Centrifuge Feeding System

The three types of feed systems for oil cleaning are gravity, pumping, and pressurized tank. Using low pressure in a pressure vessel is probably the easiest and may be seen in a video here:

http://www.youtube.com/watch?v=zFNnoQyN7Zs

Low pressure, about 20 psi is used to force the oil into the centrifuge.  A diaphragm type regulator is required. Orifice type regulators are not suited to constant pressure in a vessel and will over-pressurize the tank over time.  Pressures lower than 20 psi are difficult to regulate using available inexpensive regulators. The pressurized tank must be an actual pressure vessel such as a propane tank or compressor tank. Plastic tanks and 55 gallon drums are not considered to be pressure vessels and may explode if pressurized. A proper vessel will have an ASME tag stating maximum operating temperature and pressure for the tank.  Any cutting or welding on the tank weakens the tank from the nameplate specification.

Diaphragm Pump

Pumping is ideal if the proper pump is selected. A small pneumatic diaphragm pump is perfect for centrifuge feeding. The diaphragm pump flow is fully adjustable and does not require a bypass when the flow is restricted. It has no motor to burn out and it does not form emulsions between the oil and contaminants. Of all the feed methods, a diaphragm pump is the best because it is easy, adjustable and safe. Because absolute vacuum is limited to 14.7 psi, set the pump close to the feed tank and use the pressure side to force the oil through the hose. A 3/8 inch diameter hose is good for pump connections.

Gear pumps are the least desirable feeding method. Gear pumps cannot be turned down without a bypass, or a frequency converter. Lower than nameplate speed causes the motor to run hot and gear pumps are excellent at blending contaminants the oil as well as forming emulsions. Gravity Feed may be used; however, flow may vary considerably as the level in the feed tank changes. The temperature of the oil, height of the feed tank and internal diameter of the input hose all affect the oil flow.

Liquids flowing through a pipe experience frictional losses along the walls of the pipe. Oil flowing through a pipe has a nearly stagnant layer at the internal surface of the pipe that extends inward into the pipe. The thickness of the stagnant layer depends upon the viscosity of the oil. A small diameter pipe may have virtually no effective internal diameter if the oil is thick. Therefore, a large diameter feed hose is essential for gravity feed systems. A one inch internal diameter hose would not be too large for a gravity feed system, 5/8 inch minimum internal hose diameter may be used with fluid oil and short runs. Thinning the oil by heating will help to some degree provided the oil does not significantly cool in the hose between the tank and centrifuge.

Flow rate is affected by height of the feed tank. Oil pressure is about .38 psi for each foot of height. The pressure at the bottom of a full 55 gallon drum is about 1.08 psi. The last 10 gallons in the tank will exert a pressure of about .18 psi. At the 10 gallon mark, flow rate will be about 1/6 of the flow rate when the drum was full. Turning the tank horizontally improves the situation and lifting a horizontal tank several feet would reduce the variation in feed pressure. A horizontal drum raised 5 feet would see a 26% variation in flow rate and at 8 feet high would see an 18% variation in flow, which I would consider acceptable for general oil cleaning. Because of the variation in feed rate, gravity feed systems produce less than pump systems in a given time.