The Design and Manufacture of Fire Hydrants

Design Project #1: Casting
Manufacturing Methods
Spring 2000
Dr. K. Mazouz
Due: March 3, 2000

By:
Elissa M. Wahlstrom & John J. Barrett

Special Thanks to:
Douglas B. Watson, Driver/Engineer
Palm Beach County Fire-Rescue
West Palm Beach, Florida

Florida Atlantic University
Department of Mechanical Engineering

Abstract

Sand casting is the most widely used casting process today. Although this is the most traditional casting process, new methods and improvements on existing technology are being developed. One such process is the process of lost foam casting. In the field of fire hydrant castings, sand casting is still the most widely used process, but the ease of the lost foam casting process is beginning to become popular. It is a trend that is being researched in many foundries and universities and is becoming a common option when considering casting processes. The scope of this paper encompasses a discussion of the process of lost foam casting, it's details, and it's advantages in the manufacture of fire hydrants.

Table of Contents

Abstract 1
Introduction 3
Casting Processes 4
     History of Lost Foam Casting 4
     Figure 1. Percentage of Aluminum Market 5
     Figure 2. Percentage of Ferrous Casting Market 5
Lost Foam Casting Process 5
     Benefits of Lost Foam Casting 6
Choice of Material 7
     Figure 3. Example of Aesthetic Hydrant 7
Design Considerations 8
     Figure 4. Isometric View (Dresser Model 300) 8
Manufacturing Considerations 9
    Figure 5. Cast Sections of Fire Hydrant 9
    Figure 6. View of Valve Mechanism Seat 9
    Figure 7. Zoom In of Valve Mechanism Guide 9
    Figure 8. Bottom View 10
    Figure 9. Threaded Outlets 10
    Figure 10. Interaction of Outlet and Hydrant 10
    Figure 11. View of Parting Line 11
     Figure 12. View of Sprue 11
Conclusions 11
References 13

Introduction

When designing a process to cast fire hydrants, several factors must be taken into consideration such as materials to be used, environmental factors, cost, etc. Once a general guide for specifications is obtained from the customer, an overall design process begins to take shape. This design process is then configured in a step by step procedure to guarantee that no oversights in regards to design are made. After the design process is set, manufacturing of the mold, and core if necessary, can begin. If the design process is done properly, it will naturally lead into a step by step procedure for the manufacturing process. During all steps of the manufacturing process the hydrant will be checked against the customer specifications to ensure a quality product is being produced. This quality control process encompasses the production of hydrants from the initial design stages to finished product.

Upon receipt of the customer specifications, the design process begins. Questions of how to incorporate these specifications into existing designs or create an entirely new design are posed at this point. For example, what kind of system is needed to attach the hydrant to the water supply, what is the pressure in the water lines connected to the hydrant, and what valves are needed in regards to the above questions. All of these questions must be answered in the preliminary stages of design.

After the initial information of the first stage of preliminary design is established and a suitable existing design is found, more detailed design can begin. This will incorporate considerations such as choice of material, tolerances of parts, and designation of a process. Included in these areas are the materials available for use, cost analysis, aesthetic needs, etc. All these choices are related as well. For example, the choice of casting method must take into consideration the material being cast. A process must be chosen that will maintain the integrity of the material. Each process must be weighed for advantages and disadvantages and compared to the overall concept of design.

When preliminary design is complete and materials and a casting process are fixed, the actual detail design can begin. General guidelines of specifications now become very specific including acceptable error or tolerance ranges. Calculations for oversizing, heat transfer, pouring temperature, pouring speed, etc. are made and applied to the overall design. Fortunately, with fire hydrants, as with most castings, there is already a similar design in place that can just be modified to fit the customer specifications. When this is the case, the design process can be considerably shortened when compared with having to design a new process from scratch, which can include, but is not limited to new mold design, making a sample, etc.

After the design process is finished and a set of procedures is established, the manufacturing process can begin. It should be noted however that the manufacturing process is designed for the hydrant in the design stage. Any adjustment to in-place procedures must be prior to production. The materials for the mold have already been selected and must now be assembled to finish the construction for the fire hydrant mold. Processes for pouring and solidifying must be confirmed and documented as well.

After the casting process has been completed all post casting processes such as machining, painting, attaching fittings, etc. must be documented and completed in order to provide the customer with a total quality product.

Casting Processes

In order to gain a better understanding of the manufacture of a fire hydrant, it is useful to know the theoretical background of casting. Casting is a process by which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity. This process is approximately 6000 years old, making it one of the oldest shaping processes. Since this process can be used to produce a wide variety of parts with respect to size and composition, it is appropriate to state that casting is one of the most versatile processes in manufacturing as well. One interesting aspect of castings is that they do not have directional properties. This means that the strength, ductility and toughness are equal in all directions.7

There exists a variety of casting processes and certain characteristics are incumbent on each. To name a few, we have:

·Sand Casting
·Shell Molding
·Vacuum Molding
·Centrifugal Casting
·Die Casting
·Lost Foam Casting

Sand casting is the most widely used and it one of the few processes that can be used for metals with high melting temperatures such as steel, nickel and titanium. The simplest process of sand casting includes pouring the molten metal into a sand mold usually made as a composition of silica, water and clay, allowing the metal to solidify and then breaking up the mold to remove the casting. Subsequent operations on the cast part usually include machining, cleaning, inspecting and possible heat treatment. Traditionally, this type of casting has been used in the manufacture of fire hydrants since their invention in the early-1800's.14

Following recent developments in casting processes, some companies have been straying away from the traditional sand casting and, instead, utilizing this new technology. The development at hand is called the lost foam process and it will be discussed

History of Lost Foam Casting

In 1958, H.F. Shroyer patented the use of foam patterns for metal casting. It consisted of a pattern machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring. In this process the polystyrene is evaporated when the molten metal is introduced to allow the formation of the cast object. This is not, however, lost foam casting, but full mold process. The only difference between the two processes is the use of bonded or unbonded sand for support. It was not until 1964 that the first lost foam casting was performed by M.C. Flemmings.16

The first public acclamation of the benefits from lost foam was through a General Motors project in 1993.6 By 1998, production by use of lost foam casting had reached approximately 140,00 tons in the United States alone. And in 2005 it is forecasted that lost foam casting will account for 29% of Aluminum and 14% of Ferrous Casting markets.6

Figure 1 Percentage of Aluminum Market

Figure 2 Percentage of Ferrous Casting Market

The Lost Foam Casting Process

There are several steps to this process and each is explained in detail below.16

1.Pre-expanded beads, usually polystyrene, are blown into a mold and therefore forming the pattern sections. These sections can then be glued together to form a cluster.
2.Once the beads are in place, the mold goes through a steam cycle that results in the interfusing of the beads due to their full expansion.
3.Now a barrier must be formed that separates the molten metal from the surrounding sand. To accomplish this the hydrant pattern of polystyrene is covered with ceramic coating. This coating does not only prevent sand erosion, but also protects the structural integrity of the product. Application of this coating can be applied through either dipping, spraying, or pouring.
4.For stability, the pattern must be supported in the flask by the surrounding sand. Due to the complexity of the geometry of a hydrant, or any other cast part, the sand might not easily reach into certain areas. The proper compaction of the unbonded sand around the pattern can be ensured by using a vibration table. Care must be taken to compact the sand without distorting the somewhat flexible coating.
5.The molten metal will then be poured directly into the foam pattern, decomposing the foam and therefore forming the cast product, or hydrant.
6.The sand is removed from the flask and the casting is cleaned and inspected before further assembling operations.

Benefits And Other Aspects of Lost Foam Casting

With the increased use of this type of casting of the last decade, it is evident that the benefits of lost foam casting are indeed significant. An explanation of these benefits is listed below.

Dimensional Accuracy and Tolerances.
As with other casting processes, the accuracy and tolerances will vary according to the size and complexity of the geometry of the part. However, the manufacturer can expect to achieve tolerances that equal, if not surpass, those of shell mold and permanent mold processes. The downside to Lost Foam is that warped, distorted, and defective castings can result without proper control.

Surface Finish.
The surface finish is significantly improved over traditional sand castings due to the thin protective coating surrounding the smooth foam pattern. With the existence of a smooth surface finish after casting, the machine and assembly requirements are substantially reduced or eliminated. The table below illustrates this benefit.

Environmental Aspects.
Solid waste and environmental emissions are significantly reduced. This benefit proves useful for not only for the environment, but also the manufacturer since solid waste is an increasingly burdensome capital cost, especially in green sand and no-bake foundries.

Range of Parts.
The versatility of this process is depicted by its ability to cast parts ranging from less than one pound up to thousands of pounds, although most cast products are less than fifty pounds.

Simplicity of Process Requirements. Lost Foam Casting does not include many restrictions when compared to the traditional methods such as gating and feeding. In addition, cores are eliminated and therefore complex casting designs are applicable and the wall thickness of the casting is controlled well. As a result, soundness is normally easier to achieve. This is why lost foam castings are used in many critical applications in automotive, heavy truck, construction, railroad, and marine applications.

Elimination of The Parting Line. With some traditional casting methods, a parting line is formed from the use of a cope and drag. With Lost Foam, the molten metal is poured into a fully-symmetric patten and therefore no line exists. With this elimination, an increased aesthetic value can be achieved as well as proper gating and riser placement.

Tooling. Tooling is typically 100% machined from aluminum billet. Since the quality of the casting can be no better than the foam pattern used to produce it, the importance of expert tooling is evident. The costs of tooling range from $30,000 to $150,000 with the average being approximately $75,000. Although the cost is high, tooling life is impressive - approximately 500,000 to 3,000,000 cycles depending on the tool complexity.

Choice of Material

A major consideration when given product specifications from a customer is the type of material that will be used to produce that product. Because often times the customer is not knowledgeable about the processes to make a specific part, it is up to the manufacturer and designer to be thorough in his/her analysis of the type(s) of material(s) to use. The material chosen must satisfy all the structural considerations, quality issues, cost issues, and on a smaller scale aesthetic needs. The materials must be considered to give optimal conditions at the lowest possible cost.

For a fire hydrant some structural considerations include internal pressure, vibration effects from the environment, and abrasive wear due to external and internal environments. These considerations directly relate to a need for high compressive strength, good vibrational damping, high impact strength, and good abrasive wear resistance. While many materials may have these properties, the material which has these properties at the lowest cost to manufacture is gray cast iron.5

Figure 3. Example of Aesthetic Hydrant

Gray cast iron (GCI) comes in classes, which are dependent on tensile strength. Depending on the class and alloy composition, tensile strength can range from 12 ksi for a low class to 40-66 ksi for a high class.2 Because of its wide range of uses GCI is very inexpensive when compared to alternatives such as brass or steel.

Another attractive benefit of GCI is the ease of machinability. With processes becoming more and more technologically advanced, the ease of reducing surface porosity and creating smoother surfaces with GCI is a reality. This is also very attractive for customers who are becoming more and more interested in the aesthetic value of fire hydrants. See figure 1.

It should be noted that GCI has its disadvantages too. While GCI has good tensile and compressive strength, it does not plastically deform. This implies that the loads the hydrant is supporting should be well under the ultimate stress. Due to the inability of GCI to plastically deform, it will fail during an automobile collision if stresses rise above the ultimate stress, meaning GCI absorbs no energy and therefore cracks nucleate and grow. However, GCI is still the most common choice for fire hydrants, but the use of ductile cast iron is becoming more popular so fire hydrants can withstand collision with an automobile without failing. GCI also has a very heavy relative weight, approximately 150 lbs for a typical fire hydrant. While this is good when the hydrant is in operation to counter balance the internal pressures, it can make installation and repair difficult due to its cumbersome form.

Another important consideration for fire hydrant materials is the material from which the fittings are made. (The fittings are the brass "threaded outlets.")8 It is pertinent that the fittings withstand corrosion and wear so that they can be used properly and easily by firefighters. Brass fills this need at a relatively low cost when compared to corrosion resistant alternatives such as titanium and cobalt5.

Design Considerations

The purpose of a fire hydrant is to serve as a valve between the underground water supply and a pump aboard fire department apparatus. Keeping this in mind, several design considerations can be formulated. Aside from the structural requirements of the metal, the geometry of the hydrant itself must enable sufficient use of its intended task. While there is quite a variety in the shapes and sizes of hydrants throughout the world, they all possess a few common characteristics. Listed below are some of these traits.

Hollow cylinder to serve as thoroughfare of water from underground water supply.

Connection(s) for fire department-issued hoses.

Internal valve that controls the flow of water.

Robust design that will resist effects of environment such as corrosion and pressure.

One of the several dozens of fire hydrants in use today is the one pictured here. It was manufactured by M & H Valve Co. in Anniston, Alabama during the 1970's who's parent company was Dresser Industries.13 Notice that the common characteristics of all hydrants are captured in this design. The number, type, and size of the connection will vary for each design. It is common to have either two or three connections, with either one or two different sizes. In this case, there are three connections (third one is out of view) that consist of two different sizes. The presence of various sizes and locations raises the versatility of the hydrant by enabling firefighters to maximize its use depending on the location of the department apparatus with respect to the hydrant and the flow rate required to combat the fire.

Figure 4 Isometric View (Dresser Model 300)

Notice some minor details such as the odd shaped nuts on the caps and top valve piece. It is an unusual pentagonal shape probably meant to reduce the possibility of unauthorized use. Also, notice the built in latches under each connection. These are present to prevent misplacement or theft of the caps when a chain is in place. The chain stretches from the hole to the groove in the cap.

Manufacturing Considerations

The first thing that must be done in the manufacturing of the hydrant is to form the polystyrene pattern of the various sections. In this case, a total of five different patterns must be created using methods that have been previously defined in this report. These sections are as listed:

main cylinder

(2) auxiliary caps

(1) main cap

cast iron portion of valve assembly

Figure 5 Cast sections of fire hydrant

Since the use of Lost Foam Casting eliminates the need for cores, the inside aspects of the hydrant must included in the pattern creation. These aspects are illustrated below.

The pictures directly below show the geometry of the seat of the valve mechanism at the top of the hydrant. It is essential that the dimensions of the inner shaft be accurate to prevent leakage. By using lost foam casting, these tight tolerances are easier to obtain than traditional methods. Also, notice that the screw holes do not penetrate through to the other side of the seat. This serves as another precaution to prevent leakage. The threads will have to be machined after casting.

Figure 6 View of valve mechanism seat

Figure 7 Zoom In of valve mechanism guide

Figure 8 Bottom View

Figures 6-8 illustrate some other aspects needed in the consideration of the foam pattern. Concerning Figure 6, the bolt holes need to be created accurately to ensure good connection to the pavement. Also, the internal cylinder and adjoining gates should be smooth upon manufacture to minimize flow resistance.

Figure 9 has been inserted to show that the brass threaded outlets are not part of the main cylinder upon casting. These brass pieces are usually manufactured by another company and

then joined with the hydrant through a spinning motion with the help of the knobs located on the inside diameter.

Figure 9 Threaded Outlets

When the patterns are formed they must then be coated in order to protect it from the sand that will surround it during casting. The coating also results in a smooth surface of the cast product. However, since the operation of a fire hydrant does not require an extremely smooth surface, the finish is not a major concern. Once that is complete, the rest of the steps defined in the "Lost Foam Casting Process" will be executed.

The necessary machining will then be performed such as the screw holes for the valve mechanism and then the separately cast brass outlets will be assembled with the cast iron hydrant. The picture to the right shows a zoom-in of the hydrant-brass outlet interaction.

Figure 10 Interaction of Outlet and Hydrant

Since production by way of lost foam is fairly new concerning hydrants, pictures are hard to obtain. However, by looking at hydrants that have been cast using traditional methods such as this one, we can compare results and visualize the difference. Take a look at Figure 10 and the arrow pointing to the indentation. This is a direct result of traditional sand casting. When an internal core is used, chaplets are also used to stabilize the core and ensure correct uniform wall thickness. After casting, these chaplets are machined down and left as part of the product. By using lost foam methods, internal cores are eliminated and therefore so are the indentations. For some, this adds to the aesthetic value.

Two obvious, and not so appealing, characteristics of traditionally cast objects is the parting line and remnants of sprue. The parting line is the line at which the two halves of a cast object are joined as shown in Figure 11. Since the Lost Foam Process is completed with a single symmetric section, the parting line does not exist. In traditional casting, the sprue acts as a gateway between the runner, or funnel, and the mold. It can be seen in Figure 12 that the result is not aesthetically appealing. With the use of Lost Foam Casting, the aesthetic value of the hydrant and any other cast product can be significantly increased.

Figure 12 View of Sprue

Figure 11 View of Parting Line

Conclusion

Lost foam casting is becoming a popular alternative when casting is the desired method of production. This casting method provides for good dimensional accuracy and tolerances and good versatility in range of application. For manufacturers wishing to reduce environmental impacts and solid waste costs, lost foam casting provides an extremely attractive alternative.

When considering the casting of fire hydrants, in particular, lost foam casting is slowly becoming popular. The concept of coreless casting makes lost foam an ideal choice for hydrants. This process also eliminates the parting line caused by the cope and drag. And even the cast irons used for hydrant casting can have a good surface finish with lost foam casting.

Gray cast iron is the material of choice of fire hydrants. Gray cast iron provides good compressive strength, high ultimate strength and good vibrational damping. A fire hydrant needs to be strong enough to withstand high stress applications. Gray cast iron fills this need. Other design considerations are size and number of fittings for hoses and a robust body that will resist the effects of corrosion and pressure.

All of the above design considerations give rise to the manufacturing considerations. In the example used, the main cylinder, the caps and the cast iron portion of the valve assembly must have polystyrene patterns made. Certain characteristics for a water-tight seal must be maintained. The lost foam casting process, with its good dimensional tolerances, helps to achieve this.

While still more expensive than traditional sand casting lost foam casting is becoming a viable alternative to many casting applications. It is still relatively new in the fire hydrant industry, but with all the benefits of lost foam casting, it has a definite place in manufacturing today.

References

References:
1. Anderson, Dave. Foundry personnel. M&H Valve and Fitting Co., Anniston, AL. Interview by telephone on 23 February 2000.
2. Askeland, Donald R. The Science and Engineering of Materials. Third edtion. Boston: PWS Publishing Company.333-415.
3. Dr. Askeland, Donald R., Ph.D., Professor, Distinguished Teaching Professor. Department of Metallurgical Engineering, University of Missouri-Rolla. Interview by telephone on 28 February 2000.
4. Dr. Bates, Charles, Ph.D., Case Institute of Technology, Research Professor of Materials Science and Engineering, Director, METALS Technology Laboratory, University of Alabama Birmingham. Interview by telephone on 28 February 2000.
5. Beeley, P R. Foundry Technology. Boston: Newnes-Butterworths, 1972. 1-431.
6. Norvil Foundry. Ballarat, Victoria, Australia, http://www.australianfoundries.com/norvil/next.htm . February 2000.
7. Groover, Mikell P. Fundamentals of Modern Manufacturing: Materials, Processes, and Systems. New York: John Wiley and Sons, Inc. 240-294.
8. "Lost Foam Casting." http://www.eng.uab.edu/mte/MtE%20Labs/Metals%20Technology%20Center/Lost%20Foam%20Casting.htm. 24 February 2000.
9. "Lost Foam Foundry." http://www.hopkinton.org/water/fireflows.htm. 23 February 2000.
10. Massey, Brian. Foundry Manager. Mueller Company, Albertville, AL. Interview by telephone 22, 25 February 2000.
11. "Most Asked Questions About Lost Foam Casting." http://consoltech.net/more2.htm. 24 February 2000.
12. "Moulding Processes." http://www.dossmann.de/english/bonform.htm. 24 February 2000.
13. Quist, Jim "hydrant information." E-mail to Elissa Wahlstrom (emwah76@hotmail.com). 26 February 2000.
14. Quist, Jim "Re[2]: hydrant information." E-mail to Elissa Wahlstrom (emwah76@hotmail.com). 03 March 2000.
15. Taylor, Howard F., Merton C. Flemings, John Wulff. Foundry Engineering, New York: John Wiley & Sons, Inc.,1959. 97-104.
16. "Welcome to the AFS Homepage at the University of Missouri-Rolla." http://www.umr.edu/~foundry/first.htm. 25 February 2000.

Photo References
Figure 3. http://www.firehydrant.org/pictures/i/0003.jpg
Location: Switzerland
Photo: Copyright ©1999, David Pedrocca
More Info: http://www.wagamet.ch/english/log50.htm

Cover Sheet. 0181
Model: Traffic 300
Date: 1975
Size: 4 1/2
Location: John Anderson's Private Collection
Photo: Copyright © 1999, John Anderson

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