Flow Control Valves with operation ,advantages and disadvantages.

Control valves are used to provide a number of functions and are typically selected

on the following basis:

- Application function

- Operating conditions

- Construction

- Sizing

Application Function

This relates to the function which the valve is to perform.

- Isolation, ON-OFF valves

These are typically ball valves and are used for shut off and isolation purposes.

- Flow control

This course is primarily aimed at regulating valves for the purpose of modulation control in continuous systems.

- Directional control

Check valves are typically used for this purpose.

- Protection, overpressure

Pressure relief valves provide suitable overpressure protection.

Operating Conditions

As with all process equipment, the conditions of the system and the environment in which it is to perform are of significant importance. Such factors of consideration are:

- Process pressures

- Process temperature

- Ambient conditions

- Process material and nature of fluid

Construction

A large range of valve designs are available and provide differing performance, both with advantages and disadvantages.

- Valve body type

- Plug and stem design

- Stem seals

- Materials of construction

Sizing

The size of a valve is dependent on the flow that is required through the valve. The performance of valves is well defined which simplifies the selection process for a valve without the need to resort to complicated calculations.

Sliding Stem Valves

Valve trim designs are provided by most manufacturers to give three different flow characteristics:

- Equal percentage

- Linear

- Quick opening

The basic body styles are:

- Globe

- Cage

- Angle body

- Y pattern

- Split body

- Three way

- Single seated

- Double seated

The trim configurations are:

- Unbalanced

- Balanced

The guiding configurations are:

- Cage

- Post

- Top

- Top and bottom

- Stem

- Skirt

Globe Valves

The globe is one of the most common types of body style for sliding-stem valves.

The trim of a valve is essentially the internal parts that are in contact with the flow stream. Because the trim absorbs the pressure of the flow (with a pressure loss across the valve) the trim design is an important consideration in determining the flow characteristics of the valve.

Top-entry, top guided single-seated globe valve.

The globe body differs considerably depending on the trim used.

The main components of the valve trim are the plug and stem and the seat ring. The most widely used valve is the single-stage orifice and plug assembly. Multi-stage orifice elements are usually found in trim designs to reduce noise, erosion and cavitation.

Advantages

Minimises disasssembly for maintenance.

Streamlined flow path with a minimum of parts and no irregular cavities.

Disadvantages

Leaking of the central joint due to thermal cycles or piping loads.

Valves cannot be welded in-line since the valve body is required to be split.

Cage Valves

Cage valves use the principle of cage guiding, where the plug rides inside the cage. This is quite common in most valves, because the bearing forces on the plug are near the fluid forces. As the plug is aligned by the cage, the valve effectively self-aligns itself so that during assembly all the pieces fit together. Correct alignment reduces the problems of side loads.

Cage guiding is not recommended when the fluid is highly viscous. Such fluids that are sticky or gummy can also cause problems, as can fluids that contain solids. The problem is the possible build up between the plug and cage which can cause operational problems. This problem of build up is also referred to as fouling. Fouling can cause a restriction of travel in the valve movement, or a delayed Respons e time to a control signal.

Block and bypass valves are used for assisting in the maintenance of valves. The need for block and bypass valves is eliminated since cage valves are very rugged and have a good service life.

Whereas globe valves (also known as post-guided) are characterised by the shape and contours of the valve plug, Cage valves are characterised by the shape of the cage window.

Cage valves with clamped-in seat ring and characterised plug

Advantages

- No threaded joints.

- Suitable for many trim types to be used.

- Easy to maintain.

- Top entry.

- No threaded joints to corrode.

- Trouble-free when specified correctly.

Split Body Valves

Split body valves provide streamlined flow and reduce the number of bolted joints. These valves use one bolt to secure the valve with the seat ring clamped between the body halves.

Their original design and subsequent operation was for difficult flows with high viscosity. Fouling is minimised due to the valves simple streamlined construction.

Maintenance requires that in order to service the valve, the flange connections must be broken. The advantages from simple valve design are outweighed by the concerns over line flange leakage after maintenance.

Split body valve with removable flanger

Advantages

- Streamlined flow.

- Minimum number of parts.

- No irregular cavities.

Disadvantages

- Leakage problems with central joint.

- Inability to weld.

- Maintenance complications.

- Limitations on trim modifications.

Angle Valves

These valves can be likened to mounting a globe valve in an elbow. The exiting flow is 90 degrees to the inlet flow.

The obvious advantage is the elimination of an elbow, should one be required, however the flow does make fewer turns as it passes through the body.

The Angle valve has little restriction on the out flow, so if flashing or cavitation occurs then it tends to do so further downstream from the valve. This saves not only on the maintenance life of the valve, but also minimises any degradation in valve performance.

Angle valves are limited in use and are generally used for erosive applications requiring replaceable inserts on the out-flow piping.

Streamlined angle valve with lined Venturi outlet.

Y-Style Valves

This style of valve has the operating components tilted at a 45 degree angle to the flow path. In theory, the flow stream has fewer turns when fully open. In practice they are mainly used for drainage applications, operating at or near the closed position.

When installed in horizontal pipe, maintenance is impaired with the added difficulty of aligning and handling the components. This is true for any extraction angle other than vertical.

Another inhibiting factor is that when installed with the moving parts not vertical, the added side load due to gravity increases wear with the need for more frequent maintenance.

Y-valve fitted with vacuum jacket.

Three-Way Valves

Three-way valves are a special type of double ported valve.

Two types of Three-way valve are available:

- Mixing

- Diverting

Mixing

The mixing valve has two inlets and one outlet.

This type of valve would be used for blending of two fluids with the associated ratio control of the mix.

Three-way valve mixing flow.

Diverting

The diverting valve has one inlet and two outlets.

Diverting valves can be used for switching or for bypass operations. The relative split provides the required controlled flow with one outlet, while allowing a constant flow through the system with the other outlet. Such valves are used in chilled water systems to prevent freezing in the pipes.

Three-way valve for diverting service.

Single Seated

Single seated valves are one form of globe valve that are very common and quite simple in design. These valves have few internal parts. They are also smaller than double seated valves and provide good shut off capability.

Maintenance is simplified due to easy access with top entry to the valve components.

Because of their widespread usage, they are available in a variety of trim configurations, and therefore a greater range of flow characteristics are available. They also produce less vibration due to the reduced plug mass.

Top-entry, top guided single-seated globe valve.

Advantages

- Simple design.

- Simplified maintenance.

- Smaller and lighter.

- Good shutoff.

Disadvantages

- More complex designs required for balancing.

Double Seated

Another globe valve body design is double seated. In this approach, there are two plugs and two seats that operate within the valve body.

In a single seated valve, the forces of the flow stream can push against the plug, requiring greater actuator force to operate the valve movement. Double seated valves use opposing forces from the two plugs to minimise the actuator force required for control movement. Balancing is the term used when the net force on the stem is minimised in this way.

These valves are not truly balanced. The result of the hydrostatic forces on the plugs may not be zero due to the geometry and dynamics. They are therefore termed semi-balanced. It is important to know the combined loading due to the amount of balancing and dynamic forces when sizing the actuator.

Shutoff is poor with the double seated valve and is one of the downfalls with this type of construction. Even though manufacturing tolerances may be tight, due to different forces on the plugs it is not possible for both plugs to make contact at te same time.

Maintenance is increased with the added internal parts required. Also these valves tend to be quite heavy and large.

These valves are an older design that have fewer advantages compared with the inherent disadvantages. Although they can be found in older systems, they are seldom used in newer applications.

Double ported valve

Advantages

- Reduced actuator force due to balancing.

- Action easily changed (Direct/Reverse).

- High flow capacity.

Disadvantages

- Poor shutoff.

- Heavy and bulky.

- More parts to service.

- Only semi-balanced.

Balanced Valves

Balancing is the term used when the resultant force on a plug is neutral. This means that the plug is neither forced up or down by the pressure of the flow stream.

The advantage with balancing is that the actuator force required for controlled movement is greatly reduced. This allows for smaller and cheaper actuators.

Balancing is applied to single-seated and double-seated valves in different ways.

Double-Seated Balancing

Double-seated valves were originally designed for balancing. These valves use opposing forces from the two plugs to minimise the actuator force required for control movement. That is, the pressure of the flowstream acting on the upper plug is intended to cancel the pressure acting on the lower plug.

The force on the upper plug is in the opposite direction to that on the lower plug and as such the result should be zero. However, because the plug sizes differ, the forces are not equal and the result is an unbalanced force.

Double-seated valves are actually semi-balanced.

Single-Seated Balancing

In a single seated valve, the forces of the flow stream can push against the plug, requiring greater actuator force to operate the valve movement.

To balance a single-seated valve, balancing holes are added to equalise the pressure on both sides of the plug. This eliminates any unbalanced force on the plug, however further seals are required for the extra leakage path between the plug and the cage.

An unbalanced valve has better shut off capability because there is only the problem of leakage between the seat and the plug.

Guiding

The control valve guide is used to support and position the valve plug over the full range of travel. Various control valve guiding designs are available and should be considered as they affect the operating life and reliability of a valve.

The guide provides the support for the valve plug. Any forces on the plug are resisted by the guide. If the guide wears or fails then vibration can become a problem.

Under high bearing loads, the surface of the guide can break down causing increased friction and impeding valve performance.

In choosing suitable guides:

- Use bearing materials with different hardness levels

- Avoid nickel and unhardened stainless steel

Types of guiding designs:

- Cage

- Stem

- Post

- Top

- Top and bottom

- Port

Cage guiding

The most common type of guiding is Cage guiding. The plug moves within a cage with little tolerance between the two. This design enables the loading on the plug to be supported by the cage with a large bearing area between the two.

Maintenance is reduced as the assembly is simplified with the components self aligning.

Cage valve with clamped-in seat ring and characterised plug

Stem guiding

Stem guiding is a simple design where the stem itself is responsible for supporting and controlling the plug.

Limitations occur due to the stem's strength as the support of the stem is farther away from the load on the plug. Guiding performance is impaired but this type of valve is cheaper to manufacture and maintain.

Top-entry, top guided single seated globe valve.

Post guiding

Post guiding is mostly used if there is a risk of fouling. The post is a section of the stem from the plug that extends into the valve body. The post is smaller in diameter than the plug but larger than the stem.

The post supports the plug from bearing loads, with the narrower stem providing positioning control.

This type of guiding also helps keep the bearing surfaces out of the flow stream. This reduces the buildup of fluid.

The two types of post guiding are:

- Top guided

When the post is above the plug, the valve is termed 'Top guided'.

- Top and bottom guided

When the plug is supported from above and below, or in the case of some dual port valves, the valve is termed 'Top and bottom guided'.

Post guiding – top

Port guided

Very seldom used but still in existence is the port guided valve. In this design the port is used to align and guide the plug. The port guided design also has a relatively small bearing surface and has the same problems with fouling as with the cage guided valves.

Port or skirt guiding

Rotary Valves

Butterfly Valves

Standard butterfly valves are dampers that are shaped from discs which rotate in the flow path to regulate the rate of flow. The disc is quite narrow and occupies little space in the pipeline. The shaft is centred on the axis of the pipeline and is in line with the seal.

The disc pulls away from the seal upon opening. This minimises seal wear and reduces friction. Control of the valve near the closed position can be difficult due to the breakout torque required to pull the valve out of the seat.

The flow characteristics are essentially equal percentage, but the rotation is limited to about 60 degrees as the leading edges are hidden in the shaft area as the disc is rotated further. The Fishtail is one modification of the disc that permits effective control out to 90 degrees of rotation.

Fish tail butterfly disc

Seals

The swivel through design of seal has very poor shutoff. For tight shutoff are the adjustable or inflatable elastomeric seals. These provide bubble tight shutoff.

High Performance Butterfly Valves

The high performance butterfly valve is a development from the conventional valve where the rotation axis of the disc is offset from both the centreline of flow and the plane of the seal.

This design produces a number of advantages, including better seal performance, lower dynamic torque, and higher allowable pressure drops. The seal performance is improved because the disc cams in and out of the seat, only contacting it at closure and so wear is reduced. As the disc only approaches the seal from one side, the pressure drop across the valve can be used to provide a pressure assisted seal. This further improves performance.

The modified shape and contour of the disc are used to reduce dynamic torque and drag. This also permits higher pressure drops. As the disc is never hidden behind the shaft, good control through the 90 degrees of operation is possible with a linear characteristic.

The high performance butterfly valve is gaining greater acceptance and use due to it's increased capability and the relatively high capacity to cost ratio.

Disc shapes.

Advantages

- Low cost and weight.

- High flow capacity.

- Fire safe design.

- Low stem leakage.

Disadvantages

- Oversizing.

Ball Valves

The Ball valve is one of the most common types of Rotary valves available. The valve is named from the valve plug segment being a ball or sphere that rotates on an axis perpendicular to the flowstream. Fully open to fully closed is performed by a 90 degree rotation of the plug segment.

The full-ball valve

The full ball valve is shaped from a spherical segment with a cylindrical hole for the flow of fluid. Among the various configurations, the 'floating' ball has two seals which provide bearing support to the ball segment. This does provide simplicity in the design, however the friction levels are higher than conventional bearing designs which can affect control performance.

Top-entry pierced ball valve.

The Characterised ball valve

The Full-ball valve was originally designed for ON-OFF control. Although modulation control is possible, the flow characteristics can be difficult to work with.

The opening between the ball and the seal can be modified to provide different flow characteristics. The V-notch is one example that produces a more gradual opening to give better Rangeability and throttling capability.

Most characterised ball valves are modified so that only a portion of the ball is used. The edge of the partial ball can be shaped to obtain the desired flow characteristics. Various manufacturers promote their valves on the characteristics achieved by this design.

Apart from the V-notch, other designs can be U-notch or parabolic curve. Although favourable characteristics may be achieved with the characterisation of the ball, problems may occur due to the reduced strength of the partial ball. Bending is one such problem which occurs under operating loads.

Care also needs to be taken during installation as overtightening of the flange bolts can damage the seals.