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.

Elbows
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.

Long-Turns
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
"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
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'. 

Identifying Crosses 
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. 

Unions
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.

Couplings
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.

Reducing Couplings
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.

Bushings
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 8.15.20.1), 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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I've never trimmed a dry valve nor have I tested or drained a system myself. I'll also admit that for longer than I should have I never investigated the details of a dry valve.

Hopefully with today's post if you've been in that same boat there's enough to better understand the basic components of a dry pipe valve.

How Dry Pipe Valves Function
1. The valve is in the closed position (clapper inside the valve is horizontal) and air pressure keeps the clapper closed.
2. A sprinkler activates, releasing air pressure at the sprinkler.
3. Air pressure within the dry pipe system begins to reduce.
4. After sufficient air pressure is reduced, the water pressure below the clapper becomes greater than the pressure above the clapper, and the valve opens upward.
5. Water flows through the opening in the valve, past the clapper, and into the system.
6. After entering through the valve, the water gravitates towards the opening in the system where the air pressure is being released, and discharges through the sprinkler at the fire interface.

Clapper Assembly
The clapper is the interface between the air and the water within a dry pipe valve.

It's purpose is to remain closed when air pressure is sufficient in the system, and open when the air pressure falls.

The surface area of the clapper is disproportionately larger on the air side than the water side - which is intentional. To hold the clapper closed, the force on the clapper must be greater on the air side than the water side. Since force is pressure x area, a larger surface area on the air side means the air can be kept at a much lower pressure and still keep the clapper assembly closed - often to a ratio of 4:1 or 5:1 (allowing 40 psi of air to 120 psi of water pressure, for instance). 

External Reset
The External Reset Knob shown in this model is used in lieu of priming water, which otherwise sits on top of the clapper and helps distribute pressure across the top interface of the clapper.

The Knob incorporated in this dry valve is depressed to "unlatch" an open clapper, allowing it to reset back into its normal, closed position. It is a convenient function where a user doesn't have to open the face of the valve in order to reset it.

Low Body Drain Valve
The low-body drain valve allows the air-side of the valve to be drained completely with the clapper closed. Since the main drain valve is below the clapper, the clapper must be opened for the main drain to be used to drain the system.

Main Drain & Intermediate Chamber
The main drain is used to drain water from the system, and is located below the clapper assembly.

For dry valves, some models permit the main drain to be used to test the waterflow pressure switch without opening the clapper.

In other models, there's an intermediate chamber between the water and air side which is normally dry. When water enters the intermediate chamber after the valve opens, the waterflow pressure switch senses water pressure and activates. In this configuration, there's usually a ball valve that can be used to test the waterflow pressure switch and fill the intermediate chamber without opening the dry valve.

Pressure Gauges
The water supply pressure gauge and the system air pressure gauge are included to monitor the incoming water supply pressure and the system air pressure. These gauges are usually attached to a gauge test valve with a plug that permits removal and cleaning of the gauge orifice.

​Pressure Switches
There are typically two pressure switches on the dry pipe valve assembly. The first is a pressure switch which monitors the air supply. When the air supply on the system drops to a pre-determined level, this pressure switch will typically send a supervisory signal to the fire alarm control panel, allowing an early warning that the dry valve is about to open and flood the system.

The second pressure switch is included to activate when there is water in the intermediate chamber or is flooding the system. This is typically an alarm signal that also activates fire alarm notification in a building.

Supply Shut-Off Valve
This valve is typically separate from the dry pipe valve, but allows the system to be shut off after a fire has been sufficiently suppressed. It also can be closed when a system needs to be isolated, such as for a modification, repair or remodel.

Other Items
Outside of the dry valve assembly, an air maintenance device allows the incoming air to be regulated to a preset air pressure and for that pressure to be maintained in the system.

In order to supply the air, or in many cases now nitrogen, an air compressor or nitrogen generator are provided to supply pressured air or nitrogen into the system.

Summary
While this valve is only one model, many of the components between dry valves are designed similarly. For further reading and detail on dry valves, I'd recommend reading NFSA's Layout, Detail and Calculation of Fire Sprinkler Systems (2nd Edition by Kenneth E. Isman, P.E.) and any product data for a dry valve you're reviewing, installing, or specifying.

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When conducting or reviewing hydraulic calculations, I very often face scenarios where the initial (very first) hydraulic demand exceeds the potential for the water supply.

At that point I lose all hope and add a fire pump to the job.

Just kidding, of course - there's at least a half dozen hydraulic elements I analyze and refine to better match the capabilities of the water supply to the design of the sprinkler system.

Refining Hydraulic Calculations with K-Factors
One of the more fine-tooth aspects I look at is the k-factor used on the sprinklers.

The k-factor for a fire sprinkler is the discharge coefficient, or in normal human terms just relates to the amount of water that is permitted through the sprinkler.

The k-factor is dependent upon the orifice diameter of the sprinkler - a low k-factor (such as K2.8) restricts the flow of water, while a larger k-factor (such as K22.4, K25.2, or K28.0) permit much more water to flow through. K-factors were originally created to be multiples of the discharge of a K5.6 sprinkler. A K2.8 sprinkler, for example, is 50% discharge of a K5.6 sprinkler, while a K11.2 sprinkler is 200% of the discharge of a K5.6. NFPA 13-2016 Table 6.2.3.1 shows this well.

Use In Design
We find K5.6 sprinklers in light hazard all the time. Residential sprinklers often have k-factors less than 5.6. ESFR and CMSA require minimum K11.2 (NFPA 13-2016 6.2.3.5). ESFR are tied directly to the hazard it protects.

Back to refining the hydraulics in a system - increasing the k-factor of a sprinkler allows more water to flow through a sprinkler with less pressure loss. This becomes very important when trying to reduce pressure loss in a system.
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Light Hazard Example
A light hazard system (0.10 gpm/sqft) with widely spaced sprinklers (at 225 sqft each) would require a minimum flow through each sprinkler of 22.5 gpm (0.10 gpm/sqft x 225 sqft = 22.5 gpm).

In order to flow 22.5 gpm, a sprinkler with a k-factor of 5.6 now requires 16.1 psi to do so (Q=k√p, or rearranged, p=(Q/k)^2). This is 9.1 psi higher than 7 psi, or the minimum that NFPA 13 requires.

In order to flow 22.5 gpm, a sprinkler with k-factor of 8.0 only requires 7.9 psi to do so, or less than 1 psi more than the minimum NFPA 13 requires.

In this scenario, flowing the same amount of water (22.5 gpm) results in a 8.2 psi difference in the pressure required at the most remote sprinkler. Can 8.2 psi be important? Absolutely! 

Other Scenarios
Similarly, consider Ordinary Hazard Group 1 (0.15 gpm/sqft) and Ordinary Hazard Group 2 (0.20 gpm/sqft) systems.

For Ordinary Hazard Group 1 and sprinklers spaced at 130 sqft, a K8.0 sprinkler requires 5.1 psi less than a K5.6 sprinkler (7.0 psi vs 12.1 psi). 

This same methodology applies to extended coverage sprinkler requirements, specific densities for traditional storage design, and more.

The K-Factor Selector
Want to quickly compare fire sprinkler k-factors across different design densities and sprinkler spacing? Easy. Here's the calculator I've created that quickly compares pressure requirements and flow rates across different sprinkler k-factors.
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Toolkit Update
Want all these tools in a downloadable, printable & PDF-saving capability? Great! The MeyerFire Toolkit will include this tool as well. You can download and try it out now through September for free.

Sprinkler Database
Other than the Toolkit, users of the comprehensive Fire Sprinkler Database can sort & search among k-factors as one of the parameters when comparing sprinklers.

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Quick updates for this week - thanks to helpful recent suggestions I've updated the Friction Loss Calculator to include several NFPA 13-provided fittings as well as fluid velocity among various sizes.

With a flow rate and length of pipe, you'll see fluid velocity as it's own column in feet/second. While both NFPA 13 and FM Global do not have any limitations on fluid velocity, it's a good point of reference to reference how quickly a fluid is moving through the pipe.

The brief list of fittings at the bottom allow you to compare friction loss across a number of different assembly sizes, so you can compare runs of pipe or assemblies at various sizes all at the same time.

​Can't see the tool above? See it here:

If you don't already get updates on our latest tools and weekly articles, you can get them for free here:
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