It was a Saturday afternoon and Rob was getting ready to call it a day. He’s the new guy on staff and had decided to pick up an extra shift to bank some bucks for an upcoming vacation when… “Oh great not NOW! Why me?” he shouted.
“I wish I was on vacation right about now!” the technician bellowed. He was the only technician on call, and of course, Murphy’s Law had decided to rear its ugly head. He was getting a call from Gas Control saying that one of their stations was behaving strangely; the outlet pressure wasn’t holding steady, and guess who’s job it was to make things right? Looks like vacation wouldn’t be starting early after all…Being the new guy, Rob wanted to make sure he had a little backup so he decided to call none other than Joe - that legendary problem solver - and ask him to rendezvous to the site in question. Even though it was a Saturday afternoon, being the superhero that he is Joe responds to the call and heads to site as well.
Rob can’t figure it out, “It was working fine a few days ago…” he grumbled as he fiddled with the set screw on the regulator pilot, “Why would it all of a sudden have trouble controlling?”
“Isn’t that always how it goes?” Joe chuckled, “Has anything changed in the past few days? Anything at all that you can think of?”
“Well…” Rob thought for a moment, “I don’t know… There’s been nothing new done to this station in years from what I’ve been told…” He hesitated, but as Joe watched him intently he decided to continue “That factory down the road did just expand and add a new boiler, so maybe they’re drawing more gas? I dunno… That couldn’t affect the reg control, could it?”
Joe gave a knowing grin “Ahh young Grasshopper, you still have much to learn! Come with me…”
He went on to explain that while designers do their best to ensure that stations are optimally designed to handle both current and projected conditions, as time goes on the demand profile can change and reveal issues that otherwise weren’t causing any problems.
The simplest one of those design considerations is the placement of the sensing line for a pilot controlled regulator, and that was exactly what Joe had identified as a potential source of the problem at hand.
Rob had never encountered this concept before, and while he had dealt with regulator control issues in the past, it normally was caused by debris inside the regulator body. He didn’t realize the layout of a station could have such an impact on performance.
Luckily, there was a tap a few more feet down the pipe where a pressure gauge faithfully displayed the downstream pressure. “Let’s remove the gauge, and tube up the sense line to that tap instead to see if it helps smooth out the control,” Joe suggested.
Sense Line Placement is Crucial to Regulator Performance
The placement of the sense line on a pilot controlled regulator is crucial to ensuring the regulator will perform as it was meant to. The situation that Rob encountered has been faced by many others in the past, where a site doesn’t have any issues for years then suddenly there is a problem.
Each manufacturer has a recommendation for the geometrical layout of a pressure reduction line, and tap placement is a key element of this. Below is a schematic describing Pietro Fiorentini’s recommended tap placements in a liner pressure reduction run. Note that the sense line should always be located at least 4 pipe diameters (4xDN) downstream of any expansion joint after the regulator outlet flange.
Below is another schematic; this time it is an angled installation. Note that once again 4 pipe diameters of undisturbed flow is recommended before the sense line. Also, note that this schematic depicts a “Scorpion Tail” or “Dead Leg” pipe run being used as a take-off point for the sense line. This can be a useful design element to incorporate when space is at a premium as it can provide a tie-in point for multiple lines - however, the 4xDN rule also applies here as well.
Pietro Fiorentini Designs Stations!
The engineers at Pietro Fiorentini don’t just make fantastic regulators, they also design incredible stations just like the one in the picture below. Sometimes the solution to a control issue isn’t as simple as just switching taps like Rob and Joe did, and drawing upon the experience of expert designers can be the best course of action. Pietro Fiorentini is always available to help consult on all your station design needs.
Read our post on Pietro Fiorentini regulators- "Sometimes Things Just Work"