06292017Thu
Last updateWed, 28 Jun 2017 5pm

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Lubricants: Always Important—Sometimes Essential

Lubricants: Always Important—Sometimes Essential

Most valve professionals take more notic...

Why Air Valves are Needed in Water Applications

Why Air Valves are Needed in Water Applications

Air valves are hydromechanical devices d...

Achieving Profitability Through Maintenance Management

Achieving Profitability Through Maintenance Management

One of the distinctions between maintena...

Control Valve Positioner Performance Diagnostics

Control Valve Positioner Performance Diagnostics

There has been discussion for some years...

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Industry Headlines

U.S. Crude, Petroleum Exports Have More than Doubled Since 2010

Wednesday, 28 June 2017  |  Chris Guy

U.S. crude oil and petroleum product gross exports have more than doubled over the past six years, increasing from 2.4 million barrels per day (b/d) i...

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Industry Headlines

Emerson in Talks to Acquire Software Company Paradigm Ltd.

1 DAY AGO

According to Bloomberg , Emerson is in talks to acquire software company Paradigm Ltd. for approximately $1.5 billion.

“Paradigm specializes in seismic data interpretation and imaging equipment to help oil and gas companies know where to drill. The logic of such a deal seems sound and will help Eme...

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VMA Members Lead MDM Top Industrial PVF Distributors List

2 DAYS AGO

Modern Distribution Management (MDM) recently published its Top Distributors list for 2017. Four of the top 5 distributors in the Industrial PVF category ; MRC Global, NOW Inc., Wolseley Industrial Group and FloWorks International; are currently VMA members.

“While oil & gas markets have start...

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U.S. Crude, Petroleum Exports Have More than Doubled Since 2010

6 HOURS AGO

U.S. crude oil and petroleum product gross exports have more than doubled over the past six years, increasing from 2.4 million barrels per day (b/d) in 2010 to 5.2 million b/d in 2016. Exports of distillate, gasoline, propane, and crude oil have all increased , but at different paces and for different...

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Mountain Valley Pipeline Gets Environmental Approval

1 DAY AGO

The Federal Energy Regulatory Commission (FERC) determined that construction and operation of the Mountain Valley Pipeline would result in some adverse environmental impacts, but the majority of these impacts would be reduced to less-than-significant levels. This determination is based on a review o...

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IMF Lowers Forecast for U.S. Growth to 2.1%

8 HOURS AGO

According to the newest forecast from the International Monetary Fund (IMF), growth in the U.S. is expected to rise 2.1% both this year and next, driven by continued solid consumption growth and a cyclical rebound in private investment. In its last forecast the IMF predicted 2.3% growth this year an...

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U.S. Durable Goods Orders Fell the Most in 18 Months

1 DAY AGO

New orders for manufactured durable goods in May decreased $2.5 billion or 1.1% to $228.2 billion, the U.S. Department of Commerce announced. This decrease, down two consecutive months, followed a 0.9% April decrease. Excluding transportation, new orders increased 0.1%. Excluding defense, new orders...

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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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