CFSA Blog
Technical Document: Concepts of Hydraulics in Simple Words
Author: Hesam Tavoosi, CFPS, CWBSP, CSITMS, Approved NFPA Instructor
Introduction:
This paper explains fundamental concepts of fluid mechanics—such as flow, pressure, pressure loss, the C Factor, and internal pipe diameter—using simple, relatable examples. A clear understanding of these concepts empowers designers to perform hydraulic calculations more accurately and optimize systems more effectively.
Setting the Scene with an Analogy:
Imagine the blue circles in Figure “A” are people gathered in a saloon. Their task is to pass through a corridor to reach the yard, where they must lift and move 20-kilogram weights.
As people make their way through the corridor, they lose energy from walking, bumping into one another, and brushing against the walls. This energy loss could leave them too tired to lift and move the weights once they arrive in the yard.
This analogy highlights the importance of reducing energy loss so that people (or, in hydraulic terms, water) reach their destination with enough energy (pressure) to complete their task.
Cases Overview:
Case 1: Reducing Distance
One solution is to relocate the saloon closer to the yard by altering the architectural layout. As shown in Figure 1, shorter corridors mean people walk less and lose less energy—both from reduced exertion and from minimized contact with the walls.
Case 2: Reducing Traffic
Another solution is to reduce the number of people passing through the corridor at any given time. For example, instead of two people entering the corridor every second, only one person enters. Fewer people in the corridor reduce energy loss because there is less crowding, which means fewer collisions and less contact with the walls (Figure 2).
Case 3: Widening the Corridor
Another approach is to widen the corridor. With a wider corridor, people have more space, reducing their chances of bumping into one another or brushing against the walls. This leads to less energy loss, as shown in Figure 3.
Case 4: Adding Alternate Routes
Adding more corridors allows people to take alternate, less crowded paths to the yard. With less congestion, they encounter fewer obstacles, which minimizes energy loss. This is illustrated in Figure 4.
Case 5: Boosting Initial Energy
Offering fresh fruit or energy drinks in the saloon gives people more starting energy. With this boost, they can retain enough energy to lift weights once they reach the yard. In Figure 5, people with higher initial energy are represented by red circles.
Case 6: Smoothing the Walls
Smoother walls in the corridor reduce energy loss when people brush against them. In Figure 6, the lower image illustrates how rough walls increase energy loss compared to smoother ones.
Linking the Analogy to Hydraulic Systems
Now, let’s translate the analogy into hydraulic and water-based fire protection design concepts. Each element in the example corresponds to a key aspect of hydraulic systems, as summarized in the following table:
In a water-based fire protection system, water stored in the tank (analogous to people in the saloon) moves through a network of pipes (like a corridor) with sufficient flow rate (number of people) and pressure (energy). As water travels through the pipes, it experiences friction—both between water molecules (similar to people colliding) and between water and the pipe walls (like people brushing against the walls). This friction results in pressure loss, analogous to energy loss in our scenario.
For effective fire suppression, water must reach its destination with adequate flow rate and pressure. Without these, controlling, suppressing, or extinguishing a fire may be ineffective, similar to how people without sufficient energy struggle to lift weights.
Applying the Analogy to Design Scenarios
Let’s see how the described strategies can be integrated into the design of fire protection systems:
Case 1: Optimal Water Tank Placement
When designing a large site, placing the water tank and fire pumps at the center (on flat terrain) minimizes pipe length. Shorter pipe distances reduce friction losses, maintaining pressure throughout the system.
Case 2: Systems with Lower Water Demand
For example, when we use quick-response sprinklers in light or ordinary hazard occupancies, we are allowed to reduce the design area based on the ceiling height. A smaller design area means less water is needed and results in less pressure loss in the pipes.
Case 3: Larger Pipe Diameters
Larger pipe diameters reduce pressure losses by minimizing friction between the water and the pipe walls.
Case 4: Looped/Gridded Networks
Looped or gridded piping networks reduce overall pressure losses compared to tree systems, as water flows through multiple paths to reach its destination.
Case 5: Increasing Initial Pressure
Fire pumps or elevated water tanks increase the pressure at the outlets, ensuring stronger water discharge at sprinklers or nozzles.
Case 6: Using Smoother Pipes
Pipes with a higher C Factor, like CPVC or copper, have smoother interiors that minimize friction losses, preserving system pressure.
