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Check Valves in LNG Cryogenic Service

Check Valves in LNG Cryogenic Service

Because natural gas is currently conside...

Will Smart Machines Obsolete Human Resources?

Will Smart Machines Obsolete Human Resources?

Is artificial intelligence (AI) going to...

Is Valve Live Loading an Option?

Is Valve Live Loading an Option?

Valves leak. There’s no getting ar...

Managing Control Valve Reliability in the Arctic

Managing Control Valve Reliability in the Arctic

“A sow and polar bear cub are walk...

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Industry Headlines

U.S. Adds 228,000 Jobs in November

Friday, 08 December 2017  |  Chris Guy

Total nonfarm payroll employment increased by 228,000 in November, and the unemployment rate was unchanged at 4.1%, the U.S. Department of Labor report...

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Will Smart Machines Obsolete Human Resources?

Will Smart Machines Obsolete Human Resources?

Wednesday, 06 December 2017  |  Kate Kunkel

Is artificial intelligence (AI) going to take over manufacturing jobs and the jobs of human resources professionals? That is a question that concerns ...

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Industry Headlines

Classic Controls Certified by ITT Engineered Valves, REXA

3 DAYS AGO

Classic Controls is now certified by ITT Engineered Valves to repair Skotch brand FM approved and general purpose burner shut-off valves for oil and gas fired systems.

REXA, Inc. has also appointed Classic Controls as its exclusive authorized sales representative in the state of Florida. Classic Contr...

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Emerson Completes Paradigm Acquisition

6 DAYS AGO

Emerson has completed the purchase of Paradigm, a provider of software solutions to the oil and gas industry. Paradigm will be combined with Emerson’s Roxar software business to create a more comprehensive Exploration & Production (E&P) software portfolio offering.

Paradigm is headquarter...

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China to Become Second-Largest LNG Importer

3 DAYS AGO

The world’s no. 1 energy user, China, “is on the verge of passing South Korea to become the No. 2 buyer of LNG behind Japan, as environmental measures and winter heating needs boost demand,” Bloomberg reports .

“Tankers with total capacity of 33.6 million metric tons have visited...

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New Permian-Gulf Pipeline Proposed

5 DAYS AGO

Magellan Midstream Partners intends to develop an approximately 375-mile, 24-inch diameter pipeline from Crane to a location near Three Rivers, TX, providing shippers the option to ultimately deliver crude oil and condensate from the Three Rivers area to the Houston area via a new 200-mile pipeline or...

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U.S. Adds 228,000 Jobs in November

2 DAYS AGO

Total nonfarm payroll employment increased by 228,000 in November, and the unemployment rate was unchanged at 4.1%, the U.S. Department of Labor reported today. Employment continued to trend up in professional and business services, manufacturing and health care.

In November, manufacturing added 31,000...

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Manufacturing PMI at 58.2% in November

4 DAYS AGO

Manufacturing expanded in November as the PMI registered 58.2% , a decrease of 0.5% from the October reading of 58.7%. A reading above 50% indicates that the manufacturing economy is generally expanding.

"This indicates growth in manufacturing for the 15th consecutive month led by expansion in new orde...

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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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