Valve Sizing & Selection Technical Reference [PDF]

Warren Controls

<<A Control Valve performs a special task, controlling the flow of fluids so a process variable such as fluid pressure, fluid level or temperature can be controlled. In addition to controlling the flow, a control valve may be used to shut off flow. A control valve may be defined as a valve with a powered actuator that responds to an external signal. The signal usually comes from a controller. The controller and valve together form a basic control loop. The control valve is seldom full open or closed but in an intermediate position controlling the flow of fluid through the valve. In this dynamic service condition, the valve must withstand the erosive effects of the flowing fluid while maintaining an accurate position to maintain the process variable.>>

Flow Control With Ball Valves

Climatic Control Company, Inc.

<<When a globe valve starts to open, it will always jump open rather than move smoothly. In order to overcome linkage and internal resistance, more movement is needed by the actuator than low flow requires. Globe valves are installed with flow tending to push open the seat. As soon as flow is established, the pressure pushes on the seat, the stem rises, and more flow than needed occurs. There is a travel zone where about 10% of CV rating occurs on opening. Ball valves do not have this jump effect. Rotation is slow, rack and pinion linkage is used, the tendency of flow is to close a ball, and a high seal resistance, all contribute to smooth rotation without jumps, resulting in turndown ratios from 160:1 to 400:1, depending on quality of actuators, valves, and signal. Minimum flow is fully controllable under part load conditions. Ball valves with Belimo actuators are exceptionally good at control, and fully utilize the control signal.>>

Ball Valve Controllers

Innovatia

<<Bidirectional valve operation is needed for proper use of cryogenic ball valves. These are valves with a small drain hole drilled into the ball at right angles to the main channel. When the valve is closed, cryogenic fluid that otherwise would be trapped in the valve can drain to the exit side through the drain hole. Without a drain hole, the fluid heats at ambient temperature, pressures increases and the valve seating bursts. To keep the drain hole on the exit side, the ball must be turned back from the opened to the closed position. A drain hole on the pressurized inlet side of the valve will not drain the ball channel.>>

Valve Wars: Rising Stem vs. Rotary

PlantService.com

<<In recent years, the growing interest in rotary-action control valves resulted in at least a perceived battle between globe valves and the various styles of rotary valves. In a few industries and applications there is a clear winner. For example, globe valves are really not suitable for handling pulp stock in pulp and paper mills. As a general rule though, it cannot be said that one style of control vale is the best. If there were one "best" control valve type, it would be the only one that the valve manufacturers would make, and that has not happened. The globe valve is the most mature of the control valve types and offers a wide choice of options. High pressure and temperature ratings are available from most manufacturers as is a broad range of trim materials. A variety of cavitation and noise reduction options are available, ranging from moderately severe duty to extremely severe service. The capacity or Cv rating of a globe valve, as well as the inherent flow characteristic, can be changed simply by changing the trim. Most modern globe valves are of the top entry design. This means that the internals can be replaced while the valve is in-line (although many plant standards do not permit the repair of valves in-line). The globe valve is an extremely versatile design. It is also a very expensive valve, especially for larger line sizes.>>

Pressure Regulator Selection Based on Performance, Design

Pipe-Line.com

<<Understanding the various types and designs of pressure regulators is necessary for gas transmission and local distribution companies to make a cost-effective choice for each and every application. A pressure regulator controls the gas flow from a higher to a lower pressure system, while attempting to maintain a constant system pressure. Such regulators are used throughout the gas industry to control the many different pressures used to move gas from production wells to the end user. The gas industry employs two main types of regulators: pressure-reducing regulators, which control the gas flow to maintain downstream pressure, and back-pressure (relief) regulators, which control upstream pressure. Both types are used as overpressure protection devices. Pressure reducing regulators in series work as monitor regulators. Back pressure regulators are used as relief valves to prevent the lower pressure system from being over-pressured. These types of regulators are available in two designs, self-operated and pilot-operated. Knowledge of each type and design is important for proper application, performance, trouble-shooting, and overseeing maintenance and operation.>>

Control Valves Do What They Are Told!

MaintenanceResources.com

<<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 degrees F (538 degrees C). 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

MaintenanceResources.com

Automated Valves Descriptions C/W Drawings and Cutaway Views

MaintenanceResources.com

<<Globe and needle valves can be either manually controlled using a valve stem connected to a hand wheel or they may be automated, such that the valve stem is raised and lowered by means other than human action. With valves of this type, the valve opening and fluid flow increases almost linearly with the valve stem position. Normally, a solenoid or pressure diaphragm is used to control the valve stem position. With the solenoid type globe valve, an electrically actuated magnetic coil is used to counteract the effects of a spring acting on the valve stem and sealing disk or needle, which effectively opens and closes the valve. A diaphragm motor acts similarly to a solenoid except a pressure signal acts on the diaphragm to counteract the effects of a control spring acting on the valve stem and disk.>>

How to Select Valve Actuators For Rotary Valves

FlowBiz.com's Tech Central

<<At times it is necessary for a process engineer to choose between a pneumatically or electrically actuated valve for a system. There are advantages to both styles, and it is valuable to have data available to make the best choice.>>

Rotary Valve Designs for Control Applications

FlowBiz.com's Tech Central

<<In a process control system the valve is considered the final control element. Where once sliding stem valves such as globe or cage guided valves dominated control applications, design and materials enhancements of today's rotary valves have made them more widely accepted for such use. Heavier stems, new bearing designs, characterized control elements, higher flow capacity, and overall compactness have all contributed to this trend. Another advantage of a rotary stem control valve is the improved performance obtained because the valve stem does not move through the packing. Sliding stem movement tends to drag along some of the line fluid, creating a leak, or to react with moisture in the atmosphere, causing corrosion. In addition, improvements in actuators and positioners and the need for more valve status feedback have made rotary valve performance and economy an attractive benefit and incentive for their use. For the control valve user there are two general categories of rotary valves available for control applications: standard and characterized. For our purpose here a standard rotary valve is considered one that is used primarily for on-off service but that may be fitted with an actuator and positioner for modulating service. One of the benefits of using standard valves in control service is that they may also provide full shutoff if necessary. Care must be taken however to assure that the control service does not cause damage to the seat. A characterized valve has a specially designed closure element that may be a V-notch, segmented ball, or other such modification. The application and design of sliding stem valves in control service is well documented, so we shall limit our coverage to the above-mentioned rotary-style valves.>>

Valve Survey

Control Engineering

<<Control Engineering's survey was conducted by sending 1,500 surveys to a selected portion of our 45,000 North American readership audience. Four-hundred and six responses were returned (27%) with 95% of the respondants being those who specify, recommend, or buy valves for their own in-plant requirement, or for use in customer facilities. The latter group includes contractors, integrators, consultants, and OEM's (original equipment manufacturers).>>

Control Valve Cavitation

Control Magazine

Selecting Valves and Actuators to Optimize Process Performance

Control Magazine

Selecting a Control Valve

Chemical Processing Magazine

How to Size Control Valves

Chemical Processing Magazine