FLUID FLOW BASICS OF THROTTLING VALVES - Control

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FLUID FLOW BASICS OF THROTTLING VALVES - Control

Transcript Of FLUID FLOW BASICS OF THROTTLING VALVES - Control

01-99
FLUID FLOW BASICS OF
THROTTLING VALVES

FLUID FLOW BASICS OF THROTTLING VALVES
FLUID PARAMETERS –
The following fluid parameters are frequently associated with throttling valves –

Cond. Inlet 1

Cond. 2 Outlet

Flow Quantity
Pressures

U pstre a m

T hrottling Va lve

D ownstre a m

∆PSize = P1 - P2

∆T = T1 - T2

CONDITION 1

CONDITION 2

.

.

m. 1 ._______________________ Mass Flow Rate ___________ m. 2 .

Q1, V1 _________________ Volumetric Flow Rate _________ Q2, V2

P1, PS1, P1(Abs) ___________ Static Pressure ___________ P2, PS2, P2 (Abs)

PV1 ______________________ Velocity Pressure __________ PV2

PVC – Pressure @ Vena Contracta

∆Psize – Sizing Pressure Drop

PVP1, PSAT1

Vapor Pressure Saturation Pressure

PVP2, PSAT2

Thermodynamic
Relative Weight/Mass
Geometric
Fluid “Resistance
to Flow”

T1, 1(Abs), [email protected]°SH _______ Temperature _____________ T2, 2(Abs), T [email protected]°SH TSAT1 ________________ Saturation Temperature ________ TSAT2 H1, h1 ________________________ Enthalpy ______________ H2, h2 V 1 _______________________ Specific Volume ___________ V 2
1 ________________________ Liquid Density ____________ 2
γ1 ______________________ Actual Gas Density __________ γ2
SG1 ______________________ Specific Gravity ___________ SG2 Ø1P _______________________ Pipe Diameter ____________ Ø2P Ø1V ______________________ Valve Body Size ___________ Ø2V Z1 ________________________ Elevation Head ___________ Z2 A1 ______________________ Cross-Section Area __________ A2 v1V _____________________ Avg. Valve Velocity __________ v2V v1P ______________________ Avg. Pipe Velocity __________ v2P
µ1 ______________________ Absolute Viscosity __________ µ2
1 ______________________ Kinematic Viscosity _________ 2

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THE BASICS OF THROTTLING VALVES
THROTTLING VALVES
Valves that are utilized as fluid control devices are typically “throttling valves”.

Cond. Inlet 1
U pstre a m

T hrottling Va lve
∆ P = P1 - P2

Cond. 2 Outlet
D ownstre a m

Such valves experience internal velocity and internal pressure gradients (both positive and negative) that conclude with a permanent pressure loss (∆P) from the inlet pipe-to-outlet pipe connections. Throttling valve trim (plug-seat) experiences relatively high internal velocities nearly 100% of operating time. In comparison, ON-OFF automated or manual valves experience velocity changes ONLY when being actuated from “open-to-closed”, or vice versa; i.e. a few seconds or minutes.
Bernoulli’s Theorem is the most useful tool in analyzing what is going on physically within the walls of a throttling valve, which includes —
• velocity gradients • pressure gradients
The other important tool is the 1st Law of Thermodynamics which allows analyzing —
• fluid state • thermal effects
Bernoulli’s principles apply to the following for throttling valves —
• inlet pipe reducer • pressure drop to main orifice • pressure recovery to outlet • outlet pipe reducer
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When the pressure gradients are graphically shown, one ends up with the rather typical “vena contracta” curve —
The velocity gradients form a sort of “inverse” of the vena contracta curve —
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The depth of the vena contracta “dip” is primarily a function of a throttling valve’s geometry; globe vs. butterfly, etc. The important parameter in determining the PVC is – “FL – Liquid Pressure Recovery Factor”. As the name implies, the FL factor is a measure of the effectiveness of the reconversion of velocity pressure into static pressure from the main orifice of the throttling valve (@ vena contracta) to the valve’s outlet. The following graphic attempts to give relative representation of the four major valve styles used for throttling service.
Both butterfly and ball valves are sub-classified as “high recovery valves”. As a general rule, globe and eccentric plug (rotary globe) styles tend to make “better” throttling control valves.
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FLUID STATES

Fluid flow is classified into two basic fluid states at the inlet.

LIQUID (Non-Compressible)

GAS–VAPOR (Compressible)

As pressure changes occur within a throttling valve, it is possible to produce 2-phase flow at the valve’s outlet for either a liquid or gas-vapor at the inlet.

A “vapor” is a “gas” that is at, or relatively near, its “saturation” (boiling) conditions of pressure and temperature; i.e. saturated vapor or slightly superheated vapor. A “gas” is a fluid that does not liquify at reduced temperatures, or is a highly superheated vapor.

Throttling valves operate as “steady state, steady flow” devices. The entering and exiting mass flow rates are the same; i.e. flow is “continuous”, and the Continuity Equation is applicable —
..
m1 = m2

(EQ. #1)

1A1v1 = 2A2v2 (no phase change)

1A1v1 = 2VA2Vv2V + 2LA2Lv2L (2-phase outlet)

Vapor

Liquid

It is a thermodynamic principle that whenever there is a phase change between a throttling valve’s entering and exiting fluid state, there is also a temperature change (i.e. decrease or cooling) in all such applications —

T1 > T2 LIQUIDS. For simple “liquid-in and liquid-out” flow there is no density change of the liquid —

1= 2

This constant density results in other parameters being typically affected —

LIQUID

Cond. 1

U pstre a m

..
m1 = m2 v1 = v2

T hrottling Va lve
∆P = P1 - P2
A1 = A2 Ø1 = Ø2
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Cond. 2
D ownstre a m

LIQUID

T1 = T2 — no phase change
1= 2

GAS-VAPORS. For simple “gas-vapor-in and gas-vapor-out” flow, there is a density change (i.e. decrease) of the gas-vapor as the fluid decompresses (i.e. expands) —
1> 2
This changing density results in other parameters being typically affected —

GAS-VAPOR

Cond. 1

U pstre a m

T hrottling Va lve

..
m1 = m2

∆ P = P1 - P2

A1 < A2

T1 ≈ T2

v1 > v2

Ø1 < Ø2
1> 2

T1 > T2

Cond. 2
D ownstre a m

GAS-VAPOR

GAS or HIGHLY SUPERHEATED VAPOR
VAPOR

THERMODYNAMIC PRINCIPLES
THROTTLING PROCESS. In looking into the thermodynamic principles of a “throttling process”, we know —

Cond. Inlet 1

Cond. 2 Outlet

U pstre a m

T hrottling Va lve
∆h = h1 - h2 ∆h = 0

D ownstre a m

THE CHANGE IN ENTHALPY ACROSS A RESTRICTION IN A PIPE — ORIFICE, REGULATOR, CONTROL VALVE — IS “ZERO” FOR A THROTTLING PROCESS.

By the continuity equation — (EQ. #2)

.. m. 1 = m2. m1h1 – m2h2 = 0 OR . m(h1–h2) = 0

Parameter

h1 = h.2 = m. 1 = m2 =

Valve Inlet Enthalpy Valve Outlet Enthalpy Inlet Mass Flow Outlet Mass Flow

English Units
Btu/# Btu/# #/Hr #/Hr
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Metric Units
kJ/kg kJ/kg kg/Hr Kg/Hr

It is the use of fluid thermodynamic data and the thermodynamic principles of the “constant enthalpy throttling process” that throttling valves experience which allows an accurate determination of a fluid’s state while internal to the valve as well as at the valve’s outlet. In particular, we want to know what the fluid is physically doing at the throttling valve’s main orifice (plugseat); i.e. what is occurring at the vena contracta and elsewhere within the valve?

SATURATION STATE. A fluid is said to be “saturated” when —

• Liquid - when at the boiling temperature – Tsat – for a given pressure – Psat

Examples: Water @ Psat = 14.7 psia

Tsat = 212°F

Psat = 1.0135 BarA

Tsat = 100°C

Water @Psat = 145 psig Psat = 10 BarA

Tsat = 355.8°F Tsat = 179.9°C

• Vapor - when at the boiling temperature – Tsat – for a given pressure – Psat

Examples: Steam @ Psat = 14.7 psia

Tsat = 212°F

Psat = 1.0135 BarA

Tsat = 100°C

Steam @ Psat = 145 psig Psat = 10 BarA

Tsat = 355.8°F Tsat = 179.9°C

Restating the above examples, we have both saturated liquid water (condensate) and saturated steam at the same Psat and Tsat. Further, for any given fluid in its “saturation” state, when we have its pressure (Psat), we KNOW its temperature (Tsat). To say a fluid is “saturated” is to give a property of the fluid. Only two extensive properties of a fluid will locate the fluid in the physical universe. We know exactly where a fluid is when we say the fluid is —

• saturated water at Psat = 29 psia = 2.0 BarA, we know that Tsat = 248.4°F = 120.1°C. • saturated steam at Tsat = 212°F = 100°C, we know that Psat = 14.7 psig = 1.013 BarA.

SUPERHEATED VAPOR. A fluid is a superheated vapor when its temperature is greater than Tsat corresponding to the flowing pressure.

Examples: Steam @ P1 = 145 psia & T1 = 425°F (Tsat = 355.8°F)

69.2°F SH

P1 = 10 BarA & T1 = 219°C (Tsat = 179.9°C)

39.1°C SH

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To say a vapor is “superheated” does NOT give an extensive property of the fluid; so, a second property must also be known to physically locate a superheated vapor in the universe.

SUB-COOLED LIQUID. A liquid is sub-cooled when its temperature is less than Tsat corresponding to the flowing pressure.

Example: Water @

P = 145 psia & T = 60°F P = 10 BarA & T = 15.5°C

(Tsat = 355.8°F) (Tsat = 179.9°C)

To say a liquid is “sub-cooled” is NOT to give an extensive property of the fluid; so, a second property must also be known to physically locate a sub-cooled liquid in the universe.

THUMB CURVE – T vs. H GRAPH. The following graph is plotted using thermodynamic data for steam condensate; i.e. straight out of the “Steam Tables”.

These graphs are NOT useful for sub-cooled liquids; they are ONLY useful for analyzing boiling (vaporizing) liquid/vapors and superheated vapors. The “critical” properties of “Critical Pressure - Pc” and “Critical Temperature - Tc” are located at the “peak” of the T vs. H Curve.
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The important consideration for throttling processes is —
∆H = 0
This means that from Condition 1 -to-Condition 2 for throttling valves, you move downwards along a vertical line, as the enthalpy does not vary.

Examples:

Hot Condensate P1 = 150 psig ≈ 165 psia T1 = saturated P2 = 35 psig ≈ 50 psia

Hot Condensate

P1 = 9 Barg ≈ 10 BarA

1

T1 = saturated

P2 = 1.5 Barg ≈ 2.5 BarA

} T1 ≈ 366°F ∆T = -76°F

T2 ≈ 290°F

2

Desc2 = “Flashing”–2-phase

Steam

P1 = 250 psig ≈ 265 psia

T1 = Saturated

1

P2 = Atm = 14.7 psia

} T1 ≈ 180°C

∆T = -53°C

T2 ≈ 127°C

Desc2 = “Flashing”–2-phase

Steam P1 = 14 Barg ≈ 15 BarA T1 = Saturated P2 = Atm = 1.013 BarA

} T1 ≈ 406°F ∆T = -96°F

T2 ≈ 310°F

2

Desc2 = Superheated Steam

Steam

P1 = 435 psig ≈ 450 psia

T1 = 770°F

1

P2 = 185 psig ≈ 200 psia

} T1 ≈ 198°C

∆T = -48°C

T2 ≈ 150°C

Desc2 = Superheated Steam

Steam P1 = 29 Barg ≈ 30 BarA T1 = 410°C P2 = 14 Barg ≈ 15 BarA

Tsat @ P1 = 456°F

Desc1 = Superheated Steam

T2 ≈ 755°F

∆T ≈ -15°F

Desc2 = Superheated Steam

Tsat @ P1 = 234°C

2

Desc1 = Superheated Steam

T2 ≈ 400°C

∆T ≈ -10°C

Desc2 = Superheated Steam

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FluidTsatPsatPressureTemperature