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Last updateTue, 24 Apr 2018 7pm

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The VMA Knowledge Forum, Part One: Technology at the Forefront

The VMA Knowledge Forum, Part One: Technology at the Forefront

The first VMA Valve Industry Knowledge F...

Air Valves in Piping Systems

Air Valves in Piping Systems

Liquid piping systems are prone to colle...

Are Valves from Low-Cost Countries Getting Better?

Are Valves from Low-Cost Countries Getting Better?

The last 25 years have seen standards cr...

Managing the Generation Gap: Much Ado About Nothing?

Managing the Generation Gap: Much Ado About Nothing?

The baby boomer generation was the most ...

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Val-Matic Names Andrew Brudniak Project Manager

2 HOURS AGO
Val-Matic Names Andrew Brudniak Project Manager

After four years with Val-Matic Valve & Mfg. Corp. , Andrew Brudniak was recently promoted to project manager, after having recently held the position of sales engineer for the northeast territory.

Brudniak also has passed the exam for the Project Management Professional (PMP) certification offered...

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Weir Group Plans to Sell Flow Control Division

4 DAYS AGO

The Weir Group’s board of directors will initiate a process to sell the Flow Control division. This process will focus on maximizing value for shareholders, with all options to be considered and no fixed timetable. Proceeds will be used to further reduce leverage and to fund future investment in...

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Shale Boom Creates New Petrochemical Hub

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Canada Grants BP Approval to Drill Off Nova Scotia Coast

1 DAY AGO

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IHS Markit U.S. Manufacturing Index Rose in April

8 HOURS AGO

Manufacturers reported the greatest improvement in overall business conditions since September 2014 in April. This was highlighted by the seasonally adjusted IHS Markit Flash U.S. Manufacturing Purchasing Managers’ Index (PMI) rising from 55.6 in March to 56.5.

Stronger growth in new orders drov...

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March Construction Starts Advance 11%

1 DAY AGO

New construction starts in March increased 11% from the previous month to a seasonally adjusted annual rate of $785.2 billion, according to Dodge Data & Analytics. The substantial gain followed modest declines in January (down 2%) and February (down 3%) and brings the pace of total construction s...

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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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