A section of gas piping must carry 220 cubic feet per hour at a design length of 100 feet. Take these capacities as given data: at 7 inches water column with a 0.5 inch water column drop, 1-inch pipe carries 175 cubic feet per hour and 1-1/4 inch pipe carries 360; at a delivery pressure of 1/2 pound per square inch with a 1.0 inch water column drop, 3/4-inch pipe carries 240 cubic feet per hour and 1-inch pipe carries 450. If the system is converted to the 1/2 pound delivery pressure with line regulators ahead of the appliances, what is the smallest size for that section?
- A1-1/4 inch, because the 7 inch water column table still governs and 1-inch pipe carries only 175 there
This is the correct size for the system as it stands at 7 inches water column, but the question asks what the section needs after the conversion. Once the delivery pressure changes, the old table no longer describes the system.
- B1 inch, because a section carrying 220 cubic feet per hour may never be smaller regardless of pressure
No size floor attaches to a flow rate by itself. What a pipe will carry depends on the pressure, the allowable drop and the length, which is exactly the set of conditions the tables are organized around.
- C1/2 inch, because raising the delivery pressure doubles the capacity of every size in the table
Higher delivery pressure increases capacity, but not by any fixed multiple, and the increase has to be read from the table for that pressure. There is also no listed capacity for that size anywhere near this load.
- 3/4 inch, because at the 1/2 pound delivery pressure that size is rated for 240 cubic feet per hour
Why D is correct
At the elevated delivery pressure the governing table shows 3/4-inch pipe carrying 240 cubic feet per hour, which covers the 220 the section has to deliver, so 3/4 inch is the smallest size that works. More pressure at the meter means more capacity through the same pipe, which is why elevated pressure systems are the standard answer to a long run.
What this question is testing
This checks whether you can move between capacity tables when the delivery pressure changes, and whether you read the smallest adequate size out of the table that actually describes the system after the change.
On the job
Converting to an elevated pressure system is the usual rescue for a long run or a big tankless addition, and it costs a line regulator, a regulator vent arrangement and sometimes a permit revision, all of which is cheaper than repiping a hundred feet through a finished building. The catch is that appliances rated for low pressure inlet cannot be fed directly, so the regulator placement becomes part of the design.
Memory technique
Push harder, carry more: change the pressure, change the table.
Exam tip
More pressure at the meter buys capacity in the same pipe, but only from the table published for that pressure.
Where to look it up
Capacity tables for elevated delivery pressures and the requirements for line pressure regulators are in IFGC Chapter 4, with sizing in IFGC 402.4. In UPC jurisdictions fuel gas piping is covered by UPC Chapter 12 rather than by the IFGC; the capacity tables trace to the same source but the section and table numbers differ.