The way to calculate the water receiver tank and pumps

Hi there.
Recently there are many cases that the water receiver tank system is used because of the BCP and for natural disasters.
A Water receiver tank is used for securing the amount of water for the case of emergency.
Of course now water direct type and pump pressure type are also used especially for the small buildings to middle size of buildings.
But the BCP now is getting recognized more.
Some younger engineers do not understand how to calculate the size of the water receiver tank and pump specification.
I will explain how to calculate the size of the water receiver tank and pump specification this time.

Outline of water receiver tank system

We can divide water receiver tank systems into two types of systems.

First one is the major one.
In the case of public water supply pipes.
The public water supply pipe will provide water to the water receiver tank through the private water supply pipe.
And The water supply pump provides the water to the necessary point in the building.

Next one is in the case of no public water pipes around the site.
A Well pump will be installed and it provides the water to the water receiver tank.
We can install it only if we can get the water from the well.
It depends on the site about water quality.
Sometimes we need to install water treatment equipment for the well water.

The way of calculating the amount of water required and amount of water for the water receiver tank

The next step is how to calculate the amount of water required and the amount of the water for the water receiver tank.
I will explain the amount of water required in a day first.

It depends on the usage of buildings and rooms how much water is required.
The water for urine, toilet, wash bowl for office normally if there is no kitchenette.
We need to sum up the amount of water if there is a kitchen in the building.
Also in the case of a gym, there is sometimes a pool and the pool requires water.
What about the laboratory, some special equipment for study requires water.
In the apartment, shower and kitchen require water.
Total amount of water should be added with all water requirements.

Next is the amount of water for the water receiver tank.
Efficient amount of water is 40%~60% of the amount of water required in a day.
It is for securing the water quality.

The way of calculating the size of the water receiver tank

I will explain how to calculate the size of the water receiver tank with prior calculation.
The first one that we need to care about is that the level under the outlet pipe level does not include efficiency.
And at the design process we do not calculate the 150mm above the top of the outlet pipe.

The next one is the space between the water surface and the inlet pipe. (At least 300mm between the HWL( high level water) and the top of the water receiver tank.
The main reason space is needed is contamination.
Just in case, water in the main water pipe will be polluted.

The way of calculating the specification of the well pump

There is a lot of water in the water receiver tank as always.
So we can calculate the specification of the well pump with the average spending amount of water.
As I already explained, there are a variety of people who use its building.
The average amount of water in an hour is decided with these people.
Total amount of water required is the pump specification.

Specification of the water supply pump

Specification of the water supply pump is decided without the water receiver tank.
People who have designed the system of water pressure seem to be easy.
The way of calculating the specification of the water supply pump is to calculate the unit of load water from sanitary.
The unit of water load will be used to calculate the amount of water required for the water supply pump.

Amount of the water required (L/min) is calculated with below.
-0.00000000003 x a^4 + 0.00000022985 x a^3 -0.00063126107 x a^2 +1.0843057483 x a+140.70830490575
a= unit of load water

Unit of load water

It depends on the sanitary.
Toilet : 6 units / qty
Urine : 3 units / qty
Wash bowl : 2 units / qty
Hose tap : 5 units / qty

Calculation example

You will be understand deeply with its example below.

Refer to the figure and the condition for calculating is written.

□Calculating the amount of water in a day
Office worker : 1,000ppl x 60L/person・day = 60,000L/day
Visitor : 100ppl x 20L/person・day =   2,000L/day
Total:62,000L/day

□Calculating the amount of efficient water for the water receiver tank
The amount of efficient water for the water receiver tank is 50% amount of water in a day
62,000L ÷ 2 = 31,000L = 31m3

□Calculating the size of the water receiver tank
We suppose that the water receiver tank cubic is 31m3 x 1.3 = 40.3m3 ≒41m3
We suppose that the size of the water receiver tank is 6m x 4m x 2mH
Outlet pipe size is 100A, we do not include 150mm+100mm+150mm= 400mmH as the efficient water level.
And we do not include the 300mm from the top of the water receiver tank as the efficient water level.
Efficient water level is 2.0mH – 0.4mH – 0.3mH = 1.3mH.
6m x 4m x 1.3mH = 31.2m3 > 31.0m3
The decided water receiver size is 6m x 4m x 2.0mH .

□Calculating the average amount of water and well pump specification
The amount of water in a day is already calculated.
Office worker : 1,000ppl x 60L/person・day = 60,000L/day
Visitor : 100ppl x 20L/person・day =   2,000L/day
Office workers work for 8 hours in a day according to the condition.
60,000L/day ÷ 8h/day = 7,500L/h
Visitors stay in the building for 2 hours for each
2,000L/day ÷ 2h/day = 1,000L/h
Total is 8,500L/h = 141.7L/min.
Normally pump ability is represented with the unit of L/min
The well pump specification is 141.7L/min.

□Calculating the specification of water supply pump
Toilet:25qty x load water unit:6 = 150units 
Urine:30qty x load water unit:3 =   90units
Wash bowl:25qty x load water unit:2 =   50units
Hose tap: 5qty x load water unit:5 =   25units
Total is 315units.

We put it into below
-0.00000000003 x a^4 + 0.00000022985 x a^3 -0.00063126107 x a^2 +1.0843057483 x a+140.70830490575
a= unit of load water
Amount of water required is 427L/min.

Summary

I will explain how to calculate the size of the water receiver tank and pump specification this time.
Water pressure pipe loss is not calculated this time and if you are interested in it, you should confirm how to calculate it for deep learning.

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