Characteristics and system requirementsThe following are some general control valve terms and characteristics for butterfly valves when used for modulating service. A valve having a stated inherent characteristic may provide a different installed characteristic due to interaction with the system.
Linear. The flowrate is directly proportional to the amount of disk travel. For example, at 50% open, the flowrate is 50% of maximum flow.
Equal percentage. Equal percentage characteristic means that equal increments of valve travel produce equal percentage changes in flowrate as related to the flowrate that existed at the previous travel position.
For example, if a valve travel change from 20% open to 30% open produced a 70% change in flowrate, then a valve travel change from 30% open to 40% open would produce another 70% change in flowrate. If the flowrate at 20% open was 100 gpm, then flowrate at 30% open would be 170 gpm and the flowrate at 40% open would be 70% greater than at 30% travel or 289 gpm. The same would be true for each additional incremental travel position.
Quick opening. A quick-opening valve means exactly that. Flowrate through the valve increases very rapidly for incremental changes in valve travel when valve position is near closed. As valve position becomes more open, flowrate changes diminish with incremental changes in valve travel approaching zero change as the valve position nears full open.
Traditionally, most butterfly valves have exhibited equal percentage inherent characteristics at angles of opening from 20 Degrees to 70 Degrees . Advances in disk design have allowed the extension of the equal percent characteristic through to the 90 Degrees , full-open position.
Designs employed for the extension of the equal percentage characteristic have varied from special contouring with planned flow disturbance to wafer-thin types with almost no flow disturbance. The former causes flow restriction at intermediate travel characteristics.
There are other types of valve disks offered that exhibit inherent flow characteristics approaching linear. This deviation from the traditional characteristic is the result of very heavy disk cross sections. As these valves are used in control applications, the user must ensure suitability of the linear characteristic.
Still another type of disk exhibits a characteristic midway between linear and equal percentage. Typically, this would be a disk design for high-pressure service but with minimal available capacity.
The selection of the appropriate control valve characteristic is dependent on the needs of the system. Because there are several factors to be considered, a complete system analysis is required to determine precisely which is the optimum characteristic. Often, it is not practical to perform a system analysis; therefore, certain rules of thumb are offered:
If in doubt about the preferred characteristic, choose equal percentage. Such a choice may result in a perfect match. If the match is not perfect, it will not be as detrimental as the selection of a linear characteristic when it is not a perfect match.
Except for pressure-relief applications, the quick-opening characteristic is seldom employed for control applications.
Other rules of thumb for the selection of characteristic for liquid applications are:
If greater than 25% of system pressure drop is available to the valve at maximum flow conditions, the use of the linear characteristic provides the best results.
If less than 25% of system pressure drop is available to the valve at maximum flow conditions, the use of the equal percentage characteristic provides the best results.
If the system is for control of pressure, the use of equal percentage characteristic is preferred.
Rules of thumb for selection of characteristic for gas applications include:
For small-volume systems, the use of the equal percentage characteristic is preferred.
For large-volume systems, the use of the linear characteristic is preferred if more than 25% of the system pressure drop is available to the valve.
In the course of selecting the desired control valve characteristic, do not be misled into choosing a linear one on the basis that it will provide an overall linear system.
While it may be desirable to have a linear system, the linear valve characteristic may not provide system linearity. This is due to a difference between the inherent valve characteristic and the installed characteristic.
The inherent characteristics are what the valves provide under constant pressure drops that are typically found in test situations. Installed characteristics occur when the valve is installed in a system where pressure drops vary with changes in valve position.
Economy and reliabilityObviously, butterfly valves cannot perform in every application. Variables dependent on the service, temperature and pressure also determine which valve type to choose. The service conditions must first be determined in order to properly apply any valve.
Technological innovations combined with its simple design have made the butterfly valve a dependable, economical and flexible solution to a variety of industrial flow control needs.
By Hugh Konigsmark, product manager for Resilient Seated Products, Tyco Valves and Controls, Houston, Texas. Konigsmark is a graduate of the University of Texas.