Q. What are the criteria for determining the maximum allowable forces and moments that can act on a centrifugal pump's nozzles without damaging the pump? What part of a pump is most likely to be damaged by excessive forces and moments?
A. Several parts of a pump must be investigated relative to nozzle loads. The weakest part varies according to the unique pump design. The following are the most likely parts that might be affected by extreme nozzle loads:
Driver/Pump Coupling Alignment
The allowable radial movement of the pump shaft at the pump coupling hub due to nozzle loads shall not exceed 0.125 mm (0.005 in) parallel, relative to the initial alignment. The axial movement of the pump shaft at the pump coupling hub is not a consideration.
Internal Pump Distortion
Contact between the rotating and stationary parts is not allowed (i.e., casing and impeller).
The maximum radial movement of the pump shaft with respect to the seal chamber due to the applied nozzle loads shall not exceed 0.025 mm (0.001 in) relative to initial shaft position.
Pump Hold-Down Bolts
The maximum allowable tensile stress allowed in the pump hold-down bolts is 90 percent of ASTM A 307 Grade A fastener material yield strength. The maximum allowable shear stress allowed in the pump hold-down bolts is 25 percent of ASTM A 307 Grade A fastener material yield strength.
Fasteners used for hold-down bolts must have a yield strength greater than or equal to ASTM A 307 Grade A fastener yield strength.
The pump shall be bolted to the baseplate at both the casing feet and rear foot position(s) and sufficiently tightened to prevent slippage of the pump on the baseplate. Refer to API 686, Appendix E, for required torque values (use 1/2-in nominal bolt diameter torque value for Group 1 and 2 pumps and 3/4-in nominal bolt diameter value for Group 3 pumps). It may be necessary to arrange for periodic tightening of the bolts to maintain required torque levels.
Pump Mounting
For maximum nozzle load capability, the base must be a fully grouted metal baseplate with anchor bolts. At a minimum, the base must be designed to withstand the applied nozzle loads combined with normal operating loads (i.e., driver weight and pump weight) and be installed and grouted in accordance with ANSI/HI 1.4, Rotodynamic (Centrifugal) Pumps for Installation, Operation and Maintenance.
Nozzle Stress
The maximum stresses developed in the pump nozzles and flanges by the applied nozzle loads combined with internal pressure will not exceed 26,250 psi tensile and 13,125 psi shear. This is in accordance with the allowable stress for ASTM A351 (A 744/743), Grade CF8M per ASME Boiler and Pressure Vessel Code.
The suction nozzle stress is calculated using 3-D stress analysis methods. The discharge nozzle stress is calculated based on the method contained in the ASME Boiler and Pressure Vessel Code, 1983 Edition, Section III, NC 3653, due to its complex geometry.
Allowable nozzle loads for many pumps can be found in the Hydraulic Institute Standard ANSI/HI 9.6.2 Centrifugal and Vertical Pumps for Allowable Nozzle Loads.
Q. What are the most commonly used stainless steel alloys for pumping sulfuric acid?
A. The answer depends on other factors as well as acid type. The liquid temperature and acid concentration are important. Assuming 10 to 65 percent sulfuric acid at ambient temperature, one the following can be used:
- A743-CN-7M 20-29 chromium nickel austenitic steel with copper and molybdenum
- A series of nickel-based alloys
- A 518 Corrosion-resistant high silicon cast iron
For more complete information see HI Standard ANSI/HI 9.1-9.5, Pumps-General Guidelines.
Q. What design features should we look for in selecting a pump for handling paper stock?
A. Paper stocks are 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 usually refers to a class of products with 1 to 7 percent fiber content by weight. These paper stocks are normally handled by end-suction centrifugal pumps equipped with semi-open impellers and contoured wearplates.
Medium-consistency stocks are made of 8 to 15 percent paper fiber. The rheological properties of fiber-water suspensions in this range are dependent on the properties of the individual fibers and the viscoelastic network that they form. Special designs of centrifugal pumps are required to handle this type of paper stock. For example, 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. This arrangement provides a self-cleaning effect whereby the impeller resists clogging to improve its reliability.
High-consistency paper stocks contain more than 15 percent paper fiber, and are found in the bleaching operation. Centrifugal pumps cannot handle such high consistencies; therefore, positive-displacement 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 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.
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 an 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. Cast iron, Types 316, 317 and 317L stainless steels, and Alloy 20 are commonly used in units handling paper stocks. These materials can also be used in various combinations to provide corrosion resistance as well as mechanical strength.
