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February 20 2020

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Thermostatic steam traps

Why do SIP temperature validation alarms, or faults occur? How does thermostatic steam trap operation affect the occurrence of temperature validation faults?  In summary, most validation temperature alarms can be traced to a single root cause: condensate back up upstream of the steam trap. The condensate forms because of the operating nature of sanitary steam traps, and the design of the tubing where they are installed.
Specific answers to the questions above will be covered in this blog post, and the following three. To begin, let’s briefly review some fundamentals of process equipment steam sterilization (SIP), and the operating principles of Sanitary Balanced port thermostatic steam traps before we answer the questions above.

PART 1: Fundamentals of process equipment steam sterilization (SIP)
SIP (Sterilize, or Steam In Place) is a timed sterilization of the upstream and downsteam biopharmaceutical production train with clean steam. It is part of the 5 step sanitization routine that occurs after every production batch, and follows the final rinse after CIP. Every square cm of all process piping and vessels that comes in direct, or indirect contact with process inputs, process, and process outputs is “sterilized” to insure that there is no microbiological activity in the system.

Clean Steam (made from USP Purified Water) is circulated through all of the process tubing during this stage, and enters large vessels through spray balls (engineered nozzles) imbedded in the vessel ceiling.
SIP is a temperature validated process, meaning that the sterilization event must be proven, by measuring the temperature of the event and recording the data. The minimum sterilization regimen requires the injection of clean steam into all piping and vessels for a minimum of ½ hour after the vessels reach a minimum temperature of 121°C. See Figure 1 below.



Validation temperature sensors (RTD’s, Themocouple’s or Thermister’s) are placed at the condensate outlets of process equipment to make sure that the sterilization temperature meets the specific regimen designed for the process system. The sensing elements are usually designed with integral sheathes and Tri-ClampTM connections, and are clamped to tubing Tee’s; or the element is inserted into a Tri-clampTM  Thermowell connected to a mating Tee.  The sensors are normally located 300 – 450 mm (12 to 18 inches) upstream of the clean steam trap where the condensate exits the piping or vessel. See Figure 2 below.



Recorded time/temperature data (like that in Figure 1) is stored in a PLC, Distributed Control System, or standalone database for later use by the company quality folks, and auditors. It is important to understand the information above before we discuss how thermostatic steam trap operation can affect the occurrence of temperature validation faults.

Part 2: Sanitary Balanced Port Thermostatic Steam Trap
See the trap cutaway illustration below. Thermostatic traps operate like a thermostat. As such, they are designed to close when the bellows senses saturated clean steam temperatures, preventing it from passing through the trap. Hence the name "steam trap". The closure occurs because the volatile, proprietary liquid alcohol fill inside the bellows vaporizes when it is exposed to clean steam temperatures. Pressure builds up inside the bellows expanding it and driving the attached plug (ball, or conical tip) into the orifice at the trap outlet. The trap will stay in that closed position until the bellows temperature falls below saturation temperature. When that happens, the bellows will contract as the alcohol vapor condenses, lifting the plug off the seat, releasing any clean steam condensate that has collected upstream of the trap. The steam temperature value when the bellows contracts and the trap begins to open is called it’s subcooling value.



Part 3: Validation Temperature Alarms Caused by High Subcooling Trap Operation
As discussed in a previous post, all sanitary thermostatic traps require that a minimum length of tubing be installed between the trap inlet and the validation temperature to account for this buildup of condensate. See illustration above right. The standard distance that has been adopted (evolved) in the industry is 12” – 18” ( ~300 – 450 mm). If a Thermostatic trap requires significant subcooling before the bellows begins to contract and open the trap, clean steam condensate will back up and build in the tubing upstream of the trap1 and may wet, and cool the validation temperature sensor. If that occurs during temperature maintenance (after the system heats up to > 121oC), a temp validation alarm will occur if the sensor is cooled by 1/2oC or more. High subcooling trap operation is one of the two most frequent causes of validation temperature alarms caused by condensate backup.
1 Note that condensate backup is a common occurrence during heat-up, as the amount of condensate produced can be significant. However, temperature validation does not officially begin until after the system being sterilized reaches its validated design temperature at some point above 121oC. At that point, the amount of steam required to keep the system at temperature, and the associated condensate load is dramatically less. The trick is choose a trap that has enough capacity to handle the larger heat up loads, but with a low enough subcooling operation, so that condensate is never allowed to build in the tubing during the significantly lower loads that occur once the system has reached validation temperature (temperature maintenance period when system temperature is > 121oC).



Part 4: Validation Temperature Faults Caused by Adjacent Trap Failure
Balanced port thermostatic traps fail when the SS bellows develops a leak and the alcohol fill escapes. Without its volatile alcohol fill, the bellows can never expand and close when exposed to steam temperature. The trap will remain open at all times allowing clean steam to pass through the trap. During Temperature Hold (low condensate creation), this can be problematic as clean steam will blow through the trap into the condensate header. This increase in condensate header pressure can cause one or more traps on the same condensate header to back up condensate. Back up will occur because the differential pressure across all of the traps tied into that header will be reduced. (DP = P1-P2) Reduced differential pressure will result in reduced flow in one of more of the adjacent traps. In smaller volume condensate headers, this flow reduction through the traps can cause condensate back up significant enough to wet the sensor and cause a temperature alarm.