Fluid_Flow_Rules_of_Thumb_for_Chemical

22

Rules of Thumb for Chemical Engineers

Rectangular Weir

Pitot Tube

Z~ - u2/2g

Fv - 3.33(L - 0.2H)H 3/2

Source

where

Branan, C. R., The Process Engineer's Pocket Handbook Vol. 1, Gulf Publishing Co., 1976.

F v - Flow in f t 3 / s e c L - Width of weir, ft H - Height of liquid over weir, ft

Control Valves

where

Notes"

APa]low = Maximum allowable differential pressure for sizing purposes, psi Km = Valve recovery coefficient (see Table 3) re = Critical pressure ratio (see Figures 1 and 2) P~ = Body inlet pressure, psia Pv = Vapor pressure of liquid at body inlet tempera- ture, psia This gives the maximum AP that is effective in produc- ing flow. Above this AP no additional flow will be pro- duced since flow will be restricted by flashing. Do not use a number higher than APa]low in the liquid sizing formula.

1. References 1 and 2 were used extensively for this section. The sizing procedure is generally that of Fisher Controls Company. 2. Use manufacturers' data where available. This hand- book will provide approximate parameters applicable to a wide range of manufacturers. 3. For any control valve design be sure to use one of the modem methods, such as that given here, that takes into account such things as control valve pres- sure recovery factors and gas transition to incom- pressible flow at critical pressure drop. Across a control valve the fluid is accelerated to some maximum velocity. At this point the pressure reduces to its lowest value. If this pressure is lower than the liquid's vapor pressure, flashing will produce bubbles or cavities of vapor. The pressure will rise or "recover" downstream of the lowest pressure point. If the pressure rises to above the vapor pressure, the bubbles or cavities collapse. This causes noise, vibration, and physical damage. When there is a choice, design for no flashing. When there is no choice, locate the valve to flash into a vessel if possible. If flashing or cavitation cannot be avoided, select hardware that can withstand these severe condi- tions. The downstream line will have to be sized for two phase flow. It is suggested to use a long conical adaptor Liquid Flow

Critical Pressure Ratios For Water

1.0

~U ~_ o.9 i . . -

0.8 r~

V) I,SJ o= 0.7 o.. . . J

U

0.6"

n t ,

U

-

O.5

500 1000 1500 2000 2500 3000 3500 VAPOR PRESSURE-PSIA

o

Figure 1. Enter on the abscissa at the water vapor pres- sure at the valve inlet. Proceed vertically to intersect the curve. Move horizontally to the left to read rc on the ordi- nate (Reference 1).

from the control valve to the downstream line. When sizing liquid control valves first use

nPa l l o w - K m (P1 - rc Pv )