Page 67 - Fluid mechanics, heat transfer, and mass transfer

PIPING, SEALS, AND VALVES
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excellent overall chemical compatibility and resis- & Mostcommonsoftfacematerialsaremetalizedcarbon
tant to ozone. More elastic than PTFE. withcarbongradewithmetalphase,resin-impregnated
& Perfluoro-elastomers (Kalrez and Chemraz): A wide carbon with generalduty carbonand acid gradecarbon
range of specific elastomer compounds with superior with low impregnant and ash content.
chemical compatibility and higher temperature prop- . What are the types of mechanical seals?
erties. Resistant to ozone. Elastic. & Pusher seals.
& Silicone (MVQ): Resistant to ozone, alcohols, gly- & Bellows seals.
cols, and many products used in food industry. Not . What are the reasons, other than loss of the fluid
resistant to steam, mineral oils, organic solvents and
material, that require special attention for eliminating
inorganic acids.
leakages in piping systems?
& Flexible graphite: Near universal chemical compati-
& Environmental concerns requiring minimization/
bility and high-temperature capability but no resiliency.
elimination of stream contamination/fugitive emis-
. What are the temperature limits for use of common sions to meet increasingly stringent regulations.
gasketing/mechanical seal materials? & Safety issues while handling hazardous materials
& Asbestos: Up to 550 C.

such as flammable, toxic, and radioactive fluids.
& NBR: 40–100 C . What are the causes of leakage in piping systems?

& EPDM: 40–150 C & Primarily leakages occur from pipe fittings.

& Teflon: 30–200 C. & Mechanical vibration leading to vibration fatigue,

& FPM/Viton: 20–200 C. which can be aggravated by poor metallurgical con-

& FEP: Suitable for temperatures up to 200 C. sistency within the fitting material, undue stress
imposed on the connection from side load or other

& MVQ: 50–230 .
system design characteristics, or improper installa-

& Kalrez and Chemraz: 20–250 C.
tion practices. The greater the amplitude of alternat-
& CAS: Up to 400 C.

ing stress on the piping/fitting material, the sooner it

& Grafoil: Up to 3000 . Graphite and metallic gasket will fail.
materials are used for high temperatures.
& The most critical areas contributing to leakage are as
. What is the effect of increase in temperature on the life follows:
of a gasket? ➢ Types of connecting devices used in joining
& Gasket strength deteriorates with increase in operat- piping.
ing temperature. ➢ The level of knowledge and practical experience
& Rule of thumb is that gasket life becomes half for of those installing and maintaining the piping.

every 10 C increase in temperature above the min- . What are the causes of flange leakage?
imum permissible temperatures.
& Improper flange facing.
. What are the requirements for seal faces? Name the
& Improper flange alignment.
commonly used face materials.
& Unclean or damaged flange facings.
& In most seal applications, the face materials consistof
& Excessive loads at flange locations.
a hard face and soft face. This combination has a
proven record of providing a low coefficient of & Thermal shock.
friction and best tolerance of face contact. The soft & Differential expansion between bolts and flanges
face can wear to match the profile of the hard face, (due to temperature gradients or use of different
resulting a thin fluid film and low leakage. materials for flanges and bolts, e.g., steel flanges and
& Hard faces used are almost exclusively of ceramic aluminum bolts).
materials. In some aggressive services, some seals & Uneven bolt stress.
use both faces made from hard materials. & Flange bowing, bolt hole distortion or nonparallelism
& Soft faces are blends of amorphous carbon, graphite, of flanges.
and impregnants. & Improper gasket material or size.
& Most hard face materials are tungsten carbide with & Gasket failure that is attributed to extrusion, crush-
cobalt or nickel binders, reaction bonded silicon ing, and creep relaxation. Creep is the cold flow or
carbide with free silicon, alumina with homogeneous thinning of a material due to applied pressure while
silicon materials and direct sintered silicon carbide relaxation is the loss of spring stiffness resiliency of a
with homogeneous ceramic silicon carbide materials. material.