Slurry Pump

Sealing

Slurry pumps are often subjected to severe shock loading and shaft whip due to the presence of solids and system upsets. For these reasons soft compression packing is still favored as a means of sealing at the stuffing box.

An alternative method for sealing is shown on Figure 13-5b. Here the lantern ring is positioned between packing rings. This configuration is called a “weep” seal. Again, clean liquid should be injected at a pressure higher than the prevailing slurry pressure near the stuffing box. Product dilution is significantly reduced compared to the “flush” seal design. However, the barrier so created is not very effective, causing abrasive particles to penetrate and cause wear. If the service is only mildly abrasive, then grease can be used in lieu of liquid.

It is impossible to predict the exact amount of flushing water required when the packing is “weep” type, since this is dependent on shaft deflection and gland maintenance. However, under normal operating conditions, weepage would be in the order of 5% of the values stated in Figure 13-6 for “flush” packing arrangement.

In most cases, seals and flush requirements are provided in ignorance of the real pressure prevailing at the stuffing box, which results in excessive use of gland water and increased maintenance.

3.1.3.3 Abrasive slurry supply unit

An abrasive slurry pump is used as one of the main components of the abrasive slurry circulation system to circulate abrasive slurry solution. The abrasive slurry solution consists of abrasive particles mixed with water, which also functions as cooling system for the tool and workpiece. Varying the control valve attached at the mill-module controls the flow rate of abrasive slurry. The power supply on abrasive slurry pump is controlled from main control unit. The performance of machining operations greatly depends on the slurry concentration, slurry flow rate and cooling system of the ultrasonic machine. Higher abrasive slurry concentration may increase higher material removal rate but lower level of concentration is preferable for better functioning of slurry pump.

The most commonly used abrasive is boron carbide (B₄C), which is one of the hardest materials that can be used as abrasives for USM operations. Abrasive particle is selected for USM based on hardness, size of it and also the type of workpiece materials. The other types of abrasives are aluminum oxide (Al₂O₃), silicon carbide (SiC), boron silicarbide and diamond. Water is used as common liquid media to make abrasive slurry. The other liquids such as oils, benzene and glycerol can also be used.

4.6.1.2 Pretreatment unit

15.7.2 Slurry Erosion Tests

There are a number of slurry erosion test machines discussed in the literature.^9,10 Many of these pump slurry at high velocity at the surface of a coated test plaque. A simpler test is described in ASTM G75-07 Standard Test Method for Determination of Slurry Abrasivity (Miller Number) and Slurry Abrasion Response of Materials (SAR Number). The relative effect of slurry abrasivity determined by measuring the mass loss of a block plastic elastomer after it has been driven in a reciprocating motion in a trough containing the slurry. A direct load is applied to the test plaque. The interior of the trough has a flat-bottomed or truncated “V” shape trough that forces the slurry particles to the reciprocating path taken by test specimen. The slurry may be of any material of interest such as sand in water or other liquid. The Miller number machine lifts slightly the test block and delays momentarily at the end of each stroke to allow time for fresh slurry material to flow back into the wear path. The test consists of measuring the mass loss of a part per unit time.

Fabrication and Processing Errors Can Prove Costly

With the impeller inverted so as to reduce the differential pressure across the shaft packing area, it is immediately shown that the primary thrust is from right to left. The two angular contact bearings on the extreme left are correctly oriented to take up the predominant load. However, their outer rings are completely unsupported because the fabricator had somehow decided to overbore the housing in the vicinity of these two bearings. Consequently, the entire radial load acting on the coupling end of the pump had to be absorbed by the remaining third angular contact thrust bearing. This bearing was thus overloaded to the point of rapid failure and was, of course, prone to rotate in the housing. Using a double row spherical roller bearing at the hydraulic end of the pump would normally make for a sturdy, well-designed pump. In this case, however, the spherical rotation or compliance feature tended to further increase the radial load transferred to the one remaining outboard bearing. The basic agent of the component failure mechanism was, of course, force.

An equally serious burden was imposed on this pump by the well-intentioned person who, in an effort to link the spare parts requirements of the North and South American plants of this major aluminum producer, added to the drawing the parts list partially reproduced in the lower left-hand corner of Figure 9-4. Having left off the appropriate alphanumeric coding behind the bearing identification number 7312, this plant and its sister facilities would receive thrust bearings in other than matched sets. A quick look at the bearing manufacturer's dimension tables (see insert, Figure 9-4) shows simple type 7312 bearings to have a width that may differ from the next bearing by as much as 0.010 inches. Mounting two such bearings in tandem may cause one to carry zero to 100% of the load, while the other one would simultaneously carry 100 to zero % of the load. On the other hand, matched sets intended for tandem mounting would be precision-ground for equal load sharing (50/50%) and would be furnished with code letter suffixes to indicate this design intent.

Did we go through our seven cause categories to identify the above root causes? Frankly, no. When both the fabrication sketch and the procurement documentation— “information processing”—show two very obvious errors, it is reasonable that rectification of these deviations should be a prerequisite to further fine-tuning. Which is just another way of saying that if it looks like a duck, walks like a duck, and quacks like a duck, we ought to call it a duck and dispense with further research into the ancestry of the bird.

3.8.1 Slurry shield construction method

3.4.1 Feed system

Issues for slurry feed and dry feed systems tend to be different. One main issue related to the slurry feed system is settlement of particulate matter in both storage tanks and suction pipes (especially upstream of the slurry pump during downtimes). This can be avoided by ensuring permanent motion of the slurry. Most slurry fed IGCCs employ slurry storage tanks, which can bridge unplanned downtime of the rod mills. Moreover, most plants (except Tampa) have two slurry pumps of 50–100% capacity each.

A similar decision is necessary for dry fed systems. The number and capacity of mills have a decisive influence on the availability of the fuel preparation unit. The 2×60% roller mills from Puertollano have been identified as being insufficiently robust (Peña, 2005) and to be important contributors to outages. The 3×50% mills used in Buggenum ensure increased availability of the fuel preparation unit. Dry fed IGCCs are of course concerned with common issues related to transport and storage of ground coal, e.g. bridging in sluicing devices or clogging of conveyors as experienced at Puertollano. Moreover, it is not trivial to maintain a stable fluidization and adequate pressure control in dense phase transport systems and to establish a vital and reasonable coal dust explosion prevention system.

An issue common to both feed systems is the proper blending of raw materials in order to guarantee adequate and predictable characteristics of the gasifier feedstock. This is of decisive importance, since adjustment of gasification conditions, slag removal and raw gas cooling parameters is based on predicted feedstock properties.

9.3.8 Fossil-fired flue gas and CO₂ scrubber pumps

Flue gas desulfurization scrubbers usually involve causing the flue gas to circulate through thin lime slurry in a contactor. The flow rates are large and the limestone slurry is abrasive. Depending upon customer preference, normal centrifugal or rubber-lined slurry pumps are commonly used.

CO₂ scrubbers are similar but may use one of a variety of different solvents. Ammonia and various recipes of amine are used. As above, pressures are relatively low and flow rates are high. CO₂ and water form carbonic acid so pump materials are typically 300 series SS as a minimum. The rich solvent returns from the contactor to a stripper where it is heated to drive off the gases and the solvent is recycled.

Compressors are then used to boost the CO₂ to supercritical pipeline pressures (>90 bar or 1300 psi). If the local seawater temperature is low enough, it may be more efficient to cool the CO₂ to a dense phase and pump it to the higher pressure needed for long pipelines and for injection.

CO₂ has a very low viscosity even at supercritical pressures so pipeline friction losses are relatively minimal. CO₂ has a surface tension less than 10% of that of propane, so careful attention is paid to welding and casting integrity.

The CO₂ can be injected into oilfields to enhance oil production. It can also be pumped into salt domes or hard rock for sequestration.

Understanding compressible supercritical fluid performance in pumps is important. Mechanical seal technology is also important.