We are having a professional reroute a natural gas line. The existing black iron pipe is 1" and is in a wall we are removing. To re-route, we were advised by one contractor to increase pipe size then reduce back to make the connection to original pipe. Another contractor stated that increasing pipe size would reduce pressure when going back down to smaller pipe. The re-route will require four 90 degree elbows. Is upsize of pipe the best solution or will pressure go down in the larger pipe and not come back up when returning to smaller pipe.
How to maintain pressure when adding bends to a gas line
pipepressure
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Best Answer
Pressure loss (or drop) happens when there is flow. Straight pipe incurs some loss; elbows incur loss too. For a given flow rate larger pipe incurs less loss.
A gas delivery system is designed so that at peak demand, meaning when all appliances that can operate concurrently are operating, there is at least some minimum level of pressure at each appliance. In other words: pipe size is increased until calculations show gas arriving with adequate pressure pushing it into each appliance.
Realize that no residential gas system is truly engineered. They're all built with prescriptive tables and techniques like the "longest branch length" method. These quick-and-easy approaches bake in plenty of extra margin.
The contractor who says that pressure will reduce when going back down to smaller pipe... in a very technical sense he's correct, but in a practical sense he's wrong. He's not evaluating the system as a whole.
The contractor who suggested up-sizing the pipe is neither crazy nor taking advantage. He's ultra-conservative. He probably judged the extra cost of material for up-sizing to be much less than the extra labor cost of working up a calculation for your entire gas system. And he'd be right: the added material cost likely equals an hour or less of his hourly rate. By installing larger-than-original pipe he can easily argue that his new work results in less pressure loss than the pre-existing system had.
The reality is that your system is likely to have enough margin in it already so that adding four elbows is inconsequential. Up-sizing is totally fine too. Choose between the contractors based on some other factor.