Video 3 C1: How pumps work

Question 1

for centrifugal pumps (characteristic curve): the greater the pressure (head), the ____ the flow

Answer 1

lower

Question 2

what does the ‘characteristic curve’ look like? what does it show?

Answer 2

head aka pressure (y axis)
discharge aka flow (x axis)

1. curve: exponentially downward
   - Downward sloping 📉 → As flow rate (Q) increases, the head (H) decreases
   ex. in assignment question, this was the curve that Hp was given as an equation
2. shows pump performance at different flow rates

Question 3

lets say the shape of the characteristic curve is steep. What does this mean for the flow rates?

Answer 3

steep at low flow rates

Question 4

lets say the shape of the pump characteristic curve is flat. What does this mean in regards of the flow when its flat?

Answer 4

flattens at higher flow rates

Question 5

what are the main components of characteristic curve? (head vs flow rate curve features)

Answer 5

• high head at low flow
• low head and high flow

intuitive. look at the exponentially downward graph and it denotes this relationship

Question 6

what is the ‘system curve’ from pressure x flow graphs?

Answer 6

shows the head loss changes with the flow rate in a piping system

ex. in assignment question, this curve was the HP values found from energy equation (the linear-ish line)

Question 7

what does the system curve look like?

Answer 7

Upward-sloping curve 📈 → As flow rate increases, head loss increases due to pipe friction and minor losses.

Question 8

what is the relationship between the pump characteristic curve and the system curve?

Answer 8

you plot them together to find the match (operating) point. The match point tells you the actual flow rate and pump head that the pump will deliver

Question 9

what is the system curve determined by?

Answer 9

Determined by system resistance → Factors like pipe length, diameter, roughness, and fittings affect the shape (DW eqn)

Question 10

lets say the system curve is flatter… what does this mean?

Answer 10

less constriction, where the pressure increases with more flow

Question 11

lets say the system curve is more vertical… what does this mean?

Answer 11

lots of constriction, where no matter the pressure, the flow rate doesn’t change much

Question 12

how do positive displacement pumps work?

Answer 12

they trap a fixed volume of fluid and force it into the discharge line

Question 13

how are positive displacement pumps different from centrifugal pumps?

Answer 13

• in CP, the impeller can spin without actually moving any fluid (variable flow).
• PDP directly couple the motor to a fixed volume no matter the pressure. therefore flow is at a constant rate, depicting a straight vertical line on their characteristic curve. (CONSTANT FLOW, HIGH PRESSURE)

Question 14

for a PDP, how is the systems total head decided?

Answer 14

by the system curve, since the pump curve does not vary (constant)

Question 15

for a CP, how is the total head decided?

Answer 15

by both the pump curve and system curve (they both vary)