Fluid_Flow_Rules_of_Thumb_for_Chemical

24

Rules of Thumb for Chemical Engineers

Table 4 Correlations of Control Valve Coefficients (Reference 2)

Steam (under 1,000psig)

Q s(1 + 0.00065 Ts. )

Km " - O f 2 = FL2 Cs = 1.83 CfCv Cv = 19.99 Cs/Ci

Ci = 36.59 Cf

CS ~

CI = 36.59 V~rn

P~sinI3417 ~AP ] C1 ~

Cg = C,Cv

deg.

Cg = 19.99 Cs

Values of Krn calculated from Cf agree within 10% of published data of K~. Values of Cl calculated from Krnare within 21% of published data of CI.

Steam and Vapors (all vapors, including steam under any pressure conditions)

Cs =

Qs

lOO

Camflex

C1

-~1 deg.

80

When the bracketed quantity in the equations equals or exceeds 90 degrees, critical flow is indicated. The quan- tity must be limited to 90 degrees. This then becomes $ r unity since sin 90~ - 1. .~ Explanation of terms: ~ 40 r,j t,., n C1 = Cg/Cv (some sizing methods use Ce or Y in place of C1) s ~ 6o

/

; / . , iiiiiiIi

Cg = Gas sizing coefficient Cs = Steam sizing coefficient Cv - Liquid sizing coefficient d~ = Density of steam or vapor at inlet, lbs/ft3

percentage

0 ~ 0

80

100

20

40

60

Percent of rated travel

Figure 3. These are characteristic curves of common valves (Reference 2).

Table 3 Average Valve-Recovery Coefficients, Km and C1" (Reference 2)

G = Gas specific gravity = mol. wt./29 P1 = Valve inlet pressure, psia AP = Pressure drop across valve, psi Q = Gas flow rate, SCFH Qs = Steam or vapor flow rate, lb/hr T = Absolute temperature of gas at inlet, ~ Tsh = Degrees of superheat, ~

Km

Ci

Type of Valve

Cage-trim globes: Unbalanced

0.8

33 33

Balanced

0.70

Butterfly: Fishtail

0.43 0.55

16

Conventional

24.7

Ball: Vee-ball, modified-ball, etc.

0.40 0.30

22

Full-area ball

The control valve coefficients in Table 4 are for full open conditions. The control valve must be designed to operate at partial open conditions for good control. Figure 3 shows partial open performance for a number of trim types.

Conventional globe: Single and double port (full port) Single and double port (reduced port) Angle: Flow tends to open (standard body) Flow tends to close (standard body) Flow tends to close (venturi outlet) Three way

0.75 0.65 0.75 0.85 0.50 0.20 0.72 0.46 0.80

35 35

General Control Valve Rules of Thumb

Camflex:

1. Design tolerance. Many use the greater of the following: Qsizing- 1.3 Qnormal Qsizing = 1.1 Qmaximum

Flow tends to close Flow tends to open

24.9 31.1

Split body

35

*For use only if not available from manufacturer.