Know a Fire Sprinkler, Like a Boss – Part 1


Expertise Includes:

    • Commercial Kitchen Fires
    • Fire Protection Systems
    • Industrial Process Hazards
    • Fire Suppression Systems
    • Scope of Damage/Cost of Repair
    • Fires & Explosions Analysis: Origin & Cause
    • Combustible Dust Explosions
    • Codes & Standards Analysis

In this blog we’ll take a look at the basic and some of the less-known parts of fire sprinklers, with more to come in a later post.

Here are the basic parts of the fire sprinkler, shown on a pendent glass bulb sprinkler and an upright solder element (“fusible link”) sprinkler:


Let’s take a closer look to learn about some of the less known parts, and also look at two types of sprinklers disassembled. Included in parentheses are some of the different names for some of these parts.

A wrench boss (wrench flat) is a flat face on near the bottom of the frame, above the threads, where the sprinkler wrench is placed when installing the sprinkler.

A kick spring (lodgement wire) is a wire-like spring intended to ensure the cap clears the waterway and frame when the sprinkler activates. It “kicks” the assembly out to the side. Not all sprinklers will have these.

Heat-Responsive Elements

Glass Bulb Sprinklers are similar across the board, with the main two two main variations being response time (standard or quick response) and temperature rating.


Glass bulb colors with their nominal temperature ratings. Image credit: Norbulb

These sprinkler gems are not for enhancing the décor; the color indicates the temperature rating of the bulb, which has to be specified based on the application. Don’t let their delicate appearance fool you; the minimum crush strength of a quick response bulb is commonly around 500 lbs. Standard response bulbs have a higher minimum crush strength which can approach 1000 lbs. As incredibly strong as glass bulbs are, the fusible link type of operating element is even more robust.

Fusible Link Sprinklers

In today’s fusible link sprinklers, there are different styles, as seen in the figure below.

On some of the classic fusible link sprinkler models, the linkage extended a good bit outside of the frame. The more compact design of modern fusible elements makes them less susceptible to mechanical damage than their predecessors.

Photo: This Viking Model C from the author’s collection is a link and lever style sprinkler where the linkage is well past the frame.

Sprinklers in the oven

How do you get a sprinkler to come apart so you can look at the pieces? Subject it to heat equal to or greater than its operating temperature, of course. What simpler way to do that than to put them in the oven? Unfortunately, this treatment will destroy the heat responsive element, but it does give us a look at what the sprinkler would look like after an activation. Side note:  the deflector and load screw (compression screw) are assembled with threadlocking compound, which fixes the level of compression on the heat-responsive element. This also discourages attempts at mechanical disassembly, tampering or adjustment.

Glass bulb type

Left: an intact, identical sprinkler. Right: after baking, the frame and deflector remain; the dislodged parts are below it.

Looking through the fragments of the glass bulb, a piece with partial numbers identical to an intact sprinkler could be picked out.

Fusible link type

Left: an intact, identical sprinkler. Right: after baking, the frame and deflector remain; the dislodged parts are below it.

Did you happen note the Belleville washers among our baked sprinklers’ parts? A Belleville washer, also known as a disc spring or Belleville spring, functions more as a spring and less as a washer. The Belleville washer resembles a flat washer, but it has a cupped, or cut-off cone, shape. This shape enables the Belleville washer to absorb high loads, acting as a spring. The spring action also means that it can absorb vibration. In the sprinkler application, it maintains the tension which holds the heat-responsive element of the sprinkler in place.

The heat-responsive elements of a sprinkler are designed to reliably hold back water at up to 175 psi (typically) maximum allowable working pressure (MAWP) for years of service. The sealing assembly is squeezed between the water pressure and the heat-sensitive parts up to the load screw. The Belleville washers in our two baked sprinklers are both made of a nickel alloy, an advanced material which imparts the needed strength, corrosion resistance, and resistance to stress relaxation.

As part of the sealing assembly that keeps the water in the pipe, these Belleville washers are coated, commonly with Teflon.

I hope that taking a closer look at some of the parts of sprinklers enhances your appreciation of the design and intricacies of these overlooked and not well-understood fire sentries. But wait, there’s more! Join me next time and we’ll take a closer look at k-factors and deflectors.

Amy Anderson, PE, CFEI, has a Bachelor of Science in Chemical Engineering from Clemson University and is a licensed Professional Engineer in Fire Protection, as well as a Certified Fire and Explosion Investigator. Amy has over 20 years of engineering experience including property loss prevention engineering specializing in fire protection, chemical and pharmaceutical facilities. She has partnered with clients to identify, assess, avoid, and reduce risk at their commercial and industrial properties. Additionally, she has assisted with the development of building and fire protection specifications, reviewed plans and performed site visits. She has reviewed project documents for compliance with applicable standards – construction, fire protection, process, and combustible dust hazards. Amy is a member of the National Association of Fire Investigators, the Society of Fire Protection Engineers, the National Fire Protection Association and the American Institute of Chemical Engineers.

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