---
title: "Is balancing tank necessary: 20000 L/day spring"
subject: "Water Supply Engineering"
chapter: 7
key: "semester_5/water_supply_engineering:res-k72"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-k72
site: BCT Engineering
---

# Is balancing tank necessary: 20000 L/day spring

Water Supply Engineering · Chapter 7: Reservoirs and Distribution System

- Asked in: 2072 Kartik, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-k72
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7

## Question

A village has design year demand of water 20000 liters per day. The demand is met by a continuous system of supply from a spring source with measured yield of 0.25 lps. The consumption pattern is as follows:

| Time (Hours) | 5-7 | 7-12 | 12-17 | 17-19 | 19-5 |
|---|---|---|---|---|---|
| Consumption (%) | 25 | 35 | 20 | 20 | 0 |

Is balancing storage tank necessary? Calculate its capacity if necessary. Justify your answer.

## Short answer

Daily demand 20 m3, spring yield 21.6 m3/day. Peak demand exceeds inflow, so tank needed. Capacity by mass curve method is 9.0 m3 (adopt 10 m3).

## Answer

#### Given data
- Daily demand = 20,000 L = 20 m³/day
- Spring yield = 0.25 lps = 21,600 L/day = 21.6 m³/day = 0.9 m³/h
- Demand: 5–7 h 25%, 7–12 h 35%, 12–17 h 20%, 17–19 h 20%, 19–5 h 0%

#### Check of source and need of storage
The yield (21.6 m³/day) exceeds the demand (20 m³/day), so the source is adequate. The demand rate in 5–7 h is 2.5 m³/h and in 7–12 h it is 1.4 m³/h, which are greater than the inflow rate 0.9 m³/h. The balancing tank is therefore **necessary**.


#### Mass-curve table (analytical method)
Inflow: 0.9 m³/h (spring yield). Time is counted from the start of the first demand period; a positive cumulative value means surplus in the tank.

| Time | Demand (m³) | Inflow (m³) | Inflow − demand | Cumulative |
|---|---|---|---|---|
| 05:00–07:00 | 5.00 | 1.80 | -3.20 | -3.20 |
| 07:00–12:00 | 7.00 | 4.50 | -2.50 | -5.70 |
| 12:00–17:00 | 4.00 | 4.50 | +0.50 | -5.20 |
| 17:00–19:00 | 4.00 | 1.80 | -2.20 | -7.40 |
| 19:00–05:00 | 0.00 | 9.00 | +9.00 | +1.60 |

Total demand = 20.00 m³; total inflow = 21.60 m³.

#### Capacity
- Maximum cumulative surplus = 1.60 m³
- Maximum cumulative deficit = 7.40 m³
- Balancing capacity = max. surplus + max. deficit = 1.60 + 7.40 = **9.00 m³**
- Because the inflow in a day (21.60 m³) is more than the demand, the extra water simply overflows once the tank is full. The minimum storage that still meets every demand is the largest draw-down, 7.40 m³. The textbook value 9.00 m³ is on the safe side.

**Answer: Balancing tank is necessary because peak demand exceeds the continuous inflow; capacity ≈ 9 m³ (adopt 10 m³).**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What justifies the balancing tank?
   A. Source yield is less than demand
   B. Demand rate in peak periods (2.5 and 1.4 m3/h) exceeds inflow of 0.9 m3/h
   C. Demand is constant
   D. No night demand
   Answer: B. Inflow cannot meet peak rates directly, so storage is needed. (Key point: Justify tank)

2. What is the demand volume in 5-7 h (25% of 20 m3)?
   A. 5 m3
   B. 2.5 m3
   C. 7 m3
   D. 1.8 m3
   Answer: A. 25% x 20 = 5 m3, i.e. 2.5 m3/h. (Key point: Demand 5-7 h)

3. What are the maximum surplus and maximum deficit?
   A. +9.00 and -3.20
   B. +1.60 and -7.40
   C. +7.40 and -1.60
   D. +0.5 and -5.7
   Answer: B. Cumulative peaks at +1.60 (at 05:00) and bottoms at -7.40 (at 19:00). (Key point: Cumulative values)

4. What capacity is adopted?
   A. 9 m3, practically 10 m3
   B. 7.4 m3 only
   C. 21.6 m3
   D. 1.6 m3
   Answer: A. Surplus plus deficit = 9.00 m3, adopted as 10 m3. (Key point: Capacity)

## Related in Water Supply Engineering

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