The value S is an assessment of a pump's inlet design, including both the stationary casing and the rotating impeller design elements. Higher numerical values of S are associated with better NPSH capabilities. For pumps of typical suction inlet design, values range from approximately 120 to 250 (6,000 to 13,000). In special designs, including inducers, values up to 700 (35,000) or higher are possible depending on the connected inlet piping, the pump's suction casing arrangement, the range of flow over which the pump must operate, size and power rating of the machine and other considerations.
Q. What care should be taken when designing or selecting reciprocating power pumps for slurry service?
A. The basic construction may or may not be the same as it is for clear liquid applications. The differences may be in the type of valves, addition of surge chambers or liquid injection into the lower portion of the stuffing box. Reciprocating slurry pumps are designed so that the liquid end parts that are subject to the deteriorating effects of slurries can be easily and quickly replaced without dismantling any other major pump component. These parts are usually replaced in accordance with a preventive maintenance program. The scheduled replacement time is based on the user's experience with the slurry pumped. Replacement timing should be such that the part still performs adequately and does not wear to the point of causing the failure of other parts of the machine.
Hydraulic passages should be sized so that the lowest velocity of the liquid will be above the critical carrying velocity of 1.2 to 1.8 meters per second (4 to 6 feet per second) on average. The highest velocity should be below that which causes excessive erosion. Typical average operating velocities through a reciprocating slurry pump's passages are 1.8 to 3.6 meters per second (6 to 12 feet per second). Lubrication and flushing of the packing are extremely important. Metered, clear, external injection, which is timed to the position of the plunger during its stroke, or continuous flow injection is employed to achieve this. The mode of flushing will depend on whether dilution of the liquid pumped by the flushing liquid can be tolerated.
To protect the main stuffing box packing, clear liquid is usually injected into the stuffing box between the bottom of the throat bushing and the packing. The injection lines are selected to withstand full working pressure and have a safety check valve located between the stuffing box and the injection liquid source to prevent accidental backflow of slurry into the clear liquid system.
Valves for use in slurry service are designed for velocities between 1.8 and 3.6 meters per second (6 and 12 feet per second) to reduce erosion and abrasion of the valve seat and other valve components. Valve construction usually has replaceable valve inserts that are made of an elastomer or polymer. Metal-to-metal ball valves may be used depending on the slurry, material, carrier liquid and temperature.
Special considerations must be given to the slurry abrasion, attrition, particle size and concentration. Slurry particle size has an influence on the valve lift and the ability of an elastomer valve to seat. Experience to date shows that slurry concentrations of up to 65 percent by weight can be handled successfully.
Suction pressure on a pump handling slurry is usually higher than that on one handling clear liquid. This takes into account the acceleration head of the solids and gases entrained within the liquid. Likewise, pulsation dampeners are usually larger on the suction and discharge side of the pump for the same reasons. To facilitate starting and stopping a slurry pump, it should be fitted with adequate connections so that the liquid end passages can be flushed of the slurry with clear liquid. This is especially true when the pump will be shut down for extended periods.
Rod and plunger packing require special considerations when dealing with abrasive materials. In a piston pump, the piston runs in a renewable metal cylinder or liner. The liners are made of abrasion and corrosion resistant metals to resist wear for each specific slurry. Piston rods and plungers are also coated to resist wear. If the abrasion of the slurry is not great and the pressures are below 14,000 kPa (2,000 psi), large-volume piston pumps are more suitable. The transportation of coal slurry falls into this class of service. Please see ANSI/HI 6.1-6.5 Reciprocating Power Pumps for Nomenclature, Definitions, Application, and Operation as a reference.
Pumps & Systems, July 2011