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Cast Iron Below Ground:
Is Hub & Spigot Really the Best Choice?
Reprinted with permission of The Wholesaler
May 1998

Installation histories refute old assumptions about hubless DWV couplings below ground

Until the mid-1960s nearly all soil pipe for underground drainage was of the hub & spigot (a.k.a. bell & spigot and "sv") variety. Hubless pipe was relegated mainly to in-wall and other above-ground applications.

Since then, though, no-hub has made a significant dent in the below-ground market, capturing 90% in some areas, mainly in California and the Far West. Still, specifiers elsewhere are reluctant to allow no-hub below ground for fear of possible leaks and other problems. Are these fears justified?

A California-based no-hub pipe foundry commissioned an informal survey of plumbing specifiers, wholesalers and contractors in the 11 western states. Their perceptions and policies regarding hubless vs. hub & spigot were and checked against performance data. Some important misunderstandings were uncovered. Perceptions of no-hub and sv systems varied considerably among them, with geographical location emerging as a variable.

For example, most of the California and Nevada engineers cited no major quality preference for either hubless or hub & spigot. Many had specified both types for below ground use for decades, with no major problems with either type. At least one billion no-hub joints have been installed in the U.S. since 1963, about 25% of them below ground.

The engineers indicated they're confident the joints are performing at least as well as the hub & spigot joints. They indicated that their decisions to spec no-hub or hub a & spigot were based on the dictates of a particular job, not on concerns about quality or joint integrity.

Many professionals from other states, however, expressed a definite preference for hub & spigot below ground because they believed no-hub might not offer the weight and joint quality to prevent leaks. Many engineers and other professionals interviewed said they used hub & spigot material below ground because "that's the way we've always done it." Others said they thought hubless couplings wouldn't perform as well.

Their reasons condensed down to four major concerns: corrosion, joint integrity, east of installation and overall cost.

Perceived corrosion resistance
Some engineers said they believed hub & spigot systems last longer. In fact, both systems are made of the same cast iron raw materials, with nearly the same wall thicknesses, and they corrode at the same rate.

No-hub couplings manufactured to national standards require no more protection from corrosion than does the pipe itself. Hubless couplings consist of a rubber gasket into which the hubless pipe or fitting is inserted until stopped by an internal rubber center stop. The gasket itself is surrounded circumferentially by a stainless steel shield; the resulting "sandwich" is tightened to the pipe or fittings by stainless steel, worm-drive sealing clamps. The rubber used in both the hub &spigot and no-hub gaskets contain neoprene as the sole elastomer. Both no-hub and hub &spigot gaskets have identical resistance to corrosion catalysts.

Nevertheless, to win over the skeptics, several manufacturers now produce a heavy-duty hubless coupling designed to provide added performance. These typically feature Type 304 and 305 stainless construction and more hose clamps, with greater torque force. They provide more corrosion resistance and better sealing than do standard hub couplings. Where corrosive soils are present, and easily installed, readily available and inexpensive prophylactic material ought to be placed around the piping to protect the pipe, fittings and couplings (both hubless and hub & spigot).

Integrity and resistance to deflection
Some engineers said they believed the hub & spigot joint provides greater resistance to deflection and longitudinal joint-separation forces. But this too appears to be unsupported by research data or empirical evidence.

Deflection is the forced dislocation of piping and/or fittings from the imaginary centerline gradient of the piping installation. Deflection is caused by seismic or other forces or by impact, such as from a backhoe or soil compactor, and most commonly during installation. Hub &spigot joints do resist or limit deflection, but this resistance itself can cause problems. For example, in resisting a strong deflective force, the bell end of the pipe could crack and/or the spigot end could fracture. Moreover, if the deflection takes the joint to the limit, the resulting gap between the spigot pipe and the gasket could allow leakage. Be aware that the hub &spigot gasket has little "memory," especially under extreme forces. And even when the line is returned to its former center line, the gasket may not return to its former sealing position on the spigot pipe.

In contrast, tests show hubless couplings allow greater deflection of the piping system. They show that no-hub couplings allow at least twice the degree of deflection without leaking, and this is under greater hydrostatic pressures than normally encountered by such a system. Instead of maintaining integrity by resisting deflection, the no-hub strategy is to accommodate deflection. It yields slightly rather than comes apart, so leakage paths and other openings don't form.

No-hub manufacturers respond
No-hub couplings change their inside and outside diameter dimensions within a broad tolerance when the worm-drive coupling clamps are loosened or tightened during installation. While a 3" coupling can't seal a 2" dia. pipe or fitting, hubless couplings appear to have more inherent latitude to accommodate two different diameters. Because hubless couplings don't rely on the displacement of rubber by the insertion of a "wedge" (i.e., spigot pipe) they seem to make a more positive joint. When proper torque is applied to the hubless coupling, an even seal around the circumference of the pipe and fittings is created. This band-load gives the coupling greater protection against leaking during deflection.

Heavy-duty no-hub couplings provide even greater band load because of higher torque clamps. And they generally have more clamps. So there's more resistance to separation than with hub &spigot joints and up to three times more resistance to internal hydrostatic pressure than either standard hubless or hub &spigot joints.

Ease of installation
More than one engineer said hub &spigot was easier to install because the making of joints is simply the shoving of a spigot into a gasket in the hub. What's more, no-hub couplings require a torque wrench to tighten the clamps, so there are chances for a faulty installation and perhaps a leak. But the hub &spigot isn't fool-proof either. If the spigot isn't seated far enough into the hub of the pipe or fitting, it too may leak or come apart if deflected, or pulled apart laterally or separate from internal effluent pressure.

Outwardly a spigot may appear to be well inside the hub, but there's no marking on the spigot indicating that it's inserted all the way. There's no way to tell if the spigot end is seated properly. Improper installation of hubless couplings, on the other hand, may be easier to determine. They distort if tightened down on pipe that isn't seated to the center stop, for there's no pipe beneath the clamp to seal against, and the effect is obvious to the installer and inspector. Also no-hub fittings have a cast-in-place gasket positioning lug that indicates the spigot of the fitting is fully seated inside the coupling.

No-hub couplings manufactured to U.S. standards require a pre-set torque wrench; there aren't screwdriver slots. Hub &spigot installations, however, are performed using all manner of tools and devices: hammers and boards, lead pipe, chain pulling tools, "spud bars," and "come-alongs." This leads to inconsistent pipe-in-hub seatings. To compound the chance of a hub &spigot joint coming apart, a lubricant typically is used to ease spigot installation. Until that lubricant dries, or washes away, it actually can assist an improperly seated joint in coming apart as the contractor manipulates pipe or fittings downstream, disturbing joints already installed upstream.

Regardless of the system specified, major plumbing codes require a hydrostatic pressure test on dwv system, witnessed and signed off by a plumbing official, to ensure that all failures have been repaired prior to covering it. This along with industry data on corrosion and joint integrity should mitigate professionals' personal biases and preferences for or against no-hub or hub &spigot concerning safety and reliability.

Misunderstandings on overall costs
Many of the plumbing professionals interviewed said they believed hub &spigot was faster to install than hubless, resulting in lower labor costs and a general competitive advantage. While there are occasions where hub &spigot is faster to install (e.g. long, straight runs), other factors could swing the cost advantage to no-hub. Take, for example, the cost of waste. Some engineers said as much as 35% of hub &spigot material is discarded as hub ends are cut off to fit the layout.

With hubless, by contrast, there are no telescoping lengths to consider, and the installer fabricates from different directions without the need for hubs. What's more, no-hub pipe scraps are readily usable; sv scraps are not. The result is up to 70% less waste with no-hub, a significant cost savings.

Although prices for hub &spigot and hubless pipe are about equal right now, prices of fittings are far higher for hub &spigot. A recent analysis showed hub &spigot fittings cost an average of 45% more than hubless fittings. A partial offset to the higher cost of hub &spigot fittings, joined with the sv gasket, comes in the form of an added cost for couplings when hubless is used. The amount of added cost depends on the layout and whether standard or heavy-duty couplings are used, and must be considered when comparing costs. Nevertheless, the typical hub &spigot system appears to be from 5% to 20% more costly than an equivalent hubless system when all costs are considered.

Is hub &spigot a proprietary spec?
Recently corporate consolidations have reduced the number of foundries producing hub &spigot materials. At present there are only two manufactures in the U.S. producing sv material, one east of the Rockies and one west. This has cut the availability of hub & spigot systems to where there's in a de facto proprietary specification. Some engineers reported job delays due to persistent backordering of certain sv and long lead times for shipping orders of hub &spigot complete.

Many engineers expressed concern that hub & spigot's de facto proprietary status may trigger the governmental requirement that more than one vendor be offered in publicly-funded projects. There's no way at present to satisfy the "or equal" clause in many government bids. Many of these same stipulations exist in privately funded work. In addition, the seeming proprietary status of hub & spigot material coupled with low number of domestic producers is generating concern that hub & spigot prices will continue to rise.

Foundries call for changes
Not surprisingly, no-hub system marketers would like to see changes in the way underground soil pipe is specified. Foundries in particular contend that its only fair that hubless systems be recognized as an "equal to / or equivalent" system to hub & spigot. This minor change, they argue, would go far in making material more available, reducing costs, and providing needed competition to the hub & spigot marketers. Some no-hub foundries are adding product to be more competitive with the hub & spigot producers. One intends to produce 12" and 15" hubless components, including heavy-duty bi-directionally corrugated couplings with high-torque clamps for extra performance.

Good engineering or bad news?
Regardless of personal preferences or historical bias in other regions, hubless soil pipe systems have been used below ground for decades in the Far West with no apparent major problems. Industry data and field experience show that concerns about corrosion, joint integrity, ease of installation and overall costs can be safely minimized. No-hub producers appear to have improved their products to withstand corrosive and seismic environments.

But specifiers will have to satisfy themselves that no-hub can truly do the job before the no-hub industry's request that "or equivalent" clauses embrace hubless systems is answered. Still, the success of western professionals with hubless ought to give skeptics pause. At least it's worth thinking about no-hub in a different light.

See Also:

Other:
    ASTM Reference Library (material and manufacturing specifications)

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