September 1, 2005 (Vol. 25, No. 15)
Valve Selection and Properly Sloped Fittings Improve Process Lines
Cleanliness and drainability are among the most critical issues biopharmaceutical manufacturers or owner companies must confront with their process lines. Owners face the prospect of losing millions of dollars each year because of improperly sloped process lines and entrapment in valves and fittings.
Some manufacturers run one operation continuously, but more often than not, manufacturers run multiple batches and must clean between them using SIP (steam in place), CIP (clean in place, which uses a chemical agent in the cleaning process), or both. When a valve is shut off in a properly constructed system, the entire system downstream should drain completely, minimizing residual puddles, reservoirs, or entrapment along process lines or in or around valves or fittings.
Some specific directives concerning drainability are given in ASME-BPE section SD 3.12. Although the ASME-BPE standard does not designate a specific slope, most companies abide by the commonly accepted guideline of a minimum 1/8 inch or 1/4 inch per foot. In other words, for every foot of tubing, the line should drop a minimum of 1/8 inch to 1/4 inch.
Variables that will impact a system’s drainability include system slope, deadlegs, interior surface finish of tubing, fitting-to-valve ratios, and valve and fitting selection and design. Owners and contractors alike can take steps in the design and construction phase to enhance the cleanability and drainability of a system. However, the onus rests on the owner to make it known that drainability and cleanability are top-line requirements.
Special Angled Fittings
Special angled elbow fittings, which are relatively new to the marketplace, are targeted to an ideal 88 or 92, with a tight tolerance of +/- 0.5 (Figure 1). Such angles ensure proper drainability. With standard 90 fittings, ASME-BPE permits a typical variability of +/- 1.3. Some contractors will try to use this tolerance and presort standard 90 fittings into those in the acute direction and those in the obtuse direction for use in different slope applications. This process is time consuming and inexact.
Usually, standard 90 fittings require some additional angling to obtain the desired slope. How is this additional angling attained? There are two common methods. One method requires a facing tool. A mitered cut is made just beyond the 90 bend of the elbow. Then, a straight piece of tubing is butt-welded to the mitered cut, creating a slope. Some bending may be required to adjust the slope.
A second method requires only one’s bare hands. A straight piece of tubing is butt-welded or otherwise attached to the fitting. Then, the installer grips the straight piece of tubing and forces it to the desired slope or position. In Figure 2, section A is mitered, and section B is bent into place.
Both of these manual methods are less than accurate. Some biopharmaceutical manufacturers, in fact, prohibit these methods. Further, when forcing tubing to a desired slope or position, one runs the risk of compromising the integrity of the internal surface finish and inviting contamination.
Also, tubing can spring back from its forced position with a change in temperature or when uncoupled for maintenance, upsetting the intended angle, resulting in reassembly challenges.
The alternative to these manual methods is special angled fittings bent to 88 or 92. The minimum slope of 1/8 inch to 1/4 inch per foot translates to 0.6 and 1.2 respectively, which means the ideal angled fitting will range in one direction from 90.6 to 91.2 and in the other direction between 89.4 and 88.8.
Such are the degree ranges available in special angled drainable fittings (88 and 92), if they are manufactured to close tolerances of +/- 0.5, providing at least the minimum slope. Special angled fittings, which are typically in an elbow or tee configuration, come faced for butt welding, with welded flanges for clamp end fittings, or with threads for threaded fittings.
Valve Technology
Valve selection should be deliberate. One valve is not as good as another. In critical shut-off applications, the two most common types of valves are the weir-style and radial diaphragm valves. The weir-style valve is the industry standard-bearer with a track record of solid performance in validated systems.
Yet the weir style sealing design can leave some opportunity for entrapment or contamination. The diaphragm is designed to seal on a sealing bead outside the weir area. However, in the open position the diaphragm lifts up and flexes, exposing the valve body along the perimeter of the bowl (Figure 3). As the valve closes, the diaphragm closes back toward the body of the valve, allowing small quantities of fluid to become trapped.
Newer radial diaphragm valve designs correct the imperfection in the weir-style valve. In these designs, the diaphragm seals along the edge of the valve’s bowl. At no time does the diaphragm lift beyond the edge of the bowl. As a result, entrapment does not occur.
Further, bowl shape, inlet and outlets are configured to ensure that the flow path is cleanly swept and optimized for full drainability. Radial diaphragm valves are seeing increased usage as many design engineers and biopharmaceutical companies explore alternative ways to safely direct the flow of water, steam, and product through their clean processing systems.
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).
Conclusion
Owners, designers, and contractors must make careful, informed decisions concerning fluid system components if they are to avoid significant monetary losses or validation failures because of contamination or poor drainability.
Valves, fittings, and system slope are all critical to the proper functioning of the larger system. As the industry moves forward, improved fluid system components will be integrated into biopharmaceutical manufacturing systems.
Components with innovative designs that reduce the margin for error, such as special angled elbow and tee fittings, Swagelok TS series fittings, multi-valves, and Viking point-of-use valves, will replace less precise alternatives and enhance a system’s cleanability and drainability.
