Page 65 - Fluid mechanics, heat transfer, and mass transfer

PIPING, SEALS, AND VALVES
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diameter of the pipe when converting from metal, & Polybutylene (PB):
reducing material costs, and to the use of smaller ➢ Polybutylene is flexible, yet stronger than even
pumps, saving energy. high-density polyethylene. Its strength increases
. What are the commonly used plastic piping in process at higher temperatures.
industry? ➢ Its temperature limits are 95 C for pressure appli-

& Polyvinyl chloride (PVC): cations and slightly higher for nonpressure
➢ Polyvinyl chloride (PVC) contains a good com- applications.
bination of long-term strength and high stiffness. ➢ Polybutylene is mostly used for hot effluent lines
These characteristics have made PVC the princi- and slurry transportation.
pal plastic pipe material for both pressure and & Polypropylene (PP):
nonpressure applications. An example of a non-
➢ Polypropylene shares similar properties with poly-
pressure application may be using PVC as an
ethylene. One key difference is polypropylene’s
electrical conduit.
excellent chemical resistance to organic solvents
➢ PVC cannot handle high-temperature applica-
and some other chemicals.
tions.
➢ Due to the chemical resistance of polypropylene,
& Chlorinated polyvinyl chloride (CPVC): CPVC pro-
good rigidity, good strength, and high-temperature
vides superior chemical resistance as well as a high
limits, it is used primarily for chemical waste
heat distortion temperature, due to its molecular
movement.
structure. Large chlorine atoms surround the carbon
➢ Polypropylene’s key limitation is its moderate
backbone to protect it like armor plating.
impact resistance.
➢ Due to its high heat distortion temperature;
& Acrylonitrile-butadiene-styrene (ABS):
chemical inertness; and outstanding mechanical,
➢ ABS is formed from three distinct monomer build-
dielectric, flame, and smoke properties, CPVC
ing blocks.
can serve any chemical plant. Applications
include processing operations, cleaning systems ➢ Substances of these types are usually referred to as
involving high temperatures and harsh cleaning copolymers. The proportions of each substance
agents. will determine the physical properties of the final
➢ CPVC piping can handle chemicals that cause product.
process leaks, flow restrictions and, ultimately, ➢ In this case, acrylonitrile contributes rigidity,
premature failure in metal systems. It withstands strength,hardness,andchemicalandheatresistance.
most mineral acids, bases, and salts, as well as ➢ Butadiene contributes impact resistance and sty-
aliphatic hydrocarbons. rene is added to increase the ease of processing.
& Temperature and pressure bearing capabilities of ➢ ABS is primarily used for drain, waste, and vent-
CPVC can be increased by wrapping it with ing applications.
fiberglass. ➢ However, one formulation of ABS has shown to be
& Polyvinylidene fluoride (PVDF). particularly useful for aboveground compressed
air applications.
& Polyethylene:
➢ Polyethylene retains its strength and flexibility
even at subfreezing temperatures.
3.2 GASKETS AND MECHANICAL SEALS
➢ Coiling is even possible with small diameter poly-
ethylene pipe; therefore, it is used for gas distri- . What are the methods used to prevent leaks from pipe
bution and water services.
joints and pumps?
➢ Polyethylene is also abrasion resistant.
& By the use of gaskets and seals wherever frequent
➢ Common uses also include chemical transfer, disassembly is required.
power ducts, and sewage mains. In some instances,
& Bellandspigotjoints,usedforbrittlematerials,involve
the polyethylene molecules are cross-linked in
pouringalowmeltingmaterialassealbetweentwopipe
order to raise the maximum operating temperature
sections. The pouring compound may be molten, or

up to 95 C.
chemical setting, or merely compacted.
➢ Common applications include underfloor heating & Push-on joints require diametral control of the end of
systems, melting ice and snow, and hot or cold
the pipe. They are used for brittle materials, usually
water systems.
for underground piping.