Technical Bulletin 145. Aeration Pipe Sizes. by:

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1 Technical Bulletin 145 Aeration Pipe Sizes by: Environmental Dynamics International Published: 12/2011

2 DISCLAIMER Technical Bulletins are presented as a service by Environmental Dynamics International headquartered in Columbia, Missouri USA. For additional more information regarding this particular bulletin or your specific aeration application contact Environmental Dynamics International by calling , toll free at EDI.AIR8 ( ), or via at techbulletins@wastewater.com. 1

3 TECHNICAL BULLETIN 145 AERATION PIPE SIZING For sizing of aeration piping system and the piping system between blowers to the aeration tank it is important to have rational design criteria to allow economical pipe sizing and efficient pipe sizing. EDI has established a rational design method for sizing piping for any type of pipe or any aeration application to deliver economical operating conditions. The most common criteria to size aeration piping in the industry is to use one of two methods: A. Limit the amount of pressure drop per unit of pipe length, i.e. inches of water column or kpa per 100 feet (30.5m) as an example. B. Set a maximum air velocity in the piping system, i.e. ft/sec or m/sec. EDI has reviewed these two design methods and have found the criteria for pressure loss per unit of pipe length is the consistent method that can be applied rationally and economically for any pipe size and any type pipe. EDI designs are all based on limiting a maximum pressure loss per unit of piping length for optimum performance. When designers use air velocities for sizing pipe, the tendency is to specify a maximum allowable air velocity and apply it in a broad range of pipe diameters. This is not logical and results in significant and excessive pressure losses in small diameter pipe while offering insignificant or very minor losses with oversizing in larger diameter pipe, i.e. excessive pipe cost. The use of air velocity in the design of piping systems is convertible to the pressure loss per unit of length of piping; however, the air velocity should not remain constant for all diameters of pipe. If the air velocity criteria is employed it will demonstrate the pressure loss in small diameter pipe with a typical engineering velocity specification of 12 meters per second will be as much as three times the pressure loss per unit of length vs a system that has large diameter pipe with the same velocity limit of 12 meter per second. An example of this would be appropriate: 2

4 Example: With a limiting velocity of 12 meters per second, a pipe that is 4 inches in diameter or approximately 100 mm in diameter in PVC would have a pressure loss of approximately 2.7 inches of water column per 100 feet (0.67 kpa per 30.5 meters). This is a reasonable pressure loss and is quite acceptable and typical design criteria for 100 mm diameter pipe. Now let s compare this 12 meters per second limitation in a PVC pipe that is 12 inches diameter (300 mm diameter). Pressure loss in this case is only 0.7 inches per 100 feet (0.17 kpa per 30.5 meters). Larger diameter pipes make velocity losses less significant at the same velocity. For even larger piping used in air headers the 12 m per sec can be extremely costly. This demonstrates that using an air velocity that is a constant value does not recognize the cumulative effects of pressure. Allowing the pressure loss per unit of pipe length to be variable is costing significant extra capital cost for large piping systems or creating excess pressure loss on small piping systems. A rational pipe sizing method will recognize pressure losses at any portion of the system have equal and accumulative impact on blower pressure and energy consumption. It should also be noted that the use of a single constant design air velocity does not take into account the material of construction of the piping, i.e. stainless steel versus cast iron pipe, etc. It is clear that piping systems that use plastic piping or smooth stainless steel have much lower pressure losses per unit of length than systems that might use cast iron or ductile iron piping. Air velocity also ignores age or condition of the pipe. Again, the fixed velocity limitation does not provide for a rational evaluation of all systems. EDI s design criteria using pressure limitations EDI proposes the optimum or economical design of aeration piping and aeration systems will incorporate a pressure criteria per unit length of air piping. This takes full account of the effect of diameter on pressure loss, plus it can take into account any roughness, coefficient, or pipe characteristics that are involved on any project. This is a universal and rational design procedure and has used successfully by EDI for over 20 years. EDI generally employs criteria limiting pressure loss in the piping to approximately 3 inches water column per 100 feet (0.75 kpa per 30.5 meters)! This criteria can be applied to any type pipe, any size pipe, and give a economical capital cost and a low operating cost system. This criteria is consistently applied to assign equal weight to all pressure losses in the aeration system piping. 3

5 With the EDI rational design method, consistent or rational design velocities for aeration piping range from 8 m/sec for 2 inch (DN 50) pipe to over 27m/sec for 12 inch diameter (DN300) pipe when limiting loss to 3 inch wc/100 ft (0.75kPa/3.05m). EDI uses a computer analysis to calculate pressure losses and pipe sizes accurately; however it is possible to create a guideline chart for selection of pipe diameters. The attached chart provides an EDI guideline of the maximum design air handling capacity for satisfactory plastic or stainless steel pipe systems in any aeration application. As a companion, the chart also includes the approximate velocity allowed in feet per second and meters per second for this 3 inch wc/100 ft economical and consistent pressure loss design basis. IPS Pipe Metric Pipe Typical Recommended Typical Airflow Headloss Velocity* Recommended Airflow Headloss Velocity* Pipe Size (in) Limit (scfm) (in W.C./1000 ft) (ft/sec) Pipe Size (mm) Limit (Sm 3 /h) (kpa/30.5 m) (m/sec) mm mm mm mm mm mm mm * Based on Sch10 SS piping. For other material - wall thickness and roughness coefficient variables must be considered. 4