How to Calculate Water Pressure Loss With Plumbing Pipes

Calculating the pressure loss of water moving through plumbing pipes is significantly less involved then most pressure loss calculations. Plumbing systems work similarly enough that assumptions can be made about how they operate. Plumbing pipes transport liquid water through a smooth pipe at less than two meters per second. Since almost all plumbing pipes obey these simple guidelines, you do not have to deal with issues like unusual fluid properties, odd fluid flow or determining how bumpy the pipe is.

Reynolds number vs. friction factor chart
Water viscosity table

Instructions

- 1
  Divide the density of water by the dynamic viscosity of water. The viscosity is the amount of resistance a fluid has to being deformed. You need to know the temperature of the water in the pipe to determine the dynamic viscosity. Once you have the temperature, you can look up the viscosity on a water viscosity table. Make sure you use the dynamic viscosity, not the kinematic viscosity. The density of water can be estimated at 1000 kg per cubic meter. Regardless of temperature, liquid water in a plumbing pipe is always going to be approximately this density.
- 2
  Multiply the dividend of the density and viscosity by the speed of the water through the pipe and the internal diameter of the pipe to get the Reynolds number for the pipe. The Reynolds number is a value that is used in pressure loss equations. The internal diameter of the pipe is not necessarily the same as the measurement the pipe is sold under. You need to measure the internal diameter yourself or check the pipe specifications for the internal diameter. Make sure you use consistent units. For example, if you have water going 1 meter per second in a 10 mm pipe, you should multiply 1 meter per second by 0.01 meter pipe, converting 10 mm into meters.
- 3
  Look at a chart comparing Reynolds number against the friction factor. Locate the Reynolds number you calculated on the horizontal axis of the chart. Trace that point vertically to the line on the chart. The point the line passes over the Reynolds number is used to determine the friction factor. Read the value on the vertical axis to get the friction factor for the pipe.
- 4
  Divide the length of the pipe by the internal diameter of the pipe.
- 5
  Multiply the value from the previous step by the friction factor.
- 6
  Multiply the value from the previous step by the density of water.
- 7
  Multiply the value from the previous step by the velocity of water squared. The squared velocity of water is obtained by multiplying the velocity of water with itself.
- 8
  Divide the value from the previous step by two. The number given is the pressure loss. If you used metric units, the pressure loss is in Newtons per square meter.

How to Calculate a Plumbing Vent Requirement

How to Repair Plumbing Leaks Under Concrete Slabs

Plumbing