Inlet pressure is the first number our applications team asks for. It indicates what job the regulator is doing, it determines which orifices you can use, and it affects how much protection the internal relief valve provides.
The quick answer
|
Inlet pressure at the regulator |
What it tells you |
143 orifices available (by MAOP) |
What usually limits you |
|---|---|---|---|
|
2 PSI |
Line pressure regulator, installed downstream of the service regulator |
All seven, up to 5/8" |
Capacity at low inlet pressure, and pressure loss in piping |
|
5 PSI |
Line pressure regulator on a higher-pressure customer system |
All seven, up to 5/8" |
Same as above, with more capacity per orifice |
|
60 PSI |
Service regulator on the meter set, fed from the distribution system |
1/4", 3/16", and 1/8" only |
Orifice capacity and IRV effectiveness |
You don't choose an inlet pressure to suit a regulator. Regulator selection is determined by inlet pressure, not the other way around. The system sets the pressure, and the regulator and orifice have to match it.
2 and 5 PSI are a different application from 60 PSI
These three numbers don't describe three versions of the same job.
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An inlet of 2 or 5 PSI means the regulator is working as a line pressure regulator. It sits after the service regulator, on the customer side, and cuts an elevated house-line pressure down to appliance pressure. Depending on the system, it may feed a single appliance or a group of them.
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An inlet around 60 PSI means it's a service regulator. It sits on the meter set and takes distribution pressure down to delivery pressure.
The USG 143 can do either job. The platform and the spring and orifice logic are the same, but the constraints differ. At 2 and 5 PSI, the orifice pressure rating is never the limit, and the question is whether the regulator can pass enough gas at low inlet pressure. At 60 PSI, the orifice rating is the main constraint, and relief performance needs a careful check.
Why inlet pressure drives the selection
Every 143 orifice has a maximum inlet pressure:
|
Orifice size |
Max inlet pressure |
|---|---|
|
1/8" |
125 PSI |
|
3/16" |
125 PSI |
|
1/4" |
60 PSI |
|
5/16" |
40 PSI |
|
3/8" |
40 PSI |
|
1/2" |
20 PSI |
|
5/8" |
10 PSI |
Two relationships work against each other:
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Smaller orifices have less capacity but a higher pressure rating. Larger orifices pass more gas and tolerate less inlet pressure.
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Higher inlet pressure pushes more gas through any given orifice. It also removes the larger orifices from the list.
So you can't simply pick the largest orifice for maximum capacity. Inlet pressure sets an upper limit on orifice size, and the capacity table tells you whether what's left under that limit is enough. This table is for the 143. The 496 has its own data, covered in the 496 sizing guide.
2 PSI inlet
Where you see it. Residential and light-commercial buildings piped at 2 PSI downstream of the meter. The higher house-line pressure allows smaller pipe and longer runs, and line regulators step it down to inches of water column near the appliances.
Orifice options. All of them. Even the 5/8" orifice is rated to 10 PSI, so the pressure rating is never the constraint.
What limits you. Capacity. With only 2 PSI pushing gas, each orifice passes far less than it would at service-line pressure. This tier is where the larger orifices are useful, and where a small shortfall in inlet pressure matters most (see the measurement section below).
Also check. A regulator installed indoors has different venting requirements from one on an outdoor meter set. See our article on vent-limited regulators and code requirements.
5 PSI inlet
Where you see it. Larger residential, commercial, and light-industrial customer systems where 2 PSI doesn't move enough energy through the building piping. Rooftop units, boilers, and process equipment are common loads.
Orifice options. All seven again, since 5 PSI is below every orifice rating.
What limits you. The same capacity question as at 2 PSI, with more room. Each orifice passes more gas at 5 PSI, so you can often meet the load with a smaller orifice than the same load would need at 2 PSI.
Also check. The appliance or equipment inlet ratings downstream, and the relief curves. With 5 PSI upstream, a failed regulator sends more pressure toward the equipment than a 2 PSI system would.
60 PSI and higher
Where you see it. The meter set, where a service regulator is fed from a distribution system operating around 60 PSI.
Orifice options. Only the 1/4" (60 PSI), 3/16" (125 PSI), and 1/8" (125 PSI). Everything 5/16" and larger is ruled out. Above 60 PSI, only the 3/16" and 1/8" remain, up to 125 PSI.
What limits you. High inlet pressure gives each orifice plenty of flow, but only the small orifices are permitted, and the small orifices have the least capacity. For a typical single-family load this usually isn't a problem. For a large home, a multi-unit service, or a property that has added significant load, it can become the deciding factor.
When the load needs more than the orifice can pass. Don't install an orifice above its pressure rating to gain capacity. If the largest orifice available at your inlet pressure doesn't have enough capacity, move to the next larger regulator model. Typically that means going from a 143 to a USG 243.
Overpressure protection changes with inlet pressure
Internal relief valves are less effective at higher inlet pressures. More pressure upstream means more gas comes through a failed regulator, and the IRV can relieve only so much.
An IRV that clearly protects at 5 PSI inlet may not protect with the same orifice at 60 PSI. The relief curves in the documentation show, for a given orifice and inlet pressure, whether the IRV keeps downstream pressure within limits. At the 60 PSI tier, checking them isn't optional.
Orifice choice affects this as well. A smaller orifice passes less gas in a failure, which makes the relief valve's job easier. For more on how relief and lock-up work together, see lock-up pressure and our registration and overpressure article.
Measure inlet pressure at the regulator
The most common way this selection goes wrong is measuring inlet pressure too far upstream and treating that as the regulator's inlet pressure.
Piping between the measurement point and the regulator loses pressure, and it loses the most under load. Customers often ignore this loss. They take a reading at a convenient point well upstream, carry that number to the capacity table, and end up with a regulator sized for pressure it never sees. At the 2 PSI tier the error can be a meaningful fraction of the inlet pressure, and it shows up as a regulator that can't carry the load.
Make sure you're measuring inlet pressure at the regulator. Then use each number for the right check:
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Check capacity at the lowest inlet pressure the regulator will see, under load and on a peak day.
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Check the orifice pressure rating and relief curves at the highest inlet pressure it can see, which is the system MAOP.
A four-step decision framework
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Start with inlet pressure at the regulator. Note both the minimum under load and the maximum at system MAOP. This also tells you whether you're specifying a line regulator or a service regulator.
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Limit the orifice by pressure rating. Rule out every orifice rated below your maximum inlet pressure.
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Confirm capacity for the load. From the remaining orifices, find one that passes the peak load at your minimum inlet pressure, using the capacity table. As a rule, use the smallest orifice that comfortably carries the load, because smaller orifices lock up tighter and are easier to protect.
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Check IRV protection. Verify that orifice and inlet pressure on the relief curves. If no combination passes all four steps, move up to the 243.
Spring selection runs alongside these steps and depends only on the required outlet pressure. The spring tables are in the 143 selection guide.
Worked examples
Example 1: 2 PSI line regulator. A 143 feeds a furnace and water heater at the end of a long 2 PSI run. The pressure reads 2 PSI at the meter outlet, but the regulator is far down the line. Measure at the regulator with the appliances firing and use that lower number. Every orifice clears its pressure rating, so go to the capacity table and find the orifice that carries the combined load at the measured inlet pressure. Then confirm on the relief curves.
Example 2: 5 PSI line regulator. A light-commercial building piped at 5 PSI feeds a rooftop unit. All seven orifices are available. The higher inlet pressure means a mid-size orifice may carry a load that would have needed a larger orifice at 2 PSI. Choose the smallest orifice that passes peak load at the minimum measured inlet pressure. Then check the relief curves at 5 PSI against the equipment's inlet rating.
Example 3: 60 PSI service regulator. A meter set is on a 60 PSI MAOP system. Orifice options are 1/4", 3/16", or 1/8". Check the capacity table at the lowest inlet pressure the service sees on a peak winter day, not at 60. If the 1/4" carries the peak load, check the relief curves at 60 PSI. If the curves don't show protection, try a smaller orifice that still carries the load. If the 1/4" can't carry the load, no larger orifice is available, and the application calls for a 243.
Give us your inlet pressure and we'll work through it with you
Holland Supply stocks USG 143 regulators, the full spring and orifice range, and 243s, for same- or next-day shipping across Ohio and surrounding states. If you give our applications team your inlet pressure, outlet target, and load, they'll work through the orifice, capacity, and relief checks with you. Related reading: 143 selection guide, 496 sizing guide, lock-up pressure, and Inlet Pressure, Outlet Pressure, and Droop.