09302016Fri
Last updateThu, 29 Sep 2016 6pm

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Caution: Horizontal Stem Installation Ahead

Caution: Horizontal Stem Installation Ahead

When it comes to linear valve installati...

The Critical Stem Nut: Who is Responsible for Maintenance?

The Critical Stem Nut: Who is Responsible for Maintenance?

One of the most important components of ...

An Alternative Basics Education: Valve Ed Comes to You!

An Alternative Basics Education: Valve Ed Comes to You!

For the first time in the seven-year his...

Give Your Flow Meter a Happy Home

Give Your Flow Meter a Happy Home

Increased emphasis on the need to improv...

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Industry Headlines

Propane Drove 2016 U.S. Petroleum Product Export Growth

Thursday, 29 September 2016  |  Chris Guy

In the first half of 2016, the U.S. exported 4.7 million barrels per day (b/d) of petroleum products—almost 10 times the crude oil export volume...

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Industry Headlines

Siemens Industry Partners for Self-Funded Efficiency Project

15 HOURS AGO

Montgomery County, TN has partnered with Siemens Industry Inc. to begin a nearly $5 million, self-funded efficiency project. The project will begin in late October and will be completed by fall of 2017.

Siemens has guaranteed the savings – including 2,473,263 kWh of electricity per year; 34,154 c...

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Wolseley Reports 2016 Full Year Results

2 DAYS AGO

Wolseley plc delivered an improvement in overall results for the fiscal year ending July 31, 2016, up 8.5% from last year. Revenue of $18.7 billion was 4.2% ahead at constant exchange rates and 2.4% ahead on a like-for-like basis. Price deflation, particularly in the U.S., reduced revenue by 1.5%. Imp...

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Propane Drove 2016 U.S. Petroleum Product Export Growth

13 HOURS AGO

In the first half of 2016, the U.S. exported 4.7 million barrels per day (b/d) of petroleum products—almost 10 times the crude oil export volume—an increase of 500,000 b/d over the first half of 2015. While U.S. exports of distillate and gasoline increased by 50,000 b/d and nearly 140,00...

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$900 Million Natural Gas Power Plant Planned for Ohio

13 HOURS AGO

EmberClear Corp. of Houston has announced plans for a new $900 million natural gas-fired electric power generation plant. The 1,000-MW Harrison Power Project will be built over 60-acres in Harrison County, OH. EmberClear said it will take 18 to 36 months to win approval of various state and federal ag...

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Durable Goods Orders Unchanged in August

1 DAY AGO

New orders for manufactured durable goods in August were little changed at $226.9 billion, the U.S. Department of Commerce announced, following a 3.6% July increase. Economists were predicting a 1.5% decrease in August. Excluding transportation, new orders decreased 0.4%. Excluding defense, new orders...

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U.S. Consumer Confidence Surges to Nine-Year High

2 DAYS AGO

The Conference Board Consumer Confidence Index, which had increased in August, improved further in September. The Index now stands at 104.1, up from 101.8 in August. The 104.1 figure is the highest since August 2007.

“Consumer confidence increased in September for a second consecutive month and i...

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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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