April 2017 Blog:

IMPORTANCE OF PIPE SIZING

Pipe size is a critical aspect in designing a hydronic system for cost, longevity, and noise reduction. The inside pipe diameter should be determined after the optimal flow rate is calculated. The pipe diameter is calculated with a maximum velocity limit. This velocity limit is critical to prevent damages from occurring. For HVAC applications, Hays recommends using maximum velocity of 7 fps for design. Fluid velocity (V) is directly proportional to the fluid flow (Q) by equation 1 (ASHRAE, 2013).

Q =VA                                                                                  (eq. 1)

where                                                                                                                                            Q=Flow, ft3/sec                                                                                                                                         V= Velocity, ft/sec                                                                                                                              A=Area, ft2

Problems can occur when a maximum velocity is not taken in consideration. From the flow equation above, as the flow increases if the pipe area does not increase this will create a higher fluid velocity. Flows at higher velocities can create undesirable noise levels, high erosion levels, and even higher pumping costs.

Noise in piping systems are a result from turbulence, cavitation, release of entrained air, and water hammer (ASHRAE, 2013). The Reynolds Number (Re) is a key indicator when looking at turbulent or laminar flow. Any value when calculating the Reynolds number over 10,000 is considered fully turbulent in that system. The Reynolds number is a dimensionless number found using equation 2 (ASHRAE, 2013). From this equation, the Reynolds Number is directly proportional to the fluid velocity. Any increase in velocity and turbulence increases with it creating unwanted noise.

Re=(DVρ)⁄μ                                                                                (eq.2)

where                                                                                                                                         Re=Reynolds Number, dimensionless                                                                                             D= inside diameter of pipe, ft                                                                                                      ρ=fluid density, ft2                                                                                                                                                                                                                                                      μ= dynamic viscosity, lbm/ft-s

Erosion occurs in hydronic systems by sediment, and water bubbles. Erosion in piping systems at velocities lower than 10 ft/sec. is not substantial. Sediment in the system at high velocities is where erosion transpires at a quick pace. Putting a strainer in piping systems is always a good idea to reduce sediment build up.

Water hammer is another design characteristic to be considered. The water hammer phenomenon is triggered when any moving fluid though a system stops abruptly. Large pressure spikes can be observed when fluid is moving at high velocities. Equation 3 (ASHRAE, 2013) shows that fluid velocity has a direct correlation to the pressure rise in a hydronic system. Keeping the velocity at a reasonable speed can reduce damage during any situation where water hammer occurs.

∆ph=(ρcs V)⁄gc                                                                          (eq.3)

where                                                                                                                                                        ∆ph= Pressure rise caused by water hammer, lbf/ft2                                                                                                                               cs= Velocity of sound in water, ft/sec                                                                                                gc = Gravity constant, ft/sec2

In summary, by properly sizing pipes based on flow through the system will prevent unwanted issues. Below is a chart of recommended pipe sizes given a certain flow rate. Use Hays Velocity Calculator for Pipe Sizing (Hays Fluid Controls, 2016).

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