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Last updateFri, 29 Apr 2016 5pm

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Food and Beverage Processing

Food and Beverage Processing

When wandering the grocery store aisles,...

Variable Frequency Drives in Electric Actuators

Variable Frequency Drives in Electric Actuators

Electric actuators are vital for operati...

Braided Packing: An Old Technology with a Modern Twist

Braided Packing: An Old Technology with a Modern Twist

Compression packing is the primary metho...

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Industry Headlines

Rotork Acquires Mastergear from Regal Beloit

2 DAYS AGO  |  Chris Guy

Rotork has acquired the entirety of Mastergear’s business and assets from Regal Beloit Corporation for $25 million on a cash-free and debt-free ...

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Industry Headlines

Rotork Acquires Mastergear from Regal Beloit

2 DAYS AGO

Rotork has acquired the entirety of Mastergear’s business and assets from Regal Beloit Corporation for $25 million on a cash-free and debt-free basis and a value of inventory that will be finalized at the end of a transition service agreement.

Mastergear will become part of Rotork's Gears divis...

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Emerson Opens New Helix Innovation Center

3 DAYS AGO

Emerson announced the launch of its “Helix” initiative, including the opening of The Helix innovation center, an industry-first, $35 million hub dedicated to advancing research and education for the global heating, ventilation, air conditioning and refrigeration (HVACR) industry. The new ...

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Energy Sector Weighs on Construction Starts

3 DAYS AGO

At a seasonally adjusted annual rate of $660.5 billion, new construction starts in March receded 1% from February’s pace, according to Dodge Data & Analytics. Total construction starts had jumped 13% in February, led by a huge gain for the electric utility and gas plant category. The dolla...

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Chevron Investing Billions in The Permian Basin

4 DAYS AGO

At Chevron’s annual security analyst meeting last month, chairman and CEO John Watson said that his company plans on investing billions into oil & gas projects in the Permian Basin.

According to the Hobbs News-Sun , Watson predicted they could be pumping up to 350,000 barrels a day out of West ...

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House Passes Bill To Lower Import Tariffs On Manufacturers

3 DAYS AGO

This week, the House voted 415-2 for the American Manufacturing Competitiveness Act of 2016. The bipartisan, bicameral bill reforms the way Congress considers legislation for suspending tariffs on products not made in the U.S., what some are calling, “manufacturing tax breaks.” The legis...

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Texas Manufacturing Index Shrinks More Than Forecast

5 DAYS AGO

Texas factory activity increased for a second month in a row in April, according to business executives responding to the Texas Manufacturing Outlook Survey. The production index, a key measure of state manufacturing conditions, rose from 3.3 to 5.8, suggesting a slight pickup in output growth.

Perce...

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Selection Considerations for Control Valves vs. Regulators

In any control system, the option potentially exists to select either a control valve or a regulator, so it is useful to compare the respective performance and economics of these approaches to arrive at some general selection guidelines.

Unlike regulators, control valves are not standalone products. A control valve is the final control element in a control system and needs to be evaluated in that context. A control valve is most frequently used in the control of the following parameters: temperature, pressure, flow and level.

However, in principle, any continuously varying system parameter that can be measured and compared to a set point can be controlled. Also, keep in mind that regardless of the parameter being controlled (the controlled variable), the control valve itself can only change the flow rate.

All control is to a set point, and the control system can be diagrammatically illustrated as shown in Figure 1.

spr09_bv_fig1

The essential feature of the controller (whether acting as a single-loop controller or as a component within a PLC or within a DCS or fieldbus device) is the incorporation of proportional and integral control modes capable of returning the measured variable to the set point following load changes or system disturbances. Control is generally within 5%, often within 1 to 2%.

Rising stem control valves are typically globe valves commonly used to the 2-inch size. However, globe valves can extend to at least 24 inches in size with special trims and cages for severe service and high-noise applications.

For economic reasons, rotary control valves are generally applied as line sizes increase above 2 inches and a variety of ball valves, eccentric plug and segmented ball valves, and butterfly valves exist in this segment of the market. The segmented ball valve is used in many applications and the largest size rotary control valves, such as butterfly valves, can extend to 72 inches in diameter.

Globe-style valves offer advantages in that the plug and/or cage can be more readily characterized to optimize the installed flow characteristic. Most rotary valves have an inherent flow coefficient characteristic that increases approximately exponentially with the increase in travel.

Although a range of actuators can be used, pneumatic actuation predominates. The diaphragm actuator is generally the style of choice due to minimized resistance and hysteresis for small changes in travel associated with control to within 1 to 2%. For improved control and minimized deadband, positioners are generally specified. It is also common practice for the current-to-pneumatic conversion to occur at the positioner. There is also a continuing increase in the use of intelligent or smart positioners, thereby providing automatic commissioning, higher accuracy, tight shut off, customizable characteristics and diagnostic capabilities.

There are obvious economic considerations associated with the choice of a control valve, which, at a minimum, require the incorporation of sensors and transmitters, controllers, positioners, instrument valves, tubing, wiring, calibration, tuning, etc. Generalized cost estimates can only be considered representative, at best, and can vary significantly depending upon application, size, material, accuracy and commercial circumstances.

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