In addition, buried manually-actuated valves are easier to design and install if the operating valve shaft is horizontal and the gearbox input shaft is oriented toward the ground surface.
One concern with orienting the valve shaft axis horizontally is hydrostatic torque. Hydrostatic torque is a sizable operating torque component on large valves. This phenomena is caused by a hydraulic gradient when a butterfly valve is installed with a horizontal shaft and an empty pipe line downstream of the valve. (Hydrostatic torque is graphically shown in Figure 3 of Part I.) Hydrostatic torque can hinder or aid in opening or closing the valve depending on the valve's disc rotational direction. This effect is covered in the appendix of 1987 and earlier editions of ANSI/AWWA Standard C504 "AWWA Standard for Rubber-Seated Butterfly Valves."
Disc Closure DirectionThe direction of closure of the valve disc can have a major impact on long-term valve operation. Looking at Figure 10, consider that construction debris and/or system fines will settle to the bottom of the pipe and valves. This solids buildup can cause damage to the valve seat and disc edge or prevent total shutoff. If the valve is installed so that the bottom disc edge creates a high local velocity of fluid at the bottom edge of the seat and disc, this higher velocity will sweep out built-up debris during valve closure.
Disc Swing ClearanceAWWA short body butterfly valve discs protrude beyond the valve end faces when the valve is fully opened. If this design is not accounted for, interference problems with mating pipe flanges or close proximity obstructions can occur. The valve manufacturer can supply the required disc swing clearance information to prevent installed disc interferences. However, the system designer should verify this clearance to prevent disc interference with the mating piping or other potential obstructions.
Valve Location with Respect to Fluid Treatment Chemical InjectionPerformance can be negatively affected by locating valves too close to chemical injection ports. The introduction of powdered chemical treatment media immediately upstream from the valve can cause the valve to lockup. This is due to bearing and seat friction induced by the solid chemicals that have not had adequate transit time to dissolve the solids. The powdered chemicals can be compared to grit between a rubber tire and an ice surface.
A second effect of installing valves too close to where chemicals are introduced is the chemical (whether solid or liquid) may not be adequately diluted in the water. In some instances, the high concentration of the added chemicals may even attack the valve elastomer seats. High concentrations of chlorine and chloramines are known to cause degradation of common nitrile rubber seats. Other chemical additives, such as petroleum based products, can degrade EPDM materials.
Fluid HammerFluid hammer (often referred to as water hammer) is a transient phenomenon that can occur in systems with liquids, gases or steam media. The results of fluid hammer can range from an irritating banging noise to catastrophic failure of valves, pumps or entire piping systems.
Transients can occur during initial filling of the pipeline, the starting or stopping of pumps, sudden shifting of inadequately supported pipe, inadequate air removal, or valve stroking times that are less than critical for the specific piping system.
The concept of fluid hammer is generally known to engineers and operators. However, the specific technology related to calculating transient characteristics and valve closure times is very complex and not as well known. A case study of low pressure transients in the Austin, Texas, water system has been documented (December 1994 Journal of AWWA).
Quarter turn butterfly valves are inherently easy and fast to close, therefore, the actuator and system design must be carefully accomplished to prevent water hammer. The typical way to address valve closure rates is with manual actuators that take many, sometimes hundreds, of turns to stroke the valve. It can also be done with speed controls on pneumatic, hydraulic or electric actuators. In lengthy pipe systems such as power plants and municipal water distribution systems, the required minimum stroking time to control transients can be many minutes. Chapters 8 and 9 of Hydraulics of Pipelines by Paul J. Tullis provides methods and examples for calculating closure times for butterfly and other valve types. Designers and operators of butterfly valve installations should be aware that overly rapid opening or closure of butterfly valves can cause significant system problems.