A compressed air auditor was surprised to see an air compressor under test was consuming excess power when unloaded. A check of the sump pressure (Fig. 1) showed the sump was not blowing down during unload cycles. And the installed flow meter showed the compressor was producing a small amount of air, even in the unloaded condition.
In a lubricated rotary screw compressor, the sump blowdown valve plays an important role — not just during shutdown, but every time the compressor unloads during normal operation. When working correctly, it helps protect your equipment, reduce energy use, and ensure smooth cycling.
Most compressors operate in a load/unload mode, where the unit temporarily stops compressing air once system pressure is met. During unloading, the sump blowdown valve opens briefly to vent pressure in the separator tank down to about 60 psi. This reduces energy consumption and allows for a smooth restart when the compressor reloads, reducing motor strain and improving efficiency.
If the valve fails closed, the compressor tries to restart under full sump pressure. This can cause motor overloads, high startup currents, and excess wear. You may notice long startup delays or frequent breaker trips.
If the valve fails open or leaks, air escapes continuously during unloaded operation. This leads to pressure instability, frequent cycling, and higher energy use. Oil mist may also escape, increasing lubricant consumption and risking air quality issues.
These failures are often overlooked but can have significant long-term effects: poor efficiency, damaged components, and rising maintenance costs. Fortunately, the fix is simple. Regular checks — such as listening for a brief hiss when the unit unloads — can catch problems early.
In compressed air systems, small failures can cause high operating costs; in this case, the compressor was found to be consuming 3-4 times its normal power consumption per unit output! The sump blowdown valve is one of those small parts that deserves more attention. In this case, it needs quick action.
