Image Credit: Belimo | Grainger | trade-hardware.com

 

As their name suggests, globe valves are linear motion valves with rounded globular shaped bodies. Since their shape is similar to other valve bodies, positive identification must be made based on internal piping. Recently globe valves have lost their traditional round body-shape. Globe valves have many advantages and disadvantages for users. They have excellent and precise throttling ability for high-pressure systems. The disadvantages include low-flow coefficients and a longer operating time because the operator must turn the handle and stem many times to ensure the valve is completely open or completely closed. Globe valves can be used in systems that require frequent stroking, vacuum, and systems that have a wide range of temperature extremes.

 

Advantages Disadvantages
Can be fast-acting High head loss
 Precise control Large opening for disk assembly
 Can be used in high-pressure systems Heavier than other valves
  Cantilevered mounting of the disk to the stem
   Low coefficient of flow
  Not good for clean or sterile applications

 

 

Globe Valve. Video Credit:iecSimulations

 

Selection of Globe Valves

The Globalspec database allows industrial buyers to select globe valves by classification, body design, performance specifications, and additional features.

 

Classification

Industrial Valves  can be classified in a number of different ways including method of control and valve function. Globe valves use a linear motion disk and function to start, stop, and throttle fluid flow.

 

Method of Control

Globe valves have a disk which can completely open or completely close the flow path. This is done with the perpendicular movement of the disk away from the seat. The annular space between the disk and seat ring gradually changes to allow fluid flow through the valve. As the fluid travels through the valve it changes direction many times and increases the pressure. In most cases, globe valves are installed with the stem vertical and the fluid stream connected to the pipe side above the disk. This helps to maintain a tight seal when the valve is fully closed. When the globe valve is open, the fluid flows through the space between the edge of the disk and the seat. The flow rate for the media is determined by the distance between the valve plug and the valve seat.

 

Globe Valve Function

Globe valves are commonly used as an on/off valve, but they may be used for throttling systems. The gradual change in spacing between the disk and seat ring gives the globe valve good throttling ability. These linear motion valves can be used in a variety of applications as long as the pressure and temperature limits are not exceeded, and the process does not require special materials to combat corrosion. Globe valve also have a smaller chance of damage to the seat or valve plug by the fluid, even if the seat is in the partially open position.

 

Media

Globe valves can be used for both gas and liquid systems. Globe valves are not specified for high purity or slurry systems. The valve has inherent cavities that easily promote contamination and allow slurry material to become entrapped, disabling the valve operation.

 

Globe Valve Components

Globe valves have a very distinct globe shape. The disk, valve stem, and the hand wheel are the moving parts in the valve body. The body is available in three different designs depending on the application as well as three different types of disks. 

 

Globe valve components. Image Credit: McGraw-Hill Companies, Inc.

 

Body Construction

Due to the angles in the globe valve body there is a high level of head loss. Head loss is the measure of reduction in the total head of liquid as it moves through the system. Total head loss can be calculated by summing the elevation head, velocity head and pressure head. While head loss is unavoidable in fluid systems, it is increased by obstructions and discontinuities in the flow path such as the S shape of the globe valve design. The body and flow pipes are rounded and smooth to provide system flow without creating turbulence or noise. To avoid creating additional pressure losses at high velocity the pipes should be a constant area. Globe valves are available in three main body types (although custom designs are available as well): angle design, Y-shaped, and Z- shaped.

  • Angle: Angle valves are designed so that the inlet and outlet are perpendicular. They are used for transferring flow from vertical to horizontal.
  • Y-body: Y-body design valves, also known as cross-flow globe valves, derive linear action from the incline between the axis of the inlet and outlet ports. This design reduces the high pressure drop inherent in globe valves. The seat and stem are at a 45° angel with the media flow, for a straighter flow path at full opening. The Y-design also keeps the stem, bonnet, and packing in a relatively pressure-resistant envelop. This valve design is well suited for high pressures and other severe services.

Selection Tip:  The flow passage for small Y-body globe valves is not as carefully streamlined as it is for larger valves because pressure loss is less important for small size Y-body valves.

  • Z- body: The Z-body design, also known as a straight-through globe valve,  is the simplest globe valve body. It is commonly used for water applications. Z-body globe valves have a Z-shaped diaphragm or partition across the globular body which contains the seat. Since the seat is horizontal, the disk and stem 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. The symmetrical form simplifies manufacturing, installation, and repair.

Globe valve body types. Image Credit: Valve Handbook

Bonnets retain the pressure in the valve by acting as a cap or cover for the body.

 

Trim

Trim consists of the disk and seat ring.  The disk is lowered onto a matching horizontal seat located in the center of the valve. The disk goes into the seat ring to stop flow through the system. The disk is a plug with a float or convex bottom. The location of the valve disk in relation to the valve seat allows or restricts flow.

  • Ball disks fit into a tapered, flat-surface seat. The ball disk is best used in low pressure, low temperature systems. They can be used for throttling services but they are best used to start/ stop flow.
  • Composition disks are replaceable disks designed with a hard, nonmetallic insert ring on the disk. They are best used in steam and hot water systems because they resist erosion. Composition disks are also resilient enough to close on solid particles without damaging the valve.

  • Plug disks are long tapered disk available in a variety of specific configurations. The plug disk provides better throttling than ball or composition disks.

Disk and Stem Connections

The stem connects the handwheel and the disk. It's threaded and fits into the threads in the valve bonnet. The constant turning of the stem to open or close the valve may cause a weakening in the glad seal (packing). There are two possible methods for connecting the disk and stem.

  • T-slot construction: The disk slips over the stem
  • Nut design: The disk is screwed into the stem

Design Tip: Take care to not screw the valve shaft too far because there is a possibility it could damage the seating surface.

 

For quick-acting globe stop valves, the stem is placed so the disk closes in the same direction as the fluid flow. Closing the valve is impeded by the kinetic energy of the media but opening is faster and easier.

 

Seats

The seat of a globe valve is either integrated with or screwed into the valve body. Most globe valves have a seating arrangement that provides a seal between the stem and bonnet called a backseat. The backseat prevents pressure from building against the valve packing because the disk rests against it when the valve is open. The disk to seat ring contact is close to a right angle, reducing the amount of seat leakage because a tight seal can be formed. Many globe valves feature top entry to the plug and seat to allow for easier servicing of the internal parts while the valve remains in-line. 

 

Valve Actuator

The valve actuator operates the stem and disk to open and close the valve. There are several types of actuators depending on the needs of the system such as the torque necessary to operate the valve, speed and the need for automatic actuation.

  • Manual/ hand operated actuators use a hand-wheel or crank to open or close the valve. They are not automatic but offer the user the ability to position the valve as needed.  The hand-wheel can be fixed to a stem or hammer which allows for the valve to be pounded open or closed if necessary. In manual globe valves, the stem screws the plug in to close or shut the valve. The threaded system is slower and not as smooth as an automated system, but it does offer easy operator control.   

  

Hand operated actuator. Image Credit: Direct Industry

  • Automatic actuator offer smooth control for throttling. They typically use a smooth sliding stem instead of a threaded stem.
    • Electric motor actuators permit manual, semi-automatic, and automatic operation of the valve. The motor is usually reversible and used for open and close functions. The actuator is operated either by the position of the valve or by the torque of the motor.  A limit switch can be included to automatically stop the motor at fully open and fully closed.
    • Solenoid operated valves use hydraulic fluid for automatic control of valve opening or closing. Manual valves can also be used for controlling the hydraulic fluid; thus providing semi-automatic operation. A solenoid is a designed electromagnet. Solenoid valves can be arranged such that power to the solenoid either opens or closes the valve.
    • Pneumatic operated valves can be automatic or semi-automatic. They function by translating an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. Pneumatic actuators are fast-acting for use in throttle valves and for open-close positioning.

    • Hydraulic actuators provide for semi-automatic or automatic positioning of the valve. They are used when a large force is required to open the valve, such as a main steam valve. With no fluid pressure, the spring force holds the valve in the closed position. Fluid enters the chamber, changing the pressure. When the force is greater than the spring force, the piston moves upward and the valve opens. To close the valve, hydraulic fluid (such as water or oil) is fed to either side of the piston while the other side is drained or bled.

Image Credit: Bani Instind 23

  • Self-actuated valves use the system fluid to position the valve. These are commonly found in relief valves, safety valves, check valves, and steam traps. Because these actuators use the fluid in the system, no external power is required. 

 

Speed of Power Actuators

Actuators can vary in operating speed. The speed should be selected based on the speed and power requirements of the system and availability of energy to the actuator.

  • Fast acting actuators are best used when a system must be quickly isolated or opened. Fast action is provided by hydraulic, pneumatic, and solenoid actuators. The speed of actuation is set by installing the correct orifice in the lines and the valve is closed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep the valve open.  Electrical motors can also provide fast actuation when the speed is set through the motor speed and gear ratio.

  • Slow acting actuators are best used when cold water is injected into a hot system or slower opening is needed. 

Actuator Size

Due to the wide variety and variations in valves, the actuator must be sized to the specific valve in the system. If the actuator is undersized, it will be unable to overcome the forces against it. This will cause slow and erratic stroking. If the actuator is not stiff enough to hold the close position, the closure element will slam into the seat, causing a pressure surge. If the actuator is oversized, it will cost more, weigh more, and be more sluggish in terms of speed and response. Larger actuators may also provide a higher thrust that will damage internal valve parts. Actuators tend to be oversized because of safety factors but smaller sizes function just as well when the built-in safety factors are considered.

 

Materials

Globe valves are generally available in a variety of metal and alloy constructions, as well as plastics, to cover this wide range of industrial applications. Proper material compatibility requires knowledge of the type, concentration and temperature of the media being handled. It may be necessary to consult the manufacturer of the valve for specific properties of the materials used in the valve.

 

Performance Specifications

Globe valves can handle temperature ranges from - 425°F to 1100°F depending on Globe valve construction. Valves are available in sizes from .5 to .48 inches (DN6 to 1200) and a weight of up to 13,000lbs for hand-operated manual valves. A valve can be used to stop and start as well as throttle or regulate the flow or movement of a media through a system. The given and desired properties of the flow can be used when selecting a valve.  For more information on valve performance please see the Industrial Valve selection guide.

 

Variables for flow calculations

Variable Symbol Units
Flow rate

q

gpm
Density ρ lb/ft3
Specific gravity G  
Pressure drop ΔP psi
Flow coefficient Cv  
Piping geometry Fp  
Inlet diameter d inches
Temperature T degree
Steam flow m lb/h
Inlet stem pi psia

 

Flow Coefficient

The valve flow coefficient is the number of U.S. gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. The coefficient is used to determine the size that will best allow the valve to pass the desired flow rate, while providing stable control of the process fluid. It can be used to compare flow capacities of valves of different sizes, types, and manufactures. The flow coefficient is different for gases, liquids, and steam and is also dependent on the pressure drop across the valve.  The Cv calculated will apply to either the opening or closing depending on the function.

 

For air and gases: Compressible media - The density of the gas changes with a change in pressure and therefore the flow rate changes. Low pressure is defined as P2 > P1/2 and high pressure is defined as P1 > P2/2

Low pressure: Cv= q/ (16.05) √(P12 - P22) / G * T

High pressure: Cv= q/ (13.61) √(1) / G * T

 

For liquids: Incompressible media- The flow rate only depends on the difference between the inlet and outlet pressures so the rate stays the same as long as the change in pressure remains the same.

 

Cv= q/Fp √(G/ΔP) 

 

For steam: Compressible media- The density of the steam changes with a change in pressure. There is a critical (choked1) and non-critical pressure drop. Critical is defined as the pressure dropping by 58% or more from the inlet to the outlet.

 

Critical pressure drop: Cv = m/1.61 pi

Non- critical pressure drop: m/(2.1 ( pi +  po))

 

If Cv calculated is too small the valve will be undersized and the process system may be starved for fluid. This also causes a higher pressure drop across the valve causing cavitation or flashing. If Cv is too high the valve will be too big leading to a waste of money and the machine being too difficult to maneuver. A larger Cv can also be a problem for throttling because the flow cannot be effectively controlled at the openings. The location of the closure element leads to the possibility of creating a high pressure drop and faster velocities causing cavitation, flushing or erosion.

 

Flow Characteristic

The flow characteristic describes the relationship between the flow coefficient and the valve stroke. It is inherent to the design of the selected valve. For example, as the valve is opened, the flow characteristic allows a certain amount of flow through the valve at a particular percentage of the stroke. This is especially important for throttle control because it controls the flow in a predictable manner.

 

Rangeability

Rangeability is a very important factor when selecting a valve type. It is defined as the maximum to minimum flow rate that can be controlled by a given valve type. The characteristic is affected by three factors: the geometry of the valve, the seat leakage, and the actuator's accuracy or stiffness at near closure of the valve. Geometry is inherent due to the design of the seat and closure and excessive seat leakage can cause instability in the valve as it lifts off the seat.  

 

Rangeability is easily calculated based on the geometry and actuator of the valve. If the valve is not accurate at 5 percent of stroke, then the rangeability is 20:1 (100 percent divided by 5%). As the rangeability increases, a wider range of flow rates can be controlled by the valve. It is not imperative that the valve has the highest rangeability because most systems do not have that wide of a flow rate range. V-notched ball valves have the highest rangeability at 200:1, while globe valves have a high rangeability of 100:1. Higher rangeability usually indicates the sensitivity is lower when the closing element is near closed and increases as the valve opens.  

 

Flow Rate

Flow rate is affected by the flow characteristic and the pressure drop across the valve. In globe valves, flow rate is determined by the lift of the valve plug from the seat. Manual globe valves can have equal-percentage, linear or quick-opening flow. Since flow characteristics determine the flow rate, the operator can roughly determine the flow rate by the linear position of the manual handwheel.

 

Pressure

Globe valves can handle pressure limits from 1480 to 1500psi. The limit is dependent on the material of construction, size, and temperature. High pressure lines can cause damage to the trim, stem packing, and actuators due to the pulsations, impacts and pressure drop in the system. The pressure drop across the valve is greater than in other valves because the passageway is S-shaped.

The pressure drop is the pressure change between the inlet and outlet of the system. The formula is as follows:

 

ΔP = G (q/FpCv)2

If the pressure drop is too high, a larger valve or a valve with a higher Cv can be used to lower the pressure.

 

Valve Sizing

Large valves require higher power and are especially noisy to operate in high pressure systems. Selecting the right size globe valve can be especially complicated. Generally the manufacture will size the valve for the buyer or provide software to make the process easier. The science behind valve sizing is determining the flow through the diameter of the valve. Valves may contain two different sized openings designed to take a pressure drop. This is why valve sizing is almost always done for throttling valves. Although, sizing for open/close valves should also be considered.

  • Open/Close valves are expected to pass 100% of the flow without a significant drop in pressure. They do not throttle the flow so the openings are generally the same size. If the valve is too small, the flow will be restricted, defeating the point of the on/off valve. A large valve will cost more because increasers will need to be installed.
  • Throttle valves are expected to produce a certain amount of flow at certain positions of opening to create a pressure drop. Throttle valves work best when the valve uses the full range of stroke while producing desired flow characteristics and maximum flow output. 

The use of increasers or reducers to create nonstandard piping configurations can be corrected in the Cv equation. In order to determine the piping geometry factor, Fp, the inside diameter of the pipe is required. "d" is the inside diameter and "D" is the outside diameter.

Cv/d2

di/Do (inches)

0.50

0.60

0.70

0.80

0.90

4

0.99

0.99

1.00

1.00

1.00

6

0.98

0.99

0.99

1.00

1.00

8

0.97

.098

0.99

0.99

1.00

10

0.96

0.97

0.98

0.99

1.00

12

0.94

0.95

0.97

0.98

1.00

14

0.92

0.94

0.96

0.98

0.99

16

0.90

0.92

0.95

0.97

0.99

18

0.87

0.90

0.94

0.97

0.99

20

0.85

0.89

0.92

0.96

0.99

25

0.79

0.84

0.89

0.94

0.98

30

0.73

0.79

0.85

0.91

0.97

35

0.68

0.74

0.81

0.89

0.96

40

0.63

0.69

0.77

0.86

0.95

Piping- geometry factor for valves with reducers and increasers on both ends. Table Credit: Valtek International

Non-wetted and Wetted

Non-wetted and wetted are terms used to describe the body and stem design.

  • Non-wetted valves have the stem and body isolated from the media in the system. Therefore, the stem and body do not need to be made of a corrosive resistant material
  • Wetted valves leave the stem and body exposed to the media in the line

Features

Globe valves are available with features for additional precision and safety.

  • Positioners- Positioners allow operators to identify how open or closed the valve is for throttling.
  • Limit switches- Limit switches limit how far the valve is able to open to close to protect the internal components of the valve.

Standards

ASME class II, III, IV, V or VI shut off requirements.

 

Applications

Globe valves can be used in a wide variety of industrial applications. Since globe valves used for steam applications must be welded into the system, the top-entry maintenance makes globe valves ideal for the power industry.

 

Resources

Head Loss

Wermac- Introduction to valves- Only the basics

Sixteen Considerations for Valve Selection You Can't Afford to Ignore

The Engineering ToolBox- Flow Coefficient

PDHEngineering- Valve Fundamentals

Skousen, Philip L. Valve Handbook. New York: McGraw-Hill, 1998. Print.

Types of Manual Valves

Valve Types

 

 

 

Read user Insights about Globe Valves

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