06272016Mon
Last updateMon, 27 Jun 2016 7pm

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Fugitive Emissions—Issues and Opportunities

Fugitive Emissions—Issues and Opportunities

Fugitive emissions, ppm, consent decrees...

Problem-Driven Innovation

Problem-Driven Innovation

Developing Alternative Technology to Imp...

Valve Repair Takes Center Stage in Houston

Valve Repair Takes Center Stage in Houston

Attendees gathered June 2-3 in Houston t...

What Internal Best Practices Can Do for Valve Selection

What Internal Best Practices Can Do for Valve Selection

As time goes by, technology moves forwar...

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Industry Headlines

How Will Brexit Affect the Manufacturing Industry?

Monday, 27 June 2016  |  Chris Guy

The day after the Brexit decision, National Association of Manufacturers (NAM) chief economist Chad Moutray addressed the UK vote to leave the EU and it...

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Industry Headlines

Curtiss-Wright Awarded Contracts for U.S. Naval Defense Platforms

4 DAYS AGO

Curtiss-Wright announced that it has been awarded contracts valued in excess of $80 million to provide valves for the U.S. Navy’s Virginia-class submarines and Ford-class aircraft carriers. The awards were received from Bechtel Plant Machinery, Inc. and General Dynamics Electric Boat Division ...

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DeZURIK Adds Dedicated Clean Room to Sartell Plant

6 DAYS AGO

In order to ensure proper cleaning procedures are performed on valves intended for oxygen, ozone, chlorine, hydrogen peroxide, isocyanate and other applications, DeZURIK has constructed a new dedicated clean room within its Sartell, MN manufacturing plant.

Operated by trained cleaning technicians, DeZU...

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Global Upstream Spending Slashed by $1 Trillion

4 DAYS AGO

Global upstream development spending from 2015 to 2020 has been cut by 22% or $740 billion since the oil price started to drop two years ago, according Wood Mackenzie's research . When you include cuts to conventional exploration investment, the figure increases to just over $1 trillion. Expect to see...

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EPA Bans Fracking Wastewater Disposal at Public Treatment Plants

6 DAYS AGO

The EPA has finalized a rule establishing pretreatment standards for discharges of wastewater from onshore unconventional oil and gas (UOG) extraction facilities to municipal sewage treatment plants (also known as publicly owned treatment works, or POTWs). The rule is designed to prevent the discharge...

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How Will Brexit Affect the Manufacturing Industry?

8 HOURS AGO

The day after the Brexit decision, National Association of Manufacturers (NAM) chief economist Chad Moutray addressed the UK vote to leave the EU and its effect on the manufacturing industry as well as the element of uncertainty it adds to the global marketplace.

According to Moutray, “If you are...

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U..S. Durable Goods Orders Down 2.2% in May

3 DAYS AGO

New orders for manufactured durable goods in May decreased $5.3 billion or 2.2% to $230.7 billion, the Commerce Department announced today. This decrease , down following two consecutive monthly increases, followed a 3.3% April increase. Excluding transportation, new orders decreased 0.3%. Excluding d...

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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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