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The answers that the others have given you are very good ones, in other words there is not a singular value for the velocity of a fluid inside a pipe. Most rules have been derived from both cost and operating experience. Thus, I will take liberty in using the previous answers and order them as follows:
a) What are the process requirements (pressure and flow) downstream.
b) What is the service (regular flow line, intermittent, blow-down, depressurizing)
c) Is noise an issue?
d) What are the variations in the flow and pressure with time
e) Fluid characteristics (composition, solids, Newtonian or non-Newtonian)
f) Phase behavior (single phase, two phase, does it have water?)
g) Flow regime (slug, annular, mist, bubble, etc) This is particularly important when designing pipe leading to separators because when the flow is in the mist regime gravity separators do not perform as desired.
h) Cost of energy, equipment and construction. Many rules of thumb were developed in the 1950′s and 1960′s, since then the ratio between the cost of energy, equipment and constructions costs have varied significantly and are very widespread throught the world.
i) Start-up and commissioning considerations (During construction there is an abnormally high amount of debree and other contaminants present) and the pipe must be designed to allow for this phase of the project.