PAPER STOCK PUMPS P060
Along with the solids, paper stock contains a lot of entrained air that presents a real challenge to centrifugal pumps
The stock is generally separated into three distinct consistency categories – low, medium and high. The design of a suitable pump is different for each type of stock.
- Low-consistency stock
- 1 to 7 percent fiber content by weight.
- Normally handled by end-suction centrifugal pumps equipped with semi-open impellers and contoured wearplates.
- Medium-consistency stocks
- 8 to 15 percent paper fiber.
- Special designs of centrifugal pumps are required to handle this type of paper stock.
- Some form of “shear generator” is needed at the inlet to create turbulence, and reduce the effective fluid viscosity.
- Special impeller design and air-extraction devices are also required to prevent air-binding.
- An end-suction centrifugal pumping unit must be specifically designed to handle medium-consistency stock mixtures without clogging the device, or dewatering the stock.
- A large suction-eye and unobstructed waterways can be provided by an overhung, semi-open impeller design. This keeps the suction velocity low to promote smooth flow, avoid air binding and prevent separation of stock fibers from water.
- The contoured front surfaces of the impeller vanes, interface with the replaceable wear plate.
- Provides a self-cleaning effect whereby the impeller resists clogging to improve its reliability.
- High-consistency paper stocks
- More than 15 percent paper fiber
- Commonly found in the bleaching operation.
- Centrifugal pumps cannot handle such high consistencies; therefore, positive-displacement (PD) rotary units are used.
- Proper suction-piping design has to be included to help this high solids mixture to enter the suction cavities of the PD rotary pump.
- Two-screw or clove-rotor types of rotary pumps are normally used for handling high-consistency paper stock.
- Once the stock has been introduced into the suction area, the positive displacement principle is employed to force the product through the pump, and out the discharge opening.
Some other points to consider
- When using centrifugal pumps, special rotor designs and clearances are often used to obtain the most-efficient pumping action.
- When extraneous material and air are entrained in paper stocks, there can be serious difficulty in handling these liquids.
- A thick-wall casing design is used to allow for ample corrosion, and withstand reasonable piping loads commonly encountered in handling hot stocks.
- Large shaft diameters and heavy-duty bearings, mounted in a rigid, one-piece bearing frame, improve the unit’s reliability in difficult services.
- The impeller-shaft assembly can be moved (via the adjustment feature located on the thrust-bearing housing) to maintain close clearances between the impeller and the wear plate, thus ensuring maximum operating efficiency.
- Some designs provide adjustability in the wear plate for the same purpose.
To resist corrosion from the process chemicals, different areas of the papermaking process require different materials of construction for the equipment. These materials can also be used in various combinations to provide corrosion resistance as well as mechanical strength.
- Cast iron
- Types 316, 317 and 317L stainless steels, and Alloy 20 are commonly used in units handling paper stocks.