Abrasive Blast Nozzle Pressure and Surface Profile
A contractor is abrasive blast cleaning a steel bridge girder to SSPC-SP 10 (Near-White Metal). The specification requires a surface profile of 2.5 to 3.5 mils (63-89 µm) and a nozzle pressure of at least 90 psi (620 kPa) at the nozzle. The contractor is using a #6 (3/8 in) nozzle and a compressor rated at 185 cfm at 100 psi. The blast pot is located 150 feet from the nozzle, and the hose is 1-1/4 inch ID. The abrasive is 20/40 mesh garnet. The inspector observes that the surface profile is only 2.0 mils (50 µm) and the blast pattern appears uneven. The contractor claims the nozzle pressure is 100 psi based on the compressor gauge. The inspector notices that the compressor is cycling frequently and the blast stream seems weak.
1. Identify the likely causes of the low surface profile and uneven blast pattern. What measurements should the inspector take to confirm?
Read the worked answer
The low surface profile and uneven pattern are likely caused by insufficient nozzle pressure and/or abrasive flow issues. The contractor's claim of 100 psi is based on the compressor gauge, which does not account for pressure drop in the 150-foot hose. For a #6 nozzle, the recommended air flow is about 225 cfm at 100 psi. The compressor delivers only 185 cfm at 100 psi, which is insufficient to maintain 90 psi at the nozzle with a 150-foot hose. The pressure drop in the hose can be estimated using the formula: pressure drop = (1.25 x 10^-5) x (Q^2 x L) / (d^5), where Q is cfm, L is hose length in feet, and d is hose ID in inches. For Q=185 cfm, L=150 ft, d=1.25 in, the pressure drop is roughly: (1.25e-5)(185^2150)/(1.25^5) = (1.25e-5)(34225150)/(3.0518) = (1.25e-5)*(5,133,750)/(3.0518) = (64.17)/(3.0518) ≈ 21 psi. So the nozzle pressure would be about 100 - 21 = 79 psi, below the required 90 psi. Additionally, the abrasive may be too fine or the flow may be inconsistent due to moisture or insufficient pressure. The inspector should: 1) Use a nozzle pressure gauge (hypodermic needle gauge) to measure actual pressure at the nozzle. 2) Check abrasive size and condition (20/40 garnet should produce the required profile if pressure is adequate). 3) Inspect the blast hose for internal wear or obstructions. 4) Verify the abrasive metering valve setting and ensure steady flow. 5) Measure surface profile with a replica tape or profilometer at multiple locations.
2. What corrective actions should the inspector recommend to achieve the specified profile?
Read the worked answer
To achieve the specified profile, the inspector should recommend: 1) Increase nozzle pressure to at least 90 psi by using a larger compressor or reducing hose length. The current compressor is undersized for a #6 nozzle; a #5 nozzle (1/4 in) might be used with the existing compressor, but that would reduce productivity. Alternatively, use a shorter hose or larger ID hose to reduce pressure drop. 2) Verify abrasive size and type: 20/40 garnet is capable of producing 2.5-3.5 mils, but if pressure is low, the impact energy is reduced. Ensure the abrasive is dry and free of fines. 3) Adjust abrasive flow rate: too little abrasive can reduce profile; too much can cause uneven pattern. Set the metering valve to provide a steady, consistent flow. 4) Check nozzle wear: a worn nozzle (increased orifice) will reduce velocity and profile. Replace if worn beyond 1/16 in. 5) Ensure the blast pattern is perpendicular to the surface and that the nozzle is held at the correct distance (18-24 inches) and angle. 6) After adjustments, re-blast a test area and measure profile. Only proceed when profile meets specification.