1. Introduction of pneumatic globe valve

Pneumatic cut-off valve, widely used in the important system of mine and nuclear power plant, is an important key equipment to ensure the safety production of power plant. In the actual operation, many factors such as the setting of manual mechanism, packing failure, improper operation, insufficient air intake flow lead to many failures, which threaten the safe and stable operation of the power station. In this article, taking the pneumatic globe valve produced by SEREG as an example, analyze the working principle of typical pneumatic globe valve for power station, and describe the setting of neutral point in detail. It provides a basis for the operation and troubleshooting of SEREG pneumatic globe valve or similar pneumatic globe valve used in the power station.

2. Introduction to the working principle of pneumatic globe valve 

Working principle of pneumatic globe valve: most of the valves which uses compressed air as power source adopt remote control. The main control room manually or automatically triggers the action command to make the intake and exhaust solenoid valve act to supply and exhaust compressed air to the actuator. Compressed air acts on the piston or diaphragm to overcome the friction of valve action and the spring force on the stem. Press down (Loss of air pressure, open the valve J ) or lift up (Loss of air pressure, open the valve F1) the valve rod, driving the valve core up and down to open or close the valve. When the air pressure is lost in the valve and the driving head compresses air, the valve will be in the safe position of full opening or full closing according to the requirements of the system. The air supply pressure of the service instrument is generally 4-9 bar. Before entering the valve, filter and purify the compressed air through the filter, and then reduce the pressure through the pressure reducing valve. The reading of the inlet pressure gauge is not more than 4bar. The adjustment can be realized by adjusting the adjusting screw installed on the top of the pressure reducing valve, and the protective cover shall be loosened before adjustment.

The four most common and widely used types of pneumatic globe valves are: the valve closed by air loss of handwheel, the valve opened by air loss of handwheel, the valve closed by air loss without handwheel, and the valve opened by air loss without handwheel.

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From the name we are able to clearly observe that the first is just one seat, and the other has a double seat, as suggested by its name may be the distinction between the valve seat. Double seat control valve has two seats may be the valve body, and also the single sitting down control valve has one seat within the valve body.

What is a single seated valve?

A single-seated valve has a single seat and a single plug, in this type of valves the process pressure would reach on the bottom side of the plug and this will create a force upward in the valve stem. The actuator in this valve can control this force or it is capable to act against the force created by process pressure. So the actuator can move or hold the stem downward by controlling this force. The actuator in this valve must be able to control this force but it is a difficult task if the valve is big. If the valve is big then the created pressure will be big too and the actuator must be capable to handle this force, and because of this, the single-seated valve is not used for high-pressure applications. Single seated valves are suitable if we need a higher degree of shut off, single-seated valve or single-port valves are in many forms they are the globe, angle, bar stock, etc. The single-seated valve has top guided construction and it has good flow capacity than the top and the bottom guided valves. The selection of actuators and plugs for a single port valve must be done according to the pressure of the fluid.

Advantages of single seated valves

• It provides high rangeability and tight shut-off
• A reversible plug is available

Disadvantages of single seated valves

• A large actuator is needed
• Pressure recovery is low
• Only used for small diameter applications
• The force required to drive the stem is high

Applications of single-seated valves

These valves are used to control a large range of process parameters and because of this feature, it is used in oil and gas production, power generation.

What is a Double seated valve (double port valve)?

A double-seated valve is a top and bottom guided valve and these valves have two plugs and two seats. In this type of valve, the line pressure acts upward in one plug and downward in the other, and because of this, the force is balanced. These valves are used for high-pressure applications. Double port valves are pressure balanced valves and they can close the valve plug against higher operating pressure. Tight shut off is not possible as in single-seated valves, these valves have fast flow response and greater capacity than single-seated valves and they are used when tight shut off is not needed. In this valve, the two ports cannot be opened at the same time. Double port valves can do the diversion of the flow from a single flow line to one or more outlets.

Advantages of Double seated valves

•  It can be used for larger valves
•  It can be used for high flow and high-pressure services

Disadvantages of Double seated valves

• Tight shut off is not possible
• Due to the erosion and cavitation, the body parts could wear out

What is difference between single seated and double seated control valve?

• 1) Single seated control valve has only one trim and one seat, double seated control valve has two trims and two seats.
• 2) Single seated control valve has a small leakage, and it is easy to achieve strict sealing and cutting; Double seated control valve has a large leakage.
• 3) Single seated control valve allows a small differential pressure;Double seated control valve is subjected to a small unbalance force and allows a large differential pressure.
• 4) Single seated control valve has a small flow capacity, and double seated control valve has a large flow capacity. Compared with other control valves of the same diameter, double seated control valve can flow through more fluid, and the flow capacity of the same diameter double seated control valve is about 20%-50% larger than that of single seated control valve.
• 5) Due to the large thrust of the fluid medium to the valve trim, that is, the unbalanced force is large, therefore, in the application of high pressure difference and large diameter, it is not suitable to use a single seated control valve; Double seated control valve is not suitable for high pressure differential applications. Due to complex flow path, it is not suitable for the control of           fiber containing media and high viscosity fluids.

The pneumatic double seat control valve seat has lower and upper valve plugs, and also the fluid flows in the left side of body, goes through the valve seat and plug, and flows out of the right side. It may flow more medium than the usual single-sitting down control valve with similar nominal diameter, and also the flow capacity is elevated by about 20% to 25%. The forces functioning on the lower and upper plug can cancel one another out, therefore the imbalance pressure is small , the operating pressure difference is big.

• 1.The options from the single seat control valve: the leakage is small, the unbalanced pressure is big, and also the allowable pressure difference is smaller sized compared to double seat valve.
• 2. The characteristics of double seat control valve: large allowable pressure difference. It is almost always especially appropriate for applications with large flow rates, ruthless drops and enormous leakage. 

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What is a Diphragm Actuator ?

By themselves, valves cannot control a process. Manual valves require an operator to position them to control a process variable. Valves that must be operated remotely and automatically require special devices to move them. These devices are called actuators.

Actuators may be pneumatic, hydraulic, or electric solenoids or motors.

Diaphragm actuators are pneumatically operated and they use an air supply from the control system or other sources. Diaphragm actuators are normally used for control valves, which are a type of globe valve that is typically used to regulate the fluid in order to adjust some process variables, such as pressure, temperature, or flow rate. Diaphragm actuators are used in all sectors of the oil and gas industry except for subsea. The most common styles for diaphragm actuators are known as “direct-acting” and “reverse-acting.”

Pneumatic Diaphragm Actuators

A simplified diagram of a pneumatic actuator. It operates by a combination of force created by air and spring force. The actuator positions a control valve by transmitting its motion through the stem.

A rubber diaphragm separates the actuator housing into two air chambers. The upper chamber receives supply air through an opening in the top of the housing.

The bottom chamber contains a spring that forces the diaphragm against mechanical stops in the upper chamber. Finally, a local indicator is connected to the stem to indicate the position of the valve.

The position of the valve is controlled by varying supply air pressure in the upper chamber. This results in a varying force on the top of the diaphragm. Initially, with no supply of air, the spring forces the diaphragm upward against the mechanical stops and holds the valve fully open.

As supply air pressure is increased from zero, its force on top of the diaphragm begins to overcome the opposing force of the spring. This causes the diaphragm to move downward and the control valve to close. With an increasing supply of air pressure, the diaphragm will continue to move downward and compress the spring until the control valve is fully closed.

Conversely, if supply air pressure is decreased, the spring will begin to force the diaphragm upward and open the control valve. Additionally, if supply pressure is held constant at some value between zero and maximum, the valve will position at an intermediate position. Therefore, the valve can be positioned anywhere between fully open and fully closed in response to changes in supply air pressure.

A Positioner is a device that regulates the supply of air pressure to a pneumatic actuator. It does this by comparing the actuator’s demanded position with the control valve’s actual position.

The demanded position is transmitted by a pneumatic or electrical control signal from a controller to the positioner. The pneumatic actuator in Figure 1 is shown in Figure 2 with a controller and positioner added.

The controller generates an output signal that represents the demanded position. This signal is sent to the positioner. Externally, the positioner consists of an input connection for the control signal (4-20mA), an instrument supply air input connection, a supply air output connection, a supply air vent connection, and a feedback linkage.

Internally, it contains an intricate network of electrical transducers, air lines, valves, linkages, and necessary adjustments. Other positioners may also provide controls for local valve positioning and gauges to indicate supply air pressure and control air pressure (for pneumatic controllers – old controlling methods).

The controller responds to a deviation of a controlled variable from the setpoint and varies the control output signal accordingly to correct the deviation. The control output signal is sent to the Positioner, which responds by increasing or decreasing the supply of air to the actuator.

The positioning of the actuator and control valve is fed back to the positioner through the feedback linkage. When the valve has reached the position demanded by the controller, the positioner stops the change in supply air pressure and holds the valve at the new position. This, in turn, corrects the controlled variable’s deviation from the setpoint.

For example, as the control signal increases, a valve inside the positioner admits more supply air to the actuator. As a result, the control valve moves downward. The linkage transmits the valve position information back to the positioner.

This forms a small internal feedback loop for the actuator. When the valve reaches the position that correlates to the control signal, the linkage stops supplying airflow to the actuator.

This causes the actuator to stop. On the other hand, if the control signal decreases, another valve inside the positioner opens and allows the supply air pressure to decrease by venting the supply air. This causes the valve to move upward and open. When the valve has opened to the proper position, the positioner stops venting air from the actuator and stops the movement of the control valve.

Advantages of Pneumatic Diaphragm Actuators

The benefits of pneumatic actuators come from their simplicity.

Pneumatic actuators’ typical applications involve areas where the conditions involve extreme temperatures, a typical temperature range is -40°F to 250°F.

In terms of safety and inspection, using air and pneumatic actuators avoids using hazardous materials. They also meet explosion protection and machine safety requirements because they create no magnetic interference due to the lack of motors.

Pneumatic actuators are also lightweight, require minimal maintenance, and have durable components that make pneumatics a cost-effective method of power.

Disadvantages of Pneumatic Diaphragm Actuators

Pressure losses and compressibility of air make pneumatics less efficient than other methods. Compressor and air delivery limitations mean that operations at lower pressures will have lower forces and slower speeds.

To be truly efficient, pneumatic actuators must be sized for a specific job. Hence, they cannot be used for other applications.

Even though the air is readily available, it can be contaminated by oil or lubrication, leading to downtime and maintenance. Companies still have to pay for compressed air, making it consumable, along with the compressor and line maintenance costs.

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What is a Actuated Globe Valve?

A throttling globe valve uses a linear motion to move a closure member into and out of a seating surface. They have a body distinguished by a globular-shaped cavity around the port region and are designed to regulate process flow when used in conjunction with an actuator assembly. Many single-seated valve bodies use cage or retainer-style construction to retain the seat-ring, provide valve plug guiding, and provide a means for establishing particular valve flow characteristics. Cage or retainer-style single-seated valve bodies can also be easily modified by change of trim parts to change the flow characteristic or provide reduced-capacity flow, noise attenuation, or reduction or elimination of cavitation.

Overview of Globe Valve Actuator

Profinet as an open Industrial Ethernet communication standard is increasingly in demand especially in the water industry. Redundant Profinet in combination with AUMA electric actuators helps the Wupperverband achieve sustainable plant operation in its Marienheide wastewater treatment plant.

Marienheide wastewater treatment plant (WWTP), Germany, has experienced re- markably high plant availability since installing redundant Profinet systems to integrate field devices. The plant operator is the Wupperverband, a large German water authority that operates 11 wastewater treatment plants, 14 reservoirs, a sludge incineration plant, and numerous flood retention and rainwater catchment reservoirs. Marienheide WWTP was the first Wupperverband plant to be equipped with Profinet technology at field device level.

At Marienheide WWTP, the AUMA actuators are linked in a ring topology with built-in redundancy using Profinet’s Media Redundancy Protocol (MRP)

Increased safety and easier handling

Since autumn 2017, around 20 AUMA actuators have been operating in a Profinet network at Marienheide WWTP. AUMA’s certified Profinet interface provides direct Profinet integration without the need for gateways, and meets the latest Profinet specification V 2.3. The interface allows bandwidths up to 100 Mbit/s and supports Profinet’s built-in redundancy concept, the Media Redundancy Protocol (MRP). At Marienheide, the AUMA actuators are installed in a ring topology using MRP. They are connected to the DCS via an intelligent switch. To support MRP, the AUMA interface incorporates a switch function with two physically separate Ethernet ports that provide two distinct communication channels to the host controller. If one port fails, communication continues automatically via the second port. All actuators therefore remain accessible even if the communication path is interrupted at one point on the ring.

“We expect high communication reliability due to the consistent use of Profinet at both control system and field level”, explains Dirk Gengnagel, Group Leader Engineering and Organization in the Urban Water Management division of the Wupperverband.

“Data and commands can be directly exchanged without any conversion; and Profinet’s integral Redundant Profinet increases plant availability redundancy function provides additional safety for plant availability. Furthermore, we expect savings and reduced proneness to faults since we can eliminate one bus level and require less hardware.”

Intelligent networks and diagnostics

Using Profinet, important operating data such as temperatures and torque curves can thus be read from the actuator. This improves the chances of detecting potential issues at an early stage, allowing for condition-based predictive maintenance.

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Pneumatic control valve

The control valve can know some information from its name, and the adjustment range of the keyword adjustment is between 0 and 100%.

Careful friends should find that there is a device hanging under the head of each regulating valve. Those familiar with it must know that this is the heart of the regulating valve, the valve positioner. Through this device, the amount of air entering the head (pneumatic film) can be adjusted. Precisely control the position of the valve.

The positioner intelligent positioner and mechanical positioners, today’s discussion is the latter mechanical positioners, shown in the picture locator same.

Working principle of Valve Positioner

The air source comes from the compressed air of the air compressor station, and there is an air filter pressure reducing valve in front of the air source inlet of the valve positioner to purify the compressed air. The gas source from the outlet of the pressure-reducing valve enters from the valve positioner. As for how much gas enters the membrane head of the valve, it is determined according to the output signal of the controller.

The electrical signal output by the controller is 4-20mA, the pneumatic signal is 20Kpa-100Kpa, and the electrical signal to the pneumatic signal is carried out through an electrical converter.

When the electrical signal output by the controller is converted into a corresponding air signal, the converted air signal is then applied to the bellows. Lever 2 moves around the fulcrum, and the lower part of lever 2 moves to the right close to the nozzle. The back pressure of the nozzle increases. After being amplified by the pneumatic amplifier (the part with the symbol less than in the figure), a part of the air source is sent to the air chamber of the pneumatic membrane, and the valve stem takes the valve core downward to automatically gradually open the valve and become smaller. At this time, the feedback rod connected to the valve stem (the swing rod in the figure) moves down around the fulcrum, so that the front end of the shaft moves down, the eccentric cam connected to it rotates counterclockwise, and the roller rotates clockwise to move to the left, thus Stretch the feedback spring. Since the lower part of the feedback spring tension lever 2 moves to the left, it will be in force balance with the signal pressure acting on the bellows, so the valve is fixed at a certain position and does not move.

Knowledge expansion 1 of Valve Positioner

Pneumatic diaphragm air chamber ventilation source, the diaphragm presses down the six spring springs covered by the diaphragm, thereby pushing the valve stem downwards. The valve stem is connected with the valve core, and the valve core is installed in the forward direction, so the air source is the valve Move to the off position. Therefore, it is called a gas shutoff valve. Failure to open means that when the air supply is interrupted due to construction or the corrosion of the air pipe, the valve is reset under the reaction force of the spring, and the valve is in the fully open position again.

How to use the air shutoff valve?

How to use it is considered from the perspective of safety, which is a necessary condition for selecting air on or off.

For example; one of the core devices of the boiler is a steam drum, and a High pressure pneumatic diaphragm control valve used in the water supply system must be air-closed. why? For example, if the gas supply or power supply is suddenly interrupted, the furnace is still burning violently, continuously heating the water in the drum. If the gas is used to open the regulating valve and the energy is interrupted, the valve will be closed, and the steam drum will be boiled off within minutes without water (dry burning). This is very dangerous. It is impossible to deal with the regulating valve failure in a short time, which will cause the boiler to stop. The accident happened. Therefore, to avoid dry burning or even shutdown accidents, air shutoff valves must be used.

Although the energy is interrupted and the regulating valve is in the fully open position, the steam drum is continuously filled with water, but it will not cause the steam drum to dry up. There is still time to deal with the failure of the regulating valve and the boiler will not be stopped directly.

Knowledge expansion 2 of Valve Positioner

The output of the valve positioner is like turtle speed.

Do not open the front cover of the valve positioner; listen to the sound and see if the air source tube is cracked and leaked, which can be judged by the naked eye. And input the air chamber to hear whether there is any leakage.

Open the front cover of the valve positioner; 1. Whether the constant orifice is blocked 2. Check the position of the baffle 3. Check the elasticity of the feedback spring 4. Disassemble the Fangda device to check the diaphragm.

The output of the valve positioner is boring

• 1. Check whether the air pressure is within the specified range and whether the feedback lever has fallen off. This is the simplest step.
• 2. Check whether the wiring of the signal line is correct (problems that appear later are generally ignored)
• 3. Is there anything stuck between the coil and the armature?
• 4. Check whether the matching position of the nozzle and the baffle is appropriate
• 5. Check the coil condition of the electromagnetic component
• 6. Check whether the debugging position of the balance spring is reasonable

Subsequently, input signal, but the output pressure does not change, if there is output, the maximum value is not reached, etc. ZECO Valve supplied Siemens pneumatic control valve to power plant projects. If you are interested in our product please feel free to contact us.

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What is Globe Control Valve?

A globe control valve uses a linear motion to move a closure member into and out of a seating surface. They have a body distinguished by a globular-shaped cavity around the port region and are designed to regulate process flow when used in conjunction with an actuator assembly. Many single-seated valve bodies use a cage or retainer-style construction to retain the seat-ring, provide valve plug guiding, and provide a means for establishing particular valve flow characteristics. Cage or retainer-style single-seated valve bodies can also be easily modified by change of trim parts to change the flow characteristic or provide reduced-capacity flow, noise attenuation, or reduction or elimination of cavitation.

Excellent Accuracy with Globe Control Valve

• Accuracy and precision
  • Globe control valves provide superior control performance and high reliability in a wide range of applications. They are designed to provide accuracy, precision, and reliability in both general and more demanding services. They can be found in service in a wide range of customer industries, from oil & gas, chemical, and petrochemical to power plants and pulp & paper mills.
• Safety and sustainability
  • Globe control valves are engineered, built, and tested to be both safe and sustainable. They ensure the lowest possible emissions in accordance with ISO 15848 and in most cases certified fire safety according to API 607. Our globe valves are also SIL 3 certified. Special cleaning options are also available for oxygen and cryogenic services for example.

Single Seat Globe Control Valve

Definition

Single Seat Globe Control Valve, the trim is interchangeable in the body. It’s content to various technics liquid process by replacing the different trim assembled.

Tehnical Specification

Application

The single-seat globe control valve is the furthest common control valve type, a simple structure, is used for the strict sealed close situation. A single-seat globe control valve can be controlled the maximum flow rate by changing the size of the plug and seat ring. setting fluid characteristics by changing the curved surface. They are applicable in a wide scope of liquid services as well as steam services.

Structure

Among different types of valve bodies, single-seated valves have trim structures with the easiest collocation, for example, angle valves. As a result of high-pressure fluid is generally held the load to add the whole area of the bottom seat, making the single plug to be endured huge unbalanced force from fluid, relative to the control valve with balanced trim, the allowable pressure drops are much less for the single-seated valve on the process control.

Trim Options

Cage Guided Control Valve

Definition

Cage Guided Control Valve, cage structure, straight stroke control valve. the trim is interchangeable in the body Its content to various technics liquid processes by replacing the different trim assembled.is designed focused on high-pressure drops, effectively reducing the effects of cavitation and the noise in the high-pressure drops services. favorable Interchangeability, be reached different flow characteristics by replacing different cages, to get much higher precision of control. Can be reduced noise, and dispelled the effects of cavitation by replacing trim.

Tehnical Specification

Application

Cage Guided control valve size is in common use for 1-12, an actuator for standard diaphragm type. On a metal seat, the suitable temperature range is -196 +538, and on a soft seat, the suitable temperature range is -45 +200. It’s content with most industrial process control.

Structure

The most excellent cage-guided structure control valve is balance characteristic, for plug balance ports function, fluid pressure acted on static unbalance force of plug, it reduced unbalance force, then the valve can be configured actuator smaller than conventional single seat valve.

The most simple structure of a cage-guided control valve is twin-seat, the twin-seat structure is applicable to situations the requirement of leakage is not high. The cage-guided single-seat control valve has a sliding seal between the plugs top of the cage. by seals function, dispelling the leak that upriver fluid entered the low-pressure downriver system. So it has the tightly closed characteristic as a conventional single seat valve, moreover has balance characteristic.

Trim Options

ZECO Valve offers high-quality Single Seat and Cage Guided Globe Control Valves, please feel free to contact us if you need them.

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The GFlo Globe Control Valve

Superior performance in liquids and gaseous services is the hallmark of the GFlo Globe Control Valve by Zeco. Easy, quick, and economical maintenance makes the GFlo Globe Control Valve more user-friendly. The spring cylinder actuated GFlo imparts stiffness and maintains very high precision in positioning. A strong response, repeatability, and high-speed controlled movement are the characteristics absent in diaphragm-operated control valves. GFlo has the thrust capability to shut off against higher fluid pressures,  up to 150 Psi / 10.30 Bar air supply.

Valve closure is dependent on springs in most diaphragm actuators. The unique design of GFlo enables it to combine the spring force, fluid pressure, and supply air pressure to achieve a tight shutoff. A self-aligning seat ring adds to and further improves the shutoff capability.

Galling at the cage is the culprit in many common maintenance issues with globe valves because of the close metal-to-metal contact between plug & cage. The GFlo has two top heavy-duty stem guides. Interference between the plug and seat retainer does not exist in GFlo. Easy and quick maintenance is due to the clamped-in seat and top entry trim.

Technical specifications of GFLO Globe Control Valve

Features of GFLO Globe Control Valve:

• Reliable High-thrust Cylinder Actuator.
• Accurate Positioning.
• High Repeatability.
• Double Stem Guiding does not come in contact with operating fluid.
• Sufficient Clearance between plug head and seat retainer to avoid galling.
• Self-Aligning seat ring.
• Shutoff assisted by fluid pressure, cylinder spring and air pressure.
• Corrosion resistant construction.
• High Thrust cylinder overcomes high pressures to provide exceptional tight shutoff.
• Anti-Cavitation and Noise Attenuation trim options.
• Heavy-Duty plug stem.
• Excellent Interchangeability between sizes and other MASCOT products.
• Valve design minimizes requirements for stocking spare parts.
• Stiff, Heavy-Duty parts provide extended life.

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