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1
Instrumentation Process
Control ValveLecture Nov 26, 2014
Dr Edi Munawar
Edi Munawar 1
Dr. Edi MunawarChemical Engineering Department
Syiah Kuala UniversityContact: edi.munawar@che.unsyiah.ac.id
Valve
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Control Valves
Control valves are used to provide a number offunctions and typically selected based on thefunctions and typically selected based on thefollowing basis: Application function Operating conditions Construction
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Sizing
Application Function
In general, the main function control valve is toperformperform. Isolation (ON-OFF valves) Flow control Directional control Protection (over pressure)
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Operating Conditions
As process equipment, the conditions of thesystem and the environment in which its to performsystem and the environment in which its to performsuch factors are need to be paid attention: Process pressures Process temperature Ambient conditions
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Process material and nature of fluid
Construction
A large range of valve designs are available andprovide differing performance both withprovide differing performance, both withadvantages and disadvantages. Generally, thecontrol valve characterized based on: Valve body type Plug and stem design St l
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Stem seals Materials of construction
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Sizing
The size of a valve is dependent on the flow thatrequired through the valve The performance ofrequired through the valve. The performance ofvalves is well defined which simplifies the selectionprocess for a valve without need complicatedcalculations.
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Body Styles Valve
The basic style of control valve body are: Gl b Globe Cage Angle body Y pattern Split body
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Three way Butterfly valve Ball valve Rotary valve
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Trim Configurations Valve
The trim configurations of control valve are: U b l d Unbalanced Balanced
Valve trim designs are provided by mostmanufacturers to give three different flowh t i ti
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characteristics: Equal percentage Linear Quick opening
The Guiding Configurations Valve
The guiding configurations of control valve are: C id d Cage guided Post guided Stem guided Skirt or Port guided
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ValveHandwheel
Stem
Plug
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Globe Valves
The globe is one of the most common types of body stylecommon 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. Since the trim can absorbs the pressure of the flow, its
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design is an important consideration in determining the flow characteristics of the valve.
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Globe Valves
Advantages Minimises disassembly for Minimises disassembly for
maintenance. Streamlined flow path with a
minimum of parts and no irregular cavities.
Disadvantages
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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 is valve where the plug travel inside thethe plug travel inside the cage. 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
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the problems of side loads.
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Cage Valves
Advantages Suitable for many trim Suitable for many trim
types to be used. Easy to maintain. Top entry. No threaded joints to
corrode.
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Trouble-free when specified correctly
Disadvantages Not suitable for viscous
fluid.
Split Body Valves
Split body valves provide streamlined flow and reducestreamlined 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
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subsequent operation was for difficult flows with high viscosity.
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Split Body Valves
Advantages Streamlined flow Streamlined flow. Minimum number of parts. No irregular cavities.
Disadvantages Leakage problems with
central joint
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central joint. Inability to weld. Maintenance complications. Limitations on trim
modifications.
Angle Valves
These valves can be likened to mounting a globe valve into mounting a globe valve in an elbow. 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.
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This saves not only on the maintenance of the valve, but also minimises any degradation in valve performance.
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Angle Valves
Advantages The exiting flow is 90O to the The exiting flow is 90 to the
inlet flow. the elimination of an elbow
is required. Minimises any degradation
in valve performance.
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Disadvantages Requiring replaceable
inserts on the out-flow piping.
Y-Style Valves
This style of valve has the operating components tiltedoperating 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,
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operating at or near the closed position.
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Y-Style Valves
Advantages Suitable used for drainage Suitable used for drainage
applications. Still can be operating at
near the closed position.
Disadvantages Maintenance is impaired
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with the added difficulty of aligning and handling the components.
Need more frequent maintenance.
Three-Way Valves
Three-Way Valves are a special type of doublespecial type of double ported valve designed for Mixing or Diverting.
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Three-Way Valves
MixingThe mixing valve has twoThe 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.
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Three-Way Valves
DivertingThe diverting application ofThe diverting application of 3-way valve can be used for switching or for bypass operations such as used in chilled water systems to prevent freezing in the pipes.
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pipes.
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Butterfly Valves
The butterfly valves are equipped with discs whichequipped with discs which rotate to regulate the flow rate. The disc is very thin and occupies small space in the pipeline. The stem is centred on the axis of the pipeline. The disc pulls
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away from the seal upon opening. This minimises seal wear and reduces friction.
Butterfly Valves
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Butterfly Valves
Advantages Low cost and weight Low cost and weight. High flow capacity. Fire safe design. Low stem leakage.
Disadvantages
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g Over sizing.
Ball Valves
The Ball valve is one of the most common types ofmost 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 flow stream. Fully open
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to fully closed is performed by a 90 degree rotation of the plug segment.
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Ball Valves
Advantages Lower cost and weight Lower cost and weight. Higher flow capacity (2-3
times of the globe valve). Tight shutoff. Fire safe. Low stem leakage.
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Disadvantages Over sizing.
Single Seated Valves
Single seated valves areone form of globe valve thatone form of globe valve thatare very common and quitesimple in design. Thesevalves have few internalparts. They are also smallerthan double seated valvesand provide good shut off
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capability
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Single Seated Valves
Advantages Si l d i Simple design. Simplified maintenance. Smaller and lighter. Good shutoff.
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Disadvantages More complex designs required for balancing.
Double Seated Valves
Another globe valve bodydesign is double seated. Indesign is double seated. Inthis approach, there are twoplugs and two seats thatoperate within the valvebody
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Double Seated Valves
Advantages Reduced actuator force due to balancing Reduced actuator force due to balancing. Action easily changed (Direct/Reverse). High flow capacity.
Disadvantages Poor shutoff.
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Heavy and bulky.
Balanced Valves
Balancing is term used when the resultant force on the plug is neutral That means the plug is neitherthe plug is neutral. That means the plug is neither forced up or down by the pressure of the flow stream.The advantage of valve have neutral force on the plug is the actuator force required for controlled movement is greatly reduced The balancing is
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movement is greatly reduced. The balancing is applied to single-seated and double-seated valves in different ways.
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Single-Seated Balancing
In a single seated valve, the forces resulted of the flowforces resulted of the flow stream can push against the plug, therefore greater actuator force is required to operate the valve movement. To balance a single-seated valve,
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balancing holes are added to equalise the pressure on the plug. However, this could reduce the valve shut off capability.
Double-Seated Balancing
Double-seated valves were originally designed fororiginally designed for balancing. These valves use opposing forces from the two plugs to minimise the actuator force required for control movement. The force on the upper plug is in
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the opposite direction to that on the lower plug and as such the result should be zero.
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Guiding
The guiding term mean is to support position the valve plug over the travel route. Any forces that can causing vibrationover the travel route. Any forces that can causing vibration on the plug are resisted by the guide.Types of guiding designs: Cage Stem Post
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Port
Cage Guided
Cage guided is the most common type of guidingcommon type of guiding system. The plug moves in the cage with little tolerance. This design enables the plug to loading on supported by the cage with a large bearing area. Maintenance
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is decreased as the assembly is simplified with the components self aligning.
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Cage Guided
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Stem Guided
Stem guided is a simple design where the stem itselfdesign where the stem itself is responsible for supporting and controlling the plug. Limitations occur due to the stem strength as the support of the stem is far away from the load on the
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plug. Guiding performance is impaired, but this type of valve is cheaper to manufacture and maintain
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Post Guided
Post guided is mostly used if there is fouling risk. Theif there is fouling risk. The post guiding is a section of the stem from the plug that extends into the valve body. The diameter of post guiding is smaller than the plug but larger than the
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stem.
Port Guided
The port guided is very seldom used, but still inseldom used, but still in existence. This type guiding is used to align and guide the plug. The port design also has a relatively small bearing surface and has the same problems with fouling
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as with the cage guided valves.
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Next Lecture
Control ValveControl Valve Selection
Thank you for your attention
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y y
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