Page 98 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.48 CHAPTER TWO

8. Packing is the material that seals the stem from leaking to the outside of the valve. The
packing is contained by the packing nut on the bonnet.
9. The backseat is a seat in the bonnet used in the fully open position to seal the valve
stem against leakage into the packing. A bushing on the stem provides the mating
surface. Backseating is useful if the packing begins to leak and it provides a means to
prevent the stem from being ejected from the valve. Back-seating is not provided on
all valves.
10. The stroke of a closure member is the distance the member must travel from the fully
opened to the fully closed position.

VALVE BODY MATERIALS

Valves are manufactured in both metallic and nonmetallic materials. Nonmetallic materials
consist primarily of thermoplastics.

Metallic
Valve bodies are manufactured of the following materials:

1. Bronze valves are usually limited to the smaller sizes and are used in water service up to
450°F. Different alloys are available for higher temperature and pressure applications.
2. Cast iron is a commonly used material for water and steam up to 450°F and is gener-
ally limited to smaller size castings. High tensile strength iron may be used in large
sizes.
3. Malleable iron is characterized by pressure tightness and resistance to stress and shock.
4. Ductile iron casting has high tensile strength, good ductility, and good corrosion
resistance.
5. Steel is available in a wide variety of alloys that are recommended for high temperature
and pressure applications and conditions that may be too severe for iron and bronze
bodies.
6. Stainless steel is available in a wide variety of alloys and is often used for pure water
and other services requiring noncorrosive materials.

Thermoplastics

Thermoplastics are rapidly gaining popularity in many utility systems. They have proven
very successful for carrying corrosive fluids where conventional valves are not suitable or
special alloy metals are very costly. It is estimated that they are suitable for use in 85 to
90 percent of all utility services.
There are many different types of thermoplastic materials capable of carrying most
chemicals. All standard valve types are available. The valve materials often have addi-
tives different from those used for manufacturing piping. Generally available valve types
and their sizes are given in Table 2.6. In general, plastic valves are limited to a maximum
temperature of 250°F and pressure of 150 psig (1035 kPa). General properties of plastic
material used for valves are given in Table 2.7.

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