Experience is typically what turns a good engineer into a great
engineer. An engineer that can look at a pipe and a flowmeter and guess the
pressure drop within 5%. Someone who can at least estimate the size of a vessel
without doing any calculations. When I think of such rules, two authors come to
my mind, Walas and Branan. Dr. Walas' book, Chemical Process Equipment:
Selection and Design has been widely used in the process industry and in
chemical engineering education for years.
Mr. Branan has either helped write or
edit numerous books concerning this topic. Perhaps his most popular is
Rules of Thumb for Chemical Engineers. Here, I'll share some of these
rules with you along with some of my own. Now, be aware that these rules are
for estimation and are not necessary meant to replace rigorous calculations when
such calculations should be performed. But at many stages of analysis and
design, these rules can save you hours and hours
|
Materials of
Construction |
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|
Material |
Advantage |
|
Disadvantage |
|
Carbon Steel |
Low cost,
easy to fabricate, abundant, most common material. Resists most alkaline
environments well. |
Very poor
resistance to acids and stronger alkaline streams. More brittle than
other materials, especially at low temperatures. |
|
Stainless
Steel |
Relatively
low cost, still easy to fabricate. Resist a wider variety of
environments than carbon steel. Available is many different types. |
No
resistance to chlorides, and resistance decreases significantly at
higher temperatures. |
|
254 SMO (Avesta) |
Moderate
cost, still easy to fabricate. Resistance is better over a wider range
of concentrations and temperatures compared to stainless steel. |
Little
resistance to chlorides, and resistance at higher temperatures could be
improved. |
|
Titanium |
Very good
resistance to chlorides (widely used in seawater applications). Strength
allows it to be fabricated at smaller thicknesses. |
While the
material is moderately expensive, fabrication is difficult. Much of cost
will be in welding labor. |
|
Pd
stabilized Titanium |
Superior
resistance to chlorides, even at higher temperatures. Is often used on
sea water application where Titanium's resistance may not be acceptable. |
Very
expensive material and fabrication is again difficult and expensive. |
|
Nickel
|
Very good
resistance to high temperature caustic streams. |
Moderate to
high expense. Difficult to weld. |
|
Hastelloy
Alloy |
Very wide
range to choose from. Some have been specifically developed for acid
services where other materials have failed. |
Fairly
expensive alloys. Their use must be justified. Most are easy to weld. |
|
Graphite |
One of the
few materials capable of withstanding weak HCl streams. |
Brittle,
very expensive, and very difficult to fabricate. Some stream components
have been know to diffusion through some types of graphites. |
|
Tantalum |
Superior
resistance to very harsh services where no other material is acceptable. |
Extremely
expensive, must be absolutely necessary. |
