Clean steam sterilization is the heart of the sanitization routine for most biopharmaceutical manufacturing processes, as well as most pharmaceutical processes that produce parenteral medications (those not administered through the digestive tract such as injections and others) or diagnostic agents. Process tubing, vessels, valves, pumps, equipment and anything that comes in direct or indirect contact with the product feeds or end product must be heated with clean steam to at least 250o F (121o C) and kept at that temperature for at least 1/2 hour before the process can be considered sterilized, or free of all live microbial organisms. The process of sterilizing these process and piping components while they are installed is called sterilize-in-place, or SIP.
The Food & Drug Administration and the European Medicines Agency require all such manufacturers to verify that a successful SIP routine has occurred by keeping sterilization temperature records of the piping, vessels and components used to process these drugs. This is done by measuring the temperature of the clean steam as it exits the process via downcombers, or drip legs, before entering a sanitary steam trap. Process designers incorporate drainable temperature sensor tees into the process tubing at those points to facilitate temperature measurement.
The industry has standardized on a minimum required length of 12-18 inches of sanitary tubing between the inlet of the steam trap and the temperature sensor. The idea is to make sure the condensate does not back up enough to wet and cool the temperature sensor. Actual practice varies and tubing downcombers of up to 5 feet have been installed by engineers and process skid manufacturers to ensure that temperature sensors don’t get wet and “fault” during SIP (Figure 1).
The clean steam traps preferred by the ASME BPE (BioProcessing Equipment) standard for this application are thermostatic, balanced port bellows traps. Like most industrial thermostatic steam traps, they work by “trapping” or holding condensate upstream of the steam trap until such time as the condensate cools enough to cause contraction of the thermostatic element (in this case a bellows), which in turn pulls a ball or conical plug away from the trap seat allowing the condensate to escape (Figure 2).
The key to making a clean steam trap for faultless SIP service is to make one with a bellows that begins to open when the clean condensate cools just a couple of degrees below saturation temperature. For example, if a trap’s standard bellows assembly is designed to open when condensate is subcooled (cooled below SIP saturation temperature) only about 2o F (1.1o C) then that trap will back up a lot less condensate than a comparable sized trap whose bellows opens at 10o F (5.6o C) below saturation temperature,
Steam trap manufacturers that know the pitfalls of SIP systems can influence low subcooling operation by designing for it. There are a number of design factors that control how slight temperature change affects steam trap stem movement:
- Clean steam traps with ball plugs are generally preferred over conical plug designs. This is because the ball type plug will allow more flow through any given orifice size than a conical plug, at a fixed amount of lift (bellows contraction).
- Controlled, single-axis stem movement. The bellows wall thickness, finished shape, number of bellows corrugations, bellows welding method and bellows fill mix are some of the factors that manufactures use to regulate a smooth, controlled, single axis movement of the plug. If any of those factors are changed, the rate and axis of lift are affected.
There are always applications in some biopharm plants where condensate backup causes SIP fault problems. Plant standardization on traps with high subcool operation and low differential pressure because of steam trap blowthrough, or poor condensate tubing design, are typically the top reasons for high subcool temperature faults. Short downcombers on vessels or compact process equipment are other installations that can cause troublesome condensate backup.
One solution to these problems is to use a sanitary subcooled condenser. These devices are designed to maximize radiant and convective cooling of clean condensate before it enters the steam trap. They also minimize the installation space required downstream of the validation temperature sensor, and immediately upstream of the clean steam trap normally required for subcooling the condensate. A fully drainable design currently on the market (Figure 3) requires only 2½ inches of space between the temperature sensor and clean steam trap. While offering a much smaller installation space, this particular device has the capacity of 4 feet of ¾-inch sanitary tubing, and about five times the cooling capacity of the traditional 12-18 inch tubing downcomber. Devices like this conserve valuable biopharm real estate, and can eliminate troublesome temperature alarms caused by high subcooling.