Regulators fail. A seat gets cut, debris holds a valve open, a diaphragm ruptures. When a regulator fails open, the only thing between full inlet pressure and the downstream system is whatever overpressure protection you designed in.
There are three main ways to provide it: a monitor regulator, a relief valve, or a slam-shut. Each protects the pipe. They differ in what they do to your customers, your emissions, and your field crews while they're protecting it. This article compares the three and shows how to choose and layer them.
Why it's required
The safety case is plain. Downstream piping, meters, and customer equipment are rated for distribution pressure, not for what's upstream of the station. Pressure beyond the MAOP puts all of it at risk.
The regulatory case is just as direct. Under 49 CFR Part 192, a pipeline fed from a source that can exceed its MAOP must have pressure relieving or pressure limiting devices (§192.195). Those devices have to meet design requirements (§192.199) and have enough capacity to hold downstream pressure within set limits above MAOP (§192.201). For a system with an MAOP of 60 psig or more, that limit is MAOP plus 10%, or the pressure that produces 75% of SMYS, whichever is lower. Lower-pressure systems have their own limits in the same section. Stations must then be inspected and tested at least once each calendar year, at intervals not exceeding 15 months (§192.739).
Every station gets protected. The design decision is how.
The three strategies at a glance
|
Monitor regulator |
Relief valve |
Slam-shut |
|
|---|---|---|---|
|
What it does in a failure |
Takes over control at a slightly higher set point |
Opens and vents excess gas to atmosphere |
Closes and stops flow |
|
Customers keep gas |
Yes |
Yes |
No |
|
Gas released |
None |
Yes, methane to atmosphere |
None |
|
Field visit to restore service |
No |
No |
Yes, manual reset |
|
How capacity is confirmed |
Sized like a regulator, with a 30% capacity reduction for the set |
Relief valve documentation, against the failed regulator's full open flow |
Must close tight against full inlet pressure |
|
Best role |
Primary protection |
Secondary protection |
Last resort |
Monitor regulators
A monitor is a second regulator in series with the operating regulator, set slightly higher. It sits open during normal operation. If the operating regulator fails and outlet pressure climbs to the monitor's set point, the monitor takes control.
Our shortcut, before any technical detail: a monitor is the best choice, because you protect the downstream gas infrastructure while keeping the gas on. Nothing vents. No customer loses service. No one has to drive to the station to restore it.
Monitors also have a sizing advantage. They can be sized just like regulators, from the same capacity tables, so there's no separate relief calculation to get wrong. The cost is capacity: a regulator used in a worker and monitor set takes a 30% capacity reduction, and that has to be in the station sizing from the start. Our article on monitor and worker configurations covers placement, set points, and control lines.
Relief valves
A relief valve opens when pressure exceeds its set point and vents gas until pressure comes back down. Like a monitor, it keeps gas flowing to customers during the event.
The drawback is where the gas goes. Venting to atmosphere means methane emissions, and a large release can be dangerous depending on what's around the station. A relief valve that's fine in an open field is a harder call next to buildings or a road.
Capacity is the other question. A relief valve has to pass what the failed regulator can deliver wide open, and that number gets big quickly as inlet pressure and regulator size go up. Check the relief valve's documentation for its capacity. If it can't keep up, move to a monitor.
Slam-shuts
A slam-shut closes when it senses overpressure and stays closed until someone resets it. It protects the pipe completely. It also shuts off the utility's customers.
Choosing a slam-shut as the primary form of protection creates headaches. When it trips, a technician has to go out in the field and reset it manually. Until that happens, customers don't have gas, they aren't happy about it, and the utility is losing revenue. Restoring service can mean more than a reset, too.
That's why slam-shuts work best as a last resort. Some utilities add them and set them slightly higher than the MAOP, so they close only to prevent catastrophic failure. In that arrangement the monitor, the relief valve, or both satisfy the regulatory requirement for overpressure protection. The slam-shut is the backstop behind them.
Layering: primary, secondary, last resort
These methods aren't mutually exclusive. Many stations use two, and some use all three. Layering works when each device has its own place in the pressure sequence. It goes wrong when they don't.
The most common layering mistake is setting devices at the same pressure. Two devices at one set point will both act at once, or chatter against each other, and you lose the orderly hand-off you were after.
Set them in steps:
-
Operating regulator at normal outlet pressure.
-
Monitor regulator above it, by about 10%, and above the operating regulator's lock-up pressure.
-
Relief valve slightly higher than the monitor, above the monitor's lock-up pressure.
-
Slam-shut, if used, higher than the relief valve.
Each device only acts if the one below it has failed to hold. The monitor handles a failed worker without any gas leaving the pipe. The relief valve opens only if the monitor also can't hold. The slam-shut closes only if everything else has been overrun.
Here's an illustrative ladder for a system with an MAOP of 60 psig. Your own numbers depend on your regulators' lock-up behavior and your company standards.
|
Device |
Illustrative set point |
Role |
|---|---|---|
|
Operating regulator |
50 psig |
Normal control |
|
Monitor regulator |
55 psig |
Primary protection |
|
Relief valve |
58 psig |
Secondary protection |
|
Slam-shut |
62 psig |
Last resort, inside the §192.201 limit of 66 psig |
Capacity: size for the regulator that can fail
Whatever you choose has to handle the full open capacity of the regulator or regulators that can fail, and it has to keep downstream pressure from building beyond the MAOP limits.
Full open capacity means what the failed regulator passes wide open at maximum inlet pressure. That's usually far more than the system ever draws. People check protection against peak demand when they should be checking it against the failure case.
It's also why changes upstream matter. A larger orifice, a bigger regulator body, or a higher inlet pressure all raise the full open capacity. Protection that was adequate for the old configuration may not cover the new one.
A decision framework
Work through these in order.
-
Can customers tolerate an outage? For a station feeding a distribution system, almost never. That rules out a slam-shut as primary protection and points to a monitor.
-
Can you vent here? Look at what surrounds the station and at your company's emissions goals. If venting is a problem, a relief valve shouldn't be the first device to act.
-
What's the full open capacity of the regulator that can fail? Work it out at maximum inlet pressure. A monitor can be sized for it like any regulator. A relief valve has to be checked against its documentation. If the relief valve falls short, use a monitor.
-
Does the station have the capacity margin for a monitor? The set costs 30% of regulator capacity. Build that into the regulator sizing, or go up a body size. See USG 441 vs. 461 for what that means for station design.
-
How much redundancy does the consequence justify? For most stations a monitor as primary protection is a sound answer. Higher-consequence stations add a relief valve as secondary protection, and some add a slam-shut above that as a last resort.
On a two-stage station, run through this for the interstage section as well as the outlet.
Maintenance and testing
Protection you haven't tested is protection you're assuming. Part 192 requires each pressure limiting and regulating station to be inspected at least once each calendar year, at intervals not exceeding 15 months. Your O&M procedures set the details. In general, each method brings its own checks:
-
Monitors. Confirm the set point and that the monitor takes control when the operating regulator is made to fail. Between inspections, watch station outlet pressure. A station sitting at the monitor's set point has a failed worker and no backup.
-
Relief valves. Confirm the set point, confirm the valve opens and reseats, and keep the vent stack clear. Review capacity whenever the regulator, its orifice, or the inlet pressure changes.
-
Slam-shuts. Trip test, confirm tight shutoff, and practice the reset. Make sure the people who'd respond know where it is and how to reset it.
-
All of them. Recheck the set point ladder. Springs age and adjustments drift, and two devices that started a few psi apart can end up on top of each other.
What to check before an audit
Showing that overpressure protection exists is the easy part. Showing that it's adequate takes more. Before an audit, make sure you can answer two questions for every station:
-
Can the protection handle the full open capacity of the regulator or regulators that can fail? Have the calculation or the documentation on file, based on the equipment that's installed today.
-
Does it keep downstream pressure from building beyond the MAOP limits? That means set points in the right order, with enough separation, and records that show it.
Talk through your station with us
Holland Supply sizes and supplies USG regulators for worker and monitor service, and we stock 441 and 461 regulators that we build to configuration from the shelf. If you're choosing protection for a new station or checking an existing one against its full open capacity, our technical team will work through it with you. You can also start with the regulator sizing tool.