NPSHa Boundary Instability and Impeller Eye Cavitation

Client: Confidential Hospitality Group (High-Rise Hotel)

Location: Downtown San Jose, CA

Project Focus: Net Positive Suction Head (NPSH) Boundary Analysis, Break Tank Level Dynamics, and Impeller Erosion Diagnostics.

Background

The asset is a prominent luxury hotel in Downtown San Jose. The building’s domestic water supply is managed by a high-pressure triplex booster system fed by a sub-grade atmospheric break tank. During peak morning occupancy (high-demand period), the system began emitting a loud, rhythmic "gravel-like" grinding sound. Simultaneously, guest complaints were logged regarding fluctuating water pressure on the upper floors (Levels 15–22).

Identified Issues

1. Hydraulic Starvation (Pumping Gravel): The lead pump exhibited severe vibration and an audible "marbles in the casing" sound, symptomatic of classic cavitation.

2. Pressure Instability: The system could not maintain its 145 PSI setpoint during peak flows, despite the VFDs (Variable Frequency Drives) ramping the motors to 100% (60Hz).

3. Elevated Surface Temperatures: Infrared thermography detected a 35°F temperature spike at the mechanical seal housing compared to the ambient water temperature, indicating a loss of cooling flow.

Technical Analysis:

1. NPSH Boundary & Friction Loss Analysis

• Issue: The "Available" suction head (NPSHa) was dropping below the manufacturer’s "Required" head (NPSHr).

• Data-Driven Discovery: Manometer testing at the pump suction flange revealed that as the break tank level dropped to its lower 25% threshold, the static head was insufficient to overcome the friction losses of the aged, 6-inch suction header.

• Analysis: The math confirmed a "Safety Margin" of nearly zero. The pressure at the eye of the impeller was falling below the vapor pressure of the water, causing instantaneous vapor bubble formation and collapse (implosion).

• Action: Recalibrated the break tank's makeup valve logic to increase the "Low Level" buffer by 12 inches, ensuring a higher static column of water.

2. Vortex Entrainment & Air Ingress

• Issue: Entrapped air was being pulled into the system from the tank.

• Data: Visual inspection of the break tank during peak draw revealed a "Whirlpool" or vortex effect at the tank outlet.

• Discovery: Because the water level was too low, the velocity of the water leaving the tank was pulling air from the surface into the suction line. This entrapped air was accumulating in the eye of the impeller, causing "Air Binding" and irregular radial loading.

• Result: Installed a stainless steel anti-vortex plate over the tank outlet to break the surface tension and prevent air entrainment even at lower water levels.

3. Internal Impeller & Seal Audit

• Issue: Suspected mechanical damage from prolonged cavitation events.

• Action: Performed a teardown of the Lead Pump (Pump #1).

• Analysis: Inspection of the 316SS impeller eye revealed distinct "honeycomb" pitting—the physical fingerprint of micro-jet implosions at 100,000 PSI. The mechanical seal faces showed "heat checking" due to intermittent dry-running caused by the vapor pockets.

• Result: Replaced the damaged impeller and upgraded the mechanical seal to a Silicon Carbide/Silicon Carbide face set for better thermal resilience. Verified final alignment via laser to HI (Hydraulic Institute) standards.

Outcome

The critical domestic water infrastructure was successfully stabilized:

• Eliminated Cavitation Noise: The "gravel" sound was completely silenced across all load ranges.

• Restored Hydraulic Continuity: The system now maintains a steady 145 PSI setpoint even during peak morning "shower spikes" in the hotel.

• Energy Optimization: By eliminating vapor-lock, the pumps now move more GPM per kW, allowing the VFDs to run at lower average frequencies (approx. 48Hz vs. 60Hz), extending the life of the 20HP motors.

Recommendations

• Low-Level Pump Lockout Logic: Implement a hard-wired safety interlock in the CRM/BMS that shuts down the booster pumps if the break tank level falls below the "NPSH Critical" line.

• Annual Impeller Borescope: Conduct an annual internal inspection of the impeller eye using a borescope to monitor for "pitting" without the cost of a full pump teardown.

• Makeup Valve Maintenance: Conduct quarterly inspections of the break tank's fast-fill solenoid to ensure it can keep pace with peak hotel demand.