How Wrong Bypass Valve Operation Caused AHU Failure in a Pharma Plant

1. Background

A Regulatory (WHO-GMP/USFDA) compliant pharmaceutical manufacturing unit operates 28 AHUs under a centralized chilled water system. Environmental control (temperature, RH, pressure cascade) is critical for:

• tablet compression
• coating
• granulation
• packing rooms
• raw material sampling

Operators routinely adjust AHU valve positions based on comfort complaints from production staff.

2. Problem Observed

Multiple rooms showed:

• fluctuating room temperature (±3°C)
• high relative humidity during peak hours
• failure to maintain pressure differentials
• inconsistent product room conditions during validation checks

BMS trends showed:

• chilled water valve staying at 100% for long durations
• unexpected bypass flow
• low coil delta-T (only 2–3°C instead of 6–7°C)

3. Operator Misconception Identified

Root misconception

“Keeping bypass partially open ensures continuous chilled water flow, avoids coil choking, and improves cooling.”

Operators believed:

• bypass must never be fully closed
• closing the bypass increases risk of airlock
• keeping valve in manual mode gives faster cooling
• pressure fluctuations are harmful to AHU coils

To “solve” room complaints, they manually:

• opened bypass valves
• kept 3-way valves in MAN 100%
• adjusted balancing valves without referencing design flow
• bypassed BMS auto control to “help” the AHU cool faster

4. Impact on Pharma Operations

a. Room Temperature Instability

Excessive flow through the coil → coil gets colder → thermostat overshoots → reheat coil activates → energy waste.

b. High Relative Humidity

Due to unstable coil temperature → dehumidification cycle breaks.

c. Pressure Cascade Failure

Rooms dipped in pressure because:

• AHU supply CFM dropped due to wet coils
• doors could not maintain differential pressure
• impacted Grade D and uncontrolled areas

d. Loss of Chiller Efficiency

Return water temperature dropped due to bypass → chiller ran at low ΔT → higher energy consumption.

e. Near-Deviation Event

One batch drying operation almost failed humidity acceptance criteria.
Deviation was avoided but required documented justification.

5. Diagnostic Findings

• Bypass valves found 50–70% open in 16 AHUs.
• 3-way valves kept in manual mode at 100%.
• DP across coils was fluctuating from 0.6 to 2.2 kPa (0.06–0.22 mbar).
  Note: measured differential was very low — consistent with excessive bypass/short-circuiting of flow. Typical chilled-water coil pressure drops are roughly 5–50 kPa in normal designs; high-flow designs may approach 20–100 kPa (AHRI guidance).
• Coil ΔT was consistently low (2–3°C).
• Chiller return temperature was 9.5°C instead of 12°C.

6. Root Cause Analysis (Operational)

1. Operators believed bypass is required to ‘keep water circulating’.
2. Operators manually interfered with BMS valve commands.
3. Lack of training on 2-way/3-way valve control logic.
4. No SOP for chilled water valve operation.
5. No monitoring or alarm for valve manual override in BMS.

7. Corrective Actions Implemented

a. Restore Correct Valve Operation

• Closed all unnecessary bypasses.
• Switched all 3-way valves back to AUTO mode.
• Re-set balancing valves to design flow.

b. Implement BMS Controls

• Enabled alarms for valve manual override.
• Added trend logs for CHW flow, ΔT, valve position.

c. Operator Training

• Trained technicians on:
  • valve logic
  • effect of bypass on ΔT
  • pharma HVAC compliance requirements

d. SOP Implementation

Created an SOP:

• no manual override without Engineering Manager approval
• bypass valves must remain fully closed unless maintenance activity

8. Results (Post-Correction)

Within 1 week:

Parameter	Before	After
Coil ΔT	2–3°C	6–7°C
Room RH variation	±12%	±3%
Temp fluctuation	±3°C	±0.8°C
Pressure cascade issues	Frequent	None
Chiller power	Increased	7–9% reduction

Production rooms passed subsequent validation without deviations.

9. Key Learning for Pharma Facilities

• Bypass ≠ stability.
• Manual override leads to loss of control & GMP non-compliance.
• Operator-created instability is more common than design errors.
• Valve logic must remain under BMS with defined SOPs.