11222017Wed
Last updateWed, 22 Nov 2017 2pm

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Unique Operating Dynamics of Trunnion-Mounted Ball Valves

Unique Operating Dynamics of Trunnion-Mounted Ball Valves

For a very long time, the idea has exist...

Adopting the IIoT: Are Results Matching the Hype?

Adopting the IIoT: Are Results Matching the Hype?

Many valve, actuator and control manufac...

Where Valves Are Used: Everywhere!

Where Valves Are Used: Everywhere!

Valves can be found just about anywhere ...

Oxygen Cleaning: A Validated Process is Critical for Safety

Oxygen Cleaning: A Validated Process is Critical for Safety

From time to time, we re-publish well-re...

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Industry Headlines

Conference Board Economic Index Rose in October

Tuesday, 21 November 2017  |  Chris Guy

The Conference Board Leading Economic Index (LEI) for the U.S. increased 1.2% in October to 130.4 (2010 = 100), following a 0.1% increase in September...

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Industry Headlines

VanAire Celebrates Manufacturing with ‘Industry After Hours’

1 DAY AGO
VanAire Celebrates Manufacturing with ‘Industry After Hours’

On Nov. 8, Delta County, MI K-12 students and parents attended Industry After Hours at VanAire to learn more about the high tech, high skilled manufacturing jobs that are available. Over 276 participants were treated to a fun, entertaining and informative event that exceeded expectations and inspired ...

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Emerson Opens Solutions Center in Singapore

4 DAYS AGO

As part of its drive to make Singapore a hub for the delivery of Industrial Internet of Things (IoT) technologies and services for customers across Asia Pacific, Emerson opened a customer-focused Solutions Center at the regional headquarters of its Automation Solutions business in Singapore. To mark...

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Nebraska Approves Keystone XL Pipeline Route

1 DAY AGO

By a vote of 3-2, the Nebraska Public Service Commission approved an order giving TransCanada the go ahead to build the Keystone XL Pipeline utilizing the proposed Mainline Alternative Route. This is one of three proposed routes included in the application filed by TransCanada Keystone Pipeline. L.P...

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Energy Outlook Shows a World in Transformation

7 DAYS AGO

The resurgence in oil and gas production from the United States, deep declines in the cost of renewables and growing electrification are changing the face of the global energy system and upending traditional ways of meeting energy demand, according to the World Energy Outlook 2017 . A cleaner and more...

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Conference Board Economic Index Rose in October

22 HOURS AGO

The Conference Board Leading Economic Index (LEI) for the U.S. increased 1.2% in October to 130.4 (2010 = 100), following a 0.1% increase in September, and a 0.4% increase in August.

“The U.S. LEI increased sharply in October, as the impact of the hurricanes dissipated,” said Ataman Ozyildi...

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U.S. Industrial Output Up 0.9% in October

4 DAYS AGO

Industrial production rose 0.9% in October, and manufacturing increased 1.3%. The index for utilities rose 2.0%, but mining output fell 1.3%, as Hurricane Nate caused a sharp but short-lived decline in oil and gas drilling and extraction. Even so, industrial activity was boosted in October by a return...

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Accessories for Control Valves

spr11_b2b_introHigh-pressure, angle-style control valves with high-pressure piston actuators are equipped with digital valve positioners, airsets and volume boosters, along with other accessories.While control valves are an essential part of most flow processes today, they cannot accomplish their key tasks without help from the many accessories that aid in smooth operation.

Control valves are a ubiquitous essential in today’s industrial facilities. Whether they are managing flow or controlling process conditions such as temperature or pressure, they are key to maximizing production, maintaining product quality and ensuring safety.

However, they cannot accomplish these tasks on their own. Equally important to the processes are the accessories that work with a control valve—helping ensure accuracy, boosting output, making various control system components compatible with one another and providing added layers of safety.

In this article, we will review some of these devices, the roles they play in a control system and factors that should be considered when specifying these devices.


VALVE POSITIONER

Positioners are required in almost all control valve applications. Many positioner types and communication protocols are available, but a positioner’s primary function is always the same: to accurately position the throttling element within a control valve as directed by a signal from the process control system.

The positioner receives a sign from an external process controller, compares the valve’s actual position to the desired position, and sends a command change to the actuator to correct the valve position in a controlled feedback loop. The process controller measures the change in the process and sends further changes to the position signal as necessary.

Without a positioner, a control valve position is an open loop; the valve moves in response to the signal but nothing is providing feedback to ensure the valve is in the desired position.

Four factors make positioners vital components in today’s process control systems:

Higher-pressure actuators: Positioners make it possible to use today’s higher-pressure actuators in traditional pneumatic control systems, which can reduce costs and make the valve more responsive.

Most pneumatic control systems use an output signal range of 3 to 15 psig or 6 to 30 psig. Traditional spring-diaphragm actuators are widely available in these ranges, but the output force from a given actuator size is significantly limited. And, piston actuators almost always require higher air pressures to perform optimally.

As a result, most actuators are designed and sized to operate at air pressure ranges well above the 3 to 15 or 6 to 30 psig delivered by the control system. Therefore, a positioner or other amplifying device is required to make them compatible with the control system’s lower output pressure. The higher air pressure allows selection of smaller actuators reducing overall equipment cost.

spr11_b2b_schematicTypical placement of some of the most commonly used control valve accessories
Forces within the control valve: Friction from valve stem sealing systems (packing) can be significant, particularly in higher-temperature applications in which graphite-based materials must be used. Unbalanced forces or flow-induced forces within the valve also will affect the actuator’s ability to consistently position the throttling element or plug within the valve.

A positioner continually monitors the effects of these forces and sends the appropriate feedback to the controller, ensuring that necessary adjustments are made quickly to keep the control valve performing as expected.

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