Discharge Flow Patterns

Mechanical efficiencies of steam pumps vary with the types of pump, stroke and the pressure differential. Some representative values are 55 to 80 percent for piston pumps with strokes of 3 inches and 24 inches respectively, and pressure differential up to 300 psi. For the same strokes a plunger design varies from 50 to 78 percent, and at over 300 psi differential the efficiencies are 41 to 67 percent [9], Steam required is approximately 120 lbs/hour per BHP. 

Variation Above Mean, 1.8% Variation Below Mean, 5.2% 

---

Figure 3-68. Reciprocating pump discharge flow patterns. (Courtesy the Aldrich Pump Co.)...

Significant Features in Reciprocating Pump Arrangements, 215 Performance, 217 Discharge Flow Patterns, 218 Horsepower, 218 Pump Selection, 221. 

In most investigations the discharge flow from a Rushton turbine in a baffled tank is considered, but also the discharge flow from pitched blade turbines, propellers and open flat blade turbines have been studied. The discharge flow patterns for three different impellers are illustrated in Fig 7.7. 

Radial flow impellers may either have a disk (Rushton turbine) or be open (FBT) and may have either flat or curved blades (backswept mrbine). Impellers without the disk do not normally pump in a true radial direction since there is pressure difference between each side of the impeller. This is also true when the impellers are positioned in the tank at different off-bottom clearances. They can pump upward or downward while discharging radially. Radial discharge flow patterns can cause stratification or compartmentalization in the mixing tank. Disk-type radial impellers provide more uniform radial flow pattern and draw more power than open impellers. The disk is a baffle on the impeller, which prevents gas from rising along the mixer shaft. In addition, it allows the addition of a large number of impeller blades. Such blade addition cannot be done easily on a hub. A disk can also be used with a pitched blade turbine for use in gas-liquid mixing. 

Because mass flow bins have stable flow patterns that mimic the shape of the bin, permeabihty values can be used to calculate critical, steady-state discharge rates from mass flow hoppers. Permeabihty values can also be used to calculate the time required for fine powders to settle in bins and silos. In general, permeabihty is affected by particle size and shape, ie, permeabihty decreases as particle size decreases and the better the fit between individual particles, the lower the permeabihty moisture content, ie, as moisture content increases, many materials tend to agglomerate which increases permeabihty and temperature, ie, because the permeabihty factor, K, is inversely proportional to the viscosity of the air or gas in the void spaces, heating causes the gas to become more viscous, making the sohd less permeable. 

Most flow problems can be overcome by using a mass flow design if the mass flow pattern developed by the bin is not disturbed. Thus a properly designed feeder or discharger must be employed. A feeder is used whenever there is a requirement to transfer soflds at a controlled rate from the bin to a process or a tmck. A discharger is used when there is a need to discharge soflds, not control the rate of discharge. 

To be consistent with a mass flow pattern in the bin above it, a feeder must be designed to maintain uniform flow across the entire cross-sectional area of the hopper outlet. In addition, the loads appHed to a feeder by the bulk soHd must be minimised. Accuracy and control over discharge rate ate critical as well. Knowledge of the bulk soHd s flow properties is essential. 

The key to solving these problems is to design the vessel for a mass flow pattern. This involves consideration of both the hopper angle and surface finish, the effect of inserts used to introduce gas and control the soHds flow pattern, and sizing the outlet valve to avoid arching and discharge rate limitations. In addition, the gas or Hquid must be injected such that the soHd particles ate uniformly exposed to it, and flow instabiHties such as fluidization in localized regions are avoided. 

Numerous studies for the discharge coefficient have been pubHshed to account for the effect of Hquid properties (12), operating conditions (13), atomizer geometry (14), vortex flow pattern (15), and conservation of axial momentum (16). From one analysis (17), the foUowiag empirical equation appears to correlate weU with the actual data obtained for swid atomizers over a wide range of parameters, where the discharge coefficient is defined as — QKA (2g/ P/) typical values of range between 0.3 and 0.5. 

Jet Mixers Continuous recycle of the contents of a tank through an external pump so arranged that the pump discharge stream appropriately reenters the vessel can result in a flow pattern in the tank which will produce a slow mixing aciion [Fossett, Trans. Jnst. Chem. Eng., 29,322 (1951)]. 

When the flow pattern in a mixed tank is primarily tangential, the fluid discharge from the impeller to the surroundings and its entrainment into the impeller are small. Also, fluid transfer in the vertical direction is at a minimum. The mixing effect is lowest when the rotational velocity of the liquid approaches that of the mixer. 

There is a potential for unstable flow through pumps, which is created by both the design-flow pattern and the radial deflection caused by back-pressure in the discharge piping. Pumps tend to operate at their second-mode shape or deflection pattern. This mode of operation generates a unique vibration frequency at the second harmonic (2x) of running speed. In extreme cases, the shaft may be deflected further and operate in its third (3x) mode shape. Therefore, both of these frequencies should be monitored. 

Horizontal split-case The flow pattern through a horizontal split-case pump is radically different than that through an end-suction pump. Inlet and discharge flow are in the same plane and almost directly opposite one another. This configuration, illustrated in Figure 44.22,... 

Total head, centrifugal pumps, 180, 183 Discharge, 205 Head curve, 198 Suction head, 184, 186 Suction lift, 184, 186 Type, 184 Tubing, 63, 64 Two-phase flow, 124 Calculations, 125-127 Flow patterns, chart, 124 System pressure drop, 125 Types of flow, 124, 125 Utilities check list, process design, 34 Vacuum,... 

In vertical flow, axial symmetry exists and flow patterns tend to be somewhat more stable. However, with slug flow in particular, oscillations in the flow can occur as a result of sudden changes in pressure as liquid slugs are discharged from the end of the pipe. 

---