Number of velocity heads

In the velocity head method, the losses are reported as a number of velocity heads K. Then, the engineering Bernoulh equation for an incompressible fluid can be written... 

Here, V is the area average velocity, K is the number of velocity heads of pressure drop provided hy the uniform resistance, Ap = FCpV/2, and a is the velocity profile factor used in the mechanical energy bal-... 

Number of velocity heads for fittings, expansions, contractions, and bends... 

Part Pressure Drop in Number of Velocity Heads Equation... 

Some representative figures are given in Table 3.2 for the friction losses in various pipe fittings for mrbulent flow of fluid, and are expressed in terms of the equivalent length of straight pipe with the same resistance, and as the number of velocity heads ( 2/2g) lost. Considerable variation occurs according to the exact construction of the fittings. 

Number of pipe diameters Number of velocity heads (u2/2g)... 

As the number of velocity heads, K, lost at each fitting or valve. 

A velocity head is u2/2g, metres of the fluid, equivalent to (u2/2)p, N/m2. The total number of velocity heads lost due to all the fittings and valves is added to the pressure drop due to pipe friction. 

The number of velocity heads lost, or equivalent pipe diameter, is a characteristic of the particular fitting or type of valve used. Values can be found in handbooks and manufacturers literature. The values for a selected number of fittings and valves are given in Table 5.3. 

Fitting or valve K, number of velocity heads number of equivalent pipe diameters... 

The loss in terms of velocity heads can be estimated by counting the number of flow contractions, expansions and reversals, and using the factors for pipe fittings to estimate the number of velocity heads lost. For two tube passes, there will be two contractions, two expansions and one flow reversal. The head loss for each of these effects (see Volume 1, Chapter 3) is contraction 0.5, expansion 1.0, 180° bend 1.5 so for two passes the maximum loss will be... 

Fitting Number of equivalent pipe diameters LJdt Number of velocity heads K... 

If the frictional losses were expressed as the head loss, hf= APf/pg, then the quantity 4fLJdi would multiply u2/2g. Thus 4/Le/d, is the total number of velocity heads lost. Consequently, an alternative presentation of frictional losses for fittings is in terms of the number of velocity heads K lost for each fitting. In this case, the total frictional pressure drop may be calculated as... 

N Number of velocity heads lost through unit height of bed m-1 L-1... 

Typical values of number of velocity heads lost, 8, and equivalent lengths, L, of fittings which may sometimes be found in relief lines are given by CCPS1141. 

The total number of velocity heads for the proposed relief routes are as follows Safety valve... 

Here, V is the area average velocity, K is the number of velocity heads of pressure drop provided by the uniform resistance, Ap = KpV2/2, and a is the velocity profile factor used in the mechanical energy balance, Eq. (6-13). It is the ratio of the area average of the cube of the velocity, to the cube of the area average velocity V. The shape of the exit velocity profile appears twice in Eq. (6-154), in V%mIV and a2. Typically, K is on the order of 10, and the desired exit velocity profile... 

---