Pump Alignment & Pump Piping Related Questions and Answer

Pump Alignment & Pump Piping:

  1. What are different types of pumps?
    Basically there are two types of pumps.
    A. Centrifugal pump.
    B. Positive displacement pump.
  2. What are the different types of centrifugal pump?

Different types of centrifugal pump are: –
A. Single stage or
B. Multi-stage

  1. What is the basic difference between single stage and multi-stage centrifugal pump?

The Single stage pump has one impeller and multi-stage pump has two or more impellers in series. The discharge of one impeller is the suction of the next one and the head developed in all the stages are totaled.

  1. How many types of centrifugal pump are available based on the suction and discharge arrangement?

Based on the suction and discharge arrangement, the type of centrifugal pumps available is: –
A. End suction top discharge.
B. Top suction top discharge.
C. Side suction side discharge.

  1. What are the different types of casing?

Casings are generally of two types: volute and circular. The impellers are fitted inside the casings.

  1. Define the working mechanism of centrifugal pump?

A centrifugal pump is one the simplest pieces of equipment in any process plant. Its purpose is to convert energy of prime mover (an electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is rotating part that converts drivers energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

  1. How the kinetic energy created by centrifugal force is converted to pressure energy?

The energy created by centrifugal force is kinetic energy. The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid. This kinetic energy of the liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle. Therefore, the head (pressure in terms of height of the liquid) developed is approximately equal to the velocity energy at the periphery of the impeller expressed by the following formula as: –
H = v2/ 2g
H = Total head developed in feet.
V = Velocity at periphery of impeller in ft/sec.
G = Acceleration due to gravity-32.2ft/sec2
Formula for calculating peripheral velocity:
V = NX D
V = Peripheral velocity in impeller in ft/sec.
N = The impeller rpm (revolution per minute)
D = Impeller diameter in inches.
One fact that must always be remembered: A pump does not create pressure, it only provides flow. Pressure is a just an indication of the amount of resistance to flow.

  1. What do you mean by cavitation in pump?

A pump is designed to handle liquid, not vapor. The satisfactory operation of pump requires that vaporization of the liquid does not occur at any condition of operation. This is so desired because when a liquid vaporizes its volume increases very much. For example, 1 ft3 of water at room temperature becomes 1700 ft3 of vapor at the same temperature. The vaporization begins when vapor pressure of the liquid at the operating temperature equals the external system pressure, which in an open system is always equal to atmospheric pressure. Any decrease in external pressure or rise in operating temperature can induce vaporization.

The vapor pressure occurs right at the impeller inlet where a sharp pressure drop occurs. The impeller rapidly builds up the pressure, which collapses vapors bubbles causing cavitation and damage the pump internals. This is avoided by maintaining sufficient NPSH. (Cavitation implies cavities or holes in the fluid we are pumping. These holes can also be described as bubbles, so cavitation is really about the formation of bubbles and their collapse. Bubbles form whenever liquid boils. It can be avoided by providing sufficient NPSH.)

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