Fluid_Flow_Rules_of_Thumb_for_Chemical

Fluid Flow

9

Typical Design Vapor Velocities* (ft./sec.)

Typical Design* Velocities for Process System Applications

Line Sizes

Fluid

<_6"

8"-12"

_>14"

Velocity, ft./sec.

Service

Saturated Vapor 0 to 50 psig

4-6.5 1-5

Average liquid process Pump suction (except boiling) Pump suction (boiling) Boiler feed water (disch., pressure) Drain lines Liquid to reboiler (no pump) Vapor-liquid mixture out reboiler Vapor to condenser Gravity separator flows

30-115

50-125

60-145

Gas or Superheated Vapor 0 to 10 psig

0.5-3 4-8 1.5-4 2-7

50-140 40-115

90-190 75-165 60-150

110-250 95-225 85-165

11 to 100 psig 101 to 900 psig

30-85

15-30 15-80 0.5-1.5

*Values listed are guides, and final line sizes and flow velocities must be determined by appropriate calculations to suit circumstances. Vacuum lines are not included in the table, but usually tolerate higher velocities. High vacuum conditions require careful pressure drop evaluation.

*To be used as guide, pressure drop and system environment govern final selection of pipe size. For heavy and viscous fluids, velocities should be reduced to about values shown. Fluids not to contain suspended solid particles.

Usual Allowable Velocities for Duct and Piping Systems*

Suggested Steam Pipe Velocities in Pipe Connecting to Steam Turbines

Velocity, ft./min.

Service/Application

Typical range, ft./sec.

Service--Steam

Forced draft ducts

2,500-3,500 2,000-3,000

Induced-draft flues and breeching

Inlet to turbine

100-150 175-200 400-500

Chimneys and stacks Water lines (max.)

2,000

Exhaust, non-condensing

600

Exhaust, condensing

High pressure steam lines Low pressure steam lines Vacuum steam lines Compressed air lines Refrigerant vapor lines High pressure

10,000

12,000-15,000

25,000

Sources

2,000

1. Branan, C. R., The Process Engineer's Pocket Hand- book, Vol. 1, Gulf Publishing Co., 1976. 2. Ludwig, E. E., Applied Process Design for Chemical and Petrochemical Plants, 2nd Ed., Gulf Publishing Co. 3. Perry, R. H., Chemical Engineer's Handbook, 3rd Ed., p. 1642, McGraw-Hill Book Co.

1,000-3,000 2,000-5,000

Low pressure

Refrigerant liquid

200 400

Brine lines

Ventilating ducts Register grilles

1,200-3,000

500

*By permission, Chemical Engineer's Handbook, 3rd Ed., p. 1642, McGraw-Hill Book Co., New York, N.Y.

Two-phaseFlow

Two-phase (liquid/vapor) flow is quite complicated and even the long-winded methods do not have high accuracy. You cannot even have complete certainty as to which flow regime exists for a given situation. Volume 2 of Ludwig's design books I and the GPSA Data Book2 give methods for analyzing two-phase behavior. For our purposes, a rough estimate for general two- phase situations can be achieved with the Lockhart and Martinelli 3 correlation. Perry's 4 has a writeup on this cor- relation. To apply the method, each phase's pressure drop is calculated as though it alone was in the line. Then the following parameter is calculated:

x - [ a P c / a P o 1'/2

where: APL and APG are the phase pressure drops The X factor is then related to either YL or YG. Whichever one is chosen is multiplied by its companion pressure drop to obtain the total pressure drop. The fol- lowing equation5is based on points taken from the YL and YG curves in Perry's 4 for both phases in turbulent flow (the most common case):

YL - 4.6X-178 + 12.5X-~ + 0.65 YG- X2yL