What is the Advantage and Disadvantage of Fluid Control Solutions

Use of fluids under pressure to generate, control, and transmit power

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Illustration of force multiplication by linked hydraulic cylinders, a fundamental feature of fluid power.

Fluid power is the use of fluids under pressure to generate, control, and transmit power. Fluid power is conventionally subdivided into hydraulics (using a liquid such as mineral oil or water) and pneumatics (using a gas such as compressed air or other gases). Although steam is also a fluid, steam power is usually classified separately from fluid power (implying hydraulics or pneumatics). Compressed-air and water-pressure systems were once used to transmit power from a central source to industrial users over extended geographic areas; fluid power systems today are usually within a single building or mobile machine.

Fluid power systems perform work by a pressurized fluid bearing directly on a piston in a cylinder or in a fluid motor. A fluid cylinder produces a force resulting in linear motion, whereas a fluid motor produces torque resulting in rotary motion. Within a fluid power system, cylinders and motors (also called actuators) do the desired work. Control components such as valves regulate the system.

Elements

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A fluid power system has a pump driven by a prime mover (such as an electric motor or internal combustion engine) that converts mechanical energy into fluid energy, Pressurized fluid is controlled and directed by valves into an actuator device such as a hydraulic cylinder or pneumatic cylinder, to provide linear motion, or a hydraulic motor or pneumatic motor, to provide rotary motion or torque. Rotary motion may be continuous or confined to less than one revolution.

Hydraulic pumps

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Dynamic (non positive displacement) pumps

This type is generally used for low-pressure, high volume flow applications. Since they are not capable of withstanding high pressures, there is little use in the fluid power field. Their maximum pressure is limited to 250-300 psi (1.7 - 2.0 MPa). This type of pump is primarily used for transporting fluids from one location to another. Centrifugal and axial flow propeller pumps are the two most common types of dynamic pumps.[1]

Positive displacement pumps

This type is universally used for fluid power systems. With this pump, a fixed amount of fluid is ejected into the hydraulic system per revolution of pump shaft rotation. These pumps are capable of overcoming the pressure resulting from the mechanical loads on the system as well as the resistance to flow due to friction. These two features are highly desirable in fluid power pumps. These pumps also have the following advantages over non positive displacement pumps:

• High-pressure capability (up to 12,000 psi, ca. 80 MPa)
• Small compact size
• High volumetric efficiency
• Small changes in efficiency throughout the design pressure range[2]

Characteristics

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Fluid power systems can produce high power and high forces in small volumes, compared with electrically-driven systems. The forces that are exerted can be easily monitored within a system by gauges and meters. In comparison to systems that provide force through electricity or fuel, fluid power systems are known to have long service lives if maintained properly. The working fluid passing through a fluid motor inherently provides cooling of the motor, which must be separately arranged for an electric motor. Fluid motors normally produce no sparks, which are a source of ignition or explosions in hazardous areas containing flammable gases or vapors.

Fluid power systems are susceptible to pressure and flow losses within pipes and control devices. Fluid power systems are equipped with filters and other measures to preserve the cleanliness of the working fluid. Any dirt in the system can cause wear of seals and leakage, or can obstruct control valves and cause erratic operation. The hydraulic fluid itself is sensitive to temperature and pressure along with being somewhat compressible. These can cause systems to not run properly. If not run properly, cavitation and aeration can occur.

Application

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A hydraulic excavator A hydraulic rescue tool is used to extract injured people from wrecked cars.

Mobile applications of fluid power are widespread. Nearly every self-propelled wheeled vehicle has either hydraulically-operated or pneumatically-operated brakes. Earthmoving equipment such as bulldozers, backhoes and others use powerful hydraulic systems for digging and also for propulsion. A very compact fluid power system is the automatic transmission found in many vehicles, which includes a hydraulic torque converter.

Fluid power is also used in automated systems, where tools or work pieces are moved or held using fluid power. Variable-flow control valves and position sensors may be included in a servomechanism system for precision machine tools. Below is a more detailed list of applications and categories that fluid power is used for:

• Industrial (also known as fixed)
  • metalworking
  • injection molding
  • controllers
  • material handling
• Aerospace
  • landing gears
  • brakes

Pneumatic and hydraulic systems compared

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Cost

Pneumatics are less expensive to build and operate. Air is used as the compressed medium, so there is no requirement to drain or recover fluid. Hydraulic systems use larger working pressures, and require larger parts than pneumatics.

Precision

Unlike liquids, gases change volume significantly when pressurized making it difficult to achieve precision.

Common hydraulic circuit application

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Synchronizing

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This circuit works off of synchronization. As a cylinder reaches a certain point another will be activated, either by a hydraulic limit switch valve or by the build-up of pressure in the cylinder. These circuits are used in manufacturing. An example of this would be on an assembly line. As a hydraulic arm is activated to grab an object. It then will reach a point of extension or retraction, where the other cylinder is activated to screw a cap or top onto the object. Hence the term synchronizing.

Regenerative

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In a regenerative circuit, a double acting cylinder is used. This cylinder has a pump that has a fixed output. The use of a regenerative circuit permits use of a smaller size pump for any given application. This works by re-routing the fluid to the cap instead of back to the tank[3][citation needed]. For example, in a drilling process a regenerative circuit will allow drilling at a consistent speed, and retraction at a much faster speed. This gives the operator faster and more precise production.[citation needed]

Electrical control

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Combinations of electrical control of fluid power elements are widespread in automated systems. A wide variety of measuring, sensing, or control elements are available in electrical form. These can be used to operate solenoid valves or servo valves that control the fluid power element. Electrical control may be used to allow, for example, remote control of a fluid power system without running long control lines to a remotely located manual control valve.

See also

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References

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• Esposito, Anthpny, Fluid Power With Applications,ISBN 0-13-513690-3
• Esposito, Anthony, Fluid Power with Applications, ISBN 0-13-010225-3
• Hydraulic Power System Analysis, A. Akers, M. Gassman, & R. Smith, Taylor & Francis, New York, 2006, ISBN 0-8247-9956-9

Control Valve: How It Works | Advantages & Disadvantages

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secureluokai

July 14, 2017

July 14, 2017

What is a control valve?

Control valve is used to regulate the fluid flow by changing its size or course as directed by the signal from a controller. It helps in directly controlling the flow rate and thus helps in regulating the other vital processes quantities such as temperature, liquid level/flow, and pressure. It is also refereed as ‘final control element’ in automatic control terminology.

In other words, control valve is used to control the flow, pressure, liquid level, and temperature of the system by completely or partially opening or closing it based on the signals received by the controllers. The electric, hydraulic and pneumatic actuators automatically controls the opening and closing of the control valve while the positioners control the closing and opening of these actuators.

How it works?

The process plant comprises of numerous control loops in order to deliver consistent quality products. These control loops have a set pressure, temperature, flow, level in order to maintain the required operating range. Each of these control loops experience internal disturbance and these disturbances are measured by sensors and transmitters. The information collected is then processed by the controllers to decide what should be done to rectify these load disturbances. Once the data collected is analyzed, measured, compared and calculated then a controlling element is implemented. This is where the control valve comes into the picture and work to reduce these disturbances.
Thus, the control valve working principle revolves around the manipulation of the flowing fluids such as water, gas, steam or chemical compounds to ensure minimizing the load disturbance and regulating the process variable to the closest value of the desired set point.

Values categorization: It’s advantages and disadvantages

On the basis of stem movement, valves can be broadly classified as linear or rotary type. Furthermore, based on the types of services it offers, it can be classified as:
For ON-OFF Services – Gate, Diaphragm Ball, Plug, Butterfly valves.
For THROTTLING Services – Globe, Diaphragm, Butterfly, Pinch
For NON-REVERSE FLOW – Check valves

Gate Valves

Gate valve comprises of flat barrier which is adjusted to effect the area of flow. It is best suited to regulate the high temperature and pressure for variety of liquid flow and has an on-off application. This design or body is used for the valves that manages the automatic emergency shut off and also for the hand-operated valves.

Advantages of Gate Valve

• • It has a good Closing feature
  • Can be used in either ways in circuit
  • It provides laminar flow and hence the loss of pressure is minimum

Disadvantages of Gate Valves

• • • • Closing and opening feature is not very prompt.
      • Assembling these valves and then erecting it, starting it and maintaining it requires large space.
      • Prone to leakage when system operate in high temperature that fluctuates periodically.
      • It creates vibration

Globe Valve

Globe valve is used for throttling purpose and the term ‘globe’ implies to the exterior shape of the valve. The globe valves have a stem that adjusts itself linearly in up-down motion in order to change the position of the plug. It has a short stem travel with high seating capability and large pressure drop and high flow controllability.

Advantages of Globe Valve

• • • • Has a good full closing feature.
      • Faster opening and closing time in comparison to gate valve as the strokes are smaller.
      • Has a good throttling feature.
      • Can work as a stop check valve with little change in shaft disconnection.

Disadvantages of Globe Valve

• • • • Higher pressure loss in comparison to gate valve.
      • It requires actuator that has a larger torque in order to close under high pressure.

Plug Valves

Plug valve comprises of plug, body, and cover and is used for on-off services basically in refinery, chemical industry, and petrochemical. Normally small in size, require less headroom and available in wide range of materials. They provide tight shut off, quick opening and low pressure drop

• • • • It comprises of few parts and has a simple design.
      • Opens and closes easily.
      • It can be maintained and repaired easily at the place of operation.
      • They have low flow resistance and has a reliable leak-proof feature

Disadvantages of Plug Valves

• • • • It creates high friction due to fast movement and hence requires large amount of force.
      • Valve transitions usually narrows flows due to conic plug.
      • Expensive than the ball valves.

Ball Valves

Ball valve is used to control pressure and flow of the corrosive fluids, normal liquids and gasses and also of slurries. It also regulates high temperature and pressure.

Advantages of Ball Valves

• • • • Offers leak-proof service.
      • Fast open and close service.
      • In comparison to gate valves has a very small dimension.
      • It is light weight in comparison to gate valves.
      • Has a multi-way design flexibility to reduce the number of valves required.
      • Provides reliable and safe service under high temperature and pressure conditions.
      • Requires less force to control in comparison to gate and globe valves.

Disadvantages of Ball Valves

• • • • Not an apt solution for permanent throttling.
      • It can cause leakage, abrasion and more such problems because the residual fluids collide and sticks to the base.

Diaphragm Valves

Diaphragm valve is used for corrosive fluids under low temperature and pressure conditions. The diaphragm valve varies the resistance to flow by deforming one surface by the force from the valve stem.

Diaphragm Valves Advantages

• • • • The diaphragm completely keeps the working parts in isolation from the process fluids.
      • Simple in construction, easier to operate and maintain.
      • It has hassle-free operation with simple construction and ease of operation.
      • It can be used for opening, throttling, and closing, purposes.
      • Based on the internal sheathing of the body, it provides good chemical resistance.
      • No shaft leakage as the fluid remains isolated from the bonnet group.
      • Suits heavy chemical, chemicals, and radioactive fluids.
      • It can prevents microbial contamination to the system as the fluid remains isolated from the bonnet group and thus can be used in food, pharma, and beer sector.

Disadvantages of Diaphragm Valves

• • • • Complete discharge of the pipeline is prevented as the valve transition has sets.
      • Operating temperature and pressure has limits based on the diaphragm material.
      • There is a limitation to the hydrostatic pressure that can be applied to the diaphragm.
      • It has limited dimensions of diaphragm valves.

Conclusion:

For more information on various control valves specifically required for your industrial plants / business, Contact Flowspec and we will be happy to answer your questions and give you all the details you need.

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