Page 62 - Fluid mechanics, heat transfer, and mass transfer

FRICTION AND PIPING 39

& When a steel pipe is coated with zinc, it is called a GI
pipe. Zinc coating protects the pipe from corrosion. It
is commonly used for water lines.
. What type of pipe is used for steam lines?
& Class C pipe, which is some times called black pipe.
. What are the generally recommended considerations in
determining utility gas piping size?
& Piping sizing should be selected so that pressure drop
at the furthest point in the system does not exceed
10% of the inlet pressure under actual flow
conditions.
& The source and the pipeline should be sized itself
Strainer. with an adequate but affordable safety factor to allow
FIGURE 3.4
for peak demand and future growth. It is not unusual
to apply a safety factor of two times the current
anticipated flow and from 1.2 to two times the
& The outside diameter of each pipe size is required pressures. This provides a margin so future
standardized. additions or changes in process requirements do not
& Therefore, a particular nominal pipe size will have a necessarily demand a totally new piping system.
different inside diameter depending on the schedule . What are the common errors in compressed air distri-
number specified. bution systems and how are they addressed?
& Schedule 40, 60, and 80 are examples while Schedule & A common error in compressed air systems is too
40 is most common. small line sizes for the desired air flow. This includes
. Which of the following has higher wall thickness, the interconnecting piping from compressor dis-
12 SWG or 18 SWG? charge, air dryer, and header. It also applies to the
distribution lines conveying air to production areas
& 12 SWG.
and within the equipment found there. Undersized
. Some times, piping is specified as Class A, Class B, and
piping restricts the flow and reduces the discharge
Class C. Which class of piping has the highest wall
pressure, thereby preventing the user of expensive
thickness?
compressed air power. Poor systems not only con-
& Class C.
sume significant energybutalso degrade productivity
. Give an equation for the estimation of internal design and quality.
pressure (IDP) for a pipe. & Use of charts showing standard pressure drop as a
function of pipe size and fittings, which sizes the
IDP ¼ðP max þ P s ÞS F ; ð3:16Þ line for the so-called acceptable pressure drop.
This practice can be misleading because the charts
where P max is the maximum steady-state operating cannot accommodate velocity- and flow-induced
pressure, P s is the surge or water hammer pressure, turbulence.
and S F is the safety factor to take care of unknowns such & The interconnecting piping is a critical element that
as external earth loads or live loads; variation of pipe must deliver air to the distribution headers with small
properties with temperature; damage that could result pressure drop.
from handling, shipping, or installing; and corrosion,
& The objective in sizing interconnecting piping is to
erosion, and other effects associated with long use. S F is
transport the maximum expected volumetric flow
usually in the range of 3–4.
from the compressor discharge, through the dryers,
. What is the maximum allowable working pressure of a
filters, and receivers, to the main distribution
material? header with minimum pressure drop (about 20 kPa
& The maximum allowable working pressure of the or less).
material selected should significantly exceed the . Excessive air velocity can be a root cause of back
maximum expected operating pressure; 20% is pressure, erratic control signals, turbulence, and turbu-
a reasonable minimum safety factor but 200% is a lence-driven pressure drop. The recommended design
desirable target. pipeline velocity for interconnecting piping and main
. What is a GI pipe? What is its application? headers is 6 m/s or less, and never to exceed 9 m/s.