Every regulator station needs an answer to one question: what happens when the regulator fails open? A monitor and worker set answers it with a second regulator. It protects the downstream system without venting gas and without shutting anyone off.
This article explains how the configuration works, where to put the monitor, how to set it, and how to pipe the control lines so it does its job.
What a monitor set is
Two regulators are installed in series. One does the work. The other stands by.
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The operating regulator, or worker, is adjusted for the normal outlet pressure. It controls the station day to day.
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The monitor regulator is adjusted slightly higher. Because the outlet pressure it senses is below its set point, it stays open during normal operation, ready to take control.
If a failure in the worker causes outlet pressure to rise past the monitor's set point, the monitor assumes control and holds the system at that slightly higher pressure. Gas keeps flowing. Nothing vents. Nobody downstream loses service.
That's why monitoring is such an effective and dependable method of overpressure protection. It protects the downstream infrastructure while keeping the gas on.
A note on terms. This article covers the standby arrangement, where the monitor sits open until it's needed. You'll also hear "working monitor" for setups where the upstream regulator takes part of the pressure cut in normal operation. That's a different configuration, closer to a two-stage design, and we don't cover it here.
Upstream or downstream: where the monitor goes
Either the upstream or the downstream regulator can be the monitor. Both arrangements work. What doesn't change is how the upstream regulator has to be built and piped:
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The upstream regulator must have a blocked throat and an external control line. With the throat blocked, the regulator can't sense pressure at its own outlet. It senses only through the control line.
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That control line connects into the outlet piping downstream of the second regulator. The upstream regulator has to read final station outlet pressure, not the pressure between the two regulators.
The reason is simple once you picture it. If the upstream regulator sensed the space between the two units, it would respond to that pressure. Whether it's the monitor or the worker, its job depends on knowing what the distribution system is seeing.
So the downstream control lines for both the operating regulator and the monitor connect downstream of both regulators. One more rule: the regulators use separate control lines, including separate control line taps. A shared tap or shared line ties the two together, and a single plugged or damaged line would then blind both the regulator and its backup.
USG 441 and 461 regulators in all three versions can serve as monitors. They respond quickly, which matters when a worker fails, and they can monitor other types of regulators too.
Setting the two set points
The worker is set to normal outlet pressure. The monitor is set higher. Two rules govern how much higher:
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Set the monitor about 10% above the worker.
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The monitor's set point must be above the worker's lock-up pressure.
The second rule is the one people forget. When demand drops to zero, the worker closes and outlet pressure settles at its lock-up pressure, a bit above set point. If the monitor is set inside that band, it starts trying to control every time the worker locks up.
As an illustration, a worker set at 60 psi puts the monitor around 66 psi. Then confirm 66 psi clears the worker's lock-up pressure and stays within what the downstream system's MAOP allows.
Set them too close together and the regulators fight each other. Both try to control the same pressure at once. You'll see outlet pressure hunt and both regulators cycle, which wears them out and makes the station hard to diagnose. Set them too far apart and the downstream system sees a larger pressure jump when the monitor takes over. About 10% is where we land.
Control lines: where monitor sets succeed or fail
A monitor set depends on external control, sometimes called external sensing. Each regulator only knows what its control line tells it. Pipe the lines badly and you have two regulators acting on bad information.
Get these right:
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Tap 8 to 10 pipe diameters downstream of the worker. Closer in, the flow is still turbulent from the regulator.
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Stay away from elbows, tees, valves, rotary meters, and other sources of turbulence. The tap belongs on a straight run.
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Tap on top of the pipe. Liquids and debris collect along the bottom.
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Use a large enough control line. An undersized line slows the regulator's response. The manual gives the size for your model.
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Run separate lines to separate taps for the worker and the monitor.
All of this takes straight pipe, so it needs to be in the station layout from the start. There's more on planning that run in USG 441 vs. 461.
The capacity cost: plan on 30% less
A monitor isn't free. Even wide open, the second regulator is a restriction in the line. When the monitor is the same pipe size and orifice size as the worker, the set passes about 70% of what the worker would pass alone. That's a 30% capacity reduction.
Miss this step and the station comes up short at peak, even though each regulator looked fine on its own. Our gate station sizing guide walks through the full calculation.
Leave piping between the two regulators
Don't bolt the worker and monitor flange to flange. Make sure they're spaced out with some piping between them. The downstream regulator needs reasonably settled flow at its inlet, and regulators installed back to back tend to interact. A spool between the two gives each one room to do its job.
Monitor vs. relief valve vs. slam-shut
All three are accepted ways to protect against overpressure. They behave very differently when the worker fails.
|
Method |
What happens in a failure |
Customers keep gas? |
Gas vented? |
Field visit to restore service? |
|---|---|---|---|---|
|
Monitor regulator |
Takes over control at a slightly higher pressure |
Yes |
No |
No |
|
Vents excess gas to atmosphere |
Yes |
Yes |
No |
|
|
Shuts off flow |
No |
No |
Yes, manual reset |
Slam-shuts aren't ideal as the main line of defense. They shut off gas to the utility's customers, and a technician has to go out and reset them by hand. Monitors give you overpressure protection without shutting off the gas and without a field visit to bring customers back. For how to combine and layer all three, see our overpressure protection guide.
Know when the monitor has taken over
The monitor's biggest strength is also something to plan around. When it takes control, nothing dramatic happens. Customers don't call. The station keeps running, about 10% above its normal set point.
That means a failed worker can sit unnoticed, and a station running on its monitor has no backup behind it. Watch station outlet pressure through telemetry, charts, or routine checks. A station holding steady at the monitor's set point is telling you the worker needs attention. Include both regulators in your periodic station inspections, and confirm the monitor actually takes control when the worker is made to fail.
Configuration pitfalls
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Set points too close. The regulators fight.
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Monitor set below the worker's lock-up pressure. The monitor interferes every time demand drops.
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Upstream regulator without a blocked throat, or with its control line tapped between the two regulators.
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Shared control line or shared tap.
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Control line tap near an elbow, tee, valve, or rotary meter, or on the bottom of the pipe.
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Undersized control line.
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Regulators installed back to back with no piping between them.
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No 30% capacity reduction in the sizing.
Configuring a monitor set? We can help
Holland Supply stocks USG 441 and 461 regulators and builds them to configuration from the shelf, including blocked-throat units with external control line connections. Tell us your inlet pressure range, outlet set point, and peak flow. We'll size the worker and monitor with the 30% reduction built in and check your set points against lock-up.
Start with the regulator sizing tool, or talk with our technical team.