Page 68 - Fluid mechanics, heat transfer, and mass transfer

GASKETS AND MECHANICAL SEALS 45

. What are the different methods of minimizing flange 6. Refrigerating or cryogenic effect of a sudden loss
leakages? of pressure on volatile fluids in determining the
& One can minimize flange leakages by the following lowest expected service temperature.
methods: . What is the hydrostatic test pressure employed in testing
➢ Reducing the distance between bolt centers. piping?
➢ Increasing the flange thickness. & Usually 1.5 times or more of the design pressure.
➢ Increasing the elastic modulus of the flange . What are the recommended practices in selecting pipe
material. sizes?
➢ Increasing the width of the flange sealing surface. & Pipe sizes are normally selected by optimizing cost of
➢ Using flat or conical washers under the bolt head. energy losses due to friction that will increase with
➢ Selecting a gasket material with a lower sealing decrease in pipe diameter and increased costs of pipe
stress. Using less compressible gasket material. that will increase with increase in pipe diameter.
➢ Decreasing the gasket area, which is one way to & If the velocity is too low, suspended solids may settle
increase flange pressure. and air may not be swept out but collect at high points
of the pipe. Velocities of over 1 m/s will be adequate
➢ Reducing gasket thickness.
for this purpose.
➢ Reducing the initial tightening torque if possible.
& The safe lower velocity limit to avoid collecting air
➢ Eliminating the lubricants on the flange face and
and sediment depends on the amount and type of
the gasket.
solid contaminants and on the pipe diameter and pipe
➢ Roughening the sealing surface.
profile. Velocities greater than about 1 m/s are usu-
➢ Use of flat washers. ally sufficient to move trapped air to air releasevalves
➢ Tightening all bolts at the same time, for example, and keep the solid contaminants in suspension.
by using a multiple head torque wrench. & Problems associated with high velocities include ero-
. What are the materials used in pipes and tubes? sion of the pipe wall or lining/coating, cavitation at
control valves and other restrictions, increased pump-
& Cast iron, carbon and low alloy steels, high alloy
ing costs, and increased risk of hydraulic transients.
stainless steels, nickel and nickel-based alloys, alu-
minum and its alloys, copper and its alloys, titanium & A typical upper velocity for many applications in-
and its alloys, tantalum, zirconium and its alloys, volving nonviscous liquids is 6 m/s.
plastic and plastic-lined/coated steels, FRP, rubber & Other considerations include problems of erosion,
and rubber lined, stoneware, cement and concrete, corrosion, noise, vibration, hammer, and cavitation.
glass and glass-lined steel, and so on. . “Sizing piping is done based on recommended
. What are the general considerations involved in the velocities.”
selection of materials for piping? (a) Among dry gas and wet gas, which possesses
& Considerations to be evaluated when selecting the higher recommended velocities?
piping materials include the following: & Dry gas.
1. Possible exposure to fire with respect to the loss of (b) What velocity is normally recommended for air/
strength, degradation temperature, melting point, dry gas?
or combustibility of the pipe or support material. & 30 m/s (100 ft/s).
2. Ability of thermal insulation to protect the pipe (c) What velocity is normally recommended for water
from fire. flow?
3. Susceptibility of the pipe to brittle failure, possi- & 1.2–2.4 m/s (4–8 ft/s).
bly resulting in fragmentation hazards, or failure (d) What velocity is normally recommended for pump
from thermal shock when exposed to fire or fire- suction line?
fighting measures. & 0.15–1 m/s (0.5–3 ft/s).
4. Susceptibility of the piping material to crevice
. “Vapor/gas velocities to be used while sizing (design-
corrosion in stagnant confined areas (screwed
ing) piping depend on line sizes and increase with
joints) or adverse electrolytic effects if the metal
increased line sizes.” True/False?
is subjected to contact a dissimilar metal.
& True.
5. Suitability of packing, seals, gaskets, and lubri-
. Suggest suitable velocities for line/duct sizing for steam
cants or sealants used on threads as well as com-
lines.
patibility with the fluid handled.