Design for drainability
Properly sloped fittings and valve selection enhance system cleanliness and drainability

Cleanliness and drainability are among the most critical issues facing designers of biopharmaceutical plants. Improperly-sloped process lines, and fluid entrapment in valves and fittings, can cost companies millions each year.

In continuously-operating biopharmaceutical plants, poor drainage in valves or pipework can cause problems with lack of sterility. The issue is even more serious in plants that operate on a batch or campaign basis, which need to be cleaned periodically using SIP (steam in place) or CIP (clean in place).
When a valve is shut off, a properly-constructed downstream system should drain completely, with no residual puddles, reservoirs or entrapment along process lines, in valves or around fittings. Unfortunately, this does not always happen. Variables affecting a system’s drainability include the slope of the pipework, the existence of dead legs, the interior surface finish of tubing, fitting-to-valve ratios, and the design of valves and fittings. Since these factors are hard to change once the plant has been built, the onus is on the owner of the plant to make drainability and cleanability top-line requirements at the design stage. The most important ways to promote drainability are to slope pipework correctly and to choose suitable valves and fittings. ASME-BPE section SD 3.12 gives some guidelines. Although the ASME-BPE standard does not specify a figure for slope, most companies specify at least 1–2 percent (10–20 mm per meter).

Angled fittings for proper slope
Standard pipe elbows and tees with a nominal 90° angle create horizontal sections of pipe, which do not drain properly. The minimum slope of 10–20 mm per meter translates to an angle of 0.6°–1.2°, so the ideal angle for an elbow fitting will be 90.6°–91.2° in one direction and 89.4°–88.8° in the other. Contractors who build biopharma plants traditionally use three methods to create suitable angles, but none of these is really satisfactory.
The first method is to make a welded butt joint, at an angle slightly greater than 180°, in the pipe just downstream of the fitting (section A in Figure 4). This requires careful miter cutting, but even so,
some bending may be required to adjust the slope. Besides being time-consuming and therefore expensive, this introduces an extra weld.
A second method requires only one’s bare hands. A straight piece of tubing is attached to the fitting. Then, the installer grips the straight piece of tubing and forces it to the desired slope (section B in Figure 4). This method can damage the fitting or the tube, inviting contamination. Changes in temperature can cause movement, which upsets the drain angle, and if the joint has to be broken for maintenance, reassembly can be challenging. Both of these manual methods are less than accurate, and some biopharmaceutical manufacturers prohibit them.
The third method relies on the fact that ASME-BPE permits a typical variability of ±1.3° in standard 90-degree fittings. Some contractors therefore take a batch of fittings and sort them into two lots: those with angles of greater than 90° and those with smaller angles. Not surprisingly, this process is time-consuming and inexact.
A much better solution is to use special angled elbow fittings manufactured with angles of 88° and 92° (Figure 3). Such fittings, which are relatively new to the marketplace, are typically available as elbows and tees. They come faced for butt welding, with welded flanges for clamp end fittings, or with threads for threaded fittings. Because they are manufactured to a tight tolerance of ±0.5°, they always provide at
least the minimum angle needed to ensure proper drainability.
Valves without dead spots
Valve selection should be deliberate. One valve is not as good as another. In critical shutoff applications, the two most common valve types are weir-style and radial diaphragm valves. The weir-style valve is the industry standard, with a track record of solid performance in validated systems, yet it leaves some opportunity for 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 1). As the valve closes, the diaphragm closes back towards the body of the valve, allowing small quantities of fluid to become trapped.
Radial diaphragm valves avoid this problem because in this design the diaphragm seals along the edge of the valve bowl. At no time does the diaphragm lift beyond the edge of the bowl, so 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. As a result, radial
diaphragm valves are becoming increasingly popular.
In choosing between the weir-style valve and a radial diaphragm valve, owners should carefully consider the sensitivity of the application to issues of drainability, entrapment, potential for contamination, and flow requirements. For example, a weir-style valve provides a higher flowrate than a radial diaphragm valve of equivalent size, so it is the appropriate choice for applications requiring high flows. Radial diaphragm valves are well-suited for many applications where cleanliness is critical.
Minimizing the number of fittings and valves can improve system performance and cut costs. Quality valves are available with multiple inlets and outlets, so a single 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 reduce the number of valves, increase the fitting-to-valve ratio and, likely, reduce overall system size and dead volume.
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-degree 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, in which the two horizontal pieces of the tee formation are no longer horizontal at all (Figure 2). Rather, they descend straight down vertically and bend 45° before entering the valve. Gravity does all the work to ensure complete drainability. The distance between the two vertical drops in a Viking formation coincides with ASME-BPE recommended dimensions for “U” drops.
Fittings for smooth flow
Designers using conventional ISO 2852 fittings should be aware of potential problems with drainability and flow obstruction. As the clamp in an ISO 2852 fitting 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 TS series, prevent gasket extrusion into the flow path. They do so by utilizing an innovative design that prevents over-tightening and provides an alternative space into which the gasket may extrude (during pull-up and clamping) or expand (during thermal cycling)