WHAT IS A WATER PUMP?
A water pump is an electrical machine designed to convert electrical power into energy, which is then used to displace and move water. The energy generated by the pump facilitates the movement of water from one location to another.
All water pumps consist of two essential components: an electric motor and a hydraulic section. The motor generates the power needed to operate the pump, while the hydraulic section is responsible for facilitating the water flow. Additionally, a sturdy support is utilized to securely mount the pump to its base, ensuring stability and preventing any unwanted movement.
STEP 1: FLOW
At home, everyone needs water
The maximum theoretical requirement is given by the sum of the quantities of water delivered to the various outlets of a flat multiplied by the number of flats. In practice, it is common practice that just some outlets are used simultaneously: that's why this number must be multiplied to a contemporaneity factor
HOW TO CALCULATE YOUR PUMP'S FLOW RATE
Residential buildings consumption
Method 1
Apartments with two toilets
Apartments with one toilet
It shows values of actual delivery, which depend on the number of flats connected to the water-supply system. Seven outlets are hypothesized for one-bathroom flats and ten outlets for two-bathroom flats.
Method 2
Maximum consumption at points of demand
• In theory, the water maximum requirement derives from the sum of the litres per minute delivered to the various outlets of a flat multiplied by the n. of flats.
• In practice, just some of the outlets are used simultaneously: that's why we can consider normally 1/3 of the total requirement.
Outlet
Qu. delivered (l/min)
Sink
Wash-basin
Bath-whirlpool tub
10
10
18
Shower
WC-flush-tank type
WC-fast-feed type
12
7
90
Bidet
Washing machine
Kitchen sink
6
12
12
Dishwasher
Outlet w/ 1/2" tap
Outlet w/ 3/4" tap
8
20
25
Other buildings consumption
These buildings require quantities of water greater than those needed in residential buildings. The values are based on hypothetical numbers of persons present in these buildings. These values offer a guideline and may vary in accordance with particular requirements of projects.
Number of persons present in building
A Offices B Shopping centres C Hospitals D Hotels
STEP 2: HEAD
1) Static head:
Distance between the suction fluid surface and the maximum discharge elevation (highest outlet).
Static
discharge
head
Total
static
head
Static
suction
head
Example
Total
static
head
Static
suction
lift
Static
discharge
head
2) Friction:
(sum of the head losses in pipes)
By approximation, head losses may be quantified as follows:
• 0.5 m per floor in new systems,
• 1 m per floor in old systems.
Friction is flow rate dependent :
• x2 flow rate, x4 head loss
• ½ flow rate, ¼ head loss
Head loss can also be calculated for pipes:
By matching the flow rate and the delivery pipe diameter, in the chart below, you’ll find the head loss in a 100 m long pipe. For example, supposing you have Q=42 m3/h and delivery pipe Ø DN80.
Therefore, the head loss will be 7,5 meters.
If the pipe were 70 m long, the head loss in the system would be calculated as the following: 7,5 meters x 70 meters / 100 meters = 5,25 meters
Head loss In m for steel pipes
Q Flow
HL Head loss, m per 100 m
V = Flow velocity: max 1,5 m/s for suction
and 3 m/s for delivery
Head loss calculated on bends and valves
Head loss in cm for bends, gate valves, foot valves and check valves
Water
flow velocity
Elbows
sweep elbow
Gate
valves
Foot valves
Check valves
System curve:
Static head + friction head = total head
Static Head
Friction Head
operating point
Calculation example:
Parameters:
• (Flow) Q = 42 m3/h
• (Static head) Hg= 40 m
• 70m DN80 pipe
Total Head =
40m + 20,25m = 61.55 m
Calculation of friction:
70 m Ø 80 pipe= 5,25 m
+ 15 m of minimum residual pressure at the highest outlet for appliances
20,25m
Pipe
Length
Pipe
Length
Foot valve
STEP 3: PUMP
The golden rule is to choose a pump at the BEP!
Total head
High temp. rise
Low bearing/seal life
Reduced Motor Efficiency
Ideal selection zone
Low bearing / seal life
Cavitation/High temp.rise
Flow
1) Throttle control:
In case of a selection at the far right-end of the curve, the flow rate is easy to control and can be reduced through a valve at outflow:
this will assure the correct pump operating condition.
Flow rate
Head
Pump curve
Throttled curve
System curve
2) Variable Speed Control / Inverter:
Constant pressure at different flows
STEP 4: NPSH
Pay attention to the suction capability of the pump, the “Net Positive Suction Head” required (NPSHr).
Its value is obtained in accordance with the flow
Check the following simplified formula for free-cavitation condition:
NPSH required
NPSH available
Where:
Hb = Atmospheric pressure (10m)
h = Suction lift
Hf = Friction loss in the suction pipe (m)
Hv = Vapour pressure of the liquid (m);
Hs = Safety factor (about 0.5 m)
Vapour
pressure values