Molasses Pumps for High-Viscosity Liquids

Molasses pumping installations are divided roughly into two groups, those for transferring the lighter grades for human consumption and those for pumping the heavy blackstrap varieties mostly used in animal feed blending.  For the former group, the specifications virtually always require sanitary preparation and food packing.  Virtually all Tri-Rotor molasses pumps are of iron fitted construction and have relieved rotor groups; many are steam jacketed.  The most difficult installations are those involving blackstrap molasses because the pumping units are nearly always located outdoors or in unheated buildings and during the cold weather the already high viscosity of the product is increased even more.

The following is a summary of the important points to remember when handling molasses with a Tri-Rotor Pump:

• The capacity of the pump (and corresponding RPM) should be synchronized to the approximate viscosity of the molasses.  Generally speaking, the pump should be driven at a low speed when pumping highly viscous materials.  To determine this speed refer to the “viscosity gallonage curve.”  Our Model 200A pump is best suited for handling viscous materials because it displaces a comparatively large volume of liquid for each revolution of the pumping members.

• The rotor group should be relieved to permit lubrication and easy movement of the pumping members.  Relieving the rotor group entails machining each member a few thousandths of an inch smaller than standard, thus increasing the clearance between these members and allowing the molasses to get in between each member and so lubricate its movement.  The clearances between standard pumping members (not relieved) are not sufficient to permit adequate lubrication when handling extremely viscous liquids.

• A steam jacketed housing and/or head is recommended to maintain the constant pumping temperature of the molasses as it flows through the pump.  Maintaining a relatively high pumping temperature throughout the system permits the molasses to be pumped at reduced viscosity thereby sustaining a normal operating head pressure.  A steam jacket consists of an enclosed chamber, incorporated as an integral part of either the head or shaft housing (or both), through which steam may pass and from which, heat is transferred by conduction through the metal structure of the pump.  There is no direct contact between the pumpage and the steam.

• A steel piston should be provided to increase the strength of the pumping members against the high torques that are prevalent when the pump is in operation.

• The pump should be provided with a solid head to minimize the congealing action of the molasses within the pump.  There is no bypass arrangement incorporated on the solid head and therefore, as a safety precaution, it is necessary to place an external relief valve assembly between the discharge line and the source of the supply with the valve spring set to bypass the full volume of the molasses at a pressure approximately 10 lbs. in excess of the head pressure that is desired to operate the system.

• Sanitary PTFE food packing should be provided and the interior of the pump should have sanitary preparation done at the factory to avoid contamination of the molasses.  The food packing is located in the shaft housing and it is different from regular packing because it is food grade PTFE.  Its purpose is to prevent the molasses from leaking out of the pump through the shaft housing.

• Suction and discharge lines should be approximately two sizes larger than the pump ports and all unnecessary elbows and valves should be eliminated.  Obstructions in the lines prevent the easy flow of the molasses to and from the pump.  When possible heat should be applied to the lines to maintain the pumping temperature of the molasses.

• An oversized motor should be used to overcome high starting torques and possible heat failure in the system.  Heat failure causes excessive head pressures due to the increased viscosity of the pumpage which in turn requires greater horsepower to overcome the same.  The motor should be one or two sizes larger than necessary for normal operation.

• The supply tank should be located above and as close to the pump as possible, thus providing a flooded suction.  The successful operation of the pump depends upon the direct and continual flow of the pumpage into the suction port.

Let’s discuss which is the ideal Tri-Rotor pump for your molasses application.