Chapter Seven: Transmission Mains
- 1 Chapter Seven: Transmission Mains
- 1.1 General Objective of Transmission Mains
- 1.2 Transmission Through Pipes
- 1.3 Common Types of Water Supply Pipes
- 1.4 Problems in Transmission Mains
- 1.5 Operation and Maintenance Activities
- 1.5.1 Operation Schedule
- 1.5.2 Maintenance Schedule
- 1.5.3 Maintenance of Pipelines
- 1.5.4 Engaging Contractors for Maintenance
- 1.5.5 Records and Reports
1 Chapter Seven: Transmission Mains
1.1 General Objective of Transmission Mains
The overall objective of transmission mains is to deliver raw water and treated water from the source to the treatment plants and to the storage reservoirs respectively for supply into the distribution networks. Transmission of raw water and treated water is through pipes. Transmission through pipes can be either by gravity flow or by pumping.
The objective of O&M of transmission mains is to achieve optimum utilization of the installed capacity of the transmission system with minimum transmission losses and at minimum cost. To achieve this objective the water supply organization has to develop operational procedures to ensure that the system can operate satisfactorily, function efficiently and continuously and last as long as possible at lowest cost. District Metering Areas (DMAs) which are separated by bulk meters to monitor users and water losses are also used.
1.2 Transmission Through Pipes
All valves installed in the transmission main should be inspected regularly to ensure that there is no leakage otherwise leakage should be attended. Many water supply utilities in rural as well as urban areas, practice passive leakage control, meaning that they repair only those leaks that are visible. This is clearly not enough since 90% of the leaks are usually not visible on the surface. This means that it takes too long, often many years, until the utility is aware that there is a leak. Since awareness timing largely determines the volume of water lost from a pipe burst, utilities need a strategy to reduce the time for awareness. If attending leakage requires stoppage of flow through pipes, the same can be attended on a pre-fixed monthly shutdown day.
1.3 Common Types of Water Supply Pipes
Various pipes are generally used for water supply projects in the country. The selection, installation and specification of pipes should be based on field conditions and used according to the water supply or sanitation project requirements. Failure to abide to the specified conditions may lead to different operational problems.
1.4 Problems in Transmission Mains
Water or sewage is often wasted through leaking pipes, joints, valves and fittings of the transmission system either due to bad quality of materials used, poor workmanship, and corrosion, age of the installations or through vandalism. This leads to reduced supply and loss of pressure. Review of flow meter data will indicate possible leakages (Chapter Sixteen). The leakages can be either visible or invisible. In the case of invisible leaks, sections of pipeline can be isolated and search carried out for the location of leaks.
Most common leaks are through the valves and joints. Leaks in joints may occur where the bolts have become loose and gland packing/rubber ring is not in position. Leaks through air valves occur due to improperly seated ball either due to the damage of the liner/seal or due to abrasion of the ball, or due to entrapped sand particles.
Causes of NRW (Chapter Seventeen) differ from case to case but most often it is a leakage that appears due to improper maintenance of the network. Undetected and control of leakages in the transmission and distribution systems will result in a dissatisfaction of the water demand or leading to unrealistic higher consumption rates to the consumers. Also, high pumping costs or other operational problems may arise during delivering of water to points of use due to the need for increased water pressure and the resulting increased leakages. Therefore, leakage control is important for revenue water and for preventing environmental damages (e.g. water seepage to roads embankment leading to potholes and other infrastructure damages).
1.4.2 Air Entrapment
Air in free form in rising main collects at the top of the pipeline and then goes up to higher points. Here, it either escapes through air valves or forms an air pocket which in turn, results into an increase or head loss. Other problems associated with air entrapment are: surging, corrosion, reduced pump efficiency and malfunctioning of valves or vibration. In some cases bursting of pipes may occur due to air entrapment. Air entrapment normally reduces the effective diameter of the pipe.
There should always be air release valve chambers with cover slabs for the protection of the air valve and it should always be kept leakage free and dry. Frequent inspection should be conducted to check, whether air valves are functioning properly and to ensure that there is no leakage through air valves.
1.4.3 Water Hammer
Water hammer in water supply systems occurs due to rapid closure of valves and sudden shut off or unexpected failure of power supply to the pumps. The pressure rise due to water hammer may have sufficient magnitude to rupture the transmission pipe or damage the valves fixed on the pipeline. Care should be taken to open and close sluice valves gradually. A common remedy is to install surge protectors.
1.4.4 Lack of System Maps and Records
System maps and records are very important to the project operators and managers. Generally, maps showing the actual alignments of transmission mains and location of other pipes and the valves on the ground may not be readily available due to inadequate records keeping. The location of pipes and the valves on the ground becomes difficult in the absence of such updated maps and thus, these need to be prepared and updated from time to time. Some minimum information about the location and size of pipes and valves and the direction of opening of valves is required to operate and maintain the system efficiently.
1.5 Operation and Maintenance Activities
1.5.1 Operation Schedule
(a) Mapping and inventory of pipes and fitting: An updated transmission system map with location of valves, flow meters and pressure gauges is the primary requirement of operation schedule. The valves indicated in the map should contain direction to open; number of turn to open, make of valve and date of fixing. The hydraulic grade lines are to be marked to indicate the pressure in the transmission system. They can be used for identifying high pressure or problem areas with low pressure.
(b) System pressure: It is essential to maintain a continuous positive pressure in the main at the time of transmission of water in the pipeline. Low pressure locations have to be investigated if necessary by measuring pressure with pressure gauge and any problem detected relating to pressure should be fixed immediately.
(c) System Surveillance. The maintenance staff of the respective department/section of the water organization should go along the transmission line frequently so as to accomplish the following objectives: (i) To detect and correct any deterioration of the transmission system, (ii) To detect if there is encroachment of transmission system failures, (iii) To detect and correct if there is any unauthorized tapping of water, (iv) To detect and correct if there is damage to the system by vandalism and so on.
1.5.2 Maintenance Schedule
A maintenance schedule is required to be prepared to improve the level of maintenance of water transmission mains through improved co-ordination and planning of administrative and fieldwork accompanied by the use of adequate techniques, equipment and materials for field maintenance and/or inspection.
It is important to note that, the schedule should be flexible so that it can achieve team action with the available resources (staff, vehicles and tools). Co-ordination of activities is required for spares and fittings, quality control of materials used and services rendered.
Training of maintenance staff shall, apart from the technical skills, include training to achieve better public relations with consumers and the public in general.
220.127.116.11 Activities of Maintenance Schedule
(a) Develop and conduct a surveillance programme for leaks in pipelines, pipe joints and valves;
(b) Develop and conduct a water quality surveillance programme. The order is bacteriological parameters followed by chemical parameters and then some physical parameters as recommended by MoW guidelines (URT, 2018). For establishment of chemical dosing daily tests must be done;
(c) Develop and conduct a programme for locating and repairing leaks including rectifying cross connections if any, arrange for flushing, cleaning and disinfecting the mains;
(d) Establish procedures for setting up maintenance schedules and obtain and process the information provided by the public and the maintenance teams about the pipeline leaks;
(e) Establish repair procedures for standard services and with provision for continuous training of the team members;
(f) Procure appropriate machinery, equipment and tools for repair of leaks and replacement of pipes and valves; (g) Allocate suitable transport, tools and equipment to each maintenance team;
(h) Establish time, labour following activities are to be included in the schedule and material requirement and output expected, time required and other standards for each maintenance task, and
(i) Arrange for monitoring the productivity of each maintenance team.
A preventive maintenance schedule has to be prepared for:
(a) Maintenance of the pipelines with particulars of the tasks to be undertaken, works not completed, and inspection of works completed;
(b) Servicing of valves, expansion joints etc.;
(c) Maintenance of valve chambers;
(d) Maintenance of record of tools, materials, labour, and
(e) Review from time to time costs required in carrying out each task.
18.104.22.168 Activities for Preventive Maintenance
(a) Servicing of fittings, air release valves and chambers: Periodical servicing is required for pipelines, fittings, air release valves, pressure release valves, sluice valves, gate valves, T-joints, washout valves, expansion joints, flow meters, chambers and pressure gauges. Corrosion of valves is the main problem in some areas and can cause failure of bonnet and gland bolts. Leaks from spindle rods occur and bonnet separates from the body. Stainless steel bolts can be used for replacement and the valve can be wrapped in polythene wrap to prevent corrosion. Manufacturer’s catalogues may be referred and servicing procedure should be prepared for the periodical servicing.
(b) List of spares: List of spares procured for the transmission mains shall be prepared and the spares shall be procured and kept for use. The spares may include pipes and fittings such as check nut, spindle rods, bolt and nuts are flanged joints, gaskets for flanged joints for all sizes of sluice valves, consumables like gland rope, grease, cotton waste, jointing materials like rubber gaskets, spun yarn, pig-lead and lead wool, etc.
(c) List of tools: The maintenance staff shall be provided with necessary tools/equipment for attending to the repairs in the transmission mains. These tools may include key rods for operation of sluice valves, hooks for lifting manhole covers, pipe wrench, spanner set, ring spanner set, screw drivers, pliers, hammers, chisels, caulking tools, crow bars, spades, dewatering pumps and the like.
1.5.3 Maintenance of Pipelines
Pipeline bursts/main breaks can occur at any time and the O&M department/section shall have a plan for attending to such events. This plan must be written down, disseminated to all staff and the organization must always be in readiness to implement the plan immediately after the pipe breaks are reported.
After a pipe break is located, determine which valve is to be closed to isolate the section where the break has occurred. Some important consumers may be on the transmission mains and having an industrial process dependent on water supply which cannot be shut down as fast as the water supply lines are cut off and should be notified about the break down. These consumers have to be informed about the probable interruption in water supply and also the estimated time of resumption of water supply.
After the closure of the valve, the dewatering/mud pumps are used to drain the pipe breakpoints. The sides of trenches have to be properly protected before the workers enter the pit. The damaged pipe is removed, and the accumulated silt is removed from inside the pipe and the damaged pipe is replaced and the line is disinfected before bringing into use. A report shall be prepared following every pipe break about the cause of such break, the resource required. The report must be documented well for future reference.
22.214.171.124 Scouring of Pipeline
Scouring is done to clean the transmission lines by removing the impurities or sediment that may be present in the pipe. This is particularly essential in the case of transmission lines carrying raw water. This can be done by opening washout valves or by air scouring (but requires both more equipment and energy supply) or by using various automated or semi-automated devices supported by ICT-mediated technologies including CCTV.
126.96.36.199 Leakage Control
• Visible leaks: The maintenance staff during surveillance operation can report visible leaks found by him or third parties to his superiors. Critical areas where leaks often occur have to be identified and appropriate corrective measures have to be implemented:
• Invisible leaks: Leak detection equipment have to be procured for detection of non-visible leaks and action to control these leaks should be initiated to control the overall problem of water loss.
Global estimates of leakage come from an annual balance of the delivery and metered consumption for the whole network. Bursts of main pipes can be detected by the flow measurements at water supply points. To enable leak detection, parts of the system have to be inspected over a period of several hours or days, depending on the size of the water supply district. Some of the simple methods of finding leakages in water supply mains include:
(a) Visual observation This method is the simplest and most applicable leak detection technique for use in small water supply systems. This requires being alert to the following signs of leaks:
(i) Appearance of wet spots during the dry season,
(ii) Greening of patches of ground in areas where plants normally do not grow,
(iii) Abnormal drops in pressure.
(b) Finding exact position of leaks
(i) Flow and pressure measurements do not indicate the exact location of leaks. In the case of severe breaks, water may appear on the surface, but more often leak detection techniques have to be applied. The most popular are: acoustic (sound) method,
(ii) leak noise correlation,
(iii) tracer techniques.
For example, after finding the approximate location of leaks in the water distribution system, their exact location can be determined by using a sounding rod without damaging the pipe. Leaks in water pipes usually make sound, small leaks make more noise than large ones. The sounding rod is a pointed metal rod used to relay to the observer the sound caused by leaks in buried pipes.
Detection and control of leakages in sewerage systems
This is done as in drinking water conduits and more reference can be obtained in Section 15.5.2 of this DCOM Manual.
188.8.131.52 Chlorine Residual Testing
A minimum free chorine residual of 0.2 mg/l is needed to be maintained at the consumer or water point. Absence of residual chlorine could indicate potential presence of contamination in the transmission mains.
The following steps which are required to be taken include:
(a) Testing of residual chlorine,
(b) Checking the chlorination equipment at the start of the transmission system,
(c) Searching for source of contamination along the transmission system which has caused the increase in chlorine demands,
(d) Immediate rectification of the source of contamination.
1.5.4 Engaging Contractors for Maintenance
Due to inadequate trained O&M staff in line department/section, the operation and maintenance of transmission mains and other components of the scheme, if required, may be done by out sourcing/awarding Contracts for Comprehensive Annual Maintenance for any specified period e.g. 5 -10 years.
1.5.5 Records and Reports
The following records and reports need to be maintained properly all the time:
(a) Updated transmission mains maps with alignment plans. Longitudinal and sectional plans,
(b) Record of daily readings of flow meter at upstream and downstream end of the pipeline,
(c) Record of water level of reservoir at both upstream and downstream end of the transmission system,
(d) Pressure reading of the transmission mains,
(e) Identification of persistent low pressure locations along the pipeline,
(f) Record of age of pipes,
(g) Identified pipelines to be replaced,
(h) Identified source of leaks,
(i) Record of Bulk meter/water meter reading before the delivery into an overhead tank,
(j) Record of residual chlorine,
(k) Record on when the pipeline leaks were repaired or pipe changed and the cost of materials and labour cost thereof.