Wednesday, January 28, 2009

Compressor Washing

Special Maintenance Procedures for Equipment Protection & Cleaning

During operation, a fine gray powder-like deposit may collection on the internals of Platforming recycle gas compressors. This material (mostly ammonium chloride salts) is soluble in hot water. It is non-corrosive when dry, but when exposed to air it absorbs moisture readily and become corrosive not only to iron and carbon steel but also to all stainless chrome steels, especially if they have been hardened. For protection of the compressor internals, steps must be taken to avoid contact with air when this deposit is present. Several precautions will assist in this matter.

For centrifugal compressors, a thorough washing of the compressor internals is recommended before the machine is opened. Such washing is carried out with a 2% soda ash solution to which a small amount of non-sudsing detergent has been added. Higher carbonate concentrations are unsatisfactory because of the need to avoid damage to any aluminum labyrinths in the compressor. After several such washes have been carried out, the internals are flushed with clean water, dried and treated with a corrosion inhibitor. The following details a satisfactory procedure for carrying out this operation:

  1. Shutdown; nitrogen purge and maintain a nitrogen blanket on the compressor until ready to proceed with the following steps.
  2. Provide sufficient 2% soda ash solution with 0.1 - 0.3% non-sudsing detergent, such as Calgon, tripotassium phosphate, etc., for at least five washings, estimated at 150-300 liters (40-80 gallons) for each washing, at a temperature of 25-35 C (80-100 F). A pump for circulation of the washing solution should be available. A 3-5 m3/hr (10-20 gpm) pump is a convenient size.
  3. Break the suction and discharge piping and blind off the compressor. Make reversing connections to the inlet and outlet drains under the case so that the solution can be pumped through from either end.
  4. Pump the solution slowly into the suction drain connection with the seal oil circulating system operating as per the manufacturer’s instructions, and with the rotor turning over at approximately 60 -250 rpm run by an air motor, a small auxiliary motor, or by a rope wrapped around the coupling. In order not to take any chance of accelerating to a high speed with liquid in the case, do not use the main driver to turn the rotor over. Wash for approximately ten minutes each time, observing the color and pH of the solution.
  5. Drain and replace the wash solution with fresh solution; continue the washings until there is no change in the solution. Maintain the temperature of the solution with a steam hose, if necessary.
  6. Flush out with clean hot water at 65 C (150 F) for ten minutes; drain and repeat twice. Remove the drain plugs to the intermediate stages in order to drain all water. Note: If any discoloration of the drain water occurs, proceed with an additional soda ash treatment.
  7. If the compressor does not have individual stage drains, steam it until all of the case is up to 95 C (200 F), and blow with nitrogen or dry air to remove water from case.
  8. Provide about 200 liters (50 U.S. gallons) of reformate with four liters (one U.S. gallon) of Unicor thoroughly mixed. (For an alternate inhibitor, Gulf Agent 178 or Kontal 77 may be used.) With the stage drains closed, introduce Reformate through the suction drain with the rotor turning over slowly. Wash thoroughly for at least 30 minutes and drain.
  9. Remove all blinds and prepare for startup with nitrogen purges, or if an inspection is desired, proceed to dismantle the machine and remove the rotor in accordance with the manufacturer’s recommendations.

For non-lubricated reciprocating compressors, the procedure is somewhat different. First, the valves should be freed of salts as soon as they are removed from the machine. This is done easily by washing the valves in a bucket of hot water which will dissolve the corrosive powder.

The valves can be tested for leakage with water during this procedure. Do not allow prolonged soaking in water, since acidic compounds will buildup in the water which can also damage the parts. When the valves are removed from the hot water, they will dry very quickly and are then ready for reinstallation. If the valves are to be stored for some time, it is advisable to apply a coating of light oil to the valve faces to prevent possible rusting. This oil should be removed before the valve is reinstalled.

In order to inspect the piston and rings, it is necessary to remove the outboard head of the cylinder, remove the rod from the crosshead, and pull the piston out far enough to view the rings. The dust should be wiped from the internal surfaces with a lint-free cloth when possible.

If the piston is entirely removed, the exposed cylinder bore and valve seating surfaces should be covered with a light coat of oil to avoid contact with air and thus prevent corrosion of the honed and polished surface of the bore. All of this oil should be removed before the piston is reinstalled. The bore can be plugged with a pump cup or other similar plug to assist in isolating it from the atmosphere. A steam hose can be used to remove powder and scale from the cylinder gas passages but before doing this, the valve ports must be blocked to avoid getting steam or water on the highly finished cylinder bore surface. Extreme care must be taken if such cleaning is attempted.

If the rings are stuck, a small amount of additional clearance can be obtained by removing the rings and sanding the sides of the rings lightly on a flat surface. If carbon piston blocks are used and these are stuck together, washing with hot water may serve to free them and permit removal from the rod. When a compressor is assembled before the rest of the plant is ready for operation, it should be blanketed with gas to avoid contact with air. Close the block valves and pressure the compressor to about 0.3 kg/cm2 (5psig) with nitrogen after purging out all air in the system.

Platforming reciprocating compressors are normally installed in outdoor locations. Therefore, the proper weight and quality of lubricating oil in the crankcase must be used during the various seasons of the year and oil should be changed with the seasons, particularly in cold climates. Use the manufacturer’s recommended type of oil for the anticipated temperature.

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