Section 1 - Basics of Fluid Dynamics

Question 1

What is meant by the term Mass?

Answer 1

The mass of an object is a measure of the quantity of matter present.

Question 2

What is meant by the term Weight?

Answer 2

Weight is a measure of a force acting on an object. Usually gravity.

Question 3

What is the density of a substance?

Answer 3

The density of a substance is defined as the mass per unit of volume of the substance.

Question 4

What is the importance of Centrifugal Force, and friction in the realms of centrifugal pumps?

Answer 4

Centrifugal Force is the force exerted on a substance, which tends to throw it outwards, causing it to speed up as it approaches the edge of the disk.

Friction, is the force which attempts to drag the substance around the pump disk.

Question 5

What governs the Angel of departure for a substance which interacts with a centrifugal pump disk?

Answer 5

The Magnitude of Centrifugal force, and the magnitude of friction acting on the droplet.

Question 6

Why is Angel of departure important when looking at Centrifugal pumps?

Answer 6

Design features of centrifugal pumps take cognisance of this Angel of departure, this ensures the maximum amount of kinetic energy is transferred.

Question 7

How is kinetic energy calculated?

Answer 7

KE = 1/2M x V2.

Where;

M = Mass (KG)
V = Velocity (M/S)
KE = Joules

Question 8

What is meant by the conservation of energy equation?

Answer 8

The total energy content of a system will always remain constant.

Eg as velocity increases through an orifice, the pressure at the orifice decreases.

Question 9

What is The definition of pressure?

Answer 9

Pressure expresses the relationship between Force (or Weight) and the area it is affecting.

Question 10

What is the Calculation of pressure, in relation to force and area?

Answer 10

P = KG Force (Weight) / Area.

Question 11

What is meant by the measure 1 Bar?

Answer 11

The measure of a Bar is an SI term which equates to almost, 1kg/cm2

Question 12

What is meant by Head pressure?

Answer 12

Head pressure is the term given to a pressure developed by a Head, or column height of liquid.

Question 13

What is meant by the term “Dynamic Pump”?

Answer 13

A dynamic pump imparts kinetic energy, they do not create pressure directly; pressure results from the liquid slowing down and the kinetic energy converting to pressure energy.

Question 14

What is a key factor of pressure generation in a dynamic pump set?

Answer 14

The pressure developed will depend on the density of the fluid being pumped.

Question 15

What is the Suction Head of a Pump set?

Answer 15

The Suction Head represents the head pressure at the pump suction end.

Question 16

What is meant by the Discharge head of a pump set?

Answer 16

The discharge head represents the head pressure delivered by the pump.

Question 17

What is the term “Total head” of a pump set referring to?

Answer 17

This is the difference between the discharge and suction head of a pump, representing the additional pressure imparted to the liquid by the pump.

Discharge - Suction = Total Head

Question 18

What is meant by the term NPSH and why is this important?

Answer 18

NPSH means Net positive suction head, and refers to the minimum suction pressure for a given fluid or mixture, which will prevent gases entering their gas phase, or beyond the point in the Pressure/Temperature curves at which gas escapes the liquid in which it is dissolved.

This is important as any gas entering a pump set may cause cavitation (See cavitation slide).

Question 19

What is meant by the term static suction line pressure?

Answer 19

This is the pressure at the suction end of the pump, observed when pumping has stopped.

Question 20

How do pump manufactures assist in design of Pump sets to ensure NPSH is maintained?

Answer 20

Manufacturers will specify the NPSH and maximum operating temperature required for each pump to handle a given liquid effectively. This must be through the entire range of the pump - an adequate safety margin would be 1 metre water head, or 10% water head above NPSH for the range. Whichever is larger.

Question 21

What is Cavitation?

Answer 21

Cavitation is the effect caused by gas bubbles escaping from the liquid in which they are absorbed - followed by reabsorption as pressure is imparted on the Gas.

Question 22

Why is cavitation so dangerous?

Answer 22

As the Gas phase is reabsorbed into the pumping liquid, as pressure is exerted, tiny voids or cavities are created in the pump - liquid then rushes from all angles - and the cavity implodes. This can transmit very large forces - causing erosion, Noise and vibration.

Question 23

What is Meant by the term S.G, and how is the SG of a substance calculated?

Answer 23

The S.G, or specific gravity of a substance is equal to the mass of a certain volume of material / mass of an equal volume of reference substance.

For Solids and Liquids - this reference is usually water.

For Gases - this reference is usually Air.

SG = Mass (of Vol) / Mass (of Ref Vol)

Question 24

What condition in a piping system can also contribute to pressure loss?

Answer 24

Turbulence can give rise to increased heat energy conversion, and thus increased pressure loss in the system.

Question 25

What is the relationship between flow rate, turbulence and pressure loss?

Answer 25

At high flow rates, high turbulence will occur and as such high pressure loss will also be noted.

At low flow rates, low turbulence will occur, and as such low pressure loss will be noted.

Question 26

What is the mathematical expression, relating Differential pressure and Flow Rate?

Answer 26

F= (Sq)DP x 10

Where;

F = Flow rate as a % of maximum.
DP = differential pressure as a % of maximum.
(sq) denotes Square root of DP.

Question 27

What happens to the DP of a system as the flow rate doubles?

Answer 27

As the flow rate doubles, the DP quadruples.