New at PipingDesign.com: 01 September 2000

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Introduction to Orifice Plates
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<< An orfice plate is simply a plate with a hole in it, forming a partial
obstruction to the flow. As will be seen from Figure 1, the flowing fluid follows the same kind of path as it does in a venturi tube. However, the narrowest part of the flow stream is not in the orfice itself, but some distance downstream; this narrowest section is known as the vena contracta. Between the vena contracta and the pipe wall, numerous eddies form. These dissipate a great deal of kinetic energy in the form of heat and are responsible for the high loss of this type of meter. >>

Sonic Flow Through Orifices, Nozzles and Venturis Optimal Systems Limited

<< The vena contracta is bounded by a zone of recirculating gas immediately
downstream of the orifice. Once sonic velocity has been achieved, further reduction of the downstream pressure cannot further increase the velocity through the vena contracta, but if the orifice plate is thin, it can increase the vena contracta's size. This is because as shown in the above figure, the recirculating gas zone extends upstream of the vena contracta, and that zone's shape can be affected even though the flow is sonic. Further reductions in downstream pressure cause the vena contracta to move upstream and to consequently increase in area. Ultimately, at high pressure ratios, the vena contracta can reach the upstream edge of the orifice, when its area would equal that of the orifice and the discharge coefficient would be unity. >>

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What is the pressure at the end of a pipe which is discharging to atmosphere?
Optimal Systems Limited

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Control Valves Do What They Are Told!
Hane Training, Inc.

<< Being the Final Control Element in a system is not an easy job. To start
with, you are blamed for any and all problems that crop up in the process. You are subjected to corrosion, high velocity, cavitation, flashing liquids, cryogenic temperatures, high temperatures, abrasion, and thermal shock. You are expected not only to throttle along through all this, but most likely, you are also being asked to act as a block valve and shut off tight. As you work with control valves always keep in mind that a control valve only does what it is told to do. >>

Valve Characteristics Selection Guidelines Control Engineering

<< Control valves are called to handle all kinds of fluids at temperatures
from the cryogenic range to well over 1,000 ºF (538 ºC). So selection of a control valve body assembly requires particular consideration to provide the best available combination of valve body style, material, and trim construction design for the intended service. Capacity requirements and system operating pressure ranges must also be considered in selecting a control valve to ensure satisfactory operation without undue initial expense. Reputable control valve manufacturers are dedicated to helping customers select the control valve best suited for the existing service conditions. Frequently several correct choices may be available, thus it is important for customers to provide control valve manufacturers the following information. >>

Fluid Kinetic Energy as a Selection Criteria for Control Valves ICEWeb

<< A selection criteria is provided that assures a control valve will
perform its control function without the attendant problems of erosion, vibration, noise and short life. The criteria involves limits on the fluid kinetic energy exiting through the valve throttling area. Use of this criteria has resolved existing valve problems as demonstrated by retrofitting of the internals of many valves and vibration measurements before and after the retrofit. The selection criteria is to limit the valve throttling exit fluid kinetic energy to 70 psi (480 KPa) or less. >>

Control Valves: Bigger Isn't Necessarily Better Control Engineering

<< When control valves are mis-sized and/or misapplied, cost impact can
extend far beyond the purchase price. Big dividends in reduced maintenance cost can be obtained when control valves are engineered to match requirements of the application, and this is especially true for severe service applications. Though less than 10% of all control valves are installed in severe service applications, applying the information in this article will ensure every control valve is correctly engineered to meet the demands of the application. >>

Control Valve "Fail-Safe" Positions
Hane Training, Inc. Received on Fri Sep 01 16:34:00 2000