"Pig" cleaning water transmission pipelines.
Two parallel eleven-mile stretches of 36-in. transmission main deliver Lake Michigan water by gravity to Green Bay from a water filtration plant located 51/2 miles east of the city on high ground. One pipeline was constructed in 1955, when Green Bay converted from well water to the Lake Michigan water source. This pipeline was paralleled more than a decade later by installation of a second 36-in. pipeline (9 miles in 1966; 2 miles in 1974).
Operating conditions on the twin gravity pipelines began to change as the population grew and the area on the east side of the city developed, with resulting increases in water demand. By 1973, residential development on the east side of the city resulted in the creation of several new water pressure zones along the route of the 36-in. mains. These pressure zones are controlled by means of pressure reducing valves attached to branch connections to the transmission pipelines. The two pipelines were now required to provide minimum water pressure levels at several pressure reducing valves along the pipeline in addition to maintaining a minimum of 80 psi at the city's main pressure control station.
Loss of Carrying Capacity
The build-up of a white powder on the pipe interior walls (determined by laboratory analysis to be aluminum silicate) was discovered in 1973 when a portion of the 1955 pipeline was relocated for a highway project. Also, lower than expected water pressures in upstream pressure zones along the 36-in. pipelines warranted further investigation.
In May 1987, hydraulic tests were performed to determine the actual carrying capacity of the two mains. These tests showed that the 1955 pipeline had a Hazen-Williams flow coefficient "C" value of 89 while the 1966 pipeline had a "C" value of 100. Initial "C" values for newly installed prestressed concrete pipelines are generally in the range of 130 to 140.
The unexpectedly high head losses during the transmission of water and the discovery of a build-up of aluminum silicate on the pipe walls led to investigation of possible pipeline cleaning methods. Efforts to improve pipeline "C" values by adding an orthophosphate compound to the water proved to be inconclusive.
A review of the literature revealed that hydraulic pipeline cleaning with foam swabs, often referred to as "pigs," had been successful in many locations where water utilities had experienced pipeline capacity loss. Case studies revealed how two cities pumping alum-treated Great Lakes water (the same situation as in Green Bay) successfully restored carrying capacity to their 42-in. and 54-in. diameter pipelines. In November 1987, a meeting was held in Chicago with a pipeline cleaning expert, Joe Dulaney of Maxi-Flow (Carmi, Illinois) to discuss the feasibility of performing a "pigging" demonstration in Green Bay. This demonstration would determine the feasibility of pipeline cleaning without requiring openings to be broken into the prestressed concrete piping for entry and exit of the pigs.
Mr. Dulaney studied drawings of the Green Bay facilities and the dimensions of possible entry and exit points and advised that a demonstration was indeed feasible if pigging were carried out with a very soft pig material--a foam swab. Restrictive dimensions of a reservoir access hatch (24-in. x 36-in.), venturi meter with a throat diameter of approximately 20 in., line gate valves 30 in. in diameter, and boiler plate manhole covers (18-in. x 16-in.) on the pipeline eliminated from consideration the use of stiffer pig materials.
Following consultations with the Wisconsin Department of Natural Resources, the decision was made to proceed with a demonstration project.
Pig materials and a pig launch device termed a "Cherne Plug" were ordered and plans were formalized for two pig runs through a 41/2 mile portion of the 1955 transmission main. (The Cherne Plug was ordered as an emergency aid for pig launching, to be used if the head of water available proved to be inadequate to launch the foam swab pigs.)
On May 11, 1988, the first foam swab pig was introduced into the 36-in. pipeline through an open 36-in. diameter sluice gate in the finished water reservoir at the water plant. The pig entered at 6:50 p.m. and was retrieved some 24,600 ft down stream at 1:30 a.m. the following morning. Later that day, a second pig was introduced at 6:40 p.m. and retrieved at the exit point at 10:26 p.m.
The discharge of a chocolate brown fluid followed by a white chalky fluid that accompanied the pig (and continued during flushing after exit of the pig) was proof of the effectiveness of the cleaning method employed.
Evaluation of Pigging Demonstration
On May 13th, "C" value testing was performed to evaluate the degree of cleaning achieved in the two pig runs through the main. The "C" value was determined to be approximately 108 as compared to a pre-cleaning value of 89. The pigging demonstration was considered to have been an unqualified success due to the flow improvement of 20 percent with only two runs, using a very soft pig material.
The test was non-destructive (no permanent pipeline facilities had to be broken to carry out the program); it was carried out without loss of service to water customers; it showed the degree of cleaning possible with a very soft pig; and it cost only about $8,000, not counting water utility employee time.
A review of the literature indicated that prestressed concrete pipelines could be cleaned almost to their original flow carrying capacity by means of pig cleaning, for instance:
* London, Ontario returned one mile of 42-in. pipeline from C = 100 to C = 135 on one occasion, and C = 115 to C = 135 on a second occasion, five years later.
* Onondaga County, New York returned 21 miles of 54-in.(and smaller) transmission pipeline from C = 105 to C = 130 to 135 with four pig runs.
More thorough cleaning, however, would require the design and installation of pig launch and capture facilities to allow the use of stiffer pig materials.
Permanent Cleaning Facilities
The twin 36-in. transmission mains had to be modified before they could be internally cleaned with optimum results. Locations for entry and exit of the
hydraulic cleaning tools or pigs were chosen to preclude the need for the pigs to travel through venturi-meter throats and areas of severely reduced pipeline diameter.
The static head available from the elevation at the water treatment plant was used as the driving force for the internal hydraulic cleaning operation.
Pig Entry. Pig entry points for the two 36-in. mains were located on water plant property just downstream of the venturi flow meters, and 36-in. x 36-in. wye fittings were cut into the twin 36-in. concrete pipelines for the purpose of pig entry. Each wye fitting was equipped with a blind flange that could be bolted closed at all times except when a cleaning pig was being introduced. For pig entry, a 37.5-in. x 36-in. reducing steel pig launch tube attached to a 90
elbow would be bolted to the 36-in. flange, following excavation down to and removal of the blind flange. Figure I illustrates the piping used for pig entry into the two 36-in. pipelines at the water filtration plant. The pigs would be forced into the main line by high pressure water introduced behind the pig by means of a fire hose connected to the fitting provided on the 36-in. blind flange.
Pig Exit. The East River was a natural site for disposal of flushing water. Green Bay's main pressure control station is located adjacent to the East River and the station piping and valves at this location preclude the further passage of a cleaning pig.
A temporary settling lagoon, approximately 55,000 sq ft in area, was constructed on the flood plain of the East River to settle out particulate matter. By means of excavation and subsequent compaction of earth, a berm approximately 5 ft high, was constructed to encircle the lagoon. Exits for the pigs were secured by means of 30-in. pressure taps into the 36-in. pre-stressed concrete pipelines just upstream of the pressure control station.
A 36-in. reinforced concrete sewer pipe was installed to carry the pigs and flushing water a distance of 400 ft from the exit taps to the temporary settling lagoon. The lagoon was equipped with an adjustable height outlet weir device (fabricated from corrugated metal culvert pipe by the Green Bay Water Utility staff). The weir box was fitted with 3-in. thick wood timbers so that clear water could be decanted off the top of the settling lagoon as the water level was progressively lowered.
Following the deposition of solids in the settling lagoon, the decanted clear flushing water was allowed to flow to the East River for ultimate disposal.
Cost of Cleaning Facilities
A construction contract was prepared and publicly bid for furnishing and installation of the cleaning facilities (as well as the removal of the lagoon and restoration of the flood plain following the pig cleaning operations). The contract cost amounted to $287,500 to furnish all material and insert two 36-in. x 36-in. wye fittings, pressure tap the 36-in. pipelines, install 430 ft of 36-in. concrete sewer line, furnish 36-in. fittings, construct the lagoon (and later remove it), insert a pig entry station on a 24-in. concrete pipeline, and remove three 24-in./20-in. butterfly valves in a 24-in./20-in. pipeline in the city.
Cleaning the Mains
With Mr. Dulaney of Maxi-Flow as the on-site pipe cleaning consultant, pig cleaning of the twin 36-in. mains was carried out in the low water demand winter months of 1990-91.
The 1955 Pipeline. On February 11, 1991, at 10:30 a.m., a 36-in. foam swab pig was launched into the 1955 main at the treatment plant. The pig arrived at the settling lagoon at 6:03p.m., having travelled a distance of 11.2 miles in approximately 7.5 hours, pushed along with an average flow of approximately 10 mgd.
The next day, a "criss-cross pig" (cylinder of foam swab reinforced with externally wrapped plastic strips) was launched in tandem with a foam swab pig. The two pigs travelled the 11.2 mile distance and exited into the settling lagoon. (A third foam swab was later launched to assure the complete removal of the second foam swab). The pipeline was then flushed with finished water before being returned to service. "C" value tests were performed a few weeks later to evaluate the effectiveness of the cleaning process. Tests on March 8 determined the "C" value to be 155, while further tests on March 20 showed the "C" value to be 150.
The 1966-74 Pipeline. On March 11 at about 10:00 a.m., a 36-in. foam swab was launched into the 1966 36-in. main at the treatment plant. The pig arrived at 6:23 p.m. (in three pieces), having travelled 11.2 miles in approximately 81/3 hours.
On March 12 a 36-in. foam swab pig was launched in tandem with a criss-cross pig. The two pigs travelled the 11.2 miles and exited the pipeline approximately 9 1/2 hours later at the settling lagoon. Following three hours of clean water flushing at a 14 mgd flow rate, the pipeline was returned to service."C"
"C" value testing on March 11 of the partially cleaned main determined an interim "C" value of 115 (up from C = 100 in 1987). On March 14 more extensive testing on the finished clean pipeline showed the "C" value had risen to 149.
Four years after the determination that the twin 36-in. pipelines had degraded sufficiently in flow carrying capacity to warrant pipeline cleaning, the mains were cleaned and returned to service with "C" value coefficients of approximately 150, which is typical of new pipe values.
The pig entry and exit facilities installed for this cleaning procedure are permanent facilities that can be re-used in the future, whenever required. Only the compacted earth lagoon, used for pig exit on the twin 36-in. mains, was a temporary structure that would need to be reconstructed if the 36-in. mains are to be cleaned again in the future.
It is the conclusion of those associated with this project that the experience in Green Bay, Wisconsin in 1988 and 1991 indicates that foam rubber swabs and criss-cross pigs are efficient devices for the interior cleaning of pipelines carrying alum-treated finished drinking water taken from Lake Michigan.
Consultant to Provide Overflow Elimination Plan
The Tulsa Metropolitan Utility Authority has selected RJN Environmental Associates, Inc. (Dallas, Texas) to provide an over flow elimination plan for the Coal Creek Drainage Area in Tulsa. The project includes intensive survey activities to locate infiltration/inflow sources, engineering analysis for selection of most cost-effective solution, and design of improvements to eliminate overflows within the study area.
Intensive survey activity will include manhole inspection, smoke testing, dyed water flooding, and TV inspection with concurrent dyed-water testing.
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|Title Annotation:||Green Bay, Wisconsin|
|Author:||Shea, Richard D.|
|Date:||Jun 1, 1993|
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