Witness the Operation of the Compressor
As you approach the used unit under consideration, take a close look at the welds on the tank, if it is a tank mounted unit or a skid mounted unit with an integral tank. Look for pin holes or cracks. Pressurize the tank. Then, using a spray bottle containing a leak detection solution (or soapy water) wet the areas around welds, tank outlets and fitting access points. If you have a sonic leak detector or one of the sophisticated visual leak detectors, all the better. The next consideration is to witness the operation of the compressor. If the seller cannot or will not run the machine for you, walk away from it (or call us for the use of our test cell). Operate the compressor for at least an hour at maximum rated pressure. Cycle the unit through all available control modes (i.e. load/unload, modulation, fully unloaded as well as timed shut down) This will reveal any weaknesses such as oil leaks, overheating, inlet valve malfunction, thermal mixing valve weakness, a dirty or clogged cooler, improper functioning of the blow down valve and most other weaknesses. After the unit is shut down, check the operation of the minimum pressure check valve. Always test the pressure relief valves on the after cooler as well as the sump.
Once the unit is operating at full load and up to operating temperature, take an amp draw. Compare all three legs of the motor to the rated amperage on the motor name plate. Be sure you are taking the amp reading of only the main drive motor. If you take the amp reading at the load side of the disconnect you will be reading the cooling fan motor as well. Compare the reading of the cooling fan motor as well. Replacing either motor soon after making the purchase will be a major expense. This is a good time to check amp draw when the unit is operating fully unloaded with the inlet valve closed. The amp reading should be very close to 15% of the name plate rated amperage of the drive motor. If it is higher, chances are the blow down system is not functioning correctly or the minimum pressure valve is leaking by and allowing system pressure backflow into the sump. In the case of a drop-in separator element this pressure will result in a shortened separator service life or, in some cases. a catastrophic element failure. If the amperage is significantly lower than 15%, the bypass orifice (hole) in the inlet valve is probably clogged. Since modern rotary screw compressors circulate oil by pressure differential, a clogged bypass method will result in the air end starving for oil, resulting in bearing and rotor damage. A very old screw compressor may have an oil pump. Be sure to check for this component. Some rotary screw vacuum pumps will also have an oil pump.
Considerations For A Reciprocating (Piston) Compressor
If you are considering the purchase of a used reciprocating air cooled air compressor, insist on seeing the unit run. Close the service valve on the receiver and keep it closed throughout the test. Drain the tank to zero psig. Pay attention to the amount and color of the condensate coming from the tank drain.
Extremely rusty condensate usually indicates an internally rusted tank. This can cause issues if the installation requires a pressure vessel permit. If the condensate has a milky or yellow color, that is emulsified oil. This indicates the piston rings have failed and the unit has passed oil into the air stream. The issue could also be as simple as a clogged or inoperative crankcase breather. This will force oil past the piston rings causing carbon build-up on discharge valves as well as emulsification of condensate.
Pump-Up Test (This test will indicate the piston ring condition)
Determine the time required to bring the tank from zero psig to 100 psig. Example: A 10 h.p. compressor on a 120-gallon receiver
Start the compressor. Record the time. Record the time when the pressure gauge reads 100 psig. A compressor with sound piston rings will follow the example below. The times recorded must be accurate to the second. The use of a stopwatch or a device with a stopwatch function is very desirable.
Multiply the horsepower of the compressor by 4. Divide the tank size in gallons by the number arrived at by multiplying the compressor horsepower by 4. This will give you the time in minutes it should take to raise the tank pressure from zero psig to 100 psig.
10 x 4 = 40 (this is the cubic feet per minute a reciprocating compressor should produce)
120 gallons ÷ 40 cfm = 3 minutes
So, a 10 h.p. reciprocating compressor in good condition should take 3 minutes to raise the pressure in a 120-gallon tank from zero psig to 100 psig. Anything significantly longer indicates weak valves and/or piston rings.
Visual Inspection Points
Visually inspect the unit. Look for paint on the head around the discharge valves. Regardless of the sales and marketing hype a manufacturer puts out, any reciprocating compressor is only capable of an 80% duty cycle (8 minutes out of ten or 48 minutes each hour). Beyond this, heat will build up in the discharge valves, weakening the valve rings, reeds, or fingers, depending on the valve type. The paint on the head will usually become discolored, indicating the unit needs new valves.
Another indicator to be aware of is the intercooler pressure relief valve. Listen for the relief valve leaking air. If the relief valve lifts, the unit has bad second stage suction valves. This holds true on any multistage compressor. I have witnessed the relief valve on a 5-stage high pressure Ingersoll Rand in a dive shop lift. Not a fun occurrence.
Other than these basic considerations, if the unit runs smooth, there is no obvious piston slap or bearing noise and there is no oil mist coming from the crankcase vent/breather, the unit is probably in serviceable condition.
Read the manufacturers ASME tag on the tank. This will tell you the year of manufacture as well as the pressure rating of the unit. Compare the rated operating pressure of the tank with the pressure rating of the pressure relief valve on the tank. Lack of a relief should stop the purchase. An unprotected tank is an extreme risk. While I have never actually seen a tank “explode” I have seen many older tanks develop cracks and pin holes along the mounting welds for the legs and top plates. A pressure vessel with no ASME tag attached is illegal in locations requiring an operating permit for pressure vessels.
Rotary Vane Compressors
Rotary vane compressor considerations are simple. Vanes are NOT NOT NOT a wearing item, regardless of what a competitor tells you. Rotary vane compressors are simple as a hammer and the older Hydrovane and Mattei compressors are good units. They are inefficient but reliable, easy to service IF you know what you are doing.
The newer, vertical Hydrovane compressors have had problems with oiling, reliability as well as causing issues for anyone unfamiliar with servicing the units. They are easy to overfill with oil, resulting in cracked castings.
Scroll Compressors
When considering a scroll, if the unit runs and makes no scraping or rubbing noises, it is probably good to go. They are simple, easy to maintain and require only simple greasing of bearings on a periodic schedule. If there is no contact between the orbiting element and the stationary housing replacing the tip seals will return to original capacity. Again, scrolls are not as efficient as an oil sealed rotary screw compressor. They are quieter and much less costly to maintain.
Older Rotary Screw Compressors
If you are considering a rotary screw compressor older than about 5 years, be aware that you are considering old technology. Variable frequency drives have become much more robust in just the last few years. Prior to that time the drives were overly sensitive to heat and vibration as well as minor voltage fluctuations.
Muffin fans in the drive cabinet to control operating temperatures. These fans are subject to frequent failure. The wafer type filters used to keep dust out of the cabinets may become clogged. Frequent cleaning or replacement is required.
Isolation transformers were used to minimize the damage caused by electrical “noise”.
Some exceedingly early VFD compressors did not use VFD wound motors, resulting in motor failure. Early VFD drives did not track variations in pressure all that well, resulting in less-than-optimal operation and a less-than-anticipated lowering of operation costs and an extended ROI time frame. Inexpensive transducers were utilized in early units as a cost control measure, resulting in “sloppy” control. After many warranty calls and the damage to several big-name manufacturers’ reputations, the stupidity of utilizing cheap components was finally brought home to the accountants and quality began to appear.
Service and Repair Parts Availability
An especially important consideration when deciding on a used unit is to assure the availability of service and repair parts. This is not usually a concern IF the unit was from a major manufacturer. If the unit is made in Turkey, Afghanistan, or some other semi-industrialized nation, it is likely there will be no service components readily available. Lubricants and filter elements will usually present no issue as every manufacturer (again, no matter the marketing or sales hype) uses components from a few suppliers. Solenoid and check valves can readily be substituted but unique components such as minimum pressure valves, inlet valves, thermal mixing valves, air end castings, and other components unique to the manufacturer are not usually available.
Service Records and Hours of Service
If service records are available, that is a bonus. A well-maintained oil lubricated rotary screw air compressor will last for 70,000 to 80,000 hours of operation. A dry “oilless” screw is usually toast around 40,000 to 50,000 hours.
The efficiency of a dry screw is dismal, usually around 3 cfm per h.p. That is the reason they are usually two stage. Most have an intercooler. If you are considering one of these units, it is imperative that you witness the operation. It must operate for at least 4 hours to allow a complete prepurchase inspection. All components must be brought to operating temperatures. The intercooler and all associated piping or tubing must be thoroughly inspected for leaks and cracks.
The air ends (or elements as one manufacturer terms them) must be thoroughly inspected for cracks and (I know it sounds bizarre) oil leaks. A lubricated gear drive is utilized for timing of the air ends. The gear case uses oil. The oil must be sealed in the case. Seals sometimes leak. Therefore, an oil leak on an oilless compressor is not at all unusual and is a costly repair.
Determine Why The Unit Was Removed From Service
If the unit comes from a bankrupt company always remember, the first thing eliminated when cash flow becomes unstable is maintenance. All air compressors require a well-maintained service program. Correct oil change intervals assure oil does not lose lubricity. Filter changes eliminate dirt from the intake. Oil filters protect all-important contact surfaces as well as bearings and cooling. Ignore these things and the service life is shortened.
Normally, if a unit looks good it is good. If a unit has been detailed and painted be very careful. Paint can hide a host of issues. In this instance operating the unit for a prepurchase inspection is even more critical.
If you are considering the purchase of a used unit and the seller either cannot or will not run the machine for you, we have a test cell capable of operating up to a 75 (56 kw) horsepower complete with receiver. We can operate the unit for you and allow you to witness the amp readings, etc.
Due to Covid-19 procedures, we will need to instruct your witnessing personnel to assure they are protected.
We can provide a written record of our test findings for you as well. Call us at (866) 924-7822 or contact us here.