Let us now increase the dimensions of the 20-gallon vessel to 200,000 gallons, but without increasing the height of the water level, and we shall find that the pressures shown on each square inch at the various depths will remain the same as in the 20-gallon vessel, the pressure on each point being due to the vertical column of water above it alone, and not in any way to the mass of water around or on each side. Each foot of vertical height added to a column of water by raising a feed-tank to a higher level, or by raising the sides of the tank and filling it, adds an increase of '4335 lbs. per square inch to the effective pressure. This is roughly taken by plumbers at ^ lb. pressure for each foot in height, and for pressure on pumps in working this estimate allows a fair margin for friction losses ; but to be more accurate and yet not too abstruse, it is a good and easy rule to calculate 3 lbs. pressure per square inch for every 7 feet in height of water. Thus 14 feet gives 6 lbs. pressure, 21 feet gives 9 lbs. pressure, 28 feet gives 12 lbs. pressure, 35 feet gives 15 lbs. pressure, 42 feet gives 18 lbs. pressure, 49 feet gives 21 lbs. pressure, and so on.
<Callout type="important" title="Critical Rule">Plumbers must remember that water ever presses to attain a horizontal level in every part of a system of connected pipes and cisterns.</Callout> The sloping surface of the waves of ocean, for instance, is at every point of that surface at right angles to the resultant or combined action of the force of gravity in one direction, and of the force of the wind in another direction. If an open vessel of water is rapidly revolved, you may observe that the curve assumed by the surface of the water is regulated by the same law, each point of surface taking right angles to the resultant or combination of the force of gravity acting in one direction and centrifugal force acting in another direction.
<Callout type="risk" title="Potential Disaster">Disaster follows whenever this law is violated or neglected.</Callout> The density of cold water being greater than the density of hot water, which latter expands and becomes lighter by the action of heat, the level of the surface of the hot water in the hot cistern stands higher than the level of the cold water in the cold cistern, this difference increasing as the difference in temperature increases, and therefore in such case the surface is not maintained in a horizontal plane, because the hot and cold liquids are of unequal density. The instant the outer action of heat ceases the densities equalise, and the levels of the surfaces also equalise into one horizontal plane.
<Callout type="beginner" title="Clarification">Water will expand 1/20th of its bulk when heated from 39°F to 212°F.</Callout>
Key Takeaways
- Pressure in water pipes is directly proportional to the height of water above any point.
- Water always seeks a horizontal plane, which is crucial for plumbing design.
- Hot and cold water have different densities, affecting their levels in interconnected cisterns.
Practical Tips
- Use the rule of thumb: 3 lbs. pressure per square inch for every 7 feet of water height.
- Ensure safety overflow pipes are correctly placed to handle expansion from heated water.
Warnings & Risks
- Neglecting hydrostatic laws can lead to burst pipes and system failures.
- Improperly designed hot-water systems may result in constant wastage due to expansion.
Modern Application
While the chapter focuses on early plumbing principles, understanding these basic physics remains crucial for modern survival scenarios. Knowledge of water pressure and behavior is essential for setting up effective water supply systems in remote or disaster-stricken areas.
Frequently Asked Questions
Q: How does the height of a water tank affect the pressure in pipes?
Each foot of vertical height added to a column of water increases the pressure by '4335 lbs. per square inch, which plumbers often round up to ^ lb. for practical calculations.
Q: What is the importance of maintaining a horizontal plane in plumbing systems?
Water naturally seeks a horizontal level, and this principle ensures that water pressure remains consistent across interconnected pipes and cisterns.
Q: Why do hot-water cisterns need to be higher than cold-water ones?
Hot water expands when heated, becoming less dense. To prevent overflow, the hot-water cistern must be positioned higher to accommodate this expansion.