Miniature Ball Valves: Plastic, Brass or Stainless Steel?

Some guidelines for selecting the right material for your application

This is the first in a new series of ISM blog posts about miniature valves and ball valves in particular.

Why use a ball valve?
Ball valves are durable, reliable and securely shut off flow even after sitting idle for long periods of time. Because of this, they are frequently used as fluid and gas flow cut-off valves. Ball valves are also easy to operate. This plus their availability in a range of useful material options has led to their widespread use.

Plastic vs metal ball valve bodies
Engineered plastics provide a useful alternative to metal ball valves. Plastics allow valves to meet specification requirements that metal cannot. Metal ball valve bodies also have advantages that plastics cannot match. The right choice of ball valve body material, whether metal or plastic, starts with knowing the design requirements, particular industry regulations, pressure ratings and fluid compatibility.

Why plastic ball valves?

• Lower cost
• Lighter weight
• Longer service life
• Better corrosion resistance
• Better resistance to cavitation
• Broader chemical compatibility

Learn more about the various options available in miniature and compact plastic ball valves.

Why metal ball valves?

• Higher burst pressures
• Lower thermal expansion
• Greater stress resistance
• Better shock pressure resistance
• Able to handle higher temperatures

When higher pressure and temperature requirements make metal the best choice for a ball valve body, brass and stainless steel are common options to choose between.

Learn more about ball valves and ball valve design.

What is brass made of?
Brass is a copper and zinc alloy. It is one of the softer metals used for metal ball valve bodies, but this fact can be a little misleading. Brass valves are noted for their durability and have higher pressure ratings than all but the most highly engineered plastics. Brass can also handle and absorb more heat than plastics. Brass has good malleability or plasticity and this makes it cheaper and easier to precisely shape, stretch and mold without breaking, cracking or rupturing.

What other metals are used in brass alloys?
In addition to the zinc and copper, different recipes for brass have varying amounts of other metals added to enhance desirable properties. These include formability, strength, corrosion resistance and toughness. One of the most important of the metals added to brass alloys is lead or lead substitutes like bismuth, silicon and selenium. These make brass very easy to shape using high speed machining techniques.

When considering the effects of brass alloy ingredient's, it is important to note that the resulting characteristics usually interact in some way. Adding more of one metal may increase a brass's strength while at the same time decreasing its machinability. The resulting brass would then be more expensive to shape and modify. The result is a wide range of brasses or copper-zinc alloy types or recipes to choose from.

The brass most often used to make ball valve bodies has traditionally been free machining brass (UNS C36000 or CDA 360) or its lead-free equivalent.

Learn more about ISM's selection of brass ball valves.

What is stainless steel made of?
Stainless steel is the generic category name given to a broad range of iron-carbon alloys that contain at least 10.5% chromium by weight. By adding chromium to steel recipes, the steel becomes stronger. It also becomes more corrosion resistant. Like the brasses, small amounts of other alloy metals and elements may be added to enhance a particular stainless steel's structure and properties.

The stainless steels most often used to make ball valve bodies are 304 stainless steel and 316 stainless steel. Type 304 contains 18% chromium and 8% nickel while type 316 contains 16% chromium, 10% nickel and 2% molybdenum.

Learn more about ISM's selection of stainless steel ball valves.

Stainless steel valve body properties

• More expensive metal
• Good heat conduction
• Pressure ratings up to 10,000 psi
• Excellent chemical and corrosion resistance
• Useful for temperatures up to 1700°F (926.7°C)
• More difficult and more expensive to shape and form

Brass valve body properties

• Less expensive metal
• Excellent heat conduction
• Pressure ratings up to 3000 psi
• Good chemical and corrosion resistance
• Useful for temperatures below 400°F (204.4°C)
• Much easier and less expensive to shape and form

Choosing your ball valve body material
Both brasses and stainless steels are important and useful material choices for ball valve bodies. Each one provides distinct advantages and optimal uses. Engineered thermoplastics have also become a standard, reliable material choice for ball valves. Choosing the right ball valve for an application means knowing industry requirements, the valve's operating environment, the media that will be flowing through it and the costs, for both the initial purchase and any ongoing maintenance and repairs.

Learn more about ball valves and the difference between full port and standard port ball valves.
 

Some useful FAQs

What is thermal expansion?
Changes in temperature cause materials to expand or contract. These changes in size are called thermal expansion. Most materials expand when they are heated and contract when they are cooled. The rate of change varies for each material. The thermal expansion of a material is described by a number called its linear coefficient of thermal expansion. Plastics generally expand and contract at higher rates than metals.

Linear temperature expansion - online calculator at The Engineering ToolBox

Learn more about calculating and measuring the Coefficient of Linear Thermal Expansion of plastics at SpecialChem.

What is cavitation?
Flow creates a pressure difference across a valve. Rapid pressure drops across valves can cause cavitation. Cavitation is the rapid formation and collapse of vapor bubbles within a liquid. When this happens, the collapse of these tiny vapor bubbles could erode the valve and pipe material around them. Plastics generally resist cavitation better than metals. This video from the IET Institute for Energy Technology at HSR University of Applied Sciences Rapperswil in Switzerland provides a more detailed explanation of cavitation.

What does UNS mean?
The Unified Numbering System for Metals and Alloys (UNS) cross references different international numbering systems for identifying specific metals. UNS assigns each metal a code number consisting of a letter followed by five numbers. The most recent version of UNS was published in 2017 and includes more than 5,730 metals, 12,180 specification cross references and 18,300 trade names.

Metal Alloy UNS Number Search at the MatWeb (Material Property Data) website.

What is malleability?
Malleability is a metal’s ability to resist breaking or cracking when hammered, pressed or rolled. Malleability is measured by how much pressure or compressive stress a metal can handle. The molecules of solid metals have a very regular crystal lattice structure. The differences in each metal's crystalline structure is why the malleability of each metal or metal alloy is different.

Metallic Structures at the Chemguide website.

What is galling?
Stainless steel threaded connections can seize. This is called galling or cold welding. Some metals resist corrosion because they have a self-repairing oxide film that forms on their surfaces. When stainless steel male and female threads rub and scrape against each other, this sliding contact scrapes off the protective oxide film and the oxide free metal surfaces can weld themselves together.

Galling at Wikipedia.

Some additional reading

• UNS Standard Designations for Wrought and Cast Copper and Copper Alloys from the Copper Development Association
   
• Copper and its Alloys: Standardsfrom the Copper Development Association
   
• Copper Key or Copperkey from Deutsches Kupferinstitut
  Use Copperkey Software online to find the nearest equivalent designations for EN, old BS (British Standard), US (ASTM), German (DIN), Japanese (JIS) and Chinese (GB) copper and copper alloys.
   
• The Crystalline Structure of Metals from Corrosionpedia
   
• Substitutional Alloysat Chemistry LibreTexts
  Brass is a substitutional alloy.
   
• Alloy at Wikipedia
  This article includes a clear description of what both substitutional and interstital alloys are.
   
• What is Stainless Steel? Part I at ThermoFisher Scientific
  This is the first in a new series of ISM blog posts about miniature valves and ball valves in particular.

About the author

Steven C. Williams, BS, is the technical writer and an inbound marketing specialist at Industrial Specialties Manufacturing (ISM), an ISO 9001-2015 supplier of miniature pneumatic, vacuum and fluid circuitry components to OEM's and distributors all over the world. He writes on technical topics related to miniature pneumatic and fluidic components as well as topics of general interest at ISM.       

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