Pipe Supports, Hangers and Clamps: Spacing, Loads and Selection

Pipe supports are the least glamorous line on a piping bill of quantities and one of the most common sources of trouble: sagging drains, vibrating chilled water mains, cracked insulation at every clamp and, in the worst case, a riser clamp that was never rated for the load it carries. This article sets out how support spacing is derived, what each hanger type is for, how to size the rod and when a spring support is needed, so that a buyer can check a support schedule rather than accept it.

The standards that govern supports

Three documents do most of the work. MSS SP-58, Pipe Hangers and Supports: Materials, Design, Manufacture, Selection, Application and Installation, is the manufacturers' standard that defines hanger types by number, sets material and fabrication criteria and establishes minimum design load ratings for rigid hanger assemblies. Since the 2009 edition it has absorbed the former SP-69 (selection and application), SP-77, SP-89 (fabrication and installation) and SP-90 (terminology), so a specification that still cites SP-69 is pointing at a withdrawn document. Seismic, wind and dynamic bracing are covered separately by MSS SP-127.

ASME B31.1, Power Piping, covers piping in power stations, industrial and institutional plants and district heating and cooling, and its support rules are the origin of the span tables most engineers use. ASME B31.9, Building Services Piping, covers piping in industrial, institutional, commercial and public buildings and multi-unit residences, at the sizes, pressures and temperatures typical of HVAC and plumbing, and is the code most MEP specifications in the Gulf name. Sprinkler piping follows NFPA 13 instead, whose hanger spacing (12 ft for steel up to 1 1/4 in and 15 ft from 1 1/2 in) is different from the building services tables and must not be mixed with them.

How far apart: the span tables

The spacing tables in the codes are derived from a simple beam model: standard-weight carbon steel pipe, filled with water and insulated, supported as a fixed beam, with the bending stress limited to about 2,300 psi and the sag between supports limited to about 2.5 mm (0.1 in) so that the line still drains. Lines carrying steam, gas or air are lighter and may span further. The figures below are widely published on that basis.

NPS	Water service	Steam, gas or air	Typical rod diameter
1 in	7 ft (2.1 m)	9 ft (2.7 m)	3/8 in
2 in	10 ft (3.0 m)	13 ft (4.0 m)	3/8 in
3 in	12 ft (3.7 m)	15 ft (4.6 m)	1/2 in
4 in	14 ft (4.3 m)	17 ft (5.2 m)	1/2 in
6 in	17 ft (5.2 m)	21 ft (6.4 m)	5/8 in
8 in	19 ft (5.8 m)	24 ft (7.3 m)	3/4 in
12 in	23 ft (7.0 m)	30 ft (9.1 m)	3/4 in

Copper tube spans less: published figures run from 5 ft for 1/2 in and 3/4 in tube to 12 ft for 4 in. Thin-wall stainless, plastic and multilayer pipe each have their own tables from the pipe manufacturer, and those govern.

The tables stop applying as soon as there is a concentrated load. Valves, strainers, flanged pairs and meters need a support on each side; changes of direction should have a support close to the elbow; and risers need their own scheme. The usual practice is to shorten the span to about three-quarters of the table value where the run changes direction between supports. Where the engineer has done a stress analysis, the analysis governs and the table is only a check.

Hanger and support types

MSS SP-58 identifies each component by type number, which is the clean way to specify them. The common ones in building services and light industrial work are:

Adjustable clevis hanger (Type 1): the default for horizontal pipe on a single rod; allows vertical adjustment and limited swing. The rod should hang within about 4 degrees of vertical in the hot position.
Band and strap hangers: light-duty alternatives to the clevis for small pipe where adjustment is not needed.
Pipe clamps and riser clamps: a two-bolt clamp around the pipe, resting on the floor slab at each riser penetration or bolted to steel; the riser clamp carries the weight of the vertical run between supports.
Roller supports and roller hangers: for long hot lines that move axially; the roller lets the pipe grow without scuffing.
Pipe shoes and slides: a welded or clamped shoe lifts insulated pipe off the steel so that insulation is continuous and the shoe, not the pipe, slides on the support.
U-bolts and guides: hold the pipe in line while allowing axial movement; a U-bolt pulled tight is an anchor, not a guide, so the specification should say which is intended.
Trapeze assemblies: a channel across two rods carrying several parallel pipes, the normal solution in corridors and plant rooms; each rod is sized for its share of the total load.
Anchors: fix the pipe to structure at the point from which thermal movement is directed, and must be designed for the full thrust.
Sway braces (Type 50): spring-loaded restraints for vibration and dynamic loads.
Variable spring supports (Types 51 and 52) and constant supports (Types 54 and 55): for lines that move vertically with temperature, described below.

Insulated chilled water and refrigerant lines need an insert or an insulated clamp at every support so that the vapour barrier stays continuous. For copper and stainless pipe, use lined clamps so that the carbon steel hanger does not sit in direct contact with the pipe.

Rods, inserts and the load path

Each rigid hanger is only as good as the rod and the fixing above it. Published MSS SP-58 load ratings for carbon steel threaded rod at up to 343 C are 610 lb for 3/8 in, 1,130 lb for 1/2 in, 1,810 lb for 5/8 in, 2,710 lb for 3/4 in, 3,770 lb for 7/8 in and 4,960 lb for 1 in; at room temperature in a building the same ratings apply conservatively. The rod diameter in the span table above is the minimum for a single rod carrying a water-filled pipe of that size, so a trapeze or a support near a valve needs its own calculation.

The fixing into the structure is the part most often left to the installer. Cast-in inserts, through-bolts and approved post-installed anchors each carry a rated load in a given concrete; beam clamps have a rated load that depends on the flange thickness; and hangers from steel deck or from other services are not acceptable.

When a spring support is needed

A rigid hanger is right where the pipe does not move vertically. Hot water, steam and condensate lines grow when they heat up, and a rigid hanger near a riser or a loop will either lift off its support or be overloaded. A variable spring hanger carries the load through a coil spring so that the pipe can rise or fall; the supporting force changes as the spring compresses or extends, and that change, called variability, is normally limited to 25 percent of the operating load. Variable springs suit modest travel, typically under about 50 mm. Where the travel is larger or the connected equipment is sensitive, a constant support, which uses a lever mechanism to hold the load within about 6 percent through its travel, is used instead. Spring supports are selected from the manufacturer's tables for the calculated hot load and travel, and arrive with a travel stop that must be removed only after hydrotest.

Selection checklist for a support schedule

Code: B31.9 for building services, B31.1 for plant, NFPA 13 for sprinklers; do not mix their spacing tables.
Component: specify by MSS SP-58 type number and finish (black, zinc-plated, hot-dip galvanised or stainless).
Rod and fixing: sized for the water-filled load with the rating documented; approved anchors only.
Movement: guides, anchors and spring supports placed per the thermal movement design, not left to site.

Key takeaways

MSS SP-58 defines hanger types and load ratings; ASME B31.1 and B31.9 set the design rules; NFPA 13 has its own spacing for sprinkler pipe.
Span tables assume straight, standard-weight, water-filled steel pipe and about 2.5 mm sag; they do not apply near valves and fittings.
Water-filled steel spans run from about 7 ft at 1 in to 19 ft at 8 in; copper and plastic span less.
Choose rod diameter from the load, not from habit; 3/8 in rod is rated for about 610 lb.
Use spring supports where a line moves vertically; variable springs up to 25 percent variability, constant supports beyond.
Insulated clamps and inserts on chilled lines prevent condensation and corrosion at every support.

Frequently asked questions

Can I use the plumbing code spacing for a chilled water main?

Use the spacing in the project specification, which for HVAC piping is normally based on ASME B31.9 or the engineer's own table. Chilled water mains are heavy, insulated and often carry valves, so the straight-run figure is a starting point and the supports near equipment are placed individually.

What is the difference between a guide and an anchor?

A guide keeps the pipe on line while letting it move along its axis; an anchor stops it moving at all and takes the thrust from thermal expansion. Both may be made from the same clamp, which is why the drawing must say which is intended at each location.

Do galvanised hangers matter indoors?

In an air-conditioned building a zinc-plated hanger is usually adequate. In plant rooms, car parks, roofs and anywhere the pipe sweats or is washed down, hot-dip galvanised or stainless steel components last far longer and cost little more on a project scale.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company supplies hangers, clamps, supports, threaded rod and fixings through its fasteners, hangers, clamps and gaskets range. Send us the support schedule with pipe sizes, service and finish and we will match the components; contact us to start.