Fluid-Displacement Pumps

Fluid-Displacement Pumps In addition to pumps that depend on the mechanical ac tion of pistons, plungers, or impellers to move the liquid, other devices for this purpose employ displacement by a secondary fluid. This group includes air lifts and acid eggs. 

The ease with which the separated products leave the bowl determines the richness of the fat. Fluid whole milk enters the separator under pressure from a positive displacement pump or centrifugal pump with flow control (Fig. 1). The fat (cream) is separated and moves toward the center of the bowl, while the skimmed milk passes to the outer space. There are two spouts or oudets, one for cream and one for skimmed milk. Cream leaves the center of the bowl with the percentage of fat ( 30 40%) controlled by the adjustment of a valve, called a cream or skim milk screw, that controls the flow of the product leaving the field of centrifugal force and thus affects the separation. 

Positive-displacement pumps may be of either the reciprocating or the rotary type. In all positive-displacement pumps, a cavity or cavities are alternately filled and emptied of the pumped fluid by the action of the pump. 

Volumetric efficiency, ratio of actual pump capacity to volume displaced per unit of time Pump efficiency with water, percent Pump efficiency with viscous fluid, percent Pump efficiency, fraction Pump efficiency with water, fraction Pump efficiency with viscous fluid, fraction Volumetric efficiency, fraction Maximum safe flowng efficiency, overall pump, fraction... 

This is the energy source for hydraulic systems. It converts electrical energy into dynamic, hydraulic pressure. In almost all cases, hydraulic systems utilize positive displacement pumps as their primary power source. These are broken down into two primary sub-classifications constant-volume or variable-volume. In the former, the pumps are designed to deliver a fixed output (i.e. both volume and pressure) of hydraulic fluid. In the later, the pump delivers only the volume or pressure required for specific functions of the system or its components. 

Since changes in pump speed affect volumetric output, some pumps are rated by their displacement. Pump displacement is the amount of fluid the pump can deliver per cycle or complete rotation. Since most pumps use a rotary drive, displacement is usually expressed in terms of cubic inches per revolution. 

In most fluid power systems, the motor is required to provide actuating power in either direction. In these applications, the ports are referred to as working ports, alternating as inlet and outlet ports. Either a four-way directional control valve or a variable-displacement pump usually controls the flow to the motor. 

Obviously the NPSH must be positive, or the liquid would be vaporized and the pump would be filled with gas. Since a pump is designed to transport liquids, if this happened it would just spin in its housing and no transfer would be accomplished. There is an increase in velocity as the liquid enters most pumps. This conversion of pressure energy to kinetic energy may reduce the pressure enough to cause fluids that have a positive NPSH to vaporize. Therefore, each pump has some minimum NPSH below which it will not operate properly. For most pumps an NPSH of 14 ft (4.2 m) of fluid is adequate. Some positive displacement pumps can operate at an NPSH of 6 ft (2 m). Use equation 5 to calculate the NPSH for each pump to be specified. 

The core - flood apparatus is illustrated in Figure 1. The system consists of two positive displacement pumps with their respective metering controls which are connected through 1/8 inch stainless steel tubing to a cross joint and subsequently to the inlet end of a coreholder 35 cm. long and 4 cm. in diameter. Online filters of 7 im size were used to filter the polymer and brine solutions. A bypass line was used to inject a slug of surfactant solution. Two Validyne pressure transducers with appropriate capacity diaphragms are connected to the system. One of these measured differential pressure between the two pressure taps located about one centimeter from either end of the coreholder, and the other recorded the total pressure drop across the core and was directly connected to the inlet line. A two - channel linear strip chart recorder provided a continuous trace of the pressures. An automatic fraction collector was used to collect the effluent fluids. 

Apparatus. A constant rate displacement pump charged with mercury was used to displace the fluid of interest from steel cylinders to the core. A pressure transducer connected to the chart recorder provided the pressure history of each core flood. An automatic sampler with... 

In general positive displacement pumps have limited flow capacity but are capable of relatively high pressures. Thus these pumps operate at essentially constant flow rate, with variable head. They are appropriate for high pressure requirements, very viscous fluids, and applications that require a precisely controlled or metered flow rate. 

Fig. 2.8. Example of a flush model. Fluid is pumped into a petroleum reservoir as a stimulant, or industrial waste is pumped into a disposal well. Unreacted fluid enters the formation, displacing the fluid already there.

PSV-2 of Figure 8-6 is a relief to protect the positive displacement pump P-1. If the fluid being handled is extremely volatile and flammable, what design modifications would you make to this relief system ... 

Successful and reproducible separations require a steady buffer flow rate and this is achieved with either a constant pressure or a constant displacement pump. These pumps are designed to deliver a constant rate of fluid independent of the resistance to the flow and recent developments in pump design permit the production of a precise and pulseless flow this has contributed towards the increased analytical precision and sensitivity that can now be achieved with amino acid analysers. The choice of flow rate is dependent upon the type of resin, the dimensions of the column and overall design of the instrument and this varies between models. 

A propeller pump is another type of positive displacement pump. It operates similar to a fan blade inside of a pipe or tube. As fluid moves past the blade, the energy of the moving blade delivers fluid through the outlet at a higher pressure than delivered. 

Diaphragm pumps are used for metering small amounts of additive into a fuel or fuel oil. The cost of these pumps is low compared to other positive displacement pumps. These pumps are excellent metering pumps and are primarily designed for low-pressure, low-flow applications. Also, they are not recommended for pumping high-viscosity fluids. 

The binder solution is delivered to the nozzle port through a spray lance and tubing (Fig. 7b). The peristaltic, or positive displacement, pump is conunonly used to pump the binder solution. The pneumatically controlled nozzle needle prevents the binder liquid from dripping when fluid flow is stopped. Nozzle port openings 0.8 and 2.8 mm in diameter are most common and are interchangeable. 

For reciprocating positive pumps and compressors the fluid displacement per stroke depends on the geometry, the fluid compressibility, the polytropic coefficient, and the internal leakages and volumetric losses, but basically not on the speed of machine. 

---