The rule of thumb for pipelines is 1 to 3 m/s. This velocity range will prevent deposits from accumulating at low spots and allow for an acceptable pressure loss.
The velocities shall in general be kept low enough to prevent problems with erosion, hammer pressure surges, noise, vibration and reaction forces. In some cases a minimum velocity is required. A compromise between line size and pump power has to be taken. For laminar flow (Re<2300) flow rate will change with 4 power of diameter Q/Q1 = (D/D1)^4. For turbulent flow where friction factor is not depending on the Reynolds number flow rate will change with 2.5 power of diameter Q/Q1 = (D/D1)^2.5
With corrosion inhibited fluids in carbon steel piping, the velocity shall be limited by a wall shear stress of 40 N/m2 to maintain the corrosion inhibiting film at the pipe wall.
The viscosity test of a commodity characterizes its resistance to flow in a pipeline.. The test may exaggerate the low temperature viscosity of paraffinic crude oils. Previous testing has shown that the viscosity behavior of a crude oil in a pipeline follows a curve somewhere between the apparent viscosity (the extrapolated straight line portion of the viscosity-temperature plot) and the measured viscosity. Apparent viscosity is generally used for head loss calculations.
An approach I've seen used is the errosional equation from API-14E, 100 divided by the square root of the density which for most oils gives a maximum velocity in the mid teens (ft/sec). For short term operation, you can use a value 25% higher.
If it's a clean fluid and you aren't close to the vapor pressure, I don't get too concerned about a maximum velocity. When I worked for Aramco we had an East/West pipeline with some pressure reducing valves and the velocity in the piping downstream of these valves were about 25 ft/sec, these areas were monitored but when I talked to the inspections folks, they had not seen a problem.
What usually gets you is the pressure drop especially in a long pipeline. Water hammer becomes another consideration.
