During non-generation periods, leakage through the wicket gates of a hydroturbine often results in very poor quality water (low or zero dissolved oxygen) in the tailrace of the hydropower facility. Generally, the leakage rate is relatively small, usually about 5-10 cfs per turbine. A bottom-mounted diffuser system was designed based on laboratory-measured and manufacturer-supplied specifications about the gas transfer characteristics of the bubble plume generated by an 11- inch flexible head diffuser. The design criteria and the overall effectiveness of the system were evaluated in field tests at Lake Eufaula, Oklahoma. The analysis of field data is reported herein

Downer, Charles W

Price, Richard E

Wilhelms, Steven C

English

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
US Army Research U.S. Department of Defense 
1992 
Design of Pneumatic Diffuser System 
Steven C. Wilhelms 
US Army Engineer Waterways Experiment Station 
Charles W. Downer 
US Army Engineer Waterways Experiment Station 
Richard E. Price 
US Army Engineer Waterways Experiment Station 
Follow this and additional works at: https://digitalcommons.unl.edu/usarmyresearch 
 Part of the Operations Research, Systems Engineering and Industrial Engineering Commons 
Wilhelms, Steven C.; Downer, Charles W.; and Price, Richard E., "Design of Pneumatic Diffuser System" 
(1992). US Army Research. 100. 
https://digitalcommons.unl.edu/usarmyresearch/100 
This Article is brought to you for free and open access by the U.S. Department of Defense at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in US Army Research by an 
authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
DESIGN OF PNEUMATIC DIFFUSER SYSTEM 
Steven C. Wilhelms', Charles W. Downer', and Richard E. Price2 
Abstract 
During non-generation periods, leakage through the wicket gates 
of a hydroturbine often results in very poor quality water (low or zero 
dissolved oxygen) in the tailrace of the hydropower facility. Gener-
ally, the leakage rate is relatively small, usually about 5-10 cfs per 
turbine. A bottom-mounted diffuser system was designed based on 
laboratory-measured and manufacturer-supplied specifications about the 
gas transfer characteristics of the bubble plume generated by an 11-
inch flexible head diffuser. The design criteria and the overall 
effectiveness of the system were evaluated in field tests at Lake 
Eufaula, Oklahoma. The analysis of field data is reported herein. 
Introduction 
Low dissolved oxygen (DO) concentrations in the tailwaters of 
deep reservoirs with hydropower is not an uncommon occurrence. Even 
during non-generation periods, low DO can present a problem for manag-
ing the tailwater fishery. In most cases, the source of the low DO 
water is the hypolimnion of the reservoir. A number of alternatives 
exist that may prevent the development of low DO in the tailrace area. 
Some may be implemented in the reservoir, in the release structure, or 
in the tailrace area. The most appropriate technique must be selected 
based upon clear identification of the source and extent of the low DO 
water and upon the operat i ona 1 and water qual i ty object i ves of the 
project. 
Eufaula Reservoir, located on the Canadian River in Oklahoma, is 
a hydropower and flood control project operated by the Corps of 
'Research Hydraulic Engineer and 2Research Physical Scientist, 
Hydraulics Laboratory, US Army Engineer Waterways Experiment Station, 
Vicksburg, Mississippi 
1055 
Published in Hydraulic Engineering: Saving a Threatened Resource—In Search of Solutions: 
Proceedings of the Hydraulic Engineering sessions at Water Forum ’92. 
Baltimore, Maryland, August 2–6, 1992. Published by American Society of Civil Engineers.
1056 HYDRAULIC ENGINEERING 
Engineers. During the summer months, low DO conditions have been 
reported in the tailrace area of the project. The cause was identified 
as leakage of hypol imnetic water through the wicket gates of the hydro-
turbines during periods of non-generation. The severity of the low-DO 
problem prompted supplemental releases through a spillway sluice as a 
mitigative measure. Tailwater aeration held several advantages over 
in-reservoir enhancement techniques. Specifically, in this technique, 
only the water being released is aerated rather than treating a signi-
ficant volume of water in the hypolimnion. 
This paper reports on the design, installation, and pilot tests 
of a tailwater aeration system for Eufaula Reservoir. 
System Concept and Design 
A floating tailwater aeration system was tested at Denison Dam 
(Price 1990) in north Texas. Operational considerations dictated that 
an alternative design be developed. The concept of a bottom-mounted 
flexible-head diffuser system was developed. The diffuser heads would 
be mounted in a structural steel member and lowered to the draft tube 
bottom in the bulkhead slots (Figure 1). The system would remain in 
Bulk Head Slot ~ 
Air Supply 
--r------- lowering Cable/Chain 
I 
I 
I 
I 
I 
I 
I 
I 
I 
t 
Diffusers 
Figure 1. Conceptual design and placement of system 
place during turbine operation and only be removed for bulkhead 
placement, diffuser maintenance, and during the winter when the 
aeration system is unneeded. For Eufaula, draft tubes of the turbines 
are divided by a center pier; thus, two diffuser systems would be 
installed for each turbine, one in each bulkhead slot. 
According to Nolen et.al. (1989), the leakage rate through the 
PNEUMATIC DIFFUSER DESIGN 1057 
Eufau1 a turbines was approximately 1 m3/s. Since there are three 
turbines at the project, the total leakage was assumed to be divided 
equally among the three turbines. Since two diffuser systems would 
treat the leakage through each turbine, each diffuser system was 
designed with the capacity to treat approximately 0.17 m3/s. From DO 
observations (Nolen 1989) the following criteria were established: DO 
in the leakage - 2.0 mg/1, DO required - 5.0 mg/1, and required DO 
uptake - 3.0 mg/1. Performing a mass balance of the required DO uptake 
and the leakage rate to be treated by one diffuser system indicated 
that 510 mg/s of oxygen would have to be absorbed to provide the 
required DO. 
The depth of the diffusers would be approximately 12 m. 
Accordi ng to Pri ce and Ti 11 man (1991), the oxygen absorption effi-
ci ency3 for a diffuser in this depth ranges from 30 to 40 percent. 
Using the lower bound, the rate of oxygen supply to the diffuser system 
should be 1,700 mg/s. Because air is only 20 percent oxygen the rate 
of air supply must be approximately 8,500 mg/s. At atmospheric pres-
sure and ambient water temperature (16 DC), this translates to an air 
flow rate of 420 11m. The loading rate of the flexible-head diffuser 
selected for this test ranged up to 42 11m, indicated that at least 10 
diffuser heads would be required for each diffuser system. 
In summary, for the required DO uptake outlined earlier, the 
complete system would consist of six diffuser systems: two for each of 
the three turbi nes. Each di ffuser system woul d have at 1 east 10 
diffuser heads to deliver an air flow rate of 420 11m. 
System Fabrication and Installation 
The pilot study was conducted with only one-third of the required 
system, i.e., two diffuser systems were fabricated and installed in the 
draft tube slots of one turbine. However, the pilot study diffuser 
systems were purposefully over-designed. Each system was fitted with 
fifteen 23-cm-diameter flexible-headed diffusers permitting a maximum 
air flow rate of approximately 630 11m. The fifteen diffuser heads 
were mounted on three 5-cm-diameter PVC pipe manifolds. A 1.25-cm-
diameter high-pressure hose supplied air to each manifold. 
The channel beams, in which the diffuser heads were mounted, were 
cut to fit the width of the bulkhead slot with guides welded on each 
end. An alternative system is required because of the vibrational and 
uplift forces on the beams. Because of the experimental nature of the 
pilot study, the gantry crane was used to lower the diffusers in place, 
instead of installing a more permanent winching system. 
Operation and Observations 
During the field tests of the aeration system, the DO concen-
3The mass of oxygen absorbed as a port i on of the mass of oxygen 
avail ab1 e. 
1058 HYDRAULIC ENGINEERING 
tration of the hypolimnetic leakage was lower than that used in the 
design ranging from 0.3 mg/l to 0.9 mg/l with a water temperature of 
21.5 °C and 21.75 °C, respectively. DO concentration profiles were 
measured at several locations in the tailrace area just downstream of 
the hydropower plant. Observations were made prior to system start-up 
and are characterized by Figure 2. The very low DO observed below a 
depth of about 7 m represents the hypolimnetic leakage. The higher 
DO from the surface to a depth of 6 m was due to aerated releases 
through the sluiceway. The end sill of the tailrace area is located 
at an elevation approximately 6 m below the tailwater surface. 
Temperature °c 
20 22 24 26 28 30 
0 0 
I 
0 
I 
2 0 
Temperature I 
3 0 10 
I 
E 4 0 +' 
-I £ ..c 5 p +' +' a. , a. Q) 6 
___ --------0 20 Q) 0 0 
7 - - Dissolved Oxygen 
8 0 
0 
9 30 
10 
0 2 3 4 5 6 
Dissolved Oxygen, mg/I 
Figure 2. DO profile at tailrace end sill before aeration 
The system was operated with an air flow rate of 616 lim per 
diffuser system. After approximately 8.5 hours of operation with no 
sluiceway releases, the DO below a depth of 6 m had increased to 2.0 
mg/l, while the DO between the surface and 6 m decreased to just over 
2.0 mg/l {the result of losing the sluiceway's contribution to the 
oxygen in the surface layers. The observations are characterized by 
Figure 3. 
Conclusions and Recommendations 
The aeration system added a significant amount of oxygen to the 
leakage water, contributing approximately 1.7 mg/l. This represented 
about 36 percent of the DO required to raise the concentrations to a 
minimum of 5.0 mg/l. In other words, the system was approximately one-
third the necessary size to accomplish the objective. In fact, an 
analysis using the design procedure outlined in a previous section 
shows that the release DO should be 2.0 mg/l, if the actual DO concen-
PNEUMATIC DIFFUSER DESIGN 1059 
Temperature °c 
20 22 24 26 28 30 
0 0 
1 r;f 
I 
2 0 
3 Dissolved 10 
E 4 Oxygen +' ..... 
.£ 5 .c +' +' 
a. Temperature a. 
CD 6 20 CD 0 0 
7 
8 
9 30 
10 
0 2 3 4 5 6 
Dissolved Oxygen, mg/I 
Figure 3. DO profile at tailrace end si 11 after 
8.5 hours of aeration 
tration of the leakage is 0.3 mgll, the DO objective is 5.0 mgll, and 
the air flow rate is 616 11m. Thus, the system was performing as 
expected, verifying the applicability of the design procedure. 
The air flow requi red to bri ng the DO of the water in the 
tailrace to 5.0 mgll can be estimated at 2400 11m. At least 80 
diffuser heads would be required to deliver this air flow. From a 
practical perspective, a slightly over-designed system would be 
preferable, since it would allow more flexibility in operation and 
provide redundancy in the case of partial system malfunction. 
A redes i gned anchori ng system is a requ is ite for a permanent 
installation. The channel beam proved to be inadequate for the dynamic 
forces and vibration to which is was subjected. An alternative 
winching system for raising and lowering the diffuser systems in the 
bulkhead slots is also required. 
Acknowledgements 
The work reported herein was funded by the Water Quality Research 
Program of the US Army Engineer Waterways Experiment Station, 
Vicksburg, MS and from the US Army Engineer District, Tulsa. Permis-
sion was granted by the Chief of Engineers to publish this information. 
1060 HYDRAULIC ENGINEERING 
References 
Nolen, S. L., Carroll, J. H., and Veenstra, J. N. 1989. "Development 
of Water Release Plans for Minimizing Fish Kills Below Tulsa District, 
Corps of Engineers, Impoundments," Journal of Environmental Systems, 
Vol 18, No.4, pp 353-366. 
Price, R. E. 1990. ""Application of Aspirating Hydraulic Mixer to 
Improve Tailwater Dissolved Oxygen," Coastal and Inland Water Quality, 
Seminar No. 22, Feb 6-7, 1990, Las Vegas, NV, pp 107-114, available 
from US Army Engineer Hydrologic Engineering Center, Davis, CA 95616. 
Price, R. E., and Tillman, D. 1991 (Sep). "McCook Reservoir Water 
Quality Model," Technical Report HL-91-17, US Army Engineer Waterways 
Experiment Station, Vicksburg, MS 39180-6199 


Design of Pneumatic Diffuser System

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln US Army Research U.S. Department of Defense 1992 Design of Pneumatic Diffuser System Steven C. Wilhelms US Army Engineer Waterways Experiment Station Charles W. Downer US Army Engineer Waterways Experiment Station Richard E. Price US Army Engineer Waterways Experiment Station Follow this and additional works at: https://digitalcommons.unl.edu/usarmyresearch  Part of the Operations Research, Systems Engineering and Industrial Engineering Commons Wilhelms, Steven C.; Downer, Charles W.; and Price, Richard E., "Design of Pneumatic Diffuser System" (1992). US Army Research. 100. https://digitalcommons.unl.edu/usarmyresearch/100 This Article is brought to you for free and open access by the U.S. Department of Defense at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in US Army Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. DESIGN OF PNEUMATIC DIFFUSER SYSTEM Steven C. Wilhelms', Charles W. Downer', and Richard E. Price2 Abstract During non-generation periods, leakage through the wicket gates of a hydroturbine often results in very poor quality water (low or zero dissolved oxygen) in the tailrace of the hydropower facility. Gener-ally, the leakage rate is relatively small, usually about 5-10 cfs per turbine. A bottom-mounted diffuser system was designed based on laboratory-measured and manufacturer-supplied specifications about the gas transfer characteristics of the bubble plume generated by an 11-inch flexible head diffuser. The design criteria and the overall effectiveness of the system were evaluated in field tests at Lake Eufaula, Oklahoma. The analysis of field data is reported herein. Introduction Low dissolved oxygen (DO) concentrations in the tailwaters of deep reservoirs with hydropower is not an uncommon occurrence. Even during non-generation periods, low DO can present a problem for manag-ing the tailwater fishery. In most cases, the source of the low DO water is the hypolimnion of the reservoir. A number of alternatives exist that may prevent the development of low DO in the tailrace area. Some may be implemented in the reservoir, in the release structure, or in the tailrace area. The most appropriate technique must be selected based upon clear identification of the source and extent of the low DO water and upon the operat i ona 1 and water qual i ty object i ves of the project. Eufaula Reservoir, located on the Canadian River in Oklahoma, is a hydropower and flood control project operated by the Corps of 'Research Hydraulic Engineer and 2Research Physical Scientist, Hydraulics Laboratory, US Army Engineer Waterways Experiment Station, Vicksburg, Mississippi 1055 Published in Hydraulic Engineering: Saving a Threatened Resource—In Search of Solutions: Proceedings of the Hydraulic Engineering sessions at Water Forum ’92. Baltimore, Maryland, August 2–6, 1992. Published by American Society of Civil Engineers.1056 HYDRAULIC ENGINEERING Engineers. During the summer months, low DO conditions have been reported in the tailrace area of the project. The cause was identified as leakage of hypol imnetic water through the wicket gates of the hydro-turbines during periods of non-generation. The severity of the low-DO problem prompted supplemental releases through a spillway sluice as a mitigative measure. Tailwater aeration held several advantages over in-reservoir enhancement techniques. Specifically, in this technique, only the water being released is aerated rather than treating a signi-ficant volume of water in the hypolimnion. This paper reports on the design, installation, and pilot tests of a tailwater aeration system for Eufaula Reservoir. System Concept and Design A floating tailwater aeration system was tested at Denison Dam (Price 1990) in north Texas. Operational considerations dictated that an alternative design be developed. The concept of a bottom-mounted flexible-head diffuser system was developed. The diffuser heads would be mounted in a structural steel member and lowered to the draft tube bottom in the bulkhead slots (Figure 1). The system would remain in Bulk Head Slot ~ Air Supply --r------- lowering Cable/Chain I I I I I I I I I t Diffusers Figure 1. Conceptual design and placement of system place during turbine operation and only be removed for bulkhead placement, diffuser maintenance, and during the winter when the aeration system is unneeded. For Eufaula, draft tubes of the turbines are divided by a center pier; thus, two diffuser systems would be installed for each turbine, one in each bulkhead slot. According to Nolen et.al. (1989), the leakage rate through the PNEUMATIC DIFFUSER DESIGN 1057 Eufau1 a turbines was approximately 1 m3/s. Since there are three turbines at the project, the total leakage was assumed to be divided equally among the three turbines. Since two diffuser systems would treat the leakage through each turbine, each diffuser system was designed with the capacity to treat approximately 0.17 m3/s. From DO observations (Nolen 1989) the following criteria were established: DO in the leakage - 2.0 mg/1, DO required - 5.0 mg/1, and required DO uptake - 3.0 mg/1. Performing a mass balance of the required DO uptake and the leakage rate to be treated by one diffuser system indicated that 510 mg/s of oxygen would have to be absorbed to provide the required DO. The depth of the diffusers would be approximately 12 m. Accordi ng to Pri ce and Ti 11 man (1991), the oxygen absorption effi-ci ency3 for a diffuser in this depth ranges from 30 to 40 percent. Using the lower bound, the rate of oxygen supply to the diffuser system should be 1,700 mg/s. Because air is only 20 percent oxygen the rate of air supply must be approximately 8,500 mg/s. At atmospheric pres-sure and ambient water temperature (16 DC), this translates to an air flow rate of 420 11m. The loading rate of the flexible-head diffuser selected for this test ranged up to 42 11m, indicated that at least 10 diffuser heads would be required for each diffuser system. In summary, for the required DO uptake outlined earlier, the complete system would consist of six diffuser systems: two for each of the three turbi nes. Each di ffuser system woul d have at 1 east 10 diffuser heads to deliver an air flow rate of 420 11m. System Fabrication and Installation The pilot study was conducted with only one-third of the required system, i.e., two diffuser systems were fabricated and installed in the draft tube slots of one turbine. However, the pilot study diffuser systems were purposefully over-designed. Each system was fitted with fifteen 23-cm-diameter flexible-headed diffusers permitting a maximum air flow rate of approximately 630 11m. The fifteen diffuser heads were mounted on three 5-cm-diameter PVC pipe manifolds. A 1.25-cm-diameter high-pressure hose supplied air to each manifold. The channel beams, in which the diffuser heads were mounted, were cut to fit the width of the bulkhead slot with guides welded on each end. An alternative system is required because of the vibrational and uplift forces on the beams. Because of the experimental nature of the pilot study, the gantry crane was used to lower the diffusers in place, instead of installing a more permanent winching system. Operation and Observations During the field tests of the aeration system, the DO concen-3The mass of oxygen absorbed as a port i on of the mass of oxygen avail ab1 e. 1058 HYDRAULIC ENGINEERING tration of the hypolimnetic leakage was lower than that used in the design ranging from 0.3 mg/l to 0.9 mg/l with a water temperature of 21.5 °C and 21.75 °C, respectively. DO concentration profiles were measured at several locations in the tailrace area just downstream of the hydropower plant. Observations were made prior to system start-up and are characterized by Figure 2. The very low DO observed below a depth of about 7 m represents the hypolimnetic leakage. The higher DO from the surface to a depth of 6 m was due to aerated releases through the sluiceway. The end sill of the tailrace area is located at an elevation approximately 6 m below the tailwater surface. Temperature °c 20 22 24 26 28 30 0 0 I 0 I 2 0 Temperature I 3 0 10 I E 4 0 +' -I £ ..c 5 p +' +' a. , a. Q) 6 ___ --------0 20 Q) 0 0 7 - - Dissolved Oxygen 8 0 0 9 30 10 0 2 3 4 5 6 Dissolved Oxygen, mg/I Figure 2. DO profile at tailrace end sill before aeration The system was operated with an air flow rate of 616 lim per diffuser system. After approximately 8.5 hours of operation with no sluiceway releases, the DO below a depth of 6 m had increased to 2.0 mg/l, while the DO between the surface and 6 m decreased to just over 2.0 mg/l {the result of losing the sluiceway's contribution to the oxygen in the surface layers. The observations are characterized by Figure 3. Conclusions and Recommendations The aeration system added a significant amount of oxygen to the leakage water, contributing approximately 1.7 mg/l. This represented about 36 percent of the DO required to raise the concentrations to a minimum of 5.0 mg/l. In other words, the system was approximately one-third the necessary size to accomplish the objective. In fact, an analysis using the design procedure outlined in a previous section shows that the release DO should be 2.0 mg/l, if the actual DO concen-PNEUMATIC DIFFUSER DESIGN 1059 Temperature °c 20 22 24 26 28 30 0 0 1 r;f I 2 0 3 Dissolved 10 E 4 Oxygen +' ..... .£ 5 .c +' +' a. Temperature a. CD 6 20 CD 0 0 7 8 9 30 10 0 2 3 4 5 6 Dissolved Oxygen, mg/I Figure 3. DO profile at tailrace end si 11 after 8.5 hours of aeration tration of the leakage is 0.3 mgll, the DO objective is 5.0 mgll, and the air flow rate is 616 11m. Thus, the system was performing as expected, verifying the applicability of the design procedure. The air flow requi red to bri ng the DO of the water in the tailrace to 5.0 mgll can be estimated at 2400 11m. At least 80 diffuser heads would be required to deliver this air flow. From a practical perspective, a slightly over-designed system would be preferable, since it would allow more flexibility in operation and provide redundancy in the case of partial system malfunction. A redes i gned anchori ng system is a requ is ite for a permanent installation. The channel beam proved to be inadequate for the dynamic forces and vibration to which is was subjected. An alternative winching system for raising and lowering the diffuser systems in the bulkhead slots is also required. Acknowledgements The work reported herein was funded by the Water Quality Research Program of the US Army Engineer Waterways Experiment Station, Vicksburg, MS and from the US Army Engineer District, Tulsa. Permis-sion was granted by the Chief of Engineers to publish this information. 1060 HYDRAULIC ENGINEERING References Nolen, S. L., Carroll, J. H., and Veenstra, J. N. 1989. "Development of Water Release Plans for Minimizing Fish Kills Below Tulsa District, Corps of Engineers, Impoundments," Journal of Environmental Systems, Vol 18, No.4, pp 353-366. Price, R. E. 1990. ""Application of Aspirating Hydraulic Mixer to Improve Tailwater Dissolved Oxygen," Coastal and Inland Water Quality, Seminar No. 22, Feb 6-7, 1990, Las Vegas, NV, pp 107-114, available from US Army Engineer Hydrologic Engineering Center, Davis, CA 95616. Price, R. E., and Tillman, D. 1991 (Sep). "McCook Reservoir Water Quality Model," Technical Report HL-91-17, US Army Engineer Waterways Experiment Station, Vicksburg, MS 39180-6199 

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