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Basics of Elastomeric Seal Design

Basics of Elastomeric Seal Design

Engineers need critical design informati...

Wastewater Treatment

Wastewater Treatment

Society’s desire for a clean envir...

Controlling Our Water Systems, Part II

Controlling Our Water Systems, Part II

To better understand the actuators and c...

Controlling Our Water Systems

Controlling Our Water Systems

Actuators and controls are a critical pr...

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Industry Headlines

Pentair Reports Second Quarter 2016 Results

1 DAY AGO

Pentair plc announced second quarter 2016 sales of $1.7B. Sales were up 4% compared to sales for the same period last year. Excluding the unfavorable impact of currency translation and the positive contribution from acquisitions, core sales declined 3% in the second quarter . Second quarter 2016 earni...

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ValvTechnologies Names Bryant Holt Industry Director, Fossil Power

2 DAYS AGO
ValvTechnologies Names Bryant Holt Industry Director, Fossil Power

ValvTechnologies, Inc. has appointed Bryant Holt as industry director for the company’s fossil power division. Holt will succeed George Stover, who has served in this role since 2014.

Based in Houston, Holt will have global management responsibility for ValvTechnologies’ fossil power group ...

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ExxonMobil Expanding Beaumont, TX Facility

1 DAY AGO

ExxonMobil has plans to increase production of ultra-low sulfur fuels at its Beaumont, TX refinery by approximately 40,000 barrels per day, representing an investment of approximately $450 million.

Construction is scheduled during the second half of 2016 to install a selective cat naphtha hydrofining u...

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LNG’s Surge from Decade-Low Seen Fizzling as Supply Ramps Up

2 DAYS AGO

“LNG’s surge is running out of gas. Liquefied natural gas in Asia, which was in such over-supply that prices in Japan fell to a decade-low in April, has risen by almost half in the past three months as production outages stifled supply, demand rose in places like China and India and a co...

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Second Quarter GDP Grew Only 1.2% in Second Quarter

8 HOURS AGO

Real GDP in the U.S. increased at an annual rate of 1.2% in the second quarter of 2016, according to the advance estimate released by the U.S. Department of Commerce. In the first quarter, real GDP increased 0.8%.

The increase in real GDP in the second quarter reflected positive contributions from cons...

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U.S. Durable Goods Orders Down 4% in June

2 DAYS AGO

New orders for manufactured durable goods in June decreased $9.3 billion or 4.0% to $219.8 billion, the U.S. Department of Commerce announced. This decrease , down two consecutive months, followed a 2.8% May decrease. Excluding transportation, new orders decreased 0.5%. Excluding defense, new orders d...

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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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