Sterilizer Door Safety Features

  1. A positive action limit switch monitors the correct closure of a sterilizer door. This device is fail-safe, indicating “Door Closed” only when the door is fully closed. In any other position, the switch indicates “Door Open.” This switch interlocks sources of pressurization to the chamber and the actuation of the door gasket.
  2. A magnetic limit switch also monitors the correct closure of the power-operated door. This switch directly interlocks the sources of pressurization to the chamber and the actuation of the door gasket. This operates by interlocking the pneumatic signals to the process valves controlling these functions.
  3. The door seal is an “O” ring gasket located in a cavity at the head ring of the chamber. At the start of the process, the cavity is pressurized to force the gasket against the rear face of the door. On completion of the process, the cavity is evacuated to retract the gasket. This must occur before the
    door can be moved (the door cannot be moved with the door gasket applied). In this condition, the pressure-bearing mechanism still fully retains the door. When retracted, the chamber is completely vented to the atmosphere through the aperture between the head ring and the door. This aperture is at least 8 square inches (the smallest PRIMUS chamber).

    The gasket protects the cavity during loading and unloading and is relatively simple and inexpensive to replace. It self-compensates for wear and accommodates minor imperfections. No critical settings or adjustments are necessary after gasket replacement. Want to know how to install a gasket on a PRIMUS sterilizer?
  4. A pressure switch monitors the gasket cavity’s pressure and electrically interlocks the control signals to the process valves that pressurize the chamber (the chamber cannot be pressurized until the door is locked and sealed). This interlock operates independently of the control system and in conjunction (a “series” connection) with the pneumatic and software interlocks.
  5. A pressure transducer connected to the chamber monitors the chamber pressure and interlocks the door gasket cavity evacuation (unseal door) until the chamber is at atmospheric pressure.
  6. While closing, the control system employs a “dead-man” interlock, in which the “Door Close” button must be held depressed until the door is fully closed. If released while closing, the door will reopen. The closing mechanism design is clean, safe, and simple. There are no gears or other complicated mechanical components involved in closing the door. Simplicity leads to greater operator safety and less opportunity for component failure.
  7. The door travels behind a stainless steel fascia panel to open. When the door is open, there are no exposed heat-radiating surfaces in the operator area, eliminating the possibility of operator hazards while unloading. Where access to the maintenance space is provided through fascia panels, an optional limit switch can be fitted to the panels to isolate the door open function while the access panel is open.
  8. In the unlikely event of a steam leak from the chamber (e.g. should the door seal leak or otherwise fail), the vapor and condensate are discharged safely behind the fascia panel work, not into the operator area. This feature is useful for cycles that do not have a post-vacuum or long cooling cycle – e.g. liquids sterilization. A plume of hot vapor emitting from the chamber as the door opens typifies such cycles. In the PRIMUS design, the vapor escapes from the chamber as soon as the door seal is retracted and until the door is opened by the operator (the seal release is part of the automatic
    cycle and it may be some time before the operator arrives to open the door). As the door is opened, any remaining vapor escapes behind the panelwork. If required, air conditioning extract may be provided in this area, integrated with the panel work.

Features Specific to Vertical Doors

For chambers with internal chamber cross sections 16″ wide x 16″ high up to 26″ wide x 26″ high, the sterilizer door rises vertically to close and is either manually or hydraulically operated.

  1. The door is retained in guides welded to the sterilizer’s body. These guides are designed to hold the door in position while the chamber is pressurized. The door is suspended by cables connected to a counterbalanced weight that provides smooth door travel. The guides prevent lateral movement, allowing the door to move vertically up and down.
  2. For critical use environments (e.g., BSL3 and BSL4 laboratories), PRIMUS recommends the use of compressed air-to-gasket. (PSS6 will only utilize special valves to ensure door gaskets remain sealed in the event of a power loss.)

Features Specific to Horizontal Doors

For chambers with internal chamber cross sections 26″ wide x 36″ high and larger, the door moves horizontally left or right depending on the room layout.

  1. A single heavy-duty beam carries the door. The door is suspended by trolleys and runs on a painted steel I-beam. The design rail and hanging mechanism negate the need for close tolerances and critical settings.
  2. A pneumatic cylinder propels the door open and closed. Drive is transferred via a stainless steel rod actuator attached to the end of the door. Magnetic switches set the door movement to fully open or fully closed. In addition to the software interlock, reversing mechanism, and other safety devices mentioned previously, the door may readily be prevented from closing manually.
  3. Locking/pressure retention is activated by a lug and pin system. As the door closes, pins on the leading vertical face engage in steel lugs welded to the chamber. Similarly, lugs welded to the trailing vertical face of the door engage pins welded to the corresponding face of the chamber. The pins are designed to withstand full test pressure (1.5 times the design pressure, in accordance with ASME requirements). Furthermore, they are designed to do this before the door plate covers the door aperture. This is a fail-safe design. The door is fully retained by a system designed to withstand test pressure before the test pressure can be generated in the system.
  4. For critical use environments (e.g., BSL3 and BSL4 laboratories), PRIMUS recommends the use of compressed air-to-gasket. (PSS6 will only utilize special valves to ensure door gaskets remain sealed in the event of a power loss.)
  5. As shown below in a series of photographs, each horizontal door sterilizer is provided with a two-piece Maintenance Lockout Device. The device is mounted on one side of the sterilizer when a person needs to enter the chamber for cleaning, inspection, or other purposes. A hole is factory drilled on the I-beam approximately in line with the left-hand edge of the door when the door is fully opened to the right. For a door that slides to the left, the hole will be in line with the right-hand edge of the door. One piece of the lockout device is placed through the hole and engaged with the second piece. The operator then places his/her lock on the device. The lockout device will then prevent any closure of the door while a person is nside the chamber.

Lockout Device Mounted on the Side of the Sterilizer

Two-Piece Lockout Design

Insert One Half into the Beam and Mate with the Other Half on the Back Side of the Beam

Door safety on a primus sterilizer from spire integrated solutions


Do you have a question or are having difficulties? Talk to Dave Schall, our PRIMUS sterilizer expert, who is here to assist you anytime at 877.679.7800 extension 1212 or by email at


How to replace a gasket on a PRIMUS sterilizer

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