Positive displacement pumps characteristics

Figure 21.18 Positive displacement pump characteristic curve...

The reciprocating and rotary positive displacement pumps primary characteristic is that they have a nearly direct relationship between the motion of the pumping... 

There are a variety of positive-displacement pumps commonly used in industrial applications. Each type has unique characteristics that must be understood and monitored. However, most of the major types have common parameters that should be monitored. 

For the most part, positive displacement pumps can be classified either as rotary pumps or as reciprocating pumps. However, pumps do exist which exhibit some of the characteristics of both types. 

Positive Displacement A characteristic of a pump or motor which has the inlet port sealed from the outlet so that fluid cannot recirculate within the system. 

A Throttling characteristics of centrifugal (c) -and positive- displacement (e, reciprocating d, rotary) pumps. B Control characteristics of positive displacement pumps a,b, Systems characteristics (b, after speed control) VFluid flow Ap, Differential pressure n, Speed h, Stroke length A, B, B, Operating points. 

The experiments were carried out in a fixed-bed tubular reactor, heated by an electric furnace divided into three heating zones. Prior to each experiment the catalyst was stripped with N2 at 482°C (reaction temperature) for 20 minutes. Then the reactant (which characteristics are given in Table II) was charged at the top of the reactor by a constant-rate positive-displacement pump. The catalyst to oil ratio with respect to the zeolite content was varied between 0.09 and 0.23 g.g 1 by changing gasoil feed (4.45-1.78 g) while keeping the weight of zeolite constant (0.40 g). 

The most commonly used vacuum pumps are steam-jet ejectors and several positive-displacement pumps, which are shown in Figures 5.1 and 5.2. Some of the characteristics of vacuum pumps are given in Table 5.1. A prime consideration when selecting a vacuum pump is the compatihility of a gas with a seal fluid. To avoid these problems, there is a trend toward using dry pumps where a seal fluid or lubricant is not used [60]. 

Positive-displacement pumps are self priming, which is the ability of a pump to lift liquids from a level below the center line of the pump. This characteristic of positive-displacement pumps is attributed to the tight seal between the discharge and suction sides of the pump. Thus, at startup air is compressed and... 

Figure 5.26 Characteristic curve for a rotary positive-displacement pump.

Positive-displacement fans, blowers, and eompressors are similar in action to positive-displacement pumps, and include reciprocating compressors, two- or three-lobe blowers, and screw compressors. However, with gases, the almost vertical characteristic curves, bend to the left more than for liquids because of the greater tendency for slip. 

If filtration takes place at constant pressure then the equations presented for the tube press in Section 6.2.1.3 are also valid for the multi-element candle filter noting that = n,ndh where n, is the total number of candles in the filter. When filtration takes place at variable pressure then it is necessary to impose the pump curve characteristics to relate Ap and q and a numerical solution to equation (6.17) is generally required. For the special case of variable pressure/constant rate filtration with a positive displacement pump... 

Several types of pumps are useful for handling slurries. The selection of a pump for a specific slurry transport line is based on the discharge pressure requirement and the particle characteristics (particle size and abrasivity). The pumps that are used are either positive displacement pumps or centrifugal (rotodynamic) pumps. 

Positive displacement pumps deliver a definite volume of liquid for each cycle of pump operation. Therefore, the only factor that affects flow rate in an ideal positive displacement is the speed at which it operates. The flow resistance of the system in which the pump is operating will not affect the flow rate through the pump. Figure 21.18 shows the characteristic curve for a positive displacement pump. 

The Gladstone pipeline uses Wilson-Snyder positive displacement pumps. At the weight concentration of 60-64%, the slurry acted as a Bingham mixture, with non-Newtonian viscosity characteristics. However, it did feature clay, sand, and iron, as the materials were formulated for the manufacture of clinker cement. The pipeline operation speed was maintained at 2 m/s and the pressure drop was around 300 kPa/km. API 5LX steel with a high yield strength was used. Corrosion rates as high as 0.25 mm/year were measured during the initial phase of operation of the pipeline. 

---