An Introduction To Globe ValvesTypes , Application, And Selection

An Introduction To Globe ValvesTypes , Application, And  Selection

A globe valve is a linear motion valve used to stop, start, and regulate fluid flow. .  the globe valve disk can be totally removed from the flowpath or it can completely close the flowpath. The essential principle of globe valve operation is the perpendicular movement of the disk away from the seat.

This causes the annular space between the disk and seat ring to gradually close as the valve is closed. This characteristic gives the globe valve good throttling ability, which permits its use in regulating flow. Therefore, the globe valve may be used for both stopping and starting fluid flow

and for regulating flow.

When compared to a gate valve, a globe valve generally yields much less seat leakage. This is because the disk-to-seat ring contact is more at right angles, which permits the force of closing to tightly seat the disk.

Globe valves can be arranged so that the disk closes against or in the same direction of fluid

flow. When the disk closes against the direction of flow, the kinetic energy of the fluid impedes

closing but aids opening of the valve. When the disk closes in the same direction of flow, the

kinetic energy of the fluid aids closing but impedes opening. This characteristic is preferable

to other designs when quick-acting stop valves are necessary.

Globe valves also have drawbacks. The most evident shortcoming of the simple globe valve is

the high head loss from two or more right angle turns of flowing fluid. Obstructions and

discontinuities in the flowpath lead to head loss. In a large high pressure line, the fluid dynamic

effects from pulsations, impacts, and pressure drops can damage trim, stem packing, and

actuators. In addition, large valve sizes require considerable power to operate and are especially

noisy in high pressure applications.

Other drawbacks of globe valves are the large openings necessary for disk assembly, heavier

weight than other valves of the same flow rating, and the cantilevered mounting of the disk to

the stem.

Globe Valve Body Designs

The three primary body designs for globe valves are Z-body, Y-body, and Angle.

Z-Body Design

The simplest design and most common for water applications is the Z-body. The Z-body is illustrated in Figure 1. For this body design, the Z-shaped diaphragm or partition across the globular body contains the seat. The horizontal setting of the seat allows the stem and disk to travel at right angles to the pipe axis. The stem passes through the bonnet which is attached to a large opening at the top of the valve body. This provides a symmetrical form that simplifies manufacture, installation, and repair.

FIG 1

Y-Body Design

Figure 2 illustrates a typical Y-body globe valve. This design is a remedy for the high pressure drop inherent in globe valves. The seat and stem are angled at approximately 45°.

The angle yields a straighter flowpath (at full opening) and provides the stem, bonnet, and packing a relatively pressure resistant envelope.

Y-body globe valves are best suited for high pressure and other severe services. In small sizes for intermittent flows, the pressure loss may not be as important as the other considerations favoring the

Y-body design. Hence, the flow passage of small Y-body globe valves is not as carefully streamlined as that for larger valves.

FIG 2

Angle Valve Design

The angle body globe valve design, illustrated in Figure 3 , is a simple modification of the

basic globe valve. Having ends at right angles, the diaphragm can be a simple flat plate. Fluid is able to flow through with only a single 90° turn and discharge downward more symmetrically than the discharge from an ordinary globe. A particular advantage of the angle body design is that it can function as both a valve and a piping elbow. For moderate conditions of pressure, temperature, and flow, the angle valve closely resembles the ordinary globe. The angle valve's discharge conditions are favorable with respect to fluid dynamics and erosion.

FIG 3 

Globe Valve Disks

Most globe valves use one of three basic disk designs: the ball disk, the composition disk, and the plug disk. Figure 4

Ball Disk

The ball disk fits on a tapered, flat-surfaced seat. The ball disk design is used primarily in relatively low pressure and low temperature systems. It is capable of throttling flow, but is primarily used to stop and start flow.

Composition Disk

The composition disk design uses a hard, nonmetallic insert ring on the disk. The insert ring creates a tighter closure. Composition disks are primarily used in steam and hot water applications. They resist erosion and are sufficiently resilient to close on solid particles without damaging the valve. Composition disks are replaceable.

Plug Disk

Because of its configuration, the plug disk provides better throttling than ball or composition designs. Plug disks are available in a variety of specific configurations. In general, they are all long and tapered.

FIG 4

Globe Valve Disk and Stem Connections

Globe valves employ two methods for connecting disk and stem: T-slot construction and disk

nut construction. In the T-slot design, the disk slips over the stem. In the disk nut design, the

disk is screwed into the stem.

Globe Valve Seats

Globe valve seats are either integral with or screwed into the valve body. Many globe valves

have backseats. A backseat is a seating arrangement that provides a seal between the stem and

bonnet. When the valve is fully open, the disk seats against the backseat. The backseat design

prevents system pressure from building against the valve packing.

Globe valves may be provided with either metal seatings or soft seatings. In the case of metal seatings, the seating stress must not only be high but also circumferentially uniform to achieve the desired degree of fluid tightness. These requirements have led to a number of seating designs. The ones shown in Figure 5

FIG 5

Globe Valve Direction of Flow

For low temperature applications, globe and angle valves are ordinarily installed so that pressure

is under the disk. This promotes easy operation, helps protect the packing, and eliminates a

certain amount of erosive action to the seat and disk faces. For high temperature steam service,

globe valves are installed so that pressure is above the disk. Otherwise, the stem will contract

upon cooling and tend to lift the disk off the seat

Globe Valve Parts

Here in below image, you can see the globe valve parts such as Body, Bonnet, Stem, Seat, Disk, etc.

FIG 6

Advantages of a Globe Valve
The following summarizes the advantages of globe valves:

1. Good shut-off capability
2. Moderate to good throttling capability
3. Shorter stroke (compared to a gate valve)
4. Available in tee, wye, and angle patterns, each offering unique capabilities
5. Easy to machine or resurface the seats
6. With disc not attached to the stem, valve can be used as a stop-check valve.

Disadvantages of a Globe Valve
The following are some shortcomings inherent in globe valves:

1. Higher pressure drop (compared to a gate valve)
2. Requires greater force or a larger actuator to seat the valve (with pressure under the seat)
3. Throttling flow under the seat and shutoff flow over the seat

Typical Applications of Globe Valves
The following are some of the typical applications of globe valves:

1. Cooling water systems where flow needs to be regulated
2. Fuel oil system where flow is regulated and leak tightness is of importance.
3. High-point vents and low-point drains when leak tightness and safety are major considerations.
4. Feed water, chemical feed, condenser air extraction, and extraction drain systems.
5. Boiler vents and drains, main steam vents and drains, and heater drains.
6. Turbine seals and drains.
7. Turbine lube oil system and others.

Flow Rate Calculation Method of Valve

Flow Coefficient Cv value, Kv value 

Cv value 
Flow coefficient(Cv) for valves expresses flow rate in gallons per minute of 60℉ water with 1.0 psi pressure drop across valve.
Kv value 
Flow coefficient(Kv) for valves expresses flow rate in liter per minute of 60℉ water with 1.0 kg/cm² pressure drop across valve.

FIG 7

For liquids :

Volume flow rate in m³/h 

 Mass flow rate in kg/h

For Gases :

Volume flow rate in m³/h         ΔP⩻P₁/2                               ΔP⩼P₁/2

 Mass flow rate in kg/h            ΔP⩻P₁/2                               ΔP⩼P₁/2

For Steam

For Gases :

Volume flow rate in kg/h           ΔP⩻P₁/2                               ΔP⩼P₁/2

where:

p1  Upstream pressure [bar] Absolute pressure pabs

p2  Downstream pressure [bar] Absolute pressure pabs

𝞓p [bar] Absolute pressure pabs (differential pressure p1 – p2)

⩒G [m³/h] Flow rate of gases, related to 0 °C and 1013 mbar

W Mass flow rate in kg/h (liquids, steam)  

 𝝆  Density in kg/m³ (general, also in liquids) 

𝝆1 Density upstream of the valve in kg/m³ (in gases and vapors) 

T1 Temperature K = 273 +T in °C upstream of the valve 

𝝆G [kg/m³] Density of gases at 0 °C and 1013 mbar 

v1  [m³/kg] Specific volume (v´ found in the steam table) for p1 and T 

v2  [m³/kg] Specific volume (v´ found in the steam table) for p2 and T 

v* [m³/kg] Specific volume (v´ found in the steam table) for P₁/2  and T

Converting between Flow Coefficient Cv and Flow Factor Kv

The relationship between Cv and Kv can be expressed as:
Cv = 1.16 Kv
Kv = 0.862 Cv

Disc Seating Taper Angle:

Disc sealing force is the force which is exerted on the disc against the seating surface to have tight shut-off. The sealing force can be reduced by changing the taper seat angle from 30 to 20 degrees as the force component vector depends on the seating angle. The figure.3 depicts the disc taper angle.

F = 3.14 x Ds x W x P x SIN (A+B)

Where 

F = Seating force

Ds = Diameter of seat ring bore

W= Width of the disc contact area.

P= Fluid pressure

A= Seat angle

B= Friction angle.

Valve Rating Pressure And Temperture