The symptom is a pressure-design conflict: an unclassified skid sits beside a Class I, Division 2 area, an air lock separates the modules, and only the internal IER is marked as pressurized. Neither +0.1 in. w.c. nor +0.05 in. w.c. can be selected from adjacency alone. Before anything else, confirm how the approved area-classification basis keeps the skid unclassified.
Pressurization approaches
Two arrangements can produce the described drawing. They are not interchangeable.
| Arrangement | Protected boundary | Required design evidence | Pressure decision |
|---|---|---|---|
| Whole-module pressurization | Exterior envelope of the unclassified module | Classification drawing, HVAC basis, pressure-control narrative, door and air-lock details | Use the documented module-to-classified-area setpoint and alarm limits. |
| IER-only pressurization | Walls, ceiling, floor, doors, and penetrations of the IER | IER pressurization designation and a separate basis for classifying the surrounding module as unclassified | Control the IER against its specified reference space; do not transfer that requirement to the entire module without a design basis. |
The second arrangement matches the stated drawing: the module is unclassified while the IER carries the pressurization marking. That marking normally applies to the identified room boundary, not automatically to every surrounding space. Confirm this interpretation against the drawing legend because symbol meanings are project-specific.
IER commonly refers to an instrument/electrical room, but the project abbreviation list governs. Define the room by its documented name, occupancy, electrical equipment, and protective function before evaluating its pressure relationship.
Recommended decision path
- Confirm the area classifications. Mark the Class I, Division 2 boundary, the unclassified module boundary, the air lock, and the IER on one controlled drawing. Do not move on until each wall and doorway belongs to a defined area.
- Identify the protection method. Determine whether the whole module remains unclassified because of pressurization, physical separation, ventilation, source control, or another documented classification basis. The classification sheet alone may not contain the complete HVAC design basis.
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Interpret the pressurization symbol. Check the legend for the circled
P. If it points only to the IER, treat IER pressurization as a separate protection function unless another document extends it to the module. -
Locate the governing pressure requirement. Read the approved HVAC narrative, room data sheet, cause-and-effect document, and applicable project specification. Consult
API RP 500as part of the location-classification review, but verify any pressure value against the governing design documents rather than treating the standard reference as proof of either proposed setpoint. - Select the setpoint only after the boundary is known. If whole-module pressure is the protection method, use the specified differential, reference location, alarm threshold, and failure response. If it is not, do not create a new whole-module setpoint merely because a classified skid is adjacent.
Pressure-boundary mechanism
Positive pressure limits inward migration only while air flows outward through leakage paths and openings. A pressure reading has meaning only when its two sensing points represent the protected space and the intended reference area. Measuring the IER against the surrounding unclassified module does not demonstrate the module’s pressure against the Class I, Division 2 space.
An air lock supports separation by preventing both boundary doors from standing open together and by controlling the pressure transition between spaces. Its presence does not, by itself, establish that the whole unclassified module is pressurized. The air-lock sequence, leakage paths, door closers, ventilation supply, and exhaust balance determine the actual pressure cascade.
Open doors can temporarily collapse differential pressure even when the supply system is correctly balanced. Cable entries, conduit seals, drains, ducts, wall penetrations, and damaged door gaskets can create uncontrolled paths. A stable indication at one transmitter therefore does not prove that the full boundary performs as designed.
Document and field checks
| Check | What to establish | Acceptance evidence |
|---|---|---|
| Classification documents | Why the module is unclassified and where Class I, Division 2 begins | Approved boundary and classification basis agree |
| HVAC documents | Protected space, reference space, normal target, low-pressure limit, and failure response | Control narrative and room data agree |
| Air-lock arrangement | Door sequence and intended pressure cascade | Doors close correctly and the sequence matches the design |
| Pressure instrument | High and low sensing-port locations, range, zero, and calibration status | Ports terminate in the specified spaces and the instrument reads correctly |
| Boundary condition | Leakage through doors, penetrations, ducts, and drains | No unaccounted path defeats the protected envelope |
Before using a displayed differential, trace both impulse tubes or remote sensing lines physically. Reversed ports can make a negative relationship appear positive. A sensing port placed in a supply-air jet, near an exhaust pickup, or in a wind-affected exterior location can also produce a reading that does not represent room static pressure.
Commissioning procedure
- Set the test configuration. Place ventilation in its documented normal mode and position all doors, dampers, and exhaust systems as required by the operating narrative. Confirm each device reports the commanded state.
- Verify the pressure instrument. Check zero and calibration, then confirm the high-side and low-side connections correspond to the protected and reference spaces. Do not move on until an independent measurement agrees with the installed indication within the project acceptance criterion.
- Test the IER boundary. Measure the IER differential against the reference space named in its design. Operate the IER door and verify pressure recovers to the documented target after closure; use the specified recovery criterion rather than inventing a time allowance.
- Test the module boundary when applicable. If documents identify whole-module pressurization as a classification safeguard, measure across the module-to-classified-area boundary under normal ventilation and credible door states defined by the design.
- Exercise the air lock. Operate each door through the intended sequence. Confirm door closure, any installed interlock, and the expected pressure direction in each occupied state.
- Challenge the protection. Reduce or stop the relevant supply airflow using the approved test method. Verify any installed low-pressure alarm, annunciation, ventilation response, or electrical action against the cause-and-effect document.
- Restore normal operation. Return every override, fan, damper, alarm bypass, and door control to its normal state. Record final pressures with the associated door and ventilation states.
Recurring design pitfalls
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Choosing a convenient differential:
+0.05 in. w.c.may be “slightly positive,” but that description is not an acceptance criterion. Use the value assigned by the governing design. - Applying an IER requirement to the module: A room-specific pressurization mark defines a local boundary unless the classification and HVAC documents say otherwise.
-
Treating
+0.1 in. w.c.as a universal code value: A proposed value does not become mandatory merely because the neighboring area is Class I, Division 2. - Ignoring the reference point: A numeric differential without identified high-side and low-side locations cannot demonstrate the intended pressure cascade.
- Using normal-operation readings only: Door movement, exhaust changes, fan loss, and blocked airflow can expose a protection failure hidden by a steady-state reading.
Frequently asked questions
Why does an adjacent Class I, Division 2 skid not automatically require +0.1 in. w.c.?
Adjacency identifies a boundary to evaluate; it does not select the protection method or differential-pressure value. Confirm whether the unclassified module depends on pressure, then use the setpoint stated in its approved design basis.
Why does +0.05 in. w.c. not prove the module is acceptable?
+0.05 in. w.c. proves only the measured relationship at the selected sensing points and operating state. Acceptance also requires the correct protected boundary, reference space, instrument arrangement, and documented criterion.
Why does the IER remain pressurized inside an unclassified module?
The IER can have a dedicated environmental or hazardous-migration control boundary while the surrounding module is kept unclassified by a different documented method. Test the IER against the reference space named in its own pressure-control design.
Why does an air lock not prove whole-module pressurization?
An air lock controls movement through a doorway, but its presence does not define the HVAC protection strategy. Review its door sequence and pressure cascade together with the classification and ventilation documents.
How do I verify skid pressurization before commissioning sign-off?
Confirm the classified boundary and protection method, trace both pressure-sensing points, test normal and defined door states, challenge any installed alarm or interlock, restore all overrides, and record the final differential with the ventilation and door states.