Usually, standard 90° fittings require some additional angling to obtain the desired slope. There are two common methods to achieve this:
Using a facing tool, a mitered cut is made near the end of the 90° 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.
The second method requires only 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 3, section A is mitered and section B is bent into place.
Figure 3. Fittings angled at 88º and 92º ensure proper drainability.
Both of these manual methods aren’t particularly accurate and, in fact, are banned at some plants. Moreover, they may compromise the integrity of the internal surface finish and so foster contamination. Also, tubing can spring back from its forced position with a change in temperature or when uncoupled for maintenance, upsetting the intended angle and resulting in reassembly challenges.
In contrast, special angled fittings can ensure proper drainability. Relatively new to the marketplace and available as elbows or tees (Figure 4), they come with bends of 88° or 92 °, with a tight tolerance of ± 0.5°.
Figure 4. Two techniques commonly are used to provide a slope in a dogleg.
These angles correspond to the requirements for a minimum slope of ⅛ in. to ¼ in. per ft. Such fittings come faced for butt welding, with welded flanges for clamp-end fittings, or with threads for threaded fittings.
The common four-bolt ANSI flange is unsanitary and therefore inappropriate for biopharmaceutical and other sanitary systems. The conventional ISO-2852-type tri-clamp fitting can be employed but poses issues of drainability and flow obstruction. As the clamp tightens, the gasket will extrude into the interior flow path. With thermal cycling, the bore intrusion may increase. Computational fluid dynamics (CFD) shows that such intrusion causes turbulence in the flow path and potential hold-up when the system is drained (Figure 5).
Figure 5. CFD shows how an ISO-2852 tri-clamp fitting can suffer from gasket extrusion into the flow path.
Fittings of an alternative design, such as the TS series, preclude gasket extrusion into the flow path by preventing over-tightening and providing an alternative space into which the gasket may extrude (during pull-up and clamping) or expand (during thermal cycling).
Although there’s no generally accepted ratio of fittings to valves, use the dictum “less is more” as your guide. Minimizing the number of fittings and valve bodies will improve 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 because at least two fittings (or welds) are necessary for each valve. Smart valve choices result in fewer 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 to — or replace — each side of the horizontal tee sections, creating an elbow header. A better option is the “Viking” design, which avoids any horizontal section in the tee formation and has 45° bends before entering the valve (Figure 6).
Figure 6. This alternative to a point-of-use valve with a static tee assures full drainability.
The distance between the two vertical drops coincides with ASME-BPE recommended dimensions for “U” drops. Weir-style or radial diaphragm valves may be employed in Viking formations.
Many subtleties come into play when designing systems. The following are the most important:
- Properly sized tubing is paramount. The correct inside diameter of the tubing relates to the volume of the system and the pressure of the cleaner to be used. The cleaner on hand must provide sufficient force to clean out the tubing.
- Steam is an effective cleaning method — but systems must be designed to tolerate it. Some elastomers (in gaskets and valve diaphragms) and cast-body products (in pumps, for instance) are not optimal for steam. In selecting gaskets and diaphragms, educate yourself on the many grades of elastomers. Always specify steam grades. Black residue in a system is a sign that a low-grade elastomer has eroded during steam cleaning. The steel in pumps, valves and the like must be forged, wrought, or machined. It’s difficult to control the metallurgy in casting; the process can give off ferrite ions that cause rust in the system when they come into contact with steam. PTFE can be used in SIP systems.
- Welds must be high quality. Serious problems can occur when welding two pieces of stainless steel with dissimilar metallurgy. For example, the weld pool can become asymmetric, favoring the low sulfur side, and the root of the weld may shift away from the joint. This effect can be minimized by matching sulfur contents within ± 0.010%; greater differences require adjustments to weld set-up to achieve full penetration to the root of the joint.
- The system must be properly supported. The weight of the tubing can cause sagging, as can technicians using the tubing as a step or handrail. To eliminate sagging, which can compromise the integrity of the system, a sloped system requires support in the form of specially made sloped tubing hangers.
Drain the system not your wallet
You must make careful, informed decisions about fluid system components to avoid significant financial losses or validation failures because of contamination or poor drainability. Pay attention to valves, fittings and system slope and consider hardware designed to ensure cleanliness and drainability.
Michael Bridge is a market manager for Swagelok Biopharm Services Company, Amherst, N.Y. E-mail him at email@example.com.