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Control valves and cavitation, application ratio and multi stage control valves

When a fluid passes a valve the fluid velocity will increase and the pressure will drop according the Bernoulli equation. Cavitation

If the speed over the valve is high enough, the pressure in the liquid drop to a level where the fluid may start bubble or flash. The pressure recovers sufficiently and the bubbles collapse upon themselves.

Cavitation may be noisy but is usually of low intensity and low frequency.

This situation is extremely destructive and may wear out the trim and body parts of the valve in short time.

Application Ratio

A common way to characterize potential cavitation conditions, is to use the

A R= p-p o/ (p-p v)(1)

where

A R= Application Ratio

p= inlet pressure, absolute

p o= outlet pressure, absolute

p v= vapor pressure of the fluid, absolute

For application ratios above 1 -the fluid flashes. This is not the same as cavitation, but the closer the ratio is to 1, the higher is the potential for cavitation.

Note! With an increasing fluid temperature the danger for cavitation increases.

Example -Flashing Water

If we know the boiling point and the absolute pressure of a fluid ("Steam Table" with saturated steam properties),the minimum outlet pressure from a valve to avoid flashing can be calculated.

For an application ratio of 1 formula (1) can expressed:

1 = p-p o/ (p-p v)

p o= p v

Using "Steam Table" with saturated steam properties:

For a water temperature of 17.51 °C, absolute inlet pressure of 1 bar-minimum outlet pressure is 0.02 bar to avoid flashing

For a water temperature of 81.35 °C, absolute inlet pressure of 1 bar -minimum outlet pressure is 0.5 bar to avoid flashing.

For a water temperature of 99.63 °C, absolute inlet pressure of 1 bar -minimum outlet pressure is 1 bar to avoid flashing.

Note! Flashing is not the same as cavitation. Due to local conditions in the valves cavitation starts on much higher outlet pressures.

Multi Stage Control Valve s

Cavitation can be avoided by using more than one control valve or more convenient a multistage control valve.

The "vena contracta"is much lower for a single stage valve than a multi stage valve. Depending on the pressure drop and the temperature of the fluid its possible to avoid cavitation conditions using more than one stage.

Control Valves and Flow Characteristics

The relationship between control valve capacity and stem traveling

The relationship between control valve capacity and valve stem travel is known as

the Flow Characteristic of the Control Valve

Trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel. Because of the variation in trim design, many valves are not linear in nature. Valve trims are instead designed, or characterized, in order to meet the large variety of control application needs. Many control loops have inherent non linearity's, which may be possible to compensate selecting the control valve trim.

Inherent Control Valve Flow Characteristics

The most common characteristics are shown in the figure above. The percent of flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers. These curves are based on constant pressure drop across the valve and are called inherent flow characteristics.

Linear-flow capacity increases linearly with valve travel.

Equal percentage-flow capacity increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing Cv.

A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.

Quick opening provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.

Hyperbolic

Square Root

The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics.

Installed Control Valve Flow Characteristics

When valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel.

When the actual flow in a system is plotted against valve opening, the curve is called the Installed Flow Characteristic. In most applications, when the valve opens, and the resistance due to fluids flow decreases the pressure drop across the valve. This moves the inherent characteristic:

A linear inherent curve will in general resemble a quick opening characteristic

An equal percentage curve will in general resemble a linear curve Leakage Classifications of Control Valves

Classification of seat leakage through control valves

Control valves are designed to throttle and not necessary to close 100%.

A control valve's ability to shut off has to do with many factors as the type of valves for instance. A double seated

control valve have very poor shut off capability. The guiding, seat material, actuator thrust, pressure drop, and

the type of fluid can all play a part in how well a particular control valve shuts off.

Seat Leakage Classifications

There are actually six different seat leakage classifications as defined by ANSI/FCI 70-2 1976(R1982) .

The most common used are

CLASS IV

CLASS Vl

CLASS IV is also known as metal to metal. It is the kind of leakage rate you can expect from a valve with a

metal plug and metal seat.

CLASS Vl is known as a soft seat classification. Soft Seat Valves are those where either the plug or seat or both

are made from some kind of composition material such as Teflon or similar.

Valve Leakage Classifications

Class I -Valve Leakage Classifications

Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made. Class I is also

known as dust tight and can refer to metal or resilient seated valves.

Class II -Valve Leakage Classifications

Intended for double port or balanced singe port valves with a metal piston ring seal and metal to metal seats.

0.5% leakage of full open valve capacity.

Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 o F.

Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

Balanced, single port, single graphite piston ring, metal seat, low seat load

Balanced, double port, metal seats, high seat load

Class III -Valve Leakage Classifications

Intended for the same types of valves as in Class II.

0.1% leakage of full open valve capacity.

Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 o F.

Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

Balanced, double port, soft seats, low seat load

Balanced, single port, single graphite piston ring, lapped metal seats, medium seat load

Class IV -Valve Leakage Classifications

Intended for single port and balanced single port valves with extra tight piston seals and metal to-metal seats.

0.01% leakage of full open valve capacity.

Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 o F.

Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

Balanced, single port, Teflon piston ring, lapped metal seats, medium seat load

Balanced, single port, multiple graphite piston rings, lapped metal seats

Unbalanced, single port, lapped metal seats, medium seat load

Class IV is also known as metal to metal

Class V -Valve Leakage Classifications

Intended for the same types of valves as Class IV.

The test fluid is water at 100 psig or operating pressure.

Leakage allowed is limited to 5 x 10 ml per minute per inch of orifice diameter per psi differential.

Service dP at 50 to 125 o F.

Typical constructions:

Unbalanced, single port, lapped metal seats, high seat load

Balanced, single port, Teflon piston rings, soft seats, low seat load

Unbalanced, single port, soft metal seats, high seat load

Class Vl -Valve Leakage Classifications

Class Vl is known as a soft seat classification. Soft Seat Valves are those where the seat or shut-off disc or both

are made from some kind of resilient material such as Teflon. Intended for resilient seating valves.

The test fluid is air or nitrogen.

Pressure is the lesser of 50 psig or operating pressure.

The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through

8 inches.

Flow Coefficient Cv and Flow Factor Kv

Comparing the flow coefficient Cv with the flow factor Kv

The flow coefficient -Cv-and the flow factor -Kv-are commonly used to specify the capacity of control valves.

The Flow Coefficient -Cv

It is often convenient to express the capacities and flow characteristics of control valves in terms of the

Flow Coefficient -Cv

The flow coefficient -Cv-is based on the imperial units system and is defined as:

the flow of water through a valve at 60 o F in US gallon/minute at a pressure drop of 1 lb/in2

The flow coefficient is commonly used in the U.S.

The Flow Factor -Kv

The metric equivalent of the flow coefficient -Cv -is based on the SI-system and is called the

Flow Factor -Kv

The flow factor is defined as

the flow of water through a valve at 20 o C in cubic meters per hour with a pressure drop of 1 kg/cm2 (1 bar)

The flow factor is commonly used outside U.S.

Converting between Flow Coefficient Cv and Flow Factor Kv

The connection between Cv and Kv can be expressed as:

Cv = 1.16 Kv(1)

Kv = 0.853 Cv(2)

Testing Procedures

Required for

Establishing Rating

Test Pressure

Test

Medium

Maximum

Leakage

Allowable

Leakage Class

Designation

No test required

x

x

x

I

45 -60 psig or

maximum operating

differential whichever

is lower

45 -60 psig or

maximum

operating

differential

whichever is lower

Air or water

at 50 -125o

F

(10 -52o C)

0.5% of

rated

capacity

II

As above

As above

As above

0.1% of

rated

capacity

III

As above

As above

As above

0.01% of

rated

capacity

IV

Maximum service

pressure drop across

valve plug not to

exceed ANSI body

rating

Maximum service

pressure drop

across valve plug

not to exceed ANSI

body rating

Water at 50

to125o F (10

to 52o C)

0.0005 ml

per minute

of water per

inch of port

diameter

per psi

differential

V

Actuator should be

adjusted to operating

conditions specified

with full normal

closing thrust applied

to valve plug seat

50 psig or max

rated differential

pressure across

valve plug

whichever is lower

Air or

nitrogen at

50 to 125o F

(10 to 52o C)

Not to

exceed

amounts

shown in

the table

above

VI

ml per minute

Bubbles per minute

Port Diameter

Millimeters

inches

0.15

1

25

1

0.30

2

38

1 1/2

0.45

3

51

2

0.60

4

2 1/2

0.90

6

76

3

1.70

11

102

4

4.00

27

152

6

6.75

45

203

8

9

63

254

10

11.5

81

305

12

Control Valves and Flow Characteristics Leakage Classifications of Control Valves

Control Valves and Cavitation Leakage Classifications of Control Valves

Class Vl -Valve Leakage

Drawn by:

GTS

Date: May 2007