positive (or hydro-static) displacement pumps

POSITIVE (OR HYDRO-STATIC) DISPLACEMENT PUMPS

1. What are Positive Displacement Pumps?

• As the name implies, the functioning of a positive-displacement pump derives from changes of the volume occupied by the fluid within the pump.

• Since the operation of the pumps depend only on mechanical and hydrostatic principles, these pumps are also called as hydro-static displacement pumps.

• Positive displacement pumps are primarily used where pressure development is the prime requirement. This type of pumps is capable of delivering high pressure fluid, so it is universally used in fluid power systems.

• These pumps discharge a fixed quantity of fluid into the hydraulic system per revolution of pump shaft rotation. Since the flow of fluid is guaranteed on every revolution of the shaft, this type of pump is named as 'positive displacement pump'.

2. Types of Positive Displacement Pumps

• Positive displacement pumps can be further divided into :

1. Fixed displacement pumps, and

2. Variable displacement pumps.

1. Fixed displacement pumps : In fixed displacement pumps, the amount of fluid ejected per revolution (displacement) is constant and it cannot be varied.

2. Variable displacement pumps: In variable displacement pumps, the displacement can be varied by changing the physical relationships of various pump elements. This change in pump displacement produces a change in output of fluid flow even though pump speed remains constant.

• Examplęs : Some of the positive displacement pumps are gear, vane, piston, and screw pumps.

3. Advantages & Disadvantages of Positive Displacement Pumps

Table 4.2 presents the merits and demerits of employing the positive displacement pumps.

Table 4.2. Advantages and disadvantages of positive displacement pumps

Advantages

(i) Positive displacement pumps can operate at very high pressure (upto 700 bar).  

(ii) They are small and compact in size.

(iii) They have very high power-to-weight ratio.

(iv) They achieved high volumetric efficiency.

(v) The efficiency will not vary much for the wide range of pressure.

(vi) They have greater flexibility of performance (as they can operate over a wide range of pressure requirement and speed ranges).

Disadvantages

 (i) These pumps are more noisy when compared to non-positive displacement pumps.

(ii) The output/delivery is not continuous; it provides intermittent flow.

(iii) These pumps require over-pressure protection.

4. Applications

The positive displacement pumps are widely used in all fluid power applications.

5. Graphic Symbols for Pumps

The four different standard symbols used for hydraulic pumps are given below:

6. Comparison Between Non-positive Displacement and Positive displacement pumps.

Table 4.3 summarises the characteristics of non-positive displacement and positive

displacement pumps.

Table 4.3. Non-positive displacement pumps Vs. Positive displazçment pumps

As the study of positive displacement pumps are more important from our subject point of view, the construction and working of various positive displacement pumps are presented in the following sections.