a tankless, gas stove...ect

For Black Iron I've always used IFGC 402.4(2) unless the inlet gas pressure is above 8", 11" or 2psi

I was speaking with a very knowledgeable inspector yesterday who made me re-think my idea's on which pressure drop tables to use. Please correct me if my new understanding is wrong.

Example:
Incoming NG pressure = 7"w.c.
Min W/H input = 5" w.c.
Min Furn input = 4.5" w.c
Pressure Drop = 2" w.c.

Would that mean my allowable pressure drop is the difference between the incoming pressure and the appliance with the highest minimum supply pressure? or a calculation between the two?

Meaning, would I have a 2" allowable pressure drop for deciding which gas pipe sizing table to use?

ChrisJ

I don't feel comfortable answering the question but, I do want to point out gas meters, as well as gas regulators at the meter if equipped also have pressure drops. If you measure 7" without a load, it's likely going to drop with demand. How much will depend on how big of a load and how the meter and regulator are sized.

In my own house, I have around 7" without a load but I can pull it down to around 5.5" with everything firing and that's mostly from the meter and pressure regulator (50 PSI from the road). In my case, I am maxing the meter out and even exceeding it's capacity by a little. But Even with lighter loads it's still going to have some drop.

Just something to keep in mind.

Jamie Hall

I too am uncomfortable answering the question -- at least exactly. But in addition to @ChrisJ 's comments, may I add that regardless of which table to use, it would seem to me at least vaguely prudent to consider all of the loads which may be firing at once, and to consider what the pressure requirements are for each of the items. For example, if you have a line running from your meter to a utility room, and the utility room is occupied by a boiler and a tankless water heater, it would seem prudent to me at least to design for the condition where both the tankless and the boiler are firing at the same time.

ChrisJ

Jamie Hall said:

I too am uncomfortable answering the question -- at least exactly. But in addition to @ChrisJ 's comments, may I add that regardless of which table to use, it would seem to me at least vaguely prudent to consider all of the loads which may be firing at once, and to consider what the pressure requirements are for each of the items. For example, if you have a line running from your meter to a utility room, and the utility room is occupied by a boiler and a tankless water heater, it would seem prudent to me at least to design for the condition where both the tankless and the boiler are firing at the same time.

I believe in smaller buildings it's not only prudent, but code to make sure every appliance can fire at the same time. Imagine what could happen if someone has a pot simmering on a stove and for whatever reason, too many appliances are on and drop the pressure low enough for the flame to blow out and no one notices? That would be BAD.

Larger, much larger buildings I think they do some fancy calculating based on X amount of appliances firing. But not in typical residential etc.

Tim McElwain

Keep in mind the different tables are based on allowable pressure drop. The lower the pressure drop allowed the larger the pipe needed. That is what the tables are all about. The point of delivery pressure (the outlet of the meter should always be 7" W.C.) That is unless you are using a hybrid system which is measured in 2 or 5 pounds depending you location.

On inner city systems with the old cast iron mains in the street you could get varying pressures depending on the time of year. Those systems are operating at pressures less than roughly 14 inches water column (1/2 pound pressure) and do not have a regulator ahead of the meter. Most residential gas systems require a minimum gas pressure ahead of the gas valves on the equipment of 5" W.C. allowing for a 1" drop in pressure through the gas valve. Regulators on the valves are set to send 3.5" W.C. pressure to the burners on the equipment.

Regulators and meters should be sized for the load. If they are not then they need to be changed.

ChrisJ

Tim McElwain said:

Keep in mind the different tables are based on allowable pressure drop. The lower the pressure drop allowed the larger the pipe needed. That is what the tables are all about. The point of delivery pressure (the outlet of the meter should always be 7" W.C.) That is unless you are using a hybrid system which is measured in 2 or 5 pounds depending you location.

On inner city systems with the old cast iron mains in the street you could get varying pressures depending on the time of year. Those systems are operating at pressures less than roughly 14 inches water column (1/2 pound pressure) and do not have a regulator ahead of the meter. Most residential gas systems require a minimum gas pressure ahead of the gas valves on the equipment of 5" W.C. allowing for a 1" drop in pressure through the gas valve. Regulators on the valves are set to send 3.5" W.C. pressure to the burners on the equipment.

Regulators and meters should be sized for the load. If they are not then they need to be changed.

The AC-250 meter I'm using is rated to have a 1/2" drop at it's rated capacity on a 1/4 pound system. It is also my understanding pressure regulators also have some drop by nature, otherwise how could the diaphragm function at all? The only thing opening the valve more is pressure drop.

mattmia2

The regulator, like any control, has to have some hysteresis.

If you dig deeper in the code, there is the formula that is used to calculate the tables. Be very careful with your units, if I recall the pressures are absolute.

ratio

Regulators droop as the flow through them increases. The mfgr should have some info on it.

I got in an argument with a plumber once over it. I needed 1 lb to a unit, the plumber installed a 1 lb spring in the regulator. Unfortunately, the paperworks said that regulator with that spring at the flow I needed would droop IIRC 2". I ended up changing it to a 2 lb spring.

EBEBRATT-Ed

@niklaw1

On low pressure gas (that's usually considered 12" wc or less) all the pipe sizing tables are usually base on a pressure drop of 0.3 or 0.5" of wc and that is what you should follow.

You can't use 2" wc as your pressure drop

Zman

As Ed said, 2" may be what will "work". I believe the code says 0.5" max pressure drop under full load for pipe sizing.
If find that this calculator works well for high pressure systems and oddball layouts. https://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html