Cleanliness and drainability are among the most critical issues that manufacturers of products such as biopharmaceuticals and active pharmaceutical ingredients (APIs) must confront with their process lines. Producers face the prospect of losing millions of dollars each year because of improperly sloped process lines and entrapment in valves and fittings.
Some products suit continuous processes. Typically, however, manufacturers must run multiple batches and must clean between them using steam in place (SIP), clean in place (CIP) with a chemical agent, or both techniques. When a valve is shut off in a properly constructed system, the entire system downstream should completely drain, minimizing residual puddles, reservoirs or entrapment along process lines or in or around valves or fittings.
Variables that will impact a system’s drainability and cleanability include:
- system slope;
- interior surface finish of tubing;
- valve and fitting selection and design; and
- fitting-to-valve ratio.
Owners and contractors alike can take steps in the design and construction phase to enhance the drainability and cleanability of a system. However, the onus rests on the owner to make these top-line requirements.
System slope, deadlegs and interior surface finish
Some specific guidelines concerning system slope and drainability are given in ASME-BPE section SD 3.12. Although the ASME-BPE standard doesn’t designate a specific slope, most companies abide by the commonly accepted guideline of a minimum ⅛ or ¼ inch per foot. Or for every foot of tubing, the line should drop a minimum of ⅛ in. to ¼ in.
Deadlegs are sections of tubing, typically tees leading to valves or valve assemblies, characterized by a discontinuity of flow. During cleaning, chemicals or steam may not reach these locations, or fluid can be held up there and not fully drain, leading to contamination. Minimizing deadlegs when designing a system is vital. Deadlegs can be managed in three ways (Figure 1), listed below from least to most desirable:
Figure 1. Three options are available, with configuration C, depicting a block body assembly, the best.
In Configuration A, a tee fitting with three welds creates a horizontal line to a valve. The deadleg is the area between the tee fitting and the valve. This arrangement creates the longest deadleg of the three options because the valve’s proximity to the intersection is limited by the tee stub. When selecting this configuration, a designer’s best approach is to choose tees whose stubs are shortest in length and widest in diameter, which will facilitate access when cleaning and minimize the area for fluid hold-up.
Configuration B uses a valve instead of a tee fitting. The tee is created by boring a hole into the side or bottom of the valve. A vertical line is welded into the valve to create the tee formation. The distance to the valve on the vertical line (the dead leg) is now shorter than in Configuration A because the welded tee fitting is not part of the flow path.
Configuration C eliminates the deadleg altogether through the use of a custom valve assembly. In these assemblies, two valves actuate from the same block, one controlling vertical flow and the other horizontal flow. Block body assemblies can be produced more cost efficiently by some companies than manufacturing two valves separately.
The interior surface finish of tubing affects both drainability and cleanability. A smooth finish, with no porosity or pitting, means better performance in both areas. Cleanability can be enhanced further by electropolishing, which adds a chemically inert layer that inhibits corrosion. The finish can be checked with special tools and by visual examination.
Valve and fitting selection and design
Carefully consider valve selection. For instance, in critical shut-off applications, the most common options are weir-style and radial diaphragm valves. The weir-style valve offers a proven track record of solid performance in validated systems. Yet its 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 2).
Figure 2. Fluids may become trapped between the diaphragm and the valve when the diaphragm closes.
As the valve closes, the diaphragm closes back toward the body of the valve, trapping small quantities of fluid.
Newer radial diaphragm valve designs avoid such problems because 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 doesn’t occur. Further, bowl shape, inlets and outlets are configured to ensure that the flow path is cleanly swept and optimized for full drainability. Not surprisingly, such valves are seeing increasing usage.
In choosing between weir-style and radial diaphragm valves, assess the sensitivity of the application to drainability, entrapment, potential 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 or high-cycling is critical.
Fittings are another important factor in drainability. With standard 90° fittings, ASME-BPE permits a typical variability of ± 1.3°. Some contractors actually 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.