VALVE AND ITS APPLICATIONS

Valve is a mechanical means used to perform basically following functions.

1. To throttle the fluid, flowing in a line either as pressure regulation or flow regulation

2. To isolate the part of system to carry out the maintenance job in safe manner

3. To prevent reversal flow

4. To release excess fluid from system

VALVE DESIGN FACTORS

Pressure

Temperature

Fluid properties

Flow pattern

Line size

Pressure drop across the valve

Type Of fluid

VALVE CLASSIFICATION

Stem Movement

Rotary type

Sliding type

Application Purpose

Isolation

Control

TYPES OF VALVES

Gate valve

Globe valve

Angle valve

Diaphragm valve

Plug valve

Ball valve

Butterfly valve

Check

GATE VALVE

In a gate valve the diameter of the opening through which the fluid passes is nearly the same as that of the pipe, and the direction of flow does not change. As a result, a wide-open gate valve introduces only a small pressure drop. The disk is tapered and fits into a tapered seat; when the valve is opened, the disk rises into the bonnet, completely out of the path of the fluid. Gate valves are used only for isolation purpose.

These valves are of three types:

1. Solid wedge type

2. Flexible wedge shaped inclined seat type

3. Double disc parallel seat type Solid wedge Type

Water or sewage line

Minimum passing persist

Flexible Wedge Shaped Inclined Seat Type

All type fluid except corrosive liquid

Tight shut off

Low temperature fluid

Low-pressure fluid

Minimum galling and chattering

Plug self-adjusting

Double Disc Parallel Seat Type

Tight shut off

High temperature fluid

GLOBE VALVE

Glove valves (so called because in the earliest designs the valve body was spherical) are widely used for controlling flow. The opening increases almost linearly with stem position, and wear is evenly distributed around the disk. The fluid passes through a restricted opening and changes direction several times. As a result the pressure drop in this kind of valve is large. Moreover these valves may be single seated or double seated.

ANGLE VALVE

1. Single seated

2. High pressure drop

3. Stream lined interior face

4. Self draining design

DIAPHRAGM VALVE

Pressure limitation

Temperature limitation

Less down time

Easy installation

Easy maintenance

Simple design

Used for handling of corrosive and suspended solid containing fluid

BUTTERFLY VALVE

Less space required

Tight sealing with elastomer

Control purpose

Low cast

Lightweight

Low torque to operate

Used for handling of corrosive & suspended solid containing fluid

Plug Valve

These are used below 250C. A quarter turn of the stem takes the valve from fully open to fully closed, and when fully open, the channel through the plug may be as large as the inside of the pipe itself, and the pressure drop is minimal. They are tight shutoff type with elastomer facing. In plug valves the area of contact between moving element and seat is large, therefore they can be used in throttling service.

Ball Valve

Like Plug valves these also face temperature limitation because of elastomer facing. In a ball valve the sealing element is spherical, and the problems of alignment and freezing of the element are less than with a plug valve. Like plug valve, the area of contact between moving element and seat is large, therefore they can also be used in throttling service.

Needle Valve

Fine controlling

Used for instrument impulse line

Cryogenic Valve

Suitable for low temperature fluid

Extension bonnet

Used for cryogenic fluid

CHECK VALVE

A check valve permits flow in one direction only. It is opened by the pressure of the fluid in the desired direction; when the flow stops or tends to reverse, the valve automatically closes by gravity or by a spring pressing against the disk.

Check valves are of three types

i) Swing Check Valve

Used in horizontal line

Works under force of gravity

Passing at low pressure

ii) Lift Check Valve

Vertical lift check valve

Globe check valve

Angle check valve

iii) Tilting Check Valve

Installed in vertical upward or horizontal line

Pressure drop varies with velocity

Speedy closing

iv) Spring Loaded Check Valve

High pressure drop

Speedy closing

Used at high pressure

VALVE APPLICATION

1. On and off control

2. Regulation

3. Venting

4. Air fail to open

5. Air fail to close

6. Air to open

7. Air to close

8. Prevent reversal flow

9. Release excess fluid

CONTROL VALVE

A valve with a power positioning actuator for moving the valve closure element to any position relative to valve port or ports in response and in proportion to an external signal sent by control loop. An independent source is used to provide energy for control valve actuator.

COMPONENTS OF CONTROL VALVE

Actuator

Means, which convert the control signal into, stem force required for valve plug travel. It may be a diaphragm, piston & cylinder, electric, hydraulic, electric motor, or solenoid.

Actuator spring

A spring that moves the actuator stem in a direction opposite to that created by diaphragm pressure.

COMPONENTS OF CONTROL VALVE

Actuator Stem

A rod like extension of diaphragm plate to permit convenient external connection

Bonnet Assembly

Removable upper part of the valve body having part through witch stem moves. It contains packing housing that prevent fluid leakage through stem and provides mounting means for actuator.

COMPONENTS OF CONTROL VALVE

Diaphragm

It is a pressure responsive element that transmits force to diaphragm plate and actuator stem

Cage

It is a hallow cylindrical trim element that is guide to align the movement of the plug with seat ring. It retains the seat ring in the valve body and its wall opening determines the flow characteristics of control valve.

CONTROL VALVE TERMINOLOGY

Actuator Stem Force

The net force from an actuator that is available for positioning of the valve plug.

Actuator Travel Characteristics

Relation between percent travel of stem and diaphragm pressure.

Valve Flow Coefficient (Cv)

This is equal to number of U. S. Gallons per minute of 60 f water that will flow through a valve with 1 psi drop under stated condition.

Formulas for Cv calculations

LIQUIDS:

Cv = 1.17 Q(d/p)

Q: FLOW RATE IN M/Hr

d: Specific Gravity of Liquid in Kg/Mat atmospheric temperature

p:Pressure Drop in Bar STEAM:

1st Case: absolute outlet pressure is greater than 50% of absolute inlet pressure

Cv = Q/16( p.P1)

Q

- FLOW RATE IN Kg/Hr

P1 - Absolute inlet pressure of steam in bar abs

p - Pressure Drop in Bar IInd Case: Absolute outlet pressure is lower than 50% of absolute inlet pressure

Cv = Q/(10.P1)

Q- Flow rate in Kg/Hr

P1 - Absolute inlet pressure of steam in bar abs

Note: Formulae given above apply to saturated steam; for superheated steam correction factors are required.

GAS:

1st Case: absolute outlet pressure is greater than 50% of absolute inlet pressure

Cv = Q/380 (d.T)/( p.P2)

Q:Flow rate in Kg/Hr

P1: Absolute outlet pressure of gas in bar abs

d:Specific Gravity of Gas referred to air(Air=1)

p:Pressure Drop in Bar IInd Case: Absolute outlet pressure is lower than 50% of absolute inlet pressure (CRITICAL FLOW)

Cv = (Q/205.P1) d.T

Q:Flow rate in Kg/Hr

P1:Absolute inlet pressure of steam in bar abs

Capacity

Rate of flow through a valve under stated condition.

Clearance Flow

This is the minimum controllable flow with valve plug not seated.

Dead Band

The amount of diaphragm pressure that can be varied without initiating valve plug motion.

Diaphragm Pressure Span

This is the difference between high and low diaphragm pressure to produce rated travel of valve plug. It may be stated as inherent or installed characteristics.

Rated Travel

Linear movement of valve plug from full open to full closed condition.

Stem Unbalance

The net force produced on valve plug in any condition by fluid pressure acting upon it.

Flow Characteristics

This is the relation between flow through the valve and percent rated travel of valve plug. It should be designated as either inherent or installed flow characteristics.

Equal Percentage Flow Characteristics

An inherent flow characteristic witch for equal increment of rated travel will ideally give equal %age change of existing flow.

Linear Flow Characteristics

An inherent flow characteristics which can be represented ideally by straight line on a rectangular plot of flow versus % rated travel.

Quick Opening Flow Characteristics

An inherent flow characteristic in there is maximum flow at minimum travel of valve plug.

Modified Parabolic Flow Characteristics

An inherent flow characteristic which provide fine controlling at low valve plug travel and approximately a linear characteristic for upper portion of valve travel.

SEAT LEAKAGE CLASS:

AS PER ANSI B.16.104 - SIX CLASSES

CLASS I: No specified values mutually agreed between customer and supplier.

CLASS II: 0.5 % of the rated full open capacity.

CLASS III: 0.1 % of the rated full open capacity.

CLASS IV: 0.01 % of the rated full open capacity.

CLASS V: 5x10-4 ml/min/inch/psi pressure differential.

CLASS VI: Bubbles/ min depends on port areafor soft-seated valves.