At today's prices, fuel is the greatest operational cost on rotating and reciprocating engines and compressors.
Roughly, fuel costs for large reciprocating engines are about $0.03 per HP HOUR for a healthy engine.
As an example, a 3000 horsepower natural gas pipeline engine consumes $748,000 worth of fuel per year at a 95% utilization using manufacturers rated specifications.
Typically, the actual fuel consumption is from 15% to 25% greater than this for a variety of reasons.
The factors causing an increased fuel consumption are :
Fuel composition - BTU content - temperature affects air/fuel ratio and combustion
Air quality and temperature affects air/fuel ratio and combustion
Poor ignition system performance causes poor combustion or no combustion
improper ignition system timing causes poor engine efficiency
Running at lower than rated torque causes poor engine efficiency
(worse in 2-strokes)
Running at lower than rated RPM causes poor engine efficiency
Running at higher than rated RPM causes poor engine efficiency
(worse in 4-strokes)
Power cylinder balance causes poor engine efficiency and increased wear
The factors 3 through 8 should be closely monitored and analyzed with a performance analyzer in order to keep the machine in optimal running condition.
A quantitative example is power cylinder balance. If an engine is run with a 20% variation between peak pressures of the cylinders, the engine will consume from 10 % to 20 % more fuel than a machine with cylinders firing evenly. Also mechanical wear is dramatically increased because some cylinders are exerting much more force on the crankshaft than other cylinders in order for the engine to output the required amount of power.
The calculation below gives a good estimate of fuel savings potential of an unbalanced engine.
MAXIMUM CYLINDER DEVIATION IN % 20 % <-enter
FUEL COST PER HP HOUR 0.03 <-enter
HORSEPOWER OF MACHINE 2500 <-enter
WASTED FUEL/YEAR $98,550
Gas compressors can and frequently do waste up to 50% of the shaft power because of:
Valve and compressor losses and restrictions
Pulsation and gas flow restrictions
Compressor valve and ring leakage
Cylinder unloader failures
Leaking compressor valves and compressor cylinders that are partially loaded waste brake horsepower (fuel or electricity) but pump less gas than normal or no gas at all.
Use of an analyzer will show that indicated horsepower consumed by the compressor and also the volume of gas pumped by the compressor. The ratio of this shows the cubic feet of gas pumped per horsepower of the compressor. This number can be compared against a healthy cylinder with comparable suction and discharge pressures and temperatures to establish efficiency and lost dollars.
As an example, on a four cylinder compressor, all cylinders are loaded equally and are consuming
10.5 horsepower per MMSCF, except for head end of cylinder number 2 is running 11.5 horsepower per MMSCF.
The analyzer software will more than likely identify the cause of the inefficiency, but we will perform a cost analysis and determine the lost dollars caused by the inefficient cylinder end.
TOTAL HORSEPOWER 1500 <-enter
AVERAGE HP per MMSCF (HEALTHY ) 10.5 <-enter
AVERAGE HP per MMSCF (LEAKING) 11.5 <-enter
TOTAL CAPACITY (MMSCF) 142.86 calculated for healthy cylinders
COST PER HP HOUR 0.03 <-enter
NUMBER OF CYLINDER ENDS 8 <-enter
MMSCF PER CYLINDER END 17.86
HORSEPOWER FOR LEAKING CYLINDER 205.36
HORSEPOWER FOR HEALTHY CYLINDER 187.50
WASTED HORSEPOWER 17.86
WASTED DOLLARS / YEAR $4,693 PER LEAKING CYLINDER END
You may download these calculations in the form of a Microsoft Excel spreadsheet for your own use to help justify a program at your facility.