Advantanges and Disadvantages of Looped vs Branched Network

A looped network in a water reticulation system is a configuration where multiple pipes are interconnected to create closed loops, providing multiple pathways for water to flow from source to consumer, rather than a simple, branching layout.

Let us do a theoretical comparison on how hydraulics behave in branch and loop networks. 

Project sample below

1. Branch Network
2. Loop Network

From the project file, this is how the pipe network looks like in a branch and loop system. Both of the files have the same demand and pipe physical properties (diameter, material, wall thickness, etc). The only difference is just the addition of Pipe 8 from Node 8 to Node 2 to form a loop system.

Peak Flow Analysis #

Take example on Pipe 2 for Peak Flow calculation:

From the table above, we can see that Q, V, and h_f in the branch network is higher than in the loop network. Residual pressure in the dead-end network is lower than in the loop network.

Discussion #

Let’s break it down. Both networks are designed for a peak flow of 25 L/s (10 L/s × 2.5 peak factor). 

In the branch system, there is only one route for water to flow across the network. In other words, all the design flow of 25 L/s is forced through a single path (via Pipe 2). In this case, the full design flow of 25 L/s is carried entirely by Pipe 2 and continues unchanged through all downstream pipes. Since each pipe segment handles the same high flow rate, the network experiences greater friction loss along the entire path. This leads to a significantly higher total head loss from the reservoir to the demand node, as energy is lost to friction throughout the system.

In contrast, a looped arrangement provides multiple flow paths (via Pipe 2 and Pipe 8) for water to reach the demand node of 25 L/s. This enables the design flow to be split across different routes, reducing the flow rate through each individual pipe. As a result, velocity and friction losses are significantly lower in each pipe, which can increase residual pressure. 

This improved hydraulic efficiency is one of the key advantages of a looped network, as it can effectively deliver the same water demand but with minimal energy loss due to friction. Unlike branch systems, where the entire flow is forced through a single path, causing high losses which results in pressure drop. Looping network, would in some sense, solve the not enough pressure problem

Fire Flow Analysis #

For Hydrant node, take example on Pipe 5 for Fire Flow calculation:

The result also shows the same as in peak flow analysis, where Q, V, and h_f in the branch network is higher than in the loop network. While pressure in a looping network is higher than in a branch system.

Discussion #

In a looped layout, flow is split across multiple paths, which reduces velocity and head loss in individual pipes. From the result above, the residual pressure at node 6 improved about 40% from branch system. Therefore, we can say that during fire flow demands (where there is fire breakout), the looped system is more reliable than the branch system as it can maintain higher residual pressure at hydrants. 

Theoretical Explanation #

 Now let us delve into the basic calculation:

1. Using Flow Rate Calculation:
   • Q = AV
   • Q is proportional to V, (Q ∝ V)
   • Therefore, decrease in discharge flow will result in decreased velocity and vice versa.

2. Using Hazen William Equation, head loss is given by:
   • h_{f}= \frac{10.67\times L \times Q^{1.852}}{C^{1.852} \times D^{4.87}}
   • Head loss if proportional to discharge flow, (hf ∝ Q)
   • When discharge rate is low, velocity of water will be low (less friction loss and corrosion in the pipe) and vice versa.  
   • Therefore energy losses in the pipe can be minimized.

3. Residual pressure = Fix Head – Head Loss – Highest Supply Level (HSL)
   • With lower head loss, the remaining residual pressure will be higher.
   • This is why looping networks tend to maintain better residual pressure at the downstream ends of the pipes.

Advantages of Loop Network #

1. Provides redundancy
   • The looping network allows multiple paths for water to flow. 
   • In a series pipe (branch network), if one of the pipes is broken, then water cannot flow to the next pipe.
   • However,if one pipe in the loop network is broken/ shut down for maintenance purposes, the system can reroute the flow to another path to meet the water demand. Therefore, water can still reach consumers without having major service interruptions.

2. Reduced water stagnation
   • In a loop, water has multiple paths to flow, which helps keep the water moving even at the ends of the network.
   • This reduces water age (the time water stays stagnant in pipes), which helps maintain disinfectant residuals like chlorine.
   • Therefore, it minimizes these stagnant zones because water can circulate through different routes.

3. Much higher residual pressure at all nodes (Improved pressure)
   • It provides much higher pressure at end node compared to branch network
   • Higher residual pressure means even the farthest or highest point in the system can still get adequate pressure, avoiding complaints from end users.

4. Enhanced fire protection
   • During fire flow conditions, looped networks can maintain high pressure, ensuring hydrants are functional and sufficient flow reaches the fire site.
   • Higher and more stable flows are available during fire emergencies since water can come from multiple directions.

5. Can use smaller pipe size
   • Because of smaller head loss, we can also utilize smaller pipe diameters in looped while still achieving required flow rates, which can optimize costs in dense networks.

Disadvantages of Loop Network #

On the downside, designing a looped network also has disadvantages.

1. Higher construction cost
   • More pipes are needed to close the loops, which increases material, excavation, and installation costs compared to branch systems. Another issue is the permit– getting permits to excavate could be difficult because it touches grounds.

2. Overdesign in Low-Density Areas
   • For small rural developments with low water demand, looping networks might not be economically justifiable.
   • Branch networks may be sufficient and more cost-effective in rural areas.

3. Higher Operation & Maintenance Complexity
   • Say if we want to do maintenance on a looped system, we will need to use valves to stop the water at that particular pipe to stop the water from flowing or control the flow direction. The use of valves, fittings and interconnections between pipelines will increase when we have a looped system. 
   • In other words, if a pipe segment requires repair, multiple valves must be closed to effectively isolate the section without affecting supply to other areas.

4. Risk of Pressure Surges/ Water Hammer
   • In certain situations (i.e maintenance work), loops can create rapid pressure fluctuations (surges) due to flow reversals or sudden valve closures.
   • This would result in a loud banging or hammering noise in the pipes and potentially damaging the pipe in the system
   • Proper surge control strategies need to take into account to mitigate this issue