Fire Sprinkler Piping Basics: Pipe, Fittings and Valves Under NFPA 13

A sprinkler system is a piping system first and a fire system second: if the pipe, fittings, hangers and valves are wrong, the sprinklers at the end of the line cannot do their job. NFPA 13, Standard for the Installation of Sprinkler Systems, is the reference that most Gulf specifications and Civil Defense approvals lean on. This article explains what the standard asks of the piping materials so that a buyer can read a sprinkler bill of quantities, question a substitution and recognise a non-compliant submittal.

Which steel pipe NFPA 13 accepts

Three ASTM standards cover the steel pipe used in the great majority of wet and dry sprinkler systems: ASTM A53 (general-purpose welded and seamless pipe), ASTM A135 (electric-resistance-welded pipe) and ASTM A795 (pipe made specifically for fire protection). The differences that matter to a buyer are wall thickness and what each pipe may be used for.

Under A53 the minimum for sprinkler use is Schedule 40 standard weight, with heavier walls permitted. A135 allows thinner walls, with Schedule 10 as the minimum. A795 is written for fire protection and covers light-wall Schedule 10 through Schedule 30 and Schedule 40, black or hot-dip galvanised. Sprinkler pipe to all three standards is normally supplied painted red or with a red coating so that it is identifiable in a ceiling void.

Standard	Typical walls for sprinklers	Normal joining	Notes
ASTM A53	Schedule 40 minimum, Schedule 80 permitted	Threaded, grooved, welded	Also a general building services pipe
ASTM A135	Schedule 10 minimum	Roll grooved, welded	ERW, lighter, lower cost per metre
ASTM A795	Schedule 10, 30 and 40	Threaded (Sch 40), grooved, welded	Fire-protection specific; black or galvanised

Schedule 10 is attractive because it is lighter, cheaper and quicker to hang, but the thin wall sets its joining method. Threading removes metal from the wall, so NFPA 13 limits threading to Schedule 40 pipe unless a lighter-wall product is specifically listed for threading by a testing laboratory. Cut grooving, which also removes wall, is limited to Schedule 40 and heavier; roll grooving, which forms the groove without cutting, is allowed on any wall. Welding is permitted on all three types when a qualified welder follows the fabrication rules of the standard. A submittal that shows Schedule 10 mains with threaded malleable iron fittings is therefore an error unless the listing is attached.

Black steel is the default for wet systems. Galvanised pipe is commonly specified for dry and pre-action systems, where the pipe stands empty, and wherever the project specification calls for it; refer to the standard and the listing for the exact cases. Whatever the finish, keep fittings consistent with the pipe so that a mixed black-and-galvanised system does not set up a corrosion cell at every joint.

Fittings and joints

Threaded malleable iron and ductile iron fittings, grooved mechanical couplings and welded outlets are the three fitting families that appear on almost every sprinkler drawing. Threaded fittings are limited by the wall-thickness rule above and by the practical size at which threading becomes slow, so they dominate branch lines in small sizes. Grooved couplings have become the normal choice for mains and risers: the joint is made with an elastomeric gasket held between two coupling halves, which makes installation and later modification fast, and the rigid or flexible style is chosen to suit the restraint design. Welded outlets and fabricated headers are used in pump rooms and where the fabricator is shop-welding spool pieces.

Copper tube and listed CPVC are also permitted under NFPA 13 for defined occupancies; CPVC must be the listed product, installed with the listed hangers, so refer to the standard and the listing before substituting it for steel.

Hangers and supports

Hanger spacing in NFPA 13 depends on pipe material and size rather than on the generic building services tables used for plumbing. For steel pipe, the maximum distance between hangers is 12 ft for 1 in and 1 1/4 in pipe and 15 ft for 1 1/2 in and larger. Hard-drawn copper tube is held at 8 ft for 3/4 in and 1 in, 10 ft for 1 1/4 in and 12 ft for 1 1/2 in and larger. CPVC spacing is set by the listing and runs from about 5 ft 6 in for 3/4 in up to 10 ft for 3 in, with reductions in hot ceiling spaces.

Location rules matter as much as spacing. A hanger must be at least 3 in from a sprinkler so that the fitting can be made up and the sprinkler is not bent out of line. The unsupported length from the last hanger to the end sprinkler on a branch line is limited to 36 in for 1 in pipe, 48 in for 1 1/4 in and 60 in for 1 1/2 in and larger. Risers are supported at their base, at intermediate floors and at intervals not exceeding 25 ft. These are the checks that an inspector walks the ceiling for, so they should be on the installer's own checklist.

Hangers themselves should be listed for fire protection service and sized for the water-filled weight of the pipe plus the standard's allowance. Rod, clevis and band hangers are the usual types; seismic bracing, where the project requires it, is a separate design to the standard's seismic chapter.

Control, check and alarm valves

Every water supply to a sprinkler system passes through a control valve, and NFPA 13 requires that valve to be an indicating type, so that anyone can see at a glance whether it is open or closed. Outside-screw-and-yoke (OS&Y) gate valves, indicating butterfly valves and post indicator valves on underground supplies all meet that description. Control valves are also supervised: locked or sealed open, or fitted with a tamper switch connected to the alarm system, so that a closed valve is detected before a fire rather than after one. Refer to the standard for the exceptions and for the minimum closing time.

Downstream of the control valve, an alarm check valve or a dry-pipe valve holds the system and operates the waterflow alarm; swing or wafer check valves prevent backflow into the supply; and a fire department connection with its own check valve allows the fire service to boost the system. The valve package should be UL listed or FM approved for fire protection, with the pressure rating matched to the system's maximum static and pump churn pressures.

Fire fighting valves are a category of their own on our site; see the fire fighting valves range for the valve types above.

Pressure testing and acceptance

Before a system is accepted it is hydrostatically tested. NFPA 13 (2022 edition) requires a test at 200 psi (14 bar) held for 2 hours without loss; where the normal working pressure exceeds 150 psi, the test is 50 psi above working pressure, and where a fire pump supplies the system the test pressure is based on the pump's shutoff pressure. The pressure is read at the lowest point of the portion tested. Where an existing system is modified and more than 20 sprinklers are affected, the new portion is isolated and tested in the same way. Dry systems carry an additional air test; refer to the standard for its pressure and duration.

The acceptance package should include the hydrostatic test record, the main drain test result and the listing documents for pipe, fittings, hangers and valves.

Buying checklist

State the pipe standard and schedule for each size range, and the joining method that goes with it.
Do not accept threaded Schedule 10 without the laboratory listing attached.
Specify black or galvanised for the whole system, pipe and fittings alike.
Call for listed hangers and record the spacing rules on the installation drawings.
Require indicating, supervised control valves and listed alarm, check and dry-pipe valves with the correct pressure rating.
Make the 200 psi, 2-hour hydrostatic test record a condition of acceptance.

Key takeaways

NFPA 13 accepts steel pipe to ASTM A53, A135 and A795; A53 is Schedule 40 minimum, A135 and A795 allow Schedule 10.
Threading is limited to Schedule 40 unless the lighter pipe is listed for threading; roll grooving suits any wall.
Steel hangers are spaced at 12 ft up to 1 1/4 in and 15 ft from 1 1/2 in; keep hangers at least 3 in from sprinklers.
Control valves must be indicating and supervised; alarm, check and dry-pipe valves must be listed for fire service.
The acceptance hydrostatic test is 200 psi for 2 hours, or 50 psi above working pressure when that exceeds 150 psi.

Frequently asked questions

Can Schedule 10 pipe be used for the whole sprinkler system?

It can be used wherever the standard and the listing permit, which in practice means roll-grooved or welded mains and branch lines. It cannot be threaded unless it is specifically listed for threading, so systems with threaded branch lines use Schedule 40 for those sizes.

Why does NFPA 13 insist on indicating valves?

A closed control valve is one of the most common reasons sprinkler systems fail in a fire. An indicating valve shows its position from across the room, and supervision by lock, seal or tamper switch means a closed valve raises a signal rather than going unnoticed.

Does galvanised pipe last longer than black pipe in a wet system?

Not necessarily. In a wet system the pipe is full of water with little dissolved oxygen, and black steel performs well. Galvanised pipe is chosen for dry and pre-action systems and where the specification calls for it; refer to the standard and the project specification.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company supplies the piping side of sprinkler projects, from fire fighting valves and hangers to GI threaded fittings, and carries out installation through its fire protection services. Send us the sprinkler schedule with sizes, schedules and joining methods and we will confirm the matching materials; contact us to begin.