It has been almost one month since we were in Orlando for the Water Quality Assn. Convention & Exposition, and we keep thinking back to our...
Wastewater is being reclaimed for non-potable uses. The use of this water is alleviating water shormance of the valve.
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."
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.
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.