Fluid Machines – Pumps and Turbines


What is a fluid Machine ?
A fluid machine is a device which converts the energy stored by a fluid in to mechanical energy or work.


What are positive displacement machines ?
The machine functioning depend essentially on the change of volume of a certain amount of fluid within the machine. There is a physical displacement of the boundary of certain fluid hence it is called positive displacement machine. 

Based on the direction of fluid flow how are the fluid machines classified ?
According to the flow direction of fluid, the fluid machines are classified as
Axial flow machines – The main flow direction is parallel to the axis of the machine.
Radial flow machines – The main flow direction is perpendicular to the axis of the machine ie. in radial direction
Mixed flow machines – The fluid enters radially and leaves axially or vice-versa. 

What is a turbine ?
A turbine converts the energy of the fluid in to mechanical energy which is then utilized in running a generator of a power plant. 

State the difference between impulse and reaction turbines ?
At the inlet of the turbine the energy available is only kinetic energy, the turbine is known as impulse turbine. If the inlet of the turbine, possesses kinetic energy and pressure energy, the turbine known as reaction turbine. 

What are Homologous units ?
In utilizing scale models in designing turbo machines, geometric simililitude is necessary as well as geometrically similar velocity diagrams at entrance and exit from impellers. Two geometrically similar units having similar vector diagrams are called homologous units. 

Define specific speed based on power.
The specific speed (Ns ) of turbine is defined as the speed of some unit of series of such size that it produces unit power with unit head. . 

Define specific speed based on discharge.
The specific speed ( Ns ) of the pump is defined as the speed of some unit of the series of such size that it delivers unit discharge at unit speed 

What is the significance of specific speed ?
The specific speed inversely proportional to the head across the machine. So low specific speed corresponds to high head across it and vice-versa.
The specific speed is directly proportional to the discharge through the machine or power produced by the machine. So low specific speed therefore refers to low discharge or low power machine and vice-versa. 

Name the main parts of a radial flow reaction turbine ?
i) Casing ii) Guide vane iii) Runner iv) Draft tube. 

What is the purpose of the guide vanes in radial flow reaction turbine ?
The purpose of the guide vane is to convert a part of pressure energy of the fluid at its entrance to the kinetic energy and then direct the fluid on to the number of blades. 

What is a draft tube ? In which turbine it is mostly used.
The draft tube is a conduit which connects the runner exit to the trail race, when water is being finally discharged from the turbine. In reaction turbine , the draft tube is mostly used. 

What is the primary function of a draft tube ?
The primary function of the draft tube is to reduce the velocity of the discharged water to minimize the loss of kinetic energy at the outlet. 

Name the main parts of Kaplan turbine.
i). Scroll casing ii). Guide vanes iii). Hub with vanes iv). draft tube 

Name the main parts of pelton wheel .
i) Nozzle ii) Runner with buckets iii) Casing. 

An inward flow reaction turbine running at 250rpm has D1 = 1m, b1 = 0.2m, D2 = 0.5m and b3 = 0.3m. If the water enters the wheel radially at 3.5m/s velocity, determine the discharge and velocity of the flow at the outlet. Assume vane thickness at the extremities to be negligible.
Q = p D1 b1 Uf1 = p D2 b2 Uf2 = p x 1 x 0.2 x 3.5 = 2.199 m3/s
Uf2 = 2.99/ (p x 0.5 x 0.3) = 4.666 m/s 

What are the different performance characteristic curves ?
i) Variable speed curves (or) main characteristics
ii) Constant seed curves (or) operating characteristics
iii) Constant efficiency curves (or) Muschel characteristics 

Give an example for a low head turbine , a medium head turbine and a high head turbine.
Low head turbine – Kaplan turbine
Medium head turbine – Francis turbine
High head turbine – Pelton wheel. 

what are the purposes of casing of a centrifugal pump
i) To provide water to and from the impeller
ii) To partially convert the kinetic energy in to pressure energy 

What are the different types of casing in centrifugal pump ?
i) Volute casing ii) Turbine casing (or) casing with guide blades 

What is a positive displacement pump ?
In the case of positive displacement pumps, the fluid is physically pushed from an enclosed space. The positive displacement pumps can be either reciprocating type or rotary type. 

Where is the reciprocating pump well suited ?>
The reciprocating pump is well suited for relatively small capacities and high heads. 

what are the main components of a reciprocating pump ?
1)cylinder 2) Piston 3) suction valve 4) Delivery valve 5) Suction pipe 6) delivery pipe 7) crank shaft and connecting rod mechanism. 

Define coefficient of discharge of reciprocating pump.
The ratio between the actual discharge and theoretical discharge is known as coefficient of discharge. Cd = Qact / Qthe. 

Define slip of the reciprocating pump. When does the negative slip occurs ?
The difference between theoretical discharge and actual discharge is called the slip of the pump.
Slip = Qthe. – Qact , % of slip = (Qthe. – Qact ) x 100 / Qthe.
Negative slip occurs when delivery pipe is short suction pipe is long and pump is running at high speed. 

Differentiate between single acting and double acting reciprocating reciprocating pump.
In single acting pump, there is one suction valve and one delivery valve. On the backward stroke of the piston, the suction valve opens and water enters into the cylinder space. On the forward stroke, the suction valve closes and delivery valve opens, the water is forced through the delivery pipe.
In the double acting pump, there are two suction valves and two delivery vales one in the front and one in the rear. When the piston moves backward, the suction valve in the front opens and delivery valve in the rear opens and water is forced through it. When the piston moves forward, the suction valve in the rear opens and delivery valve in the front opens and water is forced through it. 

What is an indicator diagram of a reciprocating pump ?
The indicator diagram of a reciprocating pump is the diagram which shows the pressure head in the cylinder corresponding to any position during the suction and delivery strokes. 

A single acting single cylinder reciprocating pump has the following characteristics.
Discharge = 6 lps, suction head = 4m, Delivery head = 20m, Find the energy required to drive the pump.
Energy required = r g Q ( Hs + Hd ) Watts = (1000 x9.81 x 6×10-3 x (4 + 20) / 1000
= 1.409 kW. 

What is an air vessel and what is its purpose ?
An air vessel is a closed chamber connected on the suction or delivery or both sides of the reciprocating pump to obtain a more uniform flow. 

Name some rotary positive displacement pumps.
a. Gear pumps.
b. Vane Pumps.
c. Piston pumps.
d. Screw pumps.

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3 Comments on “Fluid Machines – Pumps and Turbines”

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