Essay: Types of Casing in Centrifugal Pump

Types of Casing in Centrifugal Pump:
1) Volute casing.
2) Vortex casing.
3) Casing with Guide Blades.
Priming in centrifugal pump:
It is the first operation in which the suction pipe casing of the pump and portion of the delivery pipe up to delivery valve are completely filled with liquid so that all the air or gas or vapour from the portion of the pump is driven out and no air pocket is left.
Working principle and parts of the centrifugal pump:
The following are the components of centrifugal pump. They are shown in figure.
(a) Impeller
The rotating wheel of a centrifugal pump which is connected with motor shaft is known as impeller. It consists of a series of backward curved vanes.
Components of Centrifugal Pump
In closed impeller blades or vanes are placed in between two circular disc. In open impeller no disc will be there. But blades are fitted on the periphery of shaft. In semi open impeller only one disc will be there. On eye side no disc will be there.
(b) Casing
It is an air tight chamber which surrounds the impeller. It has provisions to support bearing. It is designed in such a way that the kinetic energy of the water discharged at the outlet of the impeller is converted into pressure energy before the water leaves these casing and enters the delivery pipe. The following are the types of casing.
(i) Volute champer type (ii) Vortex chamber type (iii) Diffuser pump/casing with guide blades.
(i) Volute chamber type:
This type of casing is of spiral form and has a sectional area which increases uniformly from the tongue to the delivery pipe as in figure.Velocity of whirl remains constant throughout the volute chamber of all cross sections.
(ii) Vertex chamber pump:
In a vortex chamber shown in figure a uniformly increasing area is provided between the impeller outer periphery and the volute casing. This increase in area leads to increase in pressure and decrease in velocity in the outward direction.
Types of Casing
(iii) Diffuser pump/Casing Guide blades:
The guide vane are arranged at the outlet of the impeller vanes as shown in figure. Water enters the guide vanes without shock. As the guide vanes are of enlarging cross section area the velocity of water decreases and pressure increases. Since these vanes provide better guidance to flow eddy losses are reduced which increases the efficiency.
(c) Suction pipe with a foot value and a strainer
A pipe whose one end is connected to the inlet of the pump and other end submerged into water in a sump is know as suction pipe. A foot value which is a non-return valve is fitted at the lower end of the suction pipe. The foot value opens only in the upward direction. A strainer is also fitted at the lower end the suction pipe to remove strains.
(d) Delivery pipe
A pipe whose one end is connected to the outlet of the pump and the other end delivers the water at a required height is known as delivery pipe.
Working of Centrifugal pump:
Priming is the first operation in which the suction pipe, casing of the pump and the portion of the delivery pipe upto delivery valve are completely filled with liquid which is to be pumped so that all the air or gas or vapor from this portion of the pump is driven out and no air pocket is left.
The delivery valve being in closed position, the electric motor is started to rotate the impeller. When the delivery valve is opened the liquid is made to flow in an outward radial direction thereby having the vanes of the impeller, at the outer circumference with high velocity and pressure and thereby at the eye of the impeller vacuum is formed which causes for the water to enter the casing or impeller from the sump. Suitable casing is provided in order to convert velocity energy into pressure energy with gradual reduction of velocity so that loss of energy due. to eddy formation is minimum.
Let us calculate the vane angle at the inlet of a centrifugal pump impeller having 200 mm diameter at inlet and 400 mm diameter at outlet. The impeller vanes are set back at angle of 45° to the outer rim and the entry of the pump is radial. The pump runs at 1,000 rpm and the velocity of flow through the impeller is constant at 3 m/s. Also calculate the work done per kN of water and the velocity as well as direction of the water at outlet.
Given data:
D1 = 0.2, D2 = 0.4 m
φ = 45°
Entry of the pump is radical, α = 90°.
N = 1000 rpm
Velocity of flow through the impeller is constant.
Vf1 = Vf2 = 3 m/s.
For inlet velocity triangle calculating vane angle at inlet θ.
Calculating workdone per kN of water
Calculating Vw2 from outlet velocity triangle
Workdone per kN of water = 38.29 m
Calculating velocity and direction of water at outlet from outlet velocity triangle,
The impeller of a centrifugal pump having external and internal diameters 500 mm and 250 mm respectively with an outlet 50 mm and running at 1200 r.p.m. works against a head of 48 m. The velocity of flow through the impeller is constant and equal to 3.0 m/s. The vanes are set back at an angle of 45° at outlet. Let us find Inlet Vane angle and Work-done by the impeller and Manometeric efficiency.
Given:
Z2 = 500 mm = 0.5 m.
D1 = 250 mm = 0.25 m.
B = 50 mm = 0.05 m.
N = 1200 rpm.
H = 48 m.
V1 = Vf1 = Vf2 – 3m/s
= 40°
= 10° 49
= 31.42 m/s
= 6°9
in here…