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Human Factors Can Cause a Disaster—or Prevent One

Human Factors Can Cause a Disaster—or Prevent One

Process industry plants are complex and ...

Improving Valve Sealing Performance and Reliability

Improving Valve Sealing Performance and Reliability

From time to time, we are re-posting wel...

A Primer on Fugitive Emissions

A Primer on Fugitive Emissions

Fourscore and seven years ago, no one ha...

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Industry Headlines

Industry Headlines

Powell Valve Approves Flotech as Authorized Repair Facility

1 DAY AGO

The Wm. Powell Company recently approved Flotech as an authorized modification and repair facility for Powell Valves.

“Flotech has been manufacturing and repairing valves for over 40 years. Adding another high quality valve manufacturer to the list of valve companies Flotech represents speaks vol...

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CIRCOR Reports Fourth Quarter 2016 Financial Results

1 DAY AGO

CIRCOR International, Inc. reports revenue of $158 million, GAAP EPS of ($0.12) and adjusted EPS of $0.48 in the fourth quarter of 2016 . CIRCOR had an operating cash flow of $20 million and free cash flow of $17 million for the quarter. Cost control and restructuring actions continued to drive bottom...

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Southern California Updating Natural Gas Pipeline Infrastructure

1 DAY AGO

Southern California Gas Company (SoCalGas) will upgrade or replace 50 to 60 pipeline valves in 2017 to further enhance the safety of its system. The effort is part of SoCalGas' Pipeline Safety Enhancement Plan (PSEP), a multi-billion-dollar program that tests and updates the natural gas pipeline infra...

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How New U.S. Policies Will Affect the Chemical Industry

7 DAYS AGO

“In 2017, barring a recession in the U.S. and Europe or a slowdown in China, Moody’s Investor Service expects EBITDA in the chemicals industry to slip by 1 or 2% year-over-year.”

A new report from PwC predicts that the Trump administration “is likely to embrace policies that are...

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Empire State Manufacturing Expands at Fastest Rate Since 2014

1 DAY AGO

Business activity expanded at a solid clip in New York State, according to firms responding to the February 2017 Empire State Manufacturing Survey. The headline general business conditions index rose twelve points to 18.7, its highest level in more than two years. The new orders index climbed to 13.5,...

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Philly Fed Manufacturing Conditions Continued to Improve in February

6 DAYS AGO

The index for current manufacturing activity in eastern Pennsylvania, southern New Jersey and Delaware increased from a reading of 23.6 in January to 43.3 this month and has remained positive for seven consecutive months. The share of firms reporting growth continues to increase: More than 48% of the ...

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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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