---
title: "Penstock thickness (1.20 m, 17.5 kg/cm²)"
subject: "Hydro Power"
chapter: 5
key: "semester_6/hydro_power:penstock-thickness-1-2m"
source: https://www.bctengineer.com/q/semester_6/hydro_power/penstock-thickness-1-2m
site: BCT Engineering
---

# Penstock thickness (1.20 m, 17.5 kg/cm²)

Hydro Power · Chapter 5: Components of Hydropower System

- Asked in: 2080 Asoj, 4 marks
- Video: [Penstock Pipe Numerical || Hydropower Engineering](https://www.youtube.com/watch?v=N97FrIIFTr8) — OPengineer
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/penstock-thickness-1-2m
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5

## Question

Calculate the thickness of a penstock of internal diameter 1.20 m which conveys water at a pressure head of 17.5 kg/cm². There is a possibility of a 20% increase in pressure due to the water hammer. Take design stress = 1050 kg/cm² and joint efficiency = 87%.

## Short answer

Design pressure p = 1.20 × 17.5 = 21 kg/cm². Thickness t = pD/(2 σ η_j) = (21 × 120)/(2 × 1050 × 0.87) = 1.379 cm = 13.79 mm, so use 14 mm.

## Answer

**Given:** $D = 1.20$ m $= 120$ cm, pressure $p_0 = 17.5$ kg/cm², water hammer rise 20%, design stress $\sigma = 1050$ kg/cm², joint efficiency $\eta_j = 0.87$.

**Formula** (hoop stress in a thin-walled pipe):

$$
t = \frac{p\,D}{2\,\sigma\,\eta_j}
$$

#### Design pressure
$$
p = 1.20 \times 17.5 = 21\ \text{kg/cm}^2
$$

#### Thickness
$$
\begin{aligned}
t &= \frac{21 \times 120}{2 \times 1050 \times 0.87} \\
&= \frac{2520}{1827} = 1.379\ \text{cm} = 13.79\ \text{mm}
\end{aligned}
$$

**Answer:** penstock thickness $t = 13.8$ mm, say **14 mm** (a corrosion allowance of about 2 mm may be added, giving 16 mm).

## Check yourself

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

1. The design pressure for 17.5 kg/cm^2 plus 20% water hammer is
   A. 21 kg/cm^2
   B. 17.5 kg/cm^2
   C. 3.5 kg/cm^2
   D. 35 kg/cm^2
   Answer: A. 1.2 x 17.5 = 21. (Key point: Design pressure)

2. Which substitution is correct?
   A. t = 21 x 120 / (1050 x 0.87)
   B. t = 21 x 120 / (2 x 1050 x 0.87)
   C. t = 21 x 1.2 / (2 x 1050 x 0.87)
   D. t = 17.5 x 120 / (2 x 1050 x 0.87)
   Answer: B. Use design pressure, D in cm and the factor 2 for two walls. (Key point: Substitution)

3. The calculated thickness is about
   A. 6.9 mm
   B. 27.6 mm
   C. 13.8 mm
   D. 1.2 mm
   Answer: C. 2520/1827 = 1.379 cm. (Key point: Result)

4. How is corrosion allowance treated?
   A. It is subtracted
   B. It multiplies the thickness by 2
   C. It is ignored always
   D. About 2 mm may be added to the computed thickness
   Answer: D. Adding about 2 mm gives roughly 16 mm. (Key point: Corrosion allowance)

## Related in Hydro Power

- [Penstock thickness (1.3 m, 20 kg/cm²)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apenstock-thickness-1-3m) — Hydro Power, chapter 5
- [Penstock plate thickness (1.5 m, 250 m head)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apenstock-thickness-1-5m) — Hydro Power, chapter 5
- [Water hammer in 250 m penstock](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Awater-hammer-250m-penstock) — Hydro Power, chapter 1
- [Penstock thickness and economic diameter](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apenstock-thickness-economic-diameter) — Hydro Power, chapter 5

## Related in other subjects

- [Economic diameter; penstock thickness (1.25 m)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apenstock-economic-diameter-thickness-1-25m) — Hydropower Engineering, chapter 6
- [Economic diameter factors; penstock wall thickness (3 m)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aeconomic-diameter-and-wall-thickness-factors) — Hydropower Engineering, chapter 6
- [Water hammer pressure and stress, 2300 m pipeline](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Awater-hammer-2300m-pressure-stress-1725m) — Hydraulics, chapter 4
- [Water hammer in 1.2 m steel pipe, wall thickness](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Awater-hammer-steel-pipe-thickness-1200mm) — Hydraulics, chapter 4