Fluid flow recommended velocities

Piping systems should be designed for an economic flow velocity. For relatively clean fluids, a recommended velocity range where minimum corrosion can be expected is 2 to 10 fps. If piping bores exist, maximum fluid velocities may have a mean velocity of 3 fps for a 3/8-in. bore to 10 fps for an 8-in.-diameter bore. Higher flow velocities are not uncommon in situations that require uniform, constant oxygen supply to form protective films on active/passive metals. 

Magnetic flow meters are sometimes utilized in corrosive Hquid streams or slurries where a low unrecoverable pressure drop and high rangeabiHty is required. The fluid is required to be electrically conductive. Magnetic flow meters, which use Faraday s law to measure the velocity of the electrically conductive Hquid, are relatively expensive. Their use is therefore reserved for special situations where less expensive meters are not appropriate. Installation recommendations usually specify an upstream straight mn of five pipe diameters, keeping the electrodes in continuous contact with the Hquid. 

Dynamic pressure may be measured by use of a pitot tube that is a simple impact tube. These tubes measure the pressure at a point where the velocity of the fluid is brought to zero. Pitot tubes must be parallel to the flow. The pitot tube is sensitive to yaw or angle attack. In general angles of attack over 10° should be avoided. In cases where the flow direction is unknown, it is recommended to use a Kiel probe. Figure 10-3 shows a Kiel probe. This probe will read accurately to an angle of about 22° with the flow. 

In the case of particulate fouling, one of the more common types, insuring a sufficient flow velocity and minimizing areas of lower velocities and stagnant flows to help keep particles in suspension is the most common means of deahng with the problem. For water, the recommended tubeside minimum velocity is about 0.9 to 1.0 m/s. This may not always be possible for moderate to high-viscosity fluids where the resulting pressure drop can be prohibitive. 

From the ACGIH recommendations, we can say that the system is operating safely if a fluid velocity greater than or equal to the capture velocity is induced across the whole of the tank surface, and the exhaust flow rate is sufficient to capture all the fluid in the jet. Since the maximum velocity at any... 

The flow pattern of fluids in gas-liquid-solid (catalyst) reactors is often far from ideal. Special care must be taken to avoid by-passing of the catalyst particles near the reactor walls, where the packing density of the catalyst pellets is lower than in the centre of the bed. By-passing becomes negligible if the ratio of reactor to particles diameter is larger than 10 a ratio of 20 is recommended. Flow maldistributions might be serious in the case of shallow beds. Special devices must be used to equalize the velocity over the cross-section of the reactor before reactants are introduced onto the catalyst bed. 

Comparison of McMillen s results with those for Newtonian fluids is instructive in two important respects. First, the non-Newtonian entrance loss was felt 40 diameters downstream from the entrance. Since the Reynolds number of the flow was only 50, the entrance loss for a Newtonian fluid (P3) would have only been felt downstream for a distance of 3 diameters. Second, comparison of the foregoing formula with the usually recommended procedure for Newtonian fluids (P3) indicates that the non-Newtonian pressure drop was approximately six times as great as that for Newtonian fluids under the same conditions. This figure was checked reasonably closely by the later work of Mooney and Black (M16), who found entrance losses of up to seven times those for comparable Newtonian fluids. Since this entrance loss (P3) is due to the energy required to set up the velocity profile, it might appear logical that... 

These are generally in the form of a U-tube attached by flexible connectors to the inlet and outlet lines of the instrument (Fig. 6.34). The instrument has a substantial volume (ID = 0.025 m) and, where flows are small (recommended minimum velocity = 1 m/s), it is probable that the whole process fluid stream, rather than simply a sample, will be sent through the instrument. 

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