It’s critical to understand the impact of head loss when troubleshooting a pump system.
By Scott Tystad
Three critical things to consider when sizing or troubleshooting a pump are flow, head, and total dynamic head.
Dan Featherstone, a technical trainer on the Pentair customer service team, had used every trick in the book to troubleshoot a customer’s pump. The ohms test showed the windings were not compromised and a voltage test showed the pump was receiving power.
Regardless, the customer still had low shower pressure, making his showers less than ideal. So, what was causing the lack of pressure?
Based on the information provided, Dan felt the pump was sized correctly for the application. After a dead-head test confirmed the pump was hitting the curve, Dan asked the customer for more information about the discharge piping. The customer recently purchased the property and had limited knowledge of the piping. While the customer started digging, Dan continued to review the numbers.
Three Troubleshooting Considerations
Three critical things to consider when sizing or troubleshooting a pump are flow, head, and total dynamic head (TDH). Flow is the amount of water a pump can move, typically rated in gallons per minute (GPM). Head refers to the distance the pump can move the water, generally stated as feet of head. TDH is the feet of head a pump must produce for the whole system to operate correctly and includes head losses due to elevation, pipe length, pipe size, pipe material, and pipe fittings.
While waiting for the pipe information from the customer, Dan felt confident that the culprit was in the discharge piping. Reviewing his notes, Dan estimated that the TDH required (pipe run, head loss, and desired feet of head at the showerhead) was roughly 250 feet. The pump’s head rating was 275 feet, indicating a significant head loss in the discharge piping.
Dan decided to eliminate the most straightforward explanation: an elevation change. If the customer’s pipes ran up a hill, the pump would need to generate more head to overcome gravity, quickly explaining the lack of pressure in the shower. Unfortunately, the customer confirmed to Dan that there were no extreme elevation changes.
Perplexed but not discouraged, Dan turned his thoughts to all potential configurations his customer was about to uncover.
Figure 1. Water velocity increases due to a decrease in pipe diameter.
Figure 2. Example of a head loss chart.
Figure 3. Example of a friction loss through fittings chart.
Understanding Head Loss
Beyond gravity, understanding head loss in a piping system can be complex. It begins by understanding the correlation between velocity and friction and how each is affected by the pipe characteristics.
When water is flowing in a pipe, it constantly struggles against itself. This struggle is due to a thin unmovable layer of water that attaches to the pipe’s inner surface. The layer of water effectively reduces the interior diameter of the pipe, forcing the water to squeeze into a smaller space.
As the water compresses towards the center of the pipe, the water’s velocity increases (Figure 1). Velocity in a piping system is constant and is based solely on the pipe’s flow rate and diameter.
While velocity does not directly cause head loss, it can directly cause other issues such as water hammer or cavitation. To mitigate issues caused by velocity, avoid velocities over 5 feet per second in the suction piping and 7 feet per second in the discharge piping.
As the velocity inside the pipe increases, there is also an increase in friction. Like how drag on a gas-powered automobile decreases the range of the vehicle, an increase in friction reduces the water’s available head, decreasing the distance it can travel.
Where velocity is a constant, friction depends on several factors including pipe size, pipe material, pipe length, and pipe fittings. The correlation between velocity and friction is why larger pipes with smoother materials generally create less velocity and friction at a given flow rate versus smaller pipes with rough materials.
Today, the three most typical materials used for piping are steel, copper, and plastic (PVC or poly pipe). In general terms, plastic pipe has the least amount of head loss due to friction, and steel has the most.
Unfortunately, generalities do not correctly size or troubleshoot systems, and all the nuances can make seeking the correct values seem daunting. Further complicating matters, the internet is chock-full of technical articles and confusing calculations. Luckily, the process is not as hard as it seems. Two tools that can make calculations easier are a head loss due to friction chart (Figure 2) and a friction loss through pipe fitting chart (Figure 3).
The first valuable tool is the head loss due to friction chart. Head loss charts are developed for different pipe materials and sizes and are available in both U.S. and metric units of measure. The charts are widely available from sources such as pump manufacturers’ technical publications, hydraulic engineering handbooks, pipe manufacturers, online calculators, etc. They provide a head loss value for a specified length of pipe based on the pipe size, pipe material, velocity, and flow rate.
To use the chart, first identify the column with the desired pipe size. Next, follow the pipe size column down to find the desired flow rate, rounding up if the flow rates provided do not match exactly. Follow the row to the right with the flow rate selected to view the velocity column. Then continue following the flow rate row to the right to view each pipe material’s head loss value.
Make sure to review the chart to identify how the value is measured. For example, the chart in Figure 2 states that the head loss is for each 100-foot section of pipe. Therefore, a ½ column of plastic pipe flowing 4 gallons per minute will create 14.8 feet of friction loss per 100-foot section of pipe.
The second valuable tool is a friction loss through fittings chart. These are also readily available and are like using a friction loss chart. A key difference with a fittings loss chart is that the values provided convert to straight-line pipe equivalents.
For example, in Figure 3, a ½-inch 90° plastic elbow would be the equivalent of 4 additional feet of straight pipe. In this scenario, if the straight-line pipe were 100 feet, adding a plastic elbow would increase the overall pipe length to 104 feet.
Armed with the values provided in the charts and accounting for increases in elevation, figuring head loss through the piping system is relatively easy. Rudimentary addition, multiplication, and division are all that are required.
An example to try:
- 275 feet of ½-inch plastic pipe
- Flowing 4 GPM
- One plastic ½-inch elbow
- 10 feet of elevation
Step one: Figure out the total equivalent pipe length.
275 feet + 4 feet of fittings + 10 feet of elevation = 289 feet
of pipe equivalent
Step two: Divide the total pipe length by 100 feet (as specified by the head loss chart).
289 ÷ 100 feet sections of pipe = 2.89
Step three: Multiply the pipe length value and the friction loss chart value.
2.89 × 14.8 = 42.77
The value of 42.77 represents the amount of head loss in the discharge piping and must be accounted for so the system can work correctly.
Unfortunately, generalities do not correctly size or troubleshoot systems, and all the nuances can make seeking the correct values seem daunting.
For example, if the shower requires 160 feet of head, the pump must produce at least 202.77 feet of head to overcome head loss due to friction and still operate the shower correctly. Failure to account for this head loss due to friction could lead to an undersized pump, increased energy costs and maintenance costs, or worst of all, an unhappy shower user.
When Dan received the callback from the customer, his suspicions were confirmed. As the customer provided Dan with the details regarding the pipe, Dan checked off all the culprits causing the excessive head loss and low water pressure. First, the customer noted that they found a 1-inch steel pipe installed.
Culprit No. 1 – Smaller pipe diameter with rough pipe material
Second, the customer found that the pipe had several elbow fittings installed to jog around what appeared to be an old structure on the property.
Culprit No. 2 – Longer than expected pipe run and additional fittings
Using a friction loss chart and the new information provided, Dan calculated the 85-foot run to the house was closer to 130 feet after accounting for friction loss due to pipe length, materials, and fittings.
With the issue diagnosed, the customer switched to a 1½- inch plastic pipe, straightened the run, and eliminated most fittings. Then, just as Dan expected, the pump showered the customer with adequate pressure.
Scott Tystad is the education leader for Pentair’s residential and commercial business units. Tystad is a Pump Systems Matter Board member who holds a master’s degree in business security from Webster University and a bachelor’s degree in horticulture from Kansas State University. He can be reached at email@example.com.