Choosing Valve Design
In choosing between the weir-style valve and a radial diaphragm valve, owners should give careful consideration to sensitivity of the application as to drainability, entrapment, potential for contamination, and system flow requirements.
For example, the equivalent size weir-style valve would provide a higher flow rate and be the appropriate choice for applications requiring a higher flow, while radial diaphragm valves are well suited for applications where cleanliness is critical.
In choosing valve designs, owners and designers should also aim to reduce the number of fittings and valves, as a means of improving overall system efficiency, cost, and performance. Quality valves are available with multiple combinations of inlets and outlets, so one multi-valve may do the job that used to require two or more individual valves.
Such designs not only reduce the number of valve bodies, but also the number of fittings, since at least two fittings (or welds) are required for each valve.
In other words, smart valve choices result in a reduction in valves, a higher fitting-to-valve ratio and, likely, a reduction in overall system size and deadlegs.
One of the more critical valve applications occurs at point-of-use outlets. Traditionally, the point-of-use valve appears as a zero static tee. While the vertical stem of the tee may drain well, the horizontal sections may not. In some cases, 90 elbows may be added toor replaceeach side of the horizontal tee sections, creating an elbow header.
A better option is the "Viking" design, in which the two horizontal pieces of the tee formation are no longer horizontal at all (Figure 4). Rather, they descend straight down vertically and bend 45 before entering the valve. Gravity does all the work to ensure complete drainability.
In addition, the distance between the two vertical drops in a Viking formation coincides with ASME-BPE recommended dimensions for "U" drops. Weir-style or radial diaphragm valves may be employed in Viking formations.
If the conventional ISO 2852 fitting is employed, the owner should be aware of its potential issues in terms of drainability and flow obstruction. As the clamp in an ISO 2852 tightens, the gasket may extrude into the interior flow path.
With thermal cycling, the extrusion may increase. Computational Fluid Dynamics (CFD) demonstrates that such extrusion causes turbulence in the flow path and potential hold-up when the system is drained (Figure 5).
Fittings of an alternative design, such as the Swagelok (www. swagelok.com) TS series fitting, will prevent gasket extrusion into the flow path. They do so by utilizing a design to prevent overtightening and providing an alternative space into which the gasket may extrude (during pull-up and clamping) or expand (during thermal cycling).