Wastewater Conveyance - Wastewater Operators - page 201

Table 4.1 valves: MaTerials of ConsTruCTion

Cast iron Gray cast iron; also referred to as flake graphite iron

Ductile iron May be malleable iron or spheroidal graphite

(nodular) cast iron

Carbon steel May be as steel forgings, or steel castings, according

to the method of manufacture; may also be
manufactured by fabrication using wrought steels

Stainless steel May be in the form of forgings, castings, or wrought

steels for fabrication

Copper alloy May be gunmetal, bronze, or brass; aluminum

bronze may also be used

High-duty alloys Usually nickel or nickel molybdenum alloys

manufactured under various trade names

Other metals Pure metals having extreme corrosion resistance,

such as titanium, or aluminum

Nonmetals Typically plastic materials such as PVC or

polypropylene

negligible quantity by precision-machined surfaces, resulting in care-
fully controlled clearances. This is, of course, one of the very impor-
tant reasons for minimizing contamination in fluid power systems.
Contamination causes valves to stick, plugs small orifices, and causes
abrasions of the valve seating surfaces, resulting in leakage between
the valve element and valve seat when the valve is in the closed position.
Any of these can result in inefficient operation or complete stoppage of
the equipment. Valves may be controlled manually, electrically, pneu-
matically, mechanically, or hydraulically, or by combinations of two or
more of these methods. Factors that determine the method of control
include the purpose of the valve, the design and purpose of the system,
the location of the valve within the system, and the availability of the
source of power.

Valves are made from bronze, cast iron, steel, Monel ® , stainless
steel, and other metals. They are also made from plastic and glass (see
Table 4.1). Special valve trim is used where seating and sealing materi-
als are different from the basic material of construction (see Table 4.2).
( valve trim usually refers to those internal parts of a valve controlling
the flow and in physical contact with the line fluid.) Valves are made in
a full range of sizes that match pipe and tubing sizes. Actual valve size
is based on the internationally agreed-upon definition of nominal size
(DN), which is a numerical designation of size that is common to all
components in a piping system other than components designated by
outside diameters. It is a convenient number for reference purposes and
is only loosely related to manufacturing dimensions.

Valves are made for service at the same pressures and temperatures
that piping and tubing is subject to. Valve pressures are based on the
internationally agreed definition of nominal pressure (PN), which is a
pressure that is conventionally accepted or used for reference purposes.
All equipment of the same nominal size (DN) designated by the same
nominal pressure (PN) number must have the same mating dimensions
appropriate to the type of end connections. The permissible working