Autoclaves for BSL-3 Facilities—Part 2: Autoclave Door Safety

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Written by: Scott Mechler

BS Mechanical Engineering, Mechanical Engineer

This is the second of three articles describing BSL-3 autoclaves, also commonly known as biocontainment sterilizers. These types of sterilizers are essential in handling microbes that can lead to serious or potentially lethal disease through inhalation. In this article we review the important aspects of the autoclave door design that are essential for every BSL-3 autoclave. Links to our first and third article can be found here:

In the first article, we discussed the fact that most BSL-3 autoclaves have two doors where the flow of goods “pass through” the autoclave – from the contained room out to the non-contained room. We also mentioned that all BSL-3 autoclaves should be equipped with a biological sealing flange or bio-seal. A bio-seal provides separation and a positive seal between the hazardous side (contained side) and safe side (non-contained side), which is imperative for the safety of personnel.

When using a BSL-3 autoclave it is imperative that pathogens do not escape through the autoclave doors – either via poorly designed gaskets or by inadvertently opening the doors simultaneously. These BSL-3 autoclaves should be carefully specified and equipped with the proper door gaskets and door safety mechanisms.

Door Operation

All autoclaves are equipped with a safety feature that prohibits the door from opening until ambient pressure is reached. BSL-3 (double door or pass-thru) sterilizers are equipped with an additional interlock that does not allow both doors to open simultaneously.  This mechanical safety feature helps to prevent the passing of microbes from the contaminated (BSL-3) side to the non-contained side. In other words, when one door is open the other cannot open. The door interlock system is electro-mechanical in nature, therefore, it is fail safe even in the event of power or utility loss.  Preventing both doors from opening also helps to maintain a consistent air differential by preventing the bulk passage of air through the chamber.  Additionally, the control system will not initiate a cycle until both doors are electrically confirmed closed.  Furthermore, the door on the non-contained side is locked, preventing the door from opening in the case of an aborted or incomplete sterilization cycle.

autoclave door

Figure 1: In this diagram, the door on the contained end (typically the BSL-3 end) is locked shut because the door on the non-contained end is open.

Door Gaskets

Door gaskets are typically constructed with solid silicone rubber – a minimum of ¼” thickness. As a safety feature they should not depend on air or steam pressure for their sealing but rather on the pressure of the door as it closes against the gasket. This is referred to as a “passive” gasket design. Their benefits include:

  • Will not deflate if punctured
  • Does not require air or steam for inflation
  • Will not deflate if compressed air or steam is lost.
  • Creates a purely mechanical seal and is therefore fail safe

The gaskets can be changed without tools or adhesives when replacement is needed, which reduces maintenance time.

If an “active” gasket (i.e. requires air or steam to activate) is incorporated into the autoclave door design then users should verify its operation in the event of either power failure or mechanical fault. If power (steam or air) is lost, will the door open on its own? Will microbes be released? If a mechanical failure occurs, will the door be locked in its current position and not be moved until fixed? If so, what happens to the items inside the sterilizer? Can microbes escape once the door is opened after the repair?

Stay tuned for our next article on BSL-3 autoclaves where we will address other important design features. For more information on BSL-3 autoclaves please contact Consolidated Sterilizer Systems.

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