Centrifugal pump knowledge for the engineers

Centrifugal Pump

Centrifugal pumps are the most commonly used turbo machinery devices. They are used to raise the pressure or induce flow in a control volume. Centrifugal pumps are radial flow devices. Various kinds of centrifugal pumps are available in the market with different construction details. But working principle behind all of them remain same. In this video we will analyze, working principle of a centrifugal pump with single suction, semi open impeller.

Following article gives detailed description of the video lecture.

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Working of Centrifugal Pumps

One of such pump (single suction, semi-open) is shown in figure below, with one part of its casing removed for ease of understanding.

Fig.1 Single suction, semi open centrifugal pump with one portion of casing removed

Working of centrifugal pump is simple; as the impeller rotates it creates very low pressure at inlet of the impeller, called as eye of impeller. This low pressure helps in sucking fluid surrounding in. The fluid is pushed radially along the impeller to the casing. Casing collects the fluid , and it is pumped out through discharge nozzle.These processes are shown schematically in following figure. We will go through main components of a centrifugal pump in a detailed way.

Fig.2 Fluid flow in a centrifugal pump

Impeller

Impeller is the device which rotates, and transfer energy to fluid. It has got collection of vanes fitted to a hub plate. Shape and geometry of impeller blades are critical in pump performance.

Fig.3 Details of impeller

Casing

Casing collects fluid from impeller in an efficient way. The casing has got a special shape, with area of cross section increases from inlet to outlet. As the impeller ejects fluid throughout casing, along length of casing mass flow rate increases. But, increasing area of casing helps in maintaining almost same velocity. Thus volute shaped casing helps in converting dynamic part of fluid energy to static part.

Construction Details of Casing

Casing is made on 2 volute curves, which are at offset. A three dimensional volute is made from this curves. A portion is removed from volute shape, in order to accommodate the impeller in it. A discharge nozzle is fit at exit portion of the casing, most of the time discharge nozzle is diverging in shape. The steps followed are shown in following figure.

Fig.4 Construction details of volute casing

Use of Diffuser blades

For centrifugal pumps of small capacity as we discussed, impeller and casing are its main components. But for larger centrifugal pumps, there will be additional diffuser blades also present, in order to reduce velocity further. Or they aid in dynamic to static energy conversion.

Fig.5 Use of diffuser blades in large capacity centrifugal pumps

Energy Head Rise

Blade and fluid velocities at inlet and outlet are shown in the figure below.

Fig.6 Flow and Blade velocities at inlet and outlet of impeller

Here you can see fluid velocity increases from inlet to outlet due to energy addition to fluid. The work required for changing inlet velocity condition to outlet is given by following equation.

Details of such turbomachinery analysis will be discussed in a separate article. Here Q is the flow rate and Vthetarepresents tangential velocity component of flow.From here we can find what’s the head rise in meters of fluid. Please note that this is energy head rise. It comprises of both pressure head and velocity head.

For a centrifugal pump, inlet velocity will be parallel to radius. So tangential component of velocity at inlet is zero.

Outlet blade angle beta can be derived in terms of velocities.

Also flow rate through impeller is given as flow area times radial velocity.

So head rise in a centrifugal pump, can be derived in terms of flow rate.

Using this equation we can predict what’s the head rise, as we change the flow rate for particular pump geometry and for a particular impeller angular velocity. Most important parameter in this equation is, blade outlet angle, beta. There can be 3 different pump characteristics depending upon value of this angle.

Backward Curved Blades

First case, if beta is less than 90 degree. Since second term in LHS of head vs flow equation is positive in this case, pressure head decreases with increase in flow. These kinds of impellers are called backward curved.

Fig.7 Head vs Flow rate curve for a bacward curved blade impeller

Radial Blades

If beta is 90 degree, with flow rate there is no change in pressure rise. Because second term in LHS of head vs flow equation is zero here. They are called Radial type.

Fig.8 Head vs Flow rate curve for a radial blade impeller

Forward Curved Blades

If beta is more than 90 degree, pressure increases with increase in flow rate. Such blades are called forward curved blades.

Fig.9 Head vs Flow rate curve for a forward curved blade impeller

Most Suited Blade for Industrial Use

The big question is that out of these blade profiles, which one is the most suited for industrial use ?. To get answer for this question let’s see how power consumption varies with discharge for each of these cases. For backward curved blades as energy head decreases with discharge power consumption stabilizes with flow. In radial blades since head does not have any connection with flow rate, power consumption increases linearly. In forward curved blades since energy head increases with flow power consumption increases exponentially.This will make the operation unstable and will eventually lead to burnout of motor.

Fig.9 Power consumption in different blade geometries

So backward curved blades which has got self stabilizing characteristics in power consumption is the most preferred one in industry.