Page 63 - Fluid mechanics, heat transfer, and mass transfer

PIPING, SEALS, AND VALVES
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. What are the ways by which pipe sections are joined? considerable pressure and corrosion after reduction
& Welding (no leakage problems) in thickness due to threading. For threaded joints
➢ Welded joints: Used wherever feasible to elimi- more than 5 cm, assembly, size, and cost of tools
nate leaks. increase rapidly.
➢ Screwed joints. & Screwed fittings involve less leakage problems com-
➢ Flanged joints. pared to flange fittings. For example, in the case of
electrical conduits and equipment in flammable at-
& What are the different types of fittings used in joining
mospheres, screwed fittings are preferred from safety
piping?
point of view.
& Butt welded fittings: Most resistant to both vibration
& Flanges are generally used for line sizes of more than
and fatigue. Limitations are requirement on site
5 cm (2 in.).
welding equipment, time taken for installation, and
. What are the common causes for leakages in piping
accessibility for maintenance.
systems?
& Threaded fittings: Threaded or screwed fittings are
& Process piping, analytical instrumentation, and util-
very common. NPT (National Pipe Thread) fittings
ity lines involve leakages, which add significantly to
have a tapered thread on both male and female ends.
process plant operating costs.
Lubricant or sealant such as PTFE tape is used on the
male threads to prevent damage. & Common causes include system vibration, pulsation,
and thermal cycling for leakages.
& SAE (Society of Automotive Engineers) straight
➢ Vibration fatigue is a factor that can be aggravated
thread fittings: Unlike in NPT fittings, seal is not
by poor metallurgical consistency within the fit-
used. Installation or accessibility is much easier.
ting material, undue stress imposed on the con-
& ISO (International Standards Organization) parallel
nection from side load or other system design
and tapered thread fittings.
characteristics, or simply improper installation
& NPTF (National Pipe Tapered Dry Seal Fittings).
practices.
& 37 AN (Army–Navy) flare fittings.

➢ High amplitude of alternating stress on a piping
. What are the different types of tube fittings? component is primarily responsible for early
& Compression fittings: Made up of nut, body, and failure.
gasket ring or ferrule. Utilizes a friction grip. Unlike ➢ Deepness of the groove or notch made in the pipe
with pipe fittings, no special tools are required in or tubing line by the fitting as it is tightened adds to
assembling. Cannot withstand high pressures. Not intensification for the leakage.
good for systems involving thermal stresses, vibra- & Types of connecting devices used in joining process
tion, and other dynamic forces.
pipe.
& Flare fittings: Made up of nut, sleeve, and body with a
& The level of knowledge and the practical experience
flare or coed end. Can handle higher pressures and
of those installing and maintaining the system.
have larger seal area providing ease of maintenance.
. How are leakages minimized/prevented?
& Bite-typefittings:Comprisedofafittingwithanut,body,
& Proper choice of type of fitting, including compati-
and ferrule(s) having a sharp leading edge, which bites
bility of material of construction with piping and
into the skin of the tubing to achieve holding ability.
fluids handled.
& Mechanical grip-type fittings: Disassembly and re-
& The fitting connection most resistant to both vibra-
make after installation can be more successfully
tion and fatigue is a Butt welded fitting. Its ability to
accomplished without damage than bite type.
resist vibration and fatigue is determined by the
. How are flanges rated? What are the normally used
strength and integrity of the connection made.
flange ratings?
& Proper installation and taping procedures for
& Flanges are rated in terms of design pressures of
threaded connections.
piping.
& Periodic inspection and maintenance.
& Flange ratings include 10, 20, 40, 103, and 175 bar
. What is the desirable information that is to be incor-
(150, 300, 600, 1500, and 2500 psig).
porated in specifications of piping systems?
. When are screwed fittings and flanges used in joining
& Line size.
pipe sections?
& Fluid handled.
& Screwed fittings for small pipe sizes [ 5cm (2 )]:
00
The walls should be thick enough to withstand & Fluid service category.