manual para cv tablas

7/28/2019 Manual Para Cv Tablas

1/12

Valve Siz ingTechnical Bul let in

ScopeValve size often is described by the nominal size of the endconnections, but a more important measure is the flow that thevalve can provide. And determining flow through a valve canbe simple.

This technical bulletin shows how flow can be estimated wellenough to select a valve sizeeasily, and without complicated

calculations. Included are the principles of flow calculations,some basic formulas, and the effects of specific gravity andtemperature. Also given are six simple graphs for estimatingthe flow of water or air through valves and other componentsand examples of how to use them.

Sizing ValvesThe graphs cover most ordinary industrial applicationsfromthe smallest metering valves to large ball valves, at systempressures up to 10 000 psig and 1000 bar.

The water formulas and graphs apply to ordinary liquidsandnot to liquids that are boiling or flashing into vapors, to slurries(mixtures of solids and liquids), or to very viscous liquids.

The air formulas and graphs apply to gases that closely followthe ideal gas laws, in which pressure, temperature, andvolume are proportional. They do not apply to gases or vaporsthat are near the pressure and temperature at which theyliquefy, such as a cryogenic nitrogen or oxygen.

For convenience, the air flow graphs show gauge pressures,whereas the formulas use absolute pressure (gauge pressureplus one atmosphere). All the graphs are based on formulasadapted from ISA S75.01, Flow Equations for Sizing ControlValves.1

Safe Product Selection

When selecting a product, the total system design must

be considered to ensure safe, trouble-free performance.

Function, material compatibility, adequate ratings, proper

installation, operation, and maintenance are the

responsibilities of the system designer and user.

www.swagelok.com

Flow Calculation PrinciplesThe principles of flow calculations are illustrated by thecommon orifice flow meter (Fig. 1). We need to know only thesize and shape of the orifice, the diameter of the pipe, and thefluid density. With that information, we can calculate the flowrate for any value of pressure drop across the orifice (thedifference between inlet and outlet pressures).

For a valve, we also need to know the pressure drop and thefluid density. But in addition to the dimensions of pipe diameterand orifice size, we need to know all the valve passagedimensions and all the changes in size and direction of flowthrough the valve.

However, rather than doing complex calculations, we use thevalve flow coefficient, which combines the effects of all theflow restrictions in the valve into a single number (Fig. 2).

Pressure drop

Orificeshape

Pipediameter

Orifice diameterFluid density

Pressure drop

Pipediameter Size

changes

Valvepassage

size

Directionchanges

Fluiddensity

Fig. 1. The flow rate through a fixed orifice can be calculated from the

meter dimensions of pipe diameter and orifice size and shape.

Fig. 2. Calculating the flow rate through a valve is much more

complex. The valve flow coefficient (Cv) takes into account all the

dimensions and other factorsincluding size and direction changes

that affect fluid flow.
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2/12

2 Valve Sizing

Valve manufacturers determine the valve flow coefficient bytesting the valve with water at several flow rates, using a

standard test method2 developed by the Instrument Society ofAmerica for control valves and now used widely for all valves.

Flow tests are done in a straight piping system of the samesize as the valve, so that the effects of fittings and piping sizechanges are not included (Fig. 3).

Liquid Flow

Because liquids are incompressible fluids, their flow rate

depends only on the difference between the inlet and outletpressures (D p, pressure drop). The flow is the same whetherthe system pressure is low or high, so long as the differencebetween the inlet and outlet pressures is the same. Thisequation shows the relationship:

The water flow graphs (pages 6 and 7) show water flow as afunction of pressure drop for a range of Cvvalues.

Gas Flow

Gas flow calculations are slightly more complex, becausegases are compressible fluids whose density changes withpressure. In addition, there are two conditions that must beconsideredlow pressure drop flow and high pressure dropflow.

p1 p2

Cv

p= p1 p2

q

Gf

q= N1Cvp

Gf

p2p1

p

Flow controlvalve

Flow meter

Minimum lengths of straight pipe

Standard distancesfrom test valve

Flow controlvalve

Test valve

Fig. 3. Valve manufacturers determine flow

coefficients by testing the valve with water

using a standard ISA test method.

Symbols Used in Flow Equations

Cv = flow coefficient

q = flow rate

p1 = inlet pressure

p2 = outlet pressureD p = pressure drop (p1p2)Gf = liquid specific gravity (water = 1.0)

Gg = gas specific gravity (air = 1.0)

N1, N2 = constants for units

T1 = absolute upstream temperature:

K = C + 273

R = F + 460

Note: p1 and p2 are absolute pressures for gas flow.


3/12

Valve Sizing 3

This equation applies when there is low pressure drop flowoutlet pressure (p2) is greater than one half of inlet pressure

(p1):

The low pressure drop air flow graphs (pages 8 and 9) showlow pressure drop air flow for a valve with a Cvof 1.0, given asa function of inlet pressure (p1) for a range of pressure drop(D p) values.When outlet pressure (p2) is less than half of inlet pressure(p1)high pressure dropany further decrease in outlet

p1 p2

Cv

p2 > 1/2p1

T1

q

Gg

q= N2Cvp1p

p1GgT1(1 )2p

3p1

pressure does not increase the flow because the gas hasreached sonic velocity at the orifice, and it cannot break that

sound barrier.

The equation for high pressure drop flow is simpler because itdepends only on inlet pressure and temperature, valve flowcoefficient, and specific gravity of the gas:

The high pressure drop air flow graphs (pages 10 and 11)show high pressure drop air flow as a function of inletpressure for a range of flow coefficients.

p1 p2

Cv

p2 < 1/2p1

T1

q

Gg

q= 0.471 N2Cvp11

GgT1

p1 p2

High pressure drop

Sonic flow

p1

p2

Low pressure drop

p1 p2

Maximum flow

p2

< 1/2p1

q

p2

> 1/2p1

q

p2

= 1/2p1

q

Low and High Pressure Drop Gas Flow

The basic orifice meter illustrates thedifference between high and lowpressure drop flow conditions.

In low pressure drop flowwhen outletpressure (p2) is greater than half of inletpressure (p1)outlet pressure restrictsflow through the orifice: as outlet

pressure decreases, flow increases,and so does the velocity of the gasleaving the orifice.

When outlet pressure decreases to halfof inlet pressure, the gas leaves theorifice at the velocity of sound. The gascannot exceed the velocity of soundandthereforethis becomes themaximum flow rate. The maximum flowrate is also known as choked flow orcritical flow.

Any further decrease in outlet pressuredoes not increase flow, even if the outlet

pressure is reduced to zero.Consequently, high pressure drop flowonly depends on inlet pressure and notoutlet pressure.


4/12

4 Valve Sizing

Effects of Specific Gravity

The flow equations include the variables Gfand Ggliquidspecific gravity and gas specific gravitywhich are the densityof the fluid compared to the density of water (for liquids) or air(for gases).

However, specific gravity is not accounted for in the graphs, soa correction factor must be applied, which includes the squareroot of G. Taking the square root reduces the effect and bringsthe value much closer to that of water or air, 1.0.

For example, the specific gravity of sulfuric acid is 80 % higher

than that of water, yet it changes flow by just 34 %. The

specific gravity of ether is 26 % lower than that of water, yet itchanges flow by only 14 %.

Figure 4 shows how the significance of specific gravity onliquid flow is diminished by taking its square root. Only if thespecific gravity of the liquid is very low or very high will theflow change by more than 10 % from that of water.

The effect of specific gravity on gases is similar. For example,the specific gravity of hydrogen is 93 % lower than that of air,but it changes flow by just 74 %. Carbon dioxide has a specificgravity 53 % higher than that of air, yet it changes flow by only24 %. Only gases with very low or very high specific gravitychange the flow by more than 10 % from that of air.

Figure 5 shows how the effect of specific gravity on gas flow is

reduced by use of the square root.

Effects of Temperature

Temperature usually is ignored in liquid flow calculationsbecause its effect is too small.

Temperature has a greater effect on gas flow calculations,because gas volume expands with higher temperature and

P

ercentchange

+80

+60

+40

+20

0

20

40

Alcohol Oils Nitric acid Sulfuric acidEther

Change in specific gravity

Change in flow rate

Percentchange

+80

80

+40

120

40

0

Hydrogen Natural gas Oxygen Argon Carbon

dioxide

Change in specific gravity

Change in flow rate

Fig. 4. For most common liquids, the

effect of specific gravity on flow is less

than 10 %. Also, most high-density liquids

such as concentrated acids and bases

usually are diluted in water and

consequentlythe specific gravity of the

mixtures is much closer to that of water

than to that of the pure liquid.

Fig. 5. For common gases, the specific

gravity of the gas changes flow by less

than 10 % from that of air. And just as

with liquids, gases with exceptionally high

or low densities often are mixed with a

carrier gas such as nitrogen, so that the

specific gravity of the mixture is close to

that of air.


5/12

Valve Sizing 5

contracts with lower temperature. Butsimilar to specificgravitytemperature affects flow by only a square-root factor.For systems that operate between 40F (40C) and +212F(+100C), the correction factor is only +12 to 11 %.

Figure 6 shows the effect of temperature on volumetric flowover a broad range of temperatures. The plus-or-minus 10 %range covers the usual operating temperatures of mostcommon applications.

Other ServicesAs noted above, this technical bulletin covers valve sizing for

many common applications and services. What about viscousliquids, slurries, or boiling and flashing liquids? Supposevapors, steam, and liquefied gases are used? How are valvessized for these other services?

Such applications are beyond the scope of this document, butthe ISA standards S75.01 and S75.02 contain a complete set

of formulas for sizing valves that will be used in a variety ofspecial services, along with a description of flow capacity testprinciples and procedures. These and other ISA publicationsare listed in the references. Also, most standard engineering

handbooks have sections on fluid mechanics. Several of theseare given in the references, as well.

Cited References1. ISA S75.01, Flow Equations for Sizing Control Valves,

Standards and Recommended Practices forInstrumentation and Control, 10th ed., Vol. 2, 1989.

2. ISA S75.02, Control Valve Capacity Test Procedure,Standards and Recommended Practices for

Instrumentation and Control, 10th ed., Vol. 2, 1989.

Other ReferencesL. Driskell, Control-Valve Selection and Sizing, ISA, 1983.

J.W. Hutchinson, ISA Handbook of Control Valves, 2nd ed.,ISA, 1976.

Chemical Engineers Handbook, 4th ed., Robert H. Perry, CeciH. Chilton, and Sidney D. Kirkpatrick, Ed., McGraw-Hill, NewYork.

Instrument Engineers Handbook, revised ed., Bla G. Liptkand Kriszta Venczel, Ed., Chilton, Radnor, PA.

Piping Handbook, 5th ed., Reno C. King, Ed., McGraw-Hill,

New York.Standard Handbook for Mechanical Engineers, 7th ed.,Theodore Baumeister and Lionel S. Marks, Ed., McGraw-Hill,New York.

Percent

change

+20

+10

0

10

20

30

Temperature, C

Temperature, F

40 0

0

+40 +80 +120 +160 +200

40

Change in flow rate

+100 +200 +300 +400

Fig. 6. Many systems operate in the

range 40F (40C) to +212F (+100C).

Within this range, temperature changesaffect flow by little more than 10 %.

Numerical Constants for Flow Equations

Constant Units Used in Equations

N q p T 1

N1 . . . . .1.0 U.S. gal/min psia

0.833 Imperial gal/min psia

14.42 L/min bar

14.28 L/min kg/cm2

N2 . . . .22.67 std ft3/min psia R

6950 std L/min bar K

6816 std L/min kg/cm2 K


6/12

6 Valve Sizing

Example:

I Enter the vertical scale with the pressure drop across the valve ( D p= 60 psi).I Read across to the desired flow rate (q= 4 U.S. gal/min).I The diagonal line is the desired Cvvalue (Cv= 0.50).

0.001 0.002 0.003 0.006 0.01 0.02 0.03 0.04 0.06 0.1 0.2 0.3 0.4 0.6 1.0 2.00.08 0.8

10 000

6000

8000

3000

2000

1000

600

800

400

300

200

100

60

80

40

30

20

10

4000

Cv

1000

600

800

400

300

200

100

60

80

40

30

20

10

6

8

4

3

2

1

0.0010

0.0025

0.0050

0.010

0.025

0.050

0.10

0.25

0.10

0.25

0.50

1.0

2.5

5.0

10

25

50

100

250

1 2 3 6 10 20 30 40 60 100 200 300 400 600 100080 8004 8

Cv= 0.0005

Flow, U.S. gal/min

Water FlowU.S. Units

PressureDrop,psi


7/12

Valve Sizing 7

0.0010

0.0025

0.0050

0.010

0.025

0.050

0.10

0.25

0.50

0.25

1.0

0.10

2.5

5.0

10

25

50

100

250

Cv= 0.0005

0.01 0.02 0.03 0.06 0.01 0.2 0.3 0.4 0.6 1.0 2.0 3.0 4.0 6.00.80.004 0.006

1000

600

800

300

200

100

60

80

40

30

20

10

6

8

4

3

2

1

400

100

60

80

40

30

20

10

6

4

3

2

1.0

0.1

0.6

0.8

0.4

0.3

0.2

0.04

2000 3000 40006 10 20 30 40 60 100 200 300 400 600 100080 8004 8

8

Example:

I Enter the vertical scale with the pressure drop across the valve ( D p= 30 bar).I Read across to the desired flow rate (q= 0.2 L/min).I The diagonal line is the desired Cvvalue (Cv= 0.0025).

PressureDrop,

bar

Flow, L/min

Water FlowMetric Units


8/12

8 Valve Sizing

Example:

I Enter the vertical scale with the inlet pressure at the valve (p1 = 200 psig).

I Read across to the diagonal line for the pressure drop across the valve (D p= 25 psi).I Read down to the horizontal scale for the flow rate through a valve with a Cvof 1.0 (q= 65 std ft

3/min).

I Multiply that flow rate by the valve Cv to determine the actual flow rate.

500 psi

p= 5 psi

10 psi

25 psi

50 psi

100 psi

250 psi

High pressure drop flow

10 20 30 50 100 200 300 500 600 1000800

1000

600

800

300

200

100

60

80

40

30

20

400

3000

2000

40040 60 80

Flow, std ft3/min

Low Pressure Drop Air FlowU.S. Units

InletPressure,psig


9/12

Valve Sizing 9

p= 0.25 bar

0.50 bar

1.0 bar

2.5 bar

10 bar

25 bar

High pressure drop flow

5 bar

2000 3000 10 000 20 000 30 000600 1000800

100

60

80

30

20

10

6

8

4

3

2

40

50

200

5

400300 4000 6000 8000

Example:

I Enter the vertical scale with the inlet pressure at the valve (p1 = 100 bar).

I Read across to the diagonal line for the pressure drop across the valve (D p= 1 bar).I Read down to the horizontal scale for the flow rate through a valve with a Cvof 1.0 (q= 4000 std L/min).

I Multiply that flow rate by the valve Cv to determine the actual flow rate.

I

nletPressure,

bar

Flow, std L/min

Low Pressure Drop Air FlowMetric Units


10/12

10 Valve Sizing

Example:

I Enter the vertical scale with the inlet pressure at the valve (p1 = 200 psig).

I Read across to the desired flow rate (q= 10 std ft3/min).

I The diagonal line is the desired Cvvalue (Cv= 0.10).

0.10

0.25

0.50

1.0

2.5

5.0

10

25

50

100

250

0.01 0.02 0.03 0.06 0.1 0.2 0.3 0.4 0.6 1.0 2.0 3.0 4.0 6.0 10 200.8 8.0

3000

2000

1000

600

800

400

300

200

100

60

80

40

30

20

Cv

0.0010

0.0025

0.0050

0.010

0.025

0.050

0.10

0.25

0.50

500

50

1000

600

800

400

300

200

100

60

80

40

30

20

500

50

0.04 0.08

10 20 30 40 100 200 300 400 600 1000 2000 3000 4000 6000 10 00080060 80

Cv= 0.0005

Flow, std ft3/min

High Pressure Drop Air FlowU.S. Units

InletPressure,psig


11/12

Valve Sizing 11

0.25

0.50

0.050

1.0

0.10

2.5

5.0

10

25

50

100

250

0.0010

0.0025

0.0050

0.010

0.025

0.050

0.10

0.25

Cv= 0.0005

200

100

50

80

40

30

20

10

6

8

4

3

2

60

5

50

80

40

30

20

10

6

8

4

3

2

60

5

100 3000.3 0.6 1.0 2 3 4 6 10 20 30 608 80 200400.4 0.8

100 000 200 000300 600 1000 2000 3000 4000 6000 10 000 20 000 60 000200 40 000400 800

Example:

I Enter the vertical scale with the inlet pressure at the valve (p1 = 20 bar).

I Read across to the desired flow rate (q= 4000 std L/min).

I The diagonal line is the desired Cvvalue (Cv= 1.0).

InletPressure,

bar

Flow, std L/min

High Pressure Drop Air FlowMetric Units


12/12

SwagelokTM Swagelok Company 1994, 1995, 2000, 2002 Swagelok CompanyJuly 2002, R3MS-06-84-E

Safe Product Selection

When selecting a product, the total system design must

be considered to ensure safe, trouble-free performance.

Function, material compatibility, adequate ratings,

proper installation, operation, and maintenance are the

responsibilities of the system designer and user.

Caution: Do not mix or interchange parts with those of

other manufacturers.

manual para cv tablas

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