Press Fittings for Compressed Air and Process Water: When Press Beats Sweat and Thread

A compressed air drop that has to go in by Friday and a hot work permit that takes half a day: that is the job press fittings were built for. But "no flame" is only the first question. The real ones are what media the fitting is listed for, at what pressure and temperature, and whether your tool and jaw are approved for it. This guide answers them from the manufacturers' manuals and the standards they cite. ProPress is a registered trademark of Viega, used here to name that product family; "copper press fittings" covers the category made by several manufacturers. See our Viega cross-reference for compatible alternatives by part number.

What a press joint actually is

A copper press fitting carries a factory-installed elastomer seal in a bead at the socket end. A hydraulic tool closes a jaw around the bead and permanently deforms the fitting onto the tube. Viega calls the result "a secure, non-detachable, mechanical connection." NIBCO describes an inboard bead and EPDM O-ring, with a witness mark left after a proper crimp.

The governing product standard is ASME B16.51-2021; ASME's front matter lists the next edition as scheduled for 2026, so check before you write a spec. Its scope covers fittings for hard drawn ASTM B88 copper water tube "for piping systems conveying water" at a maximum of 200 psi from 32°F to 200°F. The solder-joint counterpart is ASME B16.22-2021; ASTM B88-22 is the tube spec for both.

The standard is written around water. Anything else, including compressed air, is a manufacturer listing, not a standard requirement.

Approved media: air, water, and glycol by manufacturer listing

Bonomi Press and PressMax. Stateside is an authorized Bonomi distributor, and Bonomi's 2026 technical catalog publishes the full listings. The copper Bonomi Press line runs 1/2 to 4 inch in Cu-DHP copper with EPDM seals and is listed for drinking, non-potable, and treated water at 300 psi from 32°F to 250°F, heating and cooling with up to 50 percent ethylene or propylene glycol at 300 psi, low-pressure steam to 15 psi, and compressed air at 200 psi up to 140°F with residual oil under 5 mg/m3 per ISO 8573-1, plus nitrogen, argon, and dry CO2 at 200 psi and rough vacuum to 29.2 inches of mercury. Every fitting to 4 inch carries a leak detection feature that reveals an unpressed joint during testing, with water between 15 and 85 psi or air between 1/2 and 45 psi. For carbon steel pipe, the Bonomi PressMax line runs 1/2 to 4 inch on Schedule 10 to 40 per ASTM A53, A106, A135, and A795, at 200 psi maximum operating and 600 psi test, with EPDM seals to 2 inch and FKM above, for chilled and heating water, low-pressure steam, compressed air, inert gases, and vacuum. Pull the current Bonomi catalog before specifying.

Viega ProPress (copper). The installation manual states a 300 psi maximum operating pressure. Viega's applications chart narrows that by media. Compressed air with 25 mg/m3 of oil or less is listed at 200 psig and 140°F maximum, with a footnote requiring condensate drainage. Air above 25 mg/m3 oil is listed only with EPDM or FKM seals, same 140°F limit. Chilled and hydronic water are listed with up to 50 percent ethylene or propylene glycol. Low-pressure steam is listed at 15 psig and 250°F.

Mueller Streamline PRS (copper). Mueller's submittal lists 300 psi CWP, minus 20°F to 250°F, EPDM O-ring. Compressed air under 25 mg/m3 oil is listed at 200 psi and 140°F. Glycol up to 50 percent is listed at 300 psi. Low-pressure steam is listed at 5 psi; non-medical gases at 140 psi.

NIBCO Press System (copper). NIBCO lists the copper line for potable, hot, chilled and process water applications and requires original NIBCO EPDM seals.

Three takeaways. The published compressed air numbers line up at 200 psi and 140°F, but the oil thresholds do not: Viega and Mueller publish 25 mg/m3, while Bonomi's listing is written to under 5 mg/m3 per ISO 8573-1, a tighter and more conservative spec. The manufacturers disagree on steam (Viega 15 psig, Mueller 5 psi, Bonomi 15 psi), and several publish system pressures above the 200 psi in the B16.51 scope. The fitting is rated by its listing, but a code official or insurer may hold you to the standard. Ask first.

Sealing elements decide the media

The body is copper regardless. The seal is the variable.

• EPDM is the factory default and, per Viega, the only sealing element approved for drinking water installations.
• FKM is Viega's element for solar, district heating, low-pressure steam, and compressed air, with an operating range of 14°F to 284°F and short spikes to 356°F.
• HNBR is used mainly for inert gas, liquid fuel, and lubricant oil, at minus 40°F to 180°F, and cannot be installed in drinking water applications.

One cold-end detail for chiller and glycol work: Viega's manual lists its EPDM range starting at 0°F while Mueller's submittal lists minus 20°F, a difference that only matters below freezing, which is exactly where a glycol loop runs.

Tube requirements

Viega ProPress may only be pressed onto ASTM B88 or B75 copper; on B88, types K, L, and M. Soft temper (O60, O50) is limited to 1/2 through 1-1/4 inch; hard temper (H58) runs 1/2 through 4 inch. Mueller lists the same ranges, and Bonomi's catalog matches them: B88 types K, L, and M, soft copper 1/2 to 1-1/4 inch, hard copper 1/2 to 4 inch. For potable service, look for NSF/ANSI/CAN 61, which covers contaminants imparted to drinking water by pipes, fittings, and joining materials. Viega and Mueller both list NSF 61 and 372; Bonomi's fittings are marked with both, plus IAPMO PS-Z1117 and ICC-ES LC1002.

Tool and jaw compatibility: the part people get wrong

The manufacturers do not agree here.

• Viega: use only jaws and rings designed for ProPress fittings; never mix actuators and rings across manufacturers.
• RIDGID: Standard Series ProPress jaws cover ProPress copper 1/2 to 2 inch and are "compatible only with RIDGID Standard Series pressing tools." RIDGID's cross-reference lists ProPress, MegaPress, and PureFlow, and no other fitting brands.
• Milwaukee: its compatibility document lists Viega, NIBCO, Mueller, Conbraco (ApolloPress), Elkhart, and others against M12 and M18 press tools, but "it is the responsibility of the installing contractor to determine the compatibility."
• Mueller: PRS fittings are stated compatible with Milwaukee, RIDGID, REMS, Rothenberger, NIBCO, Klauke, DeWalt, and Hilti tools.
• Bonomi: requires an electro-hydraulic or electro-mechanical tool with a V-shaped profile jaw through 2 inch (XLC profile above), and maintains an approved-tool list available from its technical staff.

Practical rule: the fitting manufacturer's listing governs the warranty. Match tool and jaw to that list, not to what fit last time. Milwaukee also flags jaw cleanliness and inspection.

Installation and inspection points

The steps are the same across brands; skipping one is the usual cause of a leak.

1. Cut the tube square. Do not deform it.
2. Deburr inside and out. Remove shavings and oxidation.
3. Confirm the correct seal is present, undamaged, and seated in the bead. No petroleum lubricants; NIBCO allows water only.
4. Mark insertion depth. Viega's minimums run from 3/4 inch at 1/2 inch nominal to 2-3/8 inch at 4 inch.
5. Insert to the mark with a slight twist, then press until the jaw closes fully.
6. Confirm the depth mark is still visible at the fitting mouth and the crimp is uniform.

Unpressed fittings are the classic failure. Viega's Smart Connect feature reveals an unpressed fitting during a leak test, but only within 15 to 85 psi on water or 1/2 to 45 psi on air; Bonomi's leak detection device works in the same two windows.

When press is not the right call

• Steam above the manufacturer's low-pressure listing. Numbers range from 5 to 15 psig depending on brand.
• Any medium not on a chart. No listing, no rating.
• Compressed air above 200 psig or 140°F per the published charts.
• Drinking water with an FKM or HNBR element.
• Unlisted tube, wrong temper, or wrong jaw.

Where press does not fit, brazing or an ASME B1.20.1 threaded connection (referenced in B16.22) is the fallback.

Bottom line for the plant

For process water, glycol loops, and dry or lightly lubricated compressed air within 200 psig and 140°F, copper press fittings carry published listings from several manufacturers and take the hot work permit off the job. Confirm three things before ordering: media listing, seal element, and the tool and jaw list for that brand.