non-positive (or hydro-dynamic) displacement pumps

NON-POSITIVE (OR HYDRO-DYNAMIC) DISPLACEMENT PUMPS

1. What are Non-Positive Displacement Pumps?

• In non-positive displacement pumps, the fluid flow is achieved using the inertia of the fluid in motion.

• These pumps are suitable for low-pressure (upto 40 bar), high-volume flow applications.

• Since these pumps cannot withstand high pressures, they are used only for transporting fluid from one place to another. For the same reason, they are not preferred in the fluid power industries.

• Examples: Some examples of such pumps are centrifugal (impeller) pumps and axial (propeller) pumps.

2. Construction and Working

• In these pumps, the clearance between the rotating and stationary elements is large and hence their output flow rate is low.

• The pump flow rate depends not only on the rotational speed but also on the resistance of the external system. As the external resistance increases, the flow rate decreases. Since the output flow varies significantly with the external circuit resistance, these pumps are not used in hydraulic systems.

• The arrangement of a typical centrifugal pump, working on the hydrodynamic principle, is presented in Fig.4.3.

• The fluid enters the pumps axially through the suction pipe via the eye of the impeller; it is discharged radially from the impeller around the entire circumference either into a ring of stationary diffuser vanes (and through them into the volute casing) or directly into the casing. The casing 'collects' the fluid, decelerates it—thus converting some of the kinetic energy into pressure energy-and finally discharges the fluid through the delivery flange.

3. Advantages and Disadvantages of Non-Positive Displacement Pumps 

Table 4.1 presents the merits and demerits of employing the non-positive displacement pumps.

Table 4.1. Advantages and disadvantages of non-positive displacement pumps

Advantages

(i) Non-positive displacement pumps have (i) fewer moving parts.

(ii) Their initial and maintenance costs are comparatively less.

(iii) They provide smooth continuous flow. 

(iv) They are suitable for handling almost any type of fluid (including sludges, slurries, etc.)

(v) They are highly reliable.

(vi) They are simple in operation.

Disadvantages

(i) These pumps are not self-priming and hence they must be positioned below the fluid level.

(ii) The output flow rate varies with the external circuit resistance.

(iii) Volumetric efficiency is very low.

4. Applications

• The non-positive displacement pumps are suitable only in fluid transport systems for transporting fluid from one location to another.