Fire extinguisher

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Contents

Executive Summary

The function of the following information is to provide details and specifications for the existing fire extinguisher. However, there are some shortcomings that are becoming more apparent in today's society. Many times a kitchen fire will develop and get out of control before anyone can notice, and by then it is too late. The number one cause of elderly patients requiring medical care for moderate to severe burns is grease fires. We plan to redesign this existing model, incorporating technology to make it an autonomous system. Our redesigned product will sense when temperatures indicative of hazardous conditions occur and automatically discharge to neutralize the threat.

Through testing and research, we have determined that the following changes would improve the effectiveness and ease of use of the standard fire extinguisher:

1) Nozzle angled down to help aim at the base of fire
2) Rectangular nozzle to achieve flat, spread out flow
3) Gun grip style handle to improve ergonomics
4) Ratcheting handle to ensure that the contents are completely discharged
5) LED indicator to ensure that user recognizes a discharged unit
6) Larger instructions with more pictures to ensure that user correctly operates extinguisher

Operation: How it Works

The basic principle behind a fire extinguisher is that there is chemical stored inside of a canister under high pressure, and is released by a human operator to cover and extinguish a fire.

The canister is filled with a combination of dry chemical (the extinguishing agent) and high pressure gas. Naturally, these substances want to expand by escaping the canister by traveling up the hose and through the nozzle. To regulate the release of chemical and gas, the system is closed by a small metallic pin, held in place by a compression spring. When in the seated position, the orifice above the hose is blocked and the contents of the extinguisher are contained.

Mounting Bracket The fire extinguisher comes equipped with a mounting bracket so that you may hang it on a wall or any other vertical surface. This plastic bracket requires only screws appropriate for the particular wall to install. The user must place the bracket on the wall with the semi-circle holding location on top. Screws must then be inserted in the provided holes and driven into the wall surface. The fire extinguisher can then be mounted by lowering the neck portion of the canister onto the semi-circle holding protrusion. To remove, the user must simply lift and pull the device towards them.

Safety Pin The safety pin works by providing interference between the handle and the pin that holds the gas in the canister. While the safety pin is in place, the handle cannot be depressed.

Handle To move the pin upward and release the contents, the user must depress the handle. The handle operates as a simple lever, pivoting on a point, giving the user a mechanical advantage, enabling operation with a minimal use of force. Depressing the handle applies a direct downward force to the pin, moving it away from the narrow orifice and allowing the contents to escape.


Diffuser Nozzle Once past the pin, the gas and chemical turn 90 degrees to travel horizontal and emerge from the nozzle. The nozzle is shaped like a diffuser on the inside. This feature serves to contract the flow, creating pressure, and then allowing for a rapid expansion that disperses the chemical outward, covering a certain area.

Pressure Gauge The amount of chemical and gas inside the canister is monitored by a pressure gauge located adjacent to the pin. When under pressure, gas travels through a small opening in the flow channel just below the pin that leads to the gauge. The gas produces a horizontal force on a small piece of metal, connected to a coil spring. The expansion of the spring is translated into a rotation in the gauge needle, indicating full. Once the extinguisher has been discharged, there is no significant pressure applied to the spring, allowing the needle to move to an area on the gauge indicating empty.

Customer Needs

Sodium bicarbonate (the extinguishing agent) can be mildly corrosive if the extinguisher is discharged onto a surface that is wet. Just as one would imagine, simple scrubbing can be employed to clean the affected area before the corrosion process beings. Additionally, the substance is only toxic after prolonged exposure.

The product should be:

  • capable of extinguishing a moderately sized fire. Large scale fires will require the assistance of a fire department, but small and moderate fires can be controlled with a household fire extinguisher.
  • capable of functioning perfectly upon use. Reliability is paramount in the operation of the extinguisher. It only needs to function once, but it must function exactly as designed.
  • capable of functioning without putting the consumer in additional danger. Releasing pressurized material may pose hazards to the consumer, so these hazards should be minimized. Recoil could very adversely effect the consumer, as would explosion of the canister.
  • aesthetically pleasing. These units are should be placed in easily accessible areas. This may include placement in plain view areas. For this reason, the product should be as pleasing to the eye as possible.
  • designed to high ergonomic standards. It should be easy to grip, maneuver and control.
  • easy for elderly and young to operate. The operator spectrum is very wide for this product. It should be designed with those who may have difficulty using the product in mind.
  • contain chemicals that are easy to be cleaned and safe for the user to be around. Inhalation of the chemical should not require medical aid, and the chemicals should not be harmful to the environment.

Use of Product

The following flowchart depicts a typical design process surrounding the operation of the product.

Parts List

In order to gain a complete understanding regarding the functionality and operation of the fire extinguisher, the product must be dissected and each part analyzed. The following table reviews the results of dissection performed September 14.

Part # Part name QTY Weight (g) Function Material Manufacturing Process Image
01 Mounting Bracket 1 32 Clasps canister, attaches to wall, straps around midsection of canister Plastic Injection Mold
02 Safety Pin 1 3 Prevents accidental discharge Pastic Injection Mold
03 Canister 1 291.5 Contains extinguishing agent Aluminum Deep Drawn
04 Spring 1 3 Holds discharge valve closed Plastic Injection Mold
05 Hose 1 16 Channels extinguising agent from bottom of canister Plastic Extrusion
06 Valve Pin 1 3 Blocks flow until displaced Plastic Injection Mold
07 Top Assembly Gasket 1 >.5 Creates seal between top assembly and canister Rubber Injection Mold
08 Hose-Top Assembly Connector 1 2 Connects hose to top assembly Plastic Injection Mold
09 Valve Pin Top Gasket 1 >.5 Creates a seal on valve Rubber Injection Mold
10 Valve Pin Bottom Gasket 1 >.5 Created a seal on valve Rubber Injection Mold
11 Diffusing Nozzle 1 2 Changes diameter of extinguishing agent flow; This is a separate part and is snapped on during manufacture Plastic Injection Mold
12 Extinguising Agent 1 1134 Extinguishes fire Sodium Bicarbonate Chemistry Lab
13 Top Assembly 1 Weight Provides handle for gripping and discharging, holds pressure gauge, provides discharge path from canister to diffuser nozzle; This is comprised of two independent parts - a single injection molded plastic piece and the included pressure gauge Plastic Injection Mold
14 Pressure Gauge 1 Weight Provides user with information about pressure of canister - "Empty" or "Full" (Assembly - Platic, Brass, Aluminum) Varies

DFX (Design for ‘X’)

The initial design step involved a product dissection and in-depth analysis to determine how the product was designed as it pertains to manufacture, assembly, usability, and the environment.


Design for Manufacture

Manufacturing processes were analyzed to determine cost and time necessary to produce the product.

Mounting Bracket: The mounting bracket is made of a single piece of injection molded plastic. Since it is not critical for this part to be made of a high level of precision, this method of manufacture is the cheapest and quickest. The part must clasp the neck of the canister and strap around the midsection to provide support when mounted on a wall or other horizontal surface.

Safety Pin: The safety pin is a single piece of injection-molded plastic. The pin serves to prevent the fire extinguisher from being accidentally discharged. This plastic need only be strong enough to withstand the force exerted on it by a human hand squeezing the handle.

Canister: The main canister is manufactured by deep drawing a piece of aluminum. A sheet of aluminum is forced through increasingly smaller holes until it resembles a cylindrical canister. The top portion is then squeezed inwards to form the neck. The formed canister resembles a fuel bottle and is rated for a pressure of up to 300psi. This form of manufacture is the most widely used method across the industry and is the most cost and time effective way to produce a reliable pressure vessel.

Spring: The spring is manufactured by drawing stainless steel into a wire and coiling it to form a spring shape. This process allows for quick manufacture when in mass production. As the wire is drawn, it is wrapped and therefore the process is continuous.

Hose: The hose is made of a translucent plastic that is extruded into a hollow cylinder. The walls of the hose are thin, making the part cheap and easy to produce. This is effective because there are no net forces acting on the hose as the pressure in the canister is uniform.

Valve Pin: The valve pin is one of the more complicated parts to manufacture in the fire extinguisher assembly. Using a hot work method, the pin is first extruded to the diameter of the lower half. Then, the top is pressed inward to form the cone shaped section. In an automated setup, the notch details are cut out with the same method as would be used on a lathe. Production of this part is costly. A less expensive method would be to cast the part using destroyable molds. This method is likely not used due to the necessary precision of the part.

Top Assembly Gasket: The gasket is manufactured by injection molding a ring like shape from a piece of rubber. This process can be automated to enable inexpensive mass production.

Hose-Top Assembly Connector: To connect the hose to the top assembly, a separate piece is required. This piece is made of a translucent plastic by the method of injection molding. This piece is then placed on the hose and crimped into a permanent position.

Valve Pin Top & Bottom Gasket: Made via injection molding a rubber material into a ring. Creates a good seal between the valve and orifice.

Diffusing Nozzle: Alters flow of outgoing chemical by narrowing the cross sectional area. This device also disperses the flow outward. The nozzle is 7/16in. diameter and is then contracted down to 1/8in. to pressurize and disperse the flow. This part is made of a hard plastic via injection molding.


Design for Assembly

Design considerations affect the assembly process, determining the associated costs and time.

In general, the fire extinguisher was designed very well for assembly. There are very few parts, and the methods used to put them together don’t get any more complicated than screwing a piece into place.

One suggestion to simplify the assembly process would be to make the nozzle and top assemble out of one piece. This would eliminate then need to press the nozzle on.

On a similar thought of simplification, the hose and hose-top assembly connector could also have been made of a single piece. The current design did not allow this to happen because of differences in the method required to manufacture the two parts. A redesign to the hose-top assembly connector making the part simpler would allow for a one-piece design.


Design For Usability

Design decisions impact how easy a product is to use by the consumer.

The simple design of the handle allows for ease of operation by the user. Without reading the instructions, the average user will instinctively be able to operate the extinguisher. The fire creates a sense of panic, to which most people will react by tensing up, or in the case of the fire extinguisher, squeeze.

Prior to this step, the user must locate and pull the safety pin. For this reason, the pin is a bright red, making it stand out from the rest of the assembly. This choice of color enables the user to quickly locate the pin and remove it.

Since most users will not be aware of the weight difference between a full and empty extinguisher, the product is equipped with a gauge, telling you if the device is full or empty. The gauge is clearly labeled, with green markings for full (the go equivalent color) and red for empty.

Instructions are clearly outlined on the canister, to provide the user with a step-by-step guide to use the product. A potential improvement would be to make the drawings larger for ease of visibility.

Another part that our team flagged for improvement is the handle. The current design requires approximately 12lbs of force to depress the handle and discharge the contents. By increasing the length of the handle, the force required to operate would be reduced, making the product more accessible to individuals with below average hand strength.


Design for Safety

Safety of the user must be considered during the design process. For our product, user safety should be addressed by analyzing material strengths and probabilities of failure.

One major concern is the structural integrity of the canister, which is rated to 300psi. Since the assumed pressure of the tank is roughly 40% of this, the likely hood of a material failure leading to an explosion is extremely low. Increasing the safety factor would be costly and unnecessary.

The red safety pin adequately prevents accidental discharge. The pin is designed well because it is secure enough that it serves its purpose, but is not too difficult to remove to operate the device.

While the instructions tell you how to operate the device, some directions arrows on the actual parts might be useful to ensure proper device operation. For example, an arrow pointing in the direction that the device is intended to point would be useful.

Design for Environment: EIO-LCA

When looking at the economic impact of this product, a few concerns come immediately to mind:

  1. Which sectors release most of the greenhouse gases and air pollutants associated with producing and using your product?
  2. Which product phase is most responsible for greenhouse gas emissions: Production, transportation, or use of your product?
  3. Which industries are responsible for the largest mass of total toxic releases associated with producing and using your product?

This product is composed of three different materials. Metals, plastics, and the chemical compressed inside. However, in order to gain the most appropriate estimate, we considered the manufacturing process of liquid oxygen tanks. This isn't a perfect estimate, but is the closest approximation to a fire extinguisher. Liquid oxygen tanks fall under the sector of Metal tank, heavy gauge, manufacturing, and we considered an economic activity of $ 1 Million Dollars. Using the EIO-LCA software provided at www.eiolca.net we were able to perform an analysis on the manufacturing sector and come to the following conclusions.

Since we only looked at one sector: Metal tank, heavy gauge, and manufacturing, will be only sector considered for the above questions.

Product phase which release most Greenhouse gas
Power Generation and Supply
Product Phase which releases most air pollution
Truck Transportation
Industries which release largest mass of total toxins
Copper, Nickel, Lead, and Zinc Mining


However, we must be able to recognize that these are based on a blanket calculations for all parts in the specified field (i.e. The sodium bicarbonate used as the dry chemical is a minor part of the entire inorganic chemical manufacturing process and was therefore left out of the analysis). Consequently, our conclusions cannot be made with certainty and this must be taken into account

In order to get a more accurate prediction of the environmental impact, a much more detailed analysis must be performed. Nonetheless, we still come away with a better idea of what areas of the environment the life cycle of this product affects.

When taking a deeper look at the various parts of the product for redesign, the first thought that comes to mind is what happens with the chemical after it has been discharged? Is it harmful to people? Does it hurt the environment?

  • The powder falls to the ground, not staying in the air.

This greatly reduces the contribution to air pollution, as well as CO2 emissions, but creates a new issue of clean up. In order to clean the chemical up, especially indoors, paper, cleaning agents, and water will have to be used, probably in great amounts. Ruined possession might have to be discarded. Either way, this will increase trash production and need of disposal. But we believe that this contribution is not too great. In addition, the chemical must be designed so that it does not have harmful effects upon human contact and require medical treatment.

More of a concern comes from the canister itself. The current design is a one-time usage, non-refillable canister that demands to be discarded immediately after use. We plan to redesign the canister so that it can be recharged, thus reducing this burden on the environment.

As a final note, the metal and plastic pieces that compose the top assembly are all capable of being recycled and reused. Energy will be necessary to transport the pieces and recycle them, but the overall gain wins out.

Failure Mode and Effects Analysis

There are many possible ways for the fire extinguisher to fail. Most failures could leave the extinguisher completely inoperable. The biggest issues regard the structural integrity of the aluminum canister or problems with the plastic top triggering mechanism. Most failures would result from misuse or misinstallation of the extinguisher.


Below is a list of these failures and the ways in which they affect the overall performance of the product.


Item & Function Failure Mode Effects of Failure S Causes of Failure O Design Controls D RPN Recmd Actions Responsibility & Deadline Actions Taken S* O* D* RPN*
Mounting Bracket Extinguisher falls off bracket Compromises structural integrity of the canister and triggering mechanism. 3 Canister not secured to bracket 5 Secure canister to bracket 5 75 Add strap to make sure canister is secured to bracket Design Engineers - 5 5 5 125
Bracket falls off wall Compromises structural integrity of the canister and triggering mechanism 5 Bracket not screwed securely to wall 5 Test mounting bracket on different types of walls 3 75 Treat motor axis to add strength Design Engineers - 5 5 3 75
Pressure Gauge May not display proper reading. Empty extinguisher could read as full. 10 Pressure from canister not correctly transmitted to gauge. 2 Test gauges by slowly discharging canister 3 50 Increase space around the pressure gauge in order to ensure that the gauge correctly reads the pressure in the canister Design Engineers - 10 2 3 50
Safety Pin Pin breaks and cannot be removed If the pin cannot be removed, the extinguisher will not discharge 10 Pin is thin, may snap if not pulled out straight 3 Test strength of pin 2 50 Make pin thicker. Use stronger plastic. Pin Manufacturer - 10 3 2 50
Top Assembly
  • Contains the triggering mechanism for the extinguisher
Top handle breaks Cannot depress pin to trigger the extinguisher 10 Extinguisher falls or is hit by an object 1 Test durability of top handle 5 50 Increase thickness of plastic. Use Stronger Plastic Design Engineers - 10 1 5 50
Bottom Handle breaks Uncomfortable to hold/use 3 Extinguisher falls or is hit by an object 1 Test durability of bottom handle 5 15 Increase thickness of plastic. Use Stronger Plastic Design Engineers - 3 1 5 15


Valve Pin Does not seal properly Could result in slow loss of pressure from the canister, leaving the extinguisher useless 10 Rubber gaskets may not fit on the pin correctly resulting in an ineffective seal 3 Test seals performance at large range of pressure 2 60 Find a better way to secure gasket to pin Design Engineers - 10 3 2 60


Canister Over pressurized Could result in unexpected rapid decompression, rendering the extinguisher useless and possibly exploding 10 Canister is over pressurized 3 Test canister to see its maximum pressure 2 60 Increase safety factor by increasing the maximum pressure rating of the canister Design Engineers - 10 3 2 60


Quantitative Analysis

Force Required to Dispense Dry Chemical

The fire extinguisher canister is rated to 300 psi. While it is reasonable to assume that the actual pressure in the canister is far below the maximum value, a precise pressure must be known in order to perform an accurate quantitative analysis. Larger models of fire extinguishers made by the same company are typically filled to 40% of the maximum pressure (a factor of safety of 2.5); we will assume this safety factor is applicable to all of the company's extinguishers. Therefore, the internal pressure of the canister is 120 psi.

As is shown in Figure 1, depression of the top assembly pushes the valve pin down and allows the sodium bicarbonate to flow out of the nozzle. Two forces contribute to holding the valve pin in place (F2 in Figure 2): the force of the spring and the internal pressure from the dry chemical.

Figure 1. Top assembly dimensions.
Figure 1. Top assembly dimensions.
Figure 2. Top assembly free body diagram.
Figure 2. Top assembly free body diagram.

Dry Chemical Contribution

Let's first look at the contribution to F2 from the dry chemical.

P = F / A

where:

P = internal pressure = 120 psi
A = cross sectional area of valve pin = 0.2485 in2
F = total force of the dry chemical on the pin

The force from the dry chemical is 29.82 lbs.

Spring Contribution

The spring's contribution to F2 was determined by hanging a known weight from it and determining the spring constant, K.

F = KX

where:

F = known weight = 3.4375 lb
X = displacement due to force = 0.5 in
K = spring constant

The spring constant is 6.875 lb/in.

Inspection of Figure 1 reveals that the valve pin needs to be depressed only 7/32 of an inch for the fire extinguisher to discharge. This translates to a force of 1.50 lbs from the spring, which is less than 5% of the dry chemical's force on the valve pin. We concluded that the spring's main purpose was to keep the valve pin in place after the extinguisher had been used or emptied. Its contribution was neglected in further calculations.

A simple moment calculation was performed to find F1 in Figure 1. The following equation was used:

∑M = ∑FD = 0

where:

M = sum of moments about a fixed point
F = force acting at some angle to the fixed point
D = the moment arm of the force

Summing moments around Freaction, F1 was found to be 12.56 lbs. This means that the fire extinguisher requires 12.56 lbs of force to dispense the dry chemical. Applying 13 pounds of force to the handle is reasonable for the majority of people that use the fire extinguisher. However, for certain users, such as the elderly and those with physical disabilities, squeezing 13 pounds may not be attainable. This design flaw will be addressed in the redesign of the fire extinguisher.

Velocity of Dry Chemical Exiting Extinguisher

The manufacture reports that the weight of the dry chemical varies from 2.33 lb to 2.77 lb and that it may take anywhere from 8 to 12 seconds to dispense the entire contents of the extinguisher. The maximum and minimum possible mass flow rate of the dry chemical was found using the following equation.

{\dot m} = F / T

where:

{\dot m} = mass flow rate
F = weight of dry chemical = 2.33 lb to 2.77 lb
T = time to dispense = 8 s to 12 s

The maximum and minimum mass flow rates are 0.346 lb/s and 0.194 lb/s, respectively. The velocity of the chemical can be found using the following equation:


{\dot m} = {\rho \cdot V\cdot A}

where:

{\dot m} = mass flow rate = 0.194 lb/s to 0.346 lb/s
ρ = density of sodium bicarbonate = 0.078 lb/in3
V = exit velocity
A = cross sectional area of the nozzle = 0.155 in2

The maximum and minimum exit velocities are 29.5 in/s and 16.54 in/s, respectively. The variability of the exit velocity may impact the effectiveness of the fire extinguisher. For example, if the chemical was traveling too fast, the dry chemical may displace the burning objects and spread the fire over a larger area. Conversely, a low exit velocity may force the user to get dangerously close to the fire in order to extinguish it.

Links

Further information detailing improvements to the existing model can be found at Fire extinguisher redesign

Acknowledgments

Prepared for: 24-441 Engineering Design Course (Fall 2007), Carnegie Mellon University


By Michael Rem, Shane McGuire, Cihan Kadipasaoglu, Adam Haag, Craig Cramer

Retrieved from "http://ddl.me.cmu.edu/ddwiki/index.php/Fire_extinguisher"