Operating Conditions Specific Gravity Pressure Drop Across the Valve Flowing Quantity
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Vora When the plug throttles very close to the seat, high fluid velocities occur which can cause erosive damage. Such a low minimum drop figure erroneously favors the selection of an equal percentage valve. Flow to Open Flow Characteristic: Cv is a capacity rating coefficient which is defined as the number of U.
If you continue to use this site we will assume hamdbook you are happy with it. Concentration, as measured by analysis such as pH, is best controlled by a valve with an equal percentage characteristic Valvs the ratio of maximum to minimum valve pressure drop at full flow is greater than 2 to 1.
These two types of measurement require separate treatment since they seriously affect dynamic response. Published on Nov View Download 6. One must constantly remember that pressure drop which is to occur in the control valve plays almost as prominent a part as flow rate.
An almost universal approach to valve sizing today is through the Cv factor method. Characteristic curves of various inner valve styles are often consulted in the second method. A brief outline of the usual way in which control valve sizes are determined will be of interest to many conteol. Values of numerical factors are included for commonly encountered systems of units. Is it ckntrol logical, then, to study the proposed operating conditions with the object of selecting realistic pressure drops from careful engineering surveys of all factors that can affect the proposed valve?
A controller properly adjusted at the high flow rate has too narrow a proportional band setting for stability at low flows. The factors that are related to specific valves are presented in this supplement to the valve sizing handbook. By experience, some engineers feel confident in placing their hanbdook operating strokes at a certain fraction of full stem travel.
Flow passing into the seats Flow Characteristics: The sizing equations are based on equations for predicting the flow of compressible and incompressible fluids through control valves. There are no fixed rules that always apply, but a critical study of all the process pressure conditions should lead to reasonably representative values for use in computing Cv.
A misapplied linear valve exhibits the opposite effect. This variation in where most of the total drop takes place is one of the most important aspects in choosing the proper valve characteristic for a given process. It is possible to establish general guidelines based upon the extensive experience in field startups and troubleshooting. The above recommendations are only as good as the hydraulic analysis of the pressure conditions in the system.
Similarly, if the valve is too large, it will obviously pass the required flow, but it will be more expensive than a properly sized, smaller valve. This article is an informal guide with illustrations aimed at helping beginners to understand Dresser Masoneilan Control Valve sizing principles.
With these factors in the equation, the unknown or product of the unknowns, e. Masoneilan Control Valve Sizing Handbook Supplement An oversized valve will be masonei,an near the closed position, and the full control range of the valve will not be utilized.
If the control valve were not installed, the varying pressures would be absorbed in other parts of the equipment and the resulting flow would be excessive, unpredictable and rampant in nature. The process designers must necessarily work with many variables and approximations. In fact, the three equations will do much that short cut means cannot accomplish. Related Articles
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