In hindsight it seems silly that early as a designer I didn't take time to understand some of the basic nuances and differences in pipe fittings.
Performance Spec Beginnings
Like a good handful of engineers in the industry, I began early in my design days doing bid/performance specification work - outlining big picture issues and project nuances - while leaving the system layout and detailing to the fire sprinkler contractor. It lends itself to understanding code surprisingly well, but lacks the hands-on experience to understand how systems are actually built.
Understanding Each Component
Why is understanding the fitting components important? When I started laying systems out there's some natural rules that develop due to availability of the materials. If you want a basic, labor efficient, and cost-effective system, then it's imperative to understand what materials are commonly available and cost effective - and materials are considered "special order" (ie: expensive and longer lead-times).
After doing shop drawing/fabrication design, finding ways to create clean designs with commonly available components is a very important part of the design process.
Overview of Components
In today's article I'm covering the basic, traditional threaded pipe fittings.
Likely the most familiar component - an elbow has two openings traditionally 90-degrees apart, with female threads on both ends. Elbows can vary in angles - while the most common are 90-degree and 45-degree, cast-iron fittings also offer a 22-1/2 degree threaded elbow.
The 'turn' on the elbow also can vary, particularly with cast-iron elbows. "Long-turn" elbows have a larger radius and make a more gradual curve, which could have hydraulic benefits should the application justify it.
"Street Elbows", typically available as a malleable-iron fitting, is an elbow with a female thread on one side and a male thread on the other. They can be particularly helpful when an elbow is needed to come directly off a welded branch line without a riser nipple in-between the branch pipe and the elbow. I don't know where "street" elbows get their name, but I like to think it comes from a dark and cloudy past of use in 1920's style gangster street battles.
Reducing Elbows & Tees
Elbows, like tees, come in reducing styles, where one opening is simply a different (reduced) size from the original opening. This is a very helpful and friendly feature with threaded fittings, as there are many different reducing elbow and reducing tee sizes that makes their use with branch lines easy.
Identifying Reducing Fittings
To label the size of a reducing tee or elbow, there's a specific order to the different openings.
A 1 x 1/2 reducing elbow, for instance, emphasizes that the primary opening is 1-inch and the smaller opening is 1/2-inch. While this terminology doesn't matter much for a traditional elbow that can be quickly spun around, it's more important for reducing street elbows and certainly for tees.
Reducing tees are labeled by their primary opening (opening A, above), then the opposite side (B, above), and then the last outlet in-between and perpendicular to the first two (opening C, above). If a branch line goes from 1-1/4" in diameter, to a 1" pipe, while serving a 1/2" threaded sprinkler at the intersection, then this reducing tee would be a 1-1/4" x 1" x 1/2" (A x B x C).
Crosses can be helpful when sprinklers split on either side of a continuous branch line. Crosses also offer a good reminder that just because a cross exists, doesn't mean it exists in the wide variety of combinations that could possibly be necessary.
Ductile iron crosses, for instance, are commonly in three sizes 2 x 2 x 1 x 1, 1-1/2 x 1-1/2 x 1 x 1, and 1-1/4 x 1-1/4 x 1 x 1. Cast iron are generally available in a wider variety, even offering sides C and D below in different sizes. It's important to caution, however, that just being available doesn't mean an item is commonly available as 'off the shelf'.
The naming convention for crosses is the primary side (largest, A above) x opposite side (B, above) x adjacent north side (C, above) x remaining south side (D, above). A cross that connects a 2-inch branch pipe to a 1-1/2-inch branch pipe while also splitting out to serve two 1" armovers would be a 2 x 1-1/2 x 1 x 1 fitting.
Riser Nipples to Avoid Crosses
One trick to avoid semi-custom crosses entirely is to consider using two tees at the intersection. Running a riser nipple from one line to another slightly above it can make use of more common reducing tees and give the designer some flexibility that crosses don't always offer.
Order of Threading
One other item to consider with crosses is the order of threading. It's important not just to select fittings that functionally work for a design, but that can physically be threaded in a sequence that can actually be accomplished in the field.
One classic situation fitters understand all too well that designers don't is the order of threading. Without a union, you can't have two risers connect into the same main drain with threaded fittings. Likewise, without a union, a gridded system can't only use threaded connections.
Why? It's all about the order of installation. Threads can only be accomplished in one circular direction (righty-tighty, lefty-loosey, right?). Because of this, threading one end will lock in pipe without the ability to then rotate the pipe on the other end.
Now introducing the union. Sent by the pipe gods, the union has a female threaded connection on both ends with a swivel disc (for lack of a better term) in-between, that allows rotation between the two female inlets. This swivel ability allows threading to occur on either side of the union without the opposite side needing to turn. As cited in the above examples, unions are used to make closed connected systems threadable.
The more basic counterpart to the union, the coupling connects two male segments by way of two female inlets.
If you ever get this mixed up and happen to order unions instead of couplings - don't worry - you'll get a call from someone in the field who will be 'happy' to straighten things out. At least that was the case for me the first and last time I accidentally swapped the two.
Also known as 'reducers' and 'bell reducers', reducing couplings connect two male threaded segments off different sizes. In the sprinkler industry these are far and away the most popular fitting used to connect a sprinkler to a sprig, drop, or armover.
The actual styles and look can vary, but in basic premise there's two different sized inlets with a hex or another flanged point to attach a wrench and turn the coupling relative to a pipe or sprinkler.
Less common in steel systems than in CPVC systems (where there's many less fitting options), bushings are similar to reducing couplings except that one side is male and the other is female.
One applications I've come across that's made good use of bushings is in a new installation installing upright sprinklers were a future ceiling will be provided. Since a minimum 1-inch outlet is required for sprinklers below a ceiling (NFPA 13-2106 126.96.36.199), the 1-inch outlet can be provided but installed with a bushing that can screw directly into the 1-inch outlet and still accommodate the 1/2-inch thread of a standard upright sprinkler.
Plug & Cap
One of the concepts that prompted this article was a discussion my wife and I had about the differences between plugs and caps (yes, I do think about this stuff all the time). In short, plugs have a male connection while caps have a female. They both generally serve the same purpose - to stop the flow of anything in the pipe network.
I don't come across caps in threaded systems much, primarily because of the availability of reducing fittings that size each component to its need. Caps are common for temporary drops in ceilings to close up a system while waiting for ceilings to be installed. Caps are also used when a branch pipe needs to be extended beyond the last sprinkler to catch a hanger.
Plugs are used quite a bit - at remote auxiliary drains that aren't piped to a discharge location, for three-way valves serving water gauges, or on tees connected to dry sprinklers.
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Joseph Meyer, PE, owns/operates his own Fire Protection Engineering practice in St. Louis, Missouri. See bio on About page.