Fluid_Flow_Rules_of_Thumb_for_Chemical

Fluid F l ow

23

Table 1 Critical Pressure of Various Fluids, Psia*

Critical Pressure Ratios For Liquids Other Than Water

I 1.0

Ammonia ........................... 1636 Argon ................................... 705.6 Butane ................................. 550.4 Carbon Di ox i de ................. 1071.6 Carbon Monoxide ............... 507.5 Chlor ine ............................. 1118.7 Dowtherm A ........................ 465 Ethane ................................. 708 Ethy lene .............................. 735 Fluorine ............................... 808.5 Helium ................................... 33.2 Hydrogen ............................. 188.2 Hydrogen Chloride ............ 1198

Isobutane ............................ 529.2 Isobutylene ......................... 580 Methane .............................. 673.3 Nitrogen ..............................492.4 Nitrous Oxide ................... 1047.6 Oxygen ............................... 736.5 Phosgene ........................... 823.2 Propane ..............................617.4 Propylene ........................... 670.3 Refrigerant 11 ..................... 635 Refrigerant 12..................... 596.9 Refrigerant 22 ..................... 716 Water ................................ 3206.2

0 . = , .

o.9

w a: 0.8 U3 U3 Lu0.7 n- O, " 0.6 r I-.

. I9O .20

.80 9 0 1.00

. 30 . 4 0 . 5 0 6 0

.70

VAPOR PRESSURE - - PS I A CR I T I CAL PRESSURE - - PS I A

*For values not listed, consult an appropriate reference book.

Figure 2. Determine the vapor pressure/critical pressure ratio by dividing the liquid vapor pressure at the valve inlet by the critical pressure of the liquid. Enter on the abscissa at the ratio just calculated and proceed vertically to inter- sect the curve. Move horizontally to the left and read rc on the ordinate (Reference 1).

Gas and Steam Flow

The gas and steam sizing formulas are Gas

Q

Cg

G-~P1 sin

~ AP

Some designers use as the minimum pressure for flash check the upstream absolute pressure minus two times control valve pressure drop. Table 1 gives critical pressures for miscellaneous fluids. Table 2 gives relative flow capacities of various types of control valves. This is a rough guide to use in lieu of manufacturer's data. The liquid sizing formula is ~G Cv - Q AP

C1

-~1 deg.

Table 2 Relative Flow Capacities of Control Valves (Reference 2)

Cd*

CdFpt

Valve Type

CdFL**

Double-seat g l obe

12

11

11

Single-seat top-guided g l obe

11.5

10.8 11.3 6-11

10 10 na

Single-seat split body

12

Sliding gate

6 - 12

Single-seat top-entry cage

13.5

12.5

11.5

where

Eccentric rotating plug (Camf lex)

14

13

12

60 ~ open but ter f l y

18 19 20 15 25

15.5

12 14 na na

Single-seat Y valve (300 & 600 Ib)

16.5

Cv - Liquid sizing coefficient Q - Flow rate in GPM

Saunders type (unlined) Saunders t ype (l ined)

17

13.5

Throttling (characterized) bal l Single-seat streamlined angle (flow-to-close)

AP - Body differential pressure, psi G - Specific gravity (water at 6 0 ~

20

15

1.0)

26

20

13

90 ~ open but ter f l y (average)

32

21.5

18

Two liquid control valve sizing rules of thumb are

Note: This table may serve as a rough guide only since actual flow capacities differ between manufacturer's products and individual valve sizes. (Source: ISA "Handbook of Control Valves" Page 17). *Valve flow coefficient Cv - Cd x d2 (d = valve dia., in.). tCv/d2 of valve when installed between pipe reducers (pipe dia. 2 x valve dia.). **Cv/d2 of valve when undergoing critical (choked) flow conditions.

1. No viscosity correction necessary if viscosity _-<20 centistokes. 2. For sizing a flashing control valve add the Cv's of the liquid and the vapor.