technical memorandum - tacomagovme.cityoftacoma.org/download/rfp/outfalleval/ctp...technical...

Technical Memorandum

TO: Eric Johnson, City of Tacoma

FROM: Jon Holland, Brown and Caldwell

PREPARED BY: Mike Britch and Jon Holland, Brown and Caldwell

REVIEWED BY: Garr Jones, Brown and Caldwell

DATE: April 6, 2004

PROJECT NO.: Brown and Caldwell Project No. 24616.706

SUBJECT: Central Wastewater Treatment Plant Outfall Evaluation Summary of Findings

CONTENTS Introduction........................................................................................................................................................1 Review of Previous Reports and Test Data ...................................................................................................2 General Summary of Findings .........................................................................................................................3 C-Values from Other Evaluations...................................................................................................................3 Summary of Air Handling Related Issues ......................................................................................................4 Flowmeter Calibration.......................................................................................................................................7 Hazen-Williams Relationship-Related Issues.................................................................................................7 Recommendations .............................................................................................................................................8

Introduction

The existing outfall for the City of Tacoma (City) Central Wastewater Treatment Plant (CTP) consists of an onshore portion (Unit A) from the CTP through the Tacoma tide flats to near the northerly end of Port of Tacoma Road, and an offshore portion (Unit B) which extends from this point to the outfall terminus in Commencement Bay at a depth of approximately 155 feet below mean lower low water. Unit A is 14,152 feet long, which includes 13,734 feet of buried 60-inch inside diameter prestressed concrete cylinder pipe (PCCP) and an elevated crossing of the Puyallup River using 418 feet of 60-inch inside diameter steel pipe (with ½-inch thick mortar lining). Unit B is 1,886 feet long and is 56-inch outside diameter, ¾-inch-thick steel pipe with a coal tar epoxy lining. Unit B consists of a 650-foot onshore section and a 1,236-foot offshore section. The offshore section includes a 298-foot diffuser section with 30 12-inch-diameter risers with a 12-inch- diameter tide-flex diffuser check valve at each outlet. The Unit A pipe was installed approximately in 1987/1988. Unit B was constructed from 1987 to 1989.

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Flow and pressure data for the outfall have previously been analyzed in the CTP Facility Plan (Parametrix, Inc., March 2002) and in the Estimation of CTP Outfall Hazen-Williams C-Value Technical Memorandum (TM) (Parametrix, Inc., May 30, 2003). These analyses were conducted in an effort to determine Hazen Williams C-values for use in evaluation of acceptable levels of flow increases in conjunction with an expansion of the CTP. The purpose of this TM is to summarize Brown and Caldwell’s review of these previous evaluations and to provide updated conclusions and recommendations.

Review of Previous Reports and Test Data

The Parametrix outfall TM concludes that the outfall “effective” C-values range from 110 to 120. It notes that “obstructions that may be present in the pipeline would act to reduce the capacity of the outfall and would be reflected in the calculation of lower C-values than would be calculated for a free-flowing pipe.” It further identifies the potential for air to be trapped in the outfall and the resulting need for additional air/vacuum release valves as important issues to be considered.

The Facility Plan includes the following excerpt:

A Hazen-Williams coefficient of C=112 was found to best fit the outfall model to the measured data, but actual Hazen-Williams coefficients for the outfall pipeline remain uncertain. A value of C=112 is lower than what would typically be used for hydraulics calculations. AWWA Manual M9, Concrete Pressure Pipe, suggests a value of C=145 for 60-inch concrete pressure pipe. A commonly recommended conservative design value for older pipe is C=120.

The Facility Plan cites American Water Works Association (AWWA) Manual M9, Concrete Pressure Pipe, as suggesting a value of C=145 for 60-inch concrete pressure pipe and goes on to note that “any air in the outfall pipeline would significantly lower the effective C-value.”

The 60-inch PCCP has a limiting operating pressure of 67 pounds per square inch according to the Facility Plan. The resulting C-values in conjunction with the pressure limitations on the outfall pipe were used by Parametrix to determine the length of parallel outfall needed to keep the pressure limitations from being exceeded by the future flow conditions.

The pressure gauge and flow data used for the Parametrix outfall TM were reviewed. Some of the data appeared erratic or unreliable; for example, some data points varied widely compared to other data or resulted in a reverse hydraulic gradient. The remaining data were evaluated and values were averaged. A C-value of 112 resulted from this analysis, the same value Parametrix derived in the Facility Plan.

The Facility Plan indicates that with C=112, 5,300 feet of new parallel outfall is required for an alternative that considers relocating the effluent pump station and for a capacity of 150 million gallons per day (mgd). The calculation of the required length of parallel pipeline is critical to meet-ing the peak flow capacity objectives.



General Summary of Findings

Our review led to the following findings:

Based on the available data, we concur with Parametrix’s determination of the cur-rent effective C-value of 112. We further concur that the expected C-value is much higher. For secondary effluent in a 17-year-old PCCP, values in the range of 130 to 145 are expected.

Inadequate air release is likely contributing to the lower effective C-value. Repeating the pressure test readings during a peak flow event may not be warranted until a de-tailed analysis of air handling needs is completed and recommendations are implemented.

In situ calibration of the CTP effluent flow meter, if not recently performed, should be done to ensure that accurate flow readings are obtained.

Use of the Hazen-Williams relationship for calculation of the capacity of the outfall, though commonly applied in the industry, may not be appropriate given the size of the existing pipeline and the history of development of the Hazen-Williams formula.

Each of these findings is discussed in greater detail below, followed by specific recommendations.

C-Values from Other Evaluations

When considering the outfall effective C-value results in light of published industry standards and other anecdotal information, much higher C-values are typically recommended. A summary of values recommended for large diameter steel pipe, ductile iron pipe, and concrete cylinder pipe, each of which have a similar cement mortar lining, are shown below.

Steel Pipe. AWWA Manual M11, Steel Pipe–A Guide for Design and Installation, recommends the following for the C-value for the Hazen Williams formula:

C = 130 + 0.16 D

The above formula includes consideration of long-term lining deterioration, slime buildup, and other factors. For the 60-inch outfall pipe, this results in a C-value of 140.

Ductile Iron Pipe. AWWA Manual M41, Ductile-Iron Pipe and Fittings, indicates that tests by the Ductile Iron Pipe Research Association suggest a C-value of 140 is reasonable for cement-lined ductile-iron pipe.



Concrete Cylinder Pipe. AWWA Manual M9, Concrete Pressure Pipe, indicates that the

C-value can be determined from the following formula:

C = 139.3 + 0.169 D

For 60-inch concrete cylinder pipe, the resulting value is 149. For 60-inch pipe, the M9 Manual recommends a maximum conservative C-value of 145.

Anecdotal information on pipelines for which C-values have been back-calculated include:

For 18-inch and 24-inch ductile iron force mains, C-values of 132 and 138 were de-termined after years of service.

For the City of Portland’s large steel water transmission mains, back-calculated C-values were generally in the range of 130 to 140 (or more in some cases).

For large diameter concrete sewer interceptors the size of the outfall in Seattle, a Manning’s “n” value equal to about 0.012 has been suggested for use. At a 38-mgd flow (current average wet season flow), the equivalent C-value for the outfall would be about 130.

In all cases above, both with the different industry guidelines for the pipe and the anecdotal infor-mation, a discrepancy with the determined effective outfall C-value exists. Possible reasons for this discrepancy include:

Unknown blockages, restrictions, or other physical impacts to the pipe Flow restrictions due to air pockets in the pipe Faulty flow measurement data Inaccuracies inherent with the use of the Hazen-Williams relationship

Summary of Air Handling Related Issues

The existing air release connections for the outfall, except for the high point at the river crossing, consist of 6-inch-diameter tangential outlet pipes that extend horizontally from the crown of the pipe to the air release valves. At the extreme high points in the outfall profile at the river crossing, the air valves were connected directly to the crown of the pipe. These two types of air release valves are shown in Figures 1 and 2.




Figure 1. 6-inch Tangential Air Release Valve

Figure 2. Air Release Valves for High Point in Outfall at River Crossing



Based on these figures, it appears air is exhausted by 1-inch air release valves.

More current ways to provide for air release is by special fittings with large tee sections at the crown of the pipe. A typical detail for this type of appurtenance, from the drawings for the City’s Leach Creek Stormwater Force Main, is shown in Figure 3.

Figure 3. Typical More Current Air Trap and Control Details

One of the benefits of the style of air trap and control detail is that it provides an air accumulation chamber at the crown of the pipe which is viewed as a better means of moving the air to the valve.

Based on information from the Facility Plan, current average day flows for the dry season and wet season are 28 mgd and 38 mgd, respectively. At these flows, the full pipe velocity would be 2.2 feet per second (fps) and 3.0 fps, respectively. Since air bubbles have a rise rate of approximately 1 fps, it is likely there is insufficient shear stress available from the forward progress of the effluent to efficiently move air accumulations to points where the air release valves can remove them. Once an air pocket has formed, a velocity of about 6 fps may be needed to create enough shear stress to move the air along the pipe.

The overall pipeline slope from the end of the river crossing high point to the end of the Unit A work (near the upstream end of the final submerged portion of the outfall) is 0.00179. Using a C-value of 112, this would suggest that about 70 mgd of capacity exists assuming the hydraulic grade


line downstream from Unit A does not influence the upstream portion (e.g., possibly with a low tide condition). It is believed that at times, portions of the outfall operate in a partially full condition. Downstream from the river crossing high point, the flow likely experiences a hydraulic jump (slope in excess of 20 percent) during such times. We expect the hydraulic jump will cause entrainment of large air bubbles that will have the effect of destabilizing the flow regime and preventing full use of the available pipe capacity. As the tide rises and/or the flow increases, the portions of the outfall that were previously partially full will become pressurized. Given the relatively low velocities in the pipeline, it is likely that large bubbles of air, trapped in the pipeline, function to throttle flow on a regular basis.

Further, we note the outfall TM indicates that air was being exhausted throughout the test. This occurred with a mean velocity in the pipe approaching 6 fps. If air was still being exhausted over many hours, strong consideration should be given to improvements that can more quickly exhaust air.

Flowmeter Calibration

Various flowmeter installations in the wastewater industry have been proven to be inaccurate. In one study, published by the U.S. Environmental Protection Agency in the 1970s, the range of inaccuracy in the group of installations examined was found to be 40 to 250 percent of actual flow. This is because flowmeter manufacturers develop meter calibration curves under laboratory condi-tions, whereas, the field installation rarely replicates the ideal conditions found in the factory laboratory. We have confirmed such findings in our experience. We have learned through this experience that the only certain means for developing accurate flow information is to calibrate the metering equipment in place, using an independent means of determining actual flow during a meter run. If numerous calibration runs are performed over the majority of the meter’s full flow range, a calibration curve unique to the specific meter installation can be developed. Several independent calibration methods are available, depending principally on the specifics of the meter installation. We have found the dye-dilution method to be one of the most accurate and repeatable, with a range of uncertainty that can be as little as ± 2 percent of actual flow possible.

Hazen-Williams Relationship-Related Issues

Use of the Hazen-Williams relationship itself should be considered carefully. A recent paper, published in the November 2003 American Society of Civil Engineers Journal of Hydraulic Engi-neering (Hydraulic Design of Large-Diameter Pipes, Bombardelli and Garcia) showed that serious errors can accrue by using the Hazen-Williams relationship as the basis of calculating the capacity of large diameter pipes. The authors note that more than 92 percent of the pipes in the field studies used as the basis for developing the formulation were less than 60 inches in diameter and that more than 72 percent of the installations in the Hazen-Williams database were operating at Reynolds numbers of less than 500,000 (by way of comparison, at 38 mgd, the Reynolds number in the 60-inch outfall is 1,400,000). Given the limited extent of that database applicable to the use of the Hazen-Williams formula for this project, it seems prudent to base any further capacity studies on the Darcy-



Weisbach formula and a range of roughness factors instead to develop a range of probable loss factors.

Recommendations

The following are our recommendations relative to the outfall evaluation:

1. The CTP effluent flow meter should be verified to determine that it is properly cali-brated against actual installation influences. We recommend a procedure that provides an independent means of calibration, such as the dye-dilution method.

2. Pressure gauges on the outfall should be inspected and calibrated if needed.

3. The elevations of the pressure gauges should be surveyed if the information is sus-pect.

4. Check to make sure that the existing air valve components are in good working order and that related isolation valves are in fully open condition. Consider replacement with more efficient devices and installation details.

5. Evaluate the location, style, and sizing of current air release valves and consider al-ternatives.

6. Evaluate whether a siphon maintenance station to eliminate the current hydraulic jump at the river crossing is appropriate. The siphon maintenance station would consist of a dome at the high point, mounted on the pipe to gather air, a vacuum pump to expel the air, and a series of sensors to control the vacuum pump when a siphon has been established. The station would shut it down when the hydraulic gra-dient exceeds the elevation of the high point.

7. To further quantify air in the pipe, a vacuum pump could be attached at an existing air valve connection to draw any air pocket out of the crown of the pipe at that loca-tion during high flows. Check to see if there is any resulting change in back-calculated hydraulic capacity of the outfall by taking readings from upstream and downstream pressure gauges.

8. Consider adding additional air handling appurtenances at key locations, including the discharge header from the effluent pump station as it exits the building and dives underground.

9. Evaluation of impacts on pressure surge and pump station operation should be con-sidered before making any air/vacuum appurtenance changes.




10. The approach condition to the effluent pump station wet well should be reviewed, as excessive turbulence can result in entrained air that the pumps move into the outfall.

11. The outfall pipe should be inspected to verify that no physical problems or debris accumulations exist and that the interior surface lining does not appear to be com-promised in some fashion. This includes inspecting the offshore portion to determine whether marine life has attached to the lining.

12. Recalculate hydraulic losses in the outfall using a more applicable relationship such as the Darcy-Weisbach formula and a range of probable roughness factors that more reasonably represents expected operating conditions. The calculations and additional tests should be done only after the air handling issues have been resolved.

13. As previously verbally discussed with the City, a thorough corrosion and condition assessment should be performed for the entire outfall, particularly related to the PCCP portions, to verify that the pipe is in suitable condition to handle the signifi-cant increase in pressures that will accompany the increased discharge capacity planned for the effluent pump station.

14. Evaluate the adequacy of existing outfall thrust restraint to ensure the increased pres-sures do not damage the pipeline at bends.