Sanitary District of Hammond / Bio Gas Engine

Compressor Surge
Updated 5/2008

The speed range of the unit is chosen (but may change) to be 900 RPM on the low end, and limited to 1200 RPM on the high end.  The engine is driving a centrifugal blower (compressor or fan) with variable inlet guide vanes.  The drive is through a gear box so the fan speed is about 4¼ times faster than the engine, or 5109 RPM maximum.  The 300 RPM speed range on the engine translates to a 1277 RPM speed range on the fan.

The Bio Gas Engine can blow the same amount of air as the electric motors at a lower speed by opening the inlet guide vanes more. The synchronous electric motors always run at 1200 rpm, and they control air flow by modulating the inlet guide vanes.  They typically run with the guide vanes open about 50%.

Air from the blower is piped to a diffuser distribution network of piping along the bottom of aeration tanks to feed aerobic micro-organisms that break down the waste.  The water level in the tanks is related to the inflow to the plant, which is related to recent rains because the facility is taking in both sewer and storm runoff.

The back pressure on the blower varies with the head pressure of water in the tanks.  If it has been raining, the water level and head pressure will increase.  The blower output needed to overcome the head pressure will vary with inflow to the facility.  If the blower output falls below the threshold needed to overcome the head pressure, the outflow from the compressor will begin to surge.  The surging compressor discharge pressure will result in surging torque which causes surging engine speed.

The engine speed surge was observed to be about 80 rpm ( ± 40 RPM ) during evaluation on 4/11/08 with the governor valve (carburetor throttle body butterfly) locked in a fixed position.  The Flo-Tech valves controlling AFR were also locked in a fixed position.  This surging cannot be controlled by the governor system.  The governor will chase the surge and amplify it.  To address the surge with a governor control, a more expensive governor with feed forward surge control would need to be implemented and tuned.  The cost of implementing such a system is not in alignment with the economics of the application, plus there are functional reasons not too operate in a surging condition, so the unit speed should be maintained above the surge threshold.

It is the facility's responsibility to vary the blower output so that it is above the varying surge threshold speed.  The current scheme is to vary the speed setpoint based on inflow to the plant, which is indirectly related to the head pressure.  There is a cost incentive to run the blower at low speeds because pumping excess air requires more fuel.