dominion road sewage pumping station and force main replacement

DOMINION ROAD SEWAGE PUMPING STATION AND FORCE MAIN REPLACEMENT Ryan Boone, EIT, R.V. Anderson Associates Limited*; Doug Pease, P. Eng., R.V. Anderson Associates Limited; Graeme Guthrie, CET, Niagara Region *R.V. Anderson Associates Limited 1 St. Paul Street, Suite 702, St. Catharines, ON L2R 7L2. [email protected] 1.0

OVERVIEW

The Niagara Region (the ‘Region’) is currently in the process of replacing the Dominion Road Sewage Pumping Station (‘PS’). The original facility was constructed in 1972 and services a population of approximately 4500 of the Town of Fort Erie. In 2009, the Region completed a condition assessment of the existing station as a precursor to a planned facility upgrade. Upon reviewing the issues identified by the condition assessment, the Niagara Region elected to replace the existing facility with a new pumping station. The new facility is to be located on the same property, to the south of the existing PS. During preliminary design, it was decided to increase the station capacity which also necessitated the replacement of significant portion of the existing force main. This paper discusses the history and condition of the existing facility, design considerations for the new facility and force main, and various constructability issues associated with the proposed works. At the time of preparation of this paper the project is under construction. 2.0

EXISTING SYSTEM

The existing Dominion Road PS was constructed in 1972, and is a dry/wet well design with an above ground generator/control building. The station is equipped with two 300 HP pump units, acting as one duty and one standby. The existing Peak Dry Weather Flow (‘PDWF’) at the station is 33 L/s, and the existing station rated capacity is 225 L/s. The additional capacity above the PDWF is helpful to convey additional flows experienced during wet weather conditions. Flows from the station are conveyed, along with flows from other facilities, to the Region’s Anger Avenue WWTP. The Dominion Road PS has the largest rated capacity of any pumping station in the Anger Avenue WWTP collection system. The PS receives flows from two local pumping stations, as well as local area sewers. The existing force main is approximately 4500m long and discharges into a gravity sewer at the intersection of Bertie Street and Concession Road in the WEAO 2013 Technical Conference, Toronto, Ontario

Page 1

Town of Fort Erie. It is 400 mm nominal diameter HDPE, with three (3) different pressure classes (Series 100, 80 and 60) over its length. 3.0

CONDITION ASSESSMENT

In 2009, the Niagara Region retained R.V. Anderson Associates Limited (‘RVA’) to complete a condition assessment as a precursor to a planned upgrade at the existing Dominion Road PS. This process was intended to identify and address maintenance and operational concerns with existing equipment. Several key areas of concern were identified during this process and are discussed below. 3.1

Dry Well Consideration

3.1.1

Working Space

The existing dry well/pump room has extremely limited working space due to its small footprint and the horizontal configuration of the existing sewage pumps. Access to the pumps, motors and discharge is difficult and requires climbing over top of various piping elements. 3.1.2

Flooding

The incoming influent sewer travels through the dry well before discharging into the wet well. This arrangement increases the risk of dry well flooding in the case of a pipe break or failure. The existing sewage pump units are horizontally mounted with non-immersible motors located just above the dry well floor and are at risk of damage due to flooding. For these reasons, the Region expressed a preference to replace the existing pump units with dry-pit submersible pumps. However, this was not feasible with the configuration of the existing intake piping. 3.1.3

Pump Removal

There are several risks associated with the removal of the existing pumps. As discussed previously, the station influent sewer runs through the dry well at an elevation above the existing pumps. Pump removal requires the use of an overhead crane located in the control room directly above. This introduces a risk of damaging the incoming sewer, which could result in flooding of the dry well. Additionally, limited space in the control room makes maneuvering of the pumps a very difficult task. One of the pumps must be lifted over top of the generator in order to reach the main doors.

WEAO 2013 Technical Conference, Toronto, Ontario

Page 2

3.1.4

Access

The only access to the dry well is a series of ladders from the electrical/control room. As a result, it is difficult to bring equipment and tools to the dry well floor. Due to space restrictions, it is not possible to install a stairwell access to the dry well within the existing structure. The only viable option to improve dry well access would be to construct a new stairwell on the exterior of the structure. Limited property and the existence of shallow bedrock at the PS site makes this option impractical. 3.2

Electrical Classification

The Ontario Electrical Safety Code (OESC, 24th Edition / 2009) includes specific rules applicable to “Sewage Lift and Treatment Plants”. Based on the OESC, the dry well space is now considered a Class I, Zone 2 hazardous area. Due to lack of physical separation from the dry well area, the electrical/control room is also considered a Class I, Zone 2 hazardous area. The existing electrical equipment located in these areas is not suitable for operation in a hazardous area. Three possible options to address this issue were considered. First, all electrical equipment in these areas could be replaced with new equipment suitable for operation in a Class I, Zone 2 hazardous area. Secondly, the area classifications could be changed to ‘unclassified’ by providing continuous positive pressure ventilation at a rate of 6 air changes per hour (‘ACPH’). In the event of failure of the ventilation equipment, the areas would revert to Class I, Zone 2 and any electrical equipment not suitable for operating in such a hazardous area, including the pump units, would have to be de-energized. Finally, a new building physically separated from the existing PS structure could be constructed to house the electrical and control equipment. All three options were considered expensive, requiring extensive upgrades to the existing facility. The risk associated with the second option was also considered unacceptable. 3.3

Generator

The existing generator was originally installed in 1972 and has reached the end of its useful life. Furthermore, it uses a single pass heat exchanger requiring municipal water for cooling. This arrangement is not considered environmentally friendly or economical. It is located in the electrical/control room with limited access for maintenance and operations. 3.4

Structural

The aging control building superstructure and roof were found to be in generally poor condition, requiring significant remediation in order to extend their useful lives.


Page 3

3.5

Preferred Solution

The condition assessment identified a number of areas of concern that would greatly increase the scope and cost of the planned upgrade. Upon further review of these issues, the Region determined it would be more cost effective to replace the existing station with a completely new facility on the same property. In 2010, the Niagara Region retained R.V. Anderson Associates Limited to undertake the design of the new facility. 4.0

DESIGN

4.1

General

The Region required the rated capacity of the new facility to be 260 L/s. This modest increase is to accommodate the built-out PDWF, maintaining the existing level of service available for wet weather flows. The new Dominion Road PS was designed to meet the Region’s current design guidelines for stations with rated capacities greater than 100 L/s. It includes an 11 m deep, 144 m2 substructure housing the pump room, pump room access stairway, inlet channel and two-cell wet well. The 5 m high, 185 m2 control building houses the electrical/control room, pump room access, generator room and building mechanical room. The control building is located partially above the pump room, extending south of the substructure. 4.2

Process

4.2.1

Wet Well

Two equal sized wet wells are configured with isolation inlet gates and an interconnecting isolation gate. During normal station operation, the inlet and interconnecting isolation gates are all open. Either wet well can be isolated and drained while the other remains in operation. The wet well is covered with a reinforced concrete slab on grade which includes a number of access hatches. Personnel access to the wet well is through a stairwell leading to the inlet channel and a mid-level platform. 4.2.2

Pumps

Consideration was given to both three pump (two duty and one standby) and four pump (three duty and one standby) configurations. It was found that two pumps equipped with 200 HP motors and operated by variable frequency drives (‘VFDs’) are capable of handling the required range of flows. During low flow situations, VFDs will be used to slow a single pump, discharging as little as 120 L/s in order to provide the minimum required scouring velocity in the force main.


Page 4

The benefits of adding a third duty pump were outweighed by the added overall cost of equipment, associated controls and the increase in building footprint. The selected pumps are dry pit submersible style, mounted in a vertical configuration. The submersible style pump was selected to ensure continued station operation in the event of an emergency in which the pump room is flooded. The pump disconnects were located at the mid-level platform above the maximum station liquid level such that they are accessible in the event of a catastrophic pump room flood. The vertically mounted configuration was selected to reduce the pump room floor area requirements. 4.2.3

Pump Controls

A Programmable Logic Controller (‘PLC’) will control the operation of the three (3) raw sewage pumps. The PLC is also programmed to alternate duty pumps. When the level in the wet well rises to the Duty Pump 1 start level, Duty Pump 1 will start at maximum speed. As the level drops, Duty Pump 1 will be slowed to match the incoming flow. The minimum allowable pump speed is based on a flow rate of 120 L/s, ensuring a minimum scouring velocity of 0.8 m/s is achieved in the force main. Duty Pump 1 will stop when the water level reaches the Low Level set point. If the level continues to rise, Duty Pump 2 will start and both pumps will maintain maximum speed while the wet well level remains above the Duty Pump 2 Start set point. As the level drops below the Duty Pump 2 Start set point, both pumps are slowed to match the incoming flow. Both pumps are stopped at a common low level elevation. 4.2.4

Suction and Discharge Piping

Although the station will have three sewage pumps, there will be four suction lines in the wet well. The centre pump will be connected to two suction lines such that it can draw from either wet well cell. This allows for two pumps to draw from one wet well cell should the other be taken offline for maintenance. Each suction line is configured with a bell mouth intake and isolation valve. The combination of the wet well benching and the addition of the fourth suction intake will mitigate dead zones in the wet wells that can often result in accumulation of solids resulting in the generation of odours. Each pump is equipped with a discharge check valve, isolation knife gate valve and discharge pipe connected to a common header at a higher elevation. The discharge check and knife gate valves are located at an elevation accessible from the pump room floor. As a result, an expensive mid-level platform in the pump room was not required. The discharge check valve was elevated above the pump volute to help minimize the effects of air-locking in the pump. Air-locking of a pump occurs when a gas becomes trapped in the volute and discharge line and cannot escape. WEAO 2013 Technical Conference, Toronto, Ontario

Page 5

Air locking typically occurs when a pump has not operated for some time and wastewater in the suction line, pump volute and discharge line releases gas. Alternatively, air can be introduced into the pump by the action of vortices forming around the suction intake when the wet well levels are allowed to get too low. The closed discharge check valve prevents the air/gas from escaping when the pump is idle. If the pump is started in this condition it may not develop enough pressure to open the check valve. In this case, the pump could run dry, resulting in damage to the seals and wear rings. One way to prevent air-locking is to alternate the duty pumps, so that each pump is operated on a regular basis. However, at the Dominion Road PS, a single pump operated by VFD will be capable of handling the incoming flows during most situations. As a result, there is the possibility that two of the three pumps may be at rest for some duration, increasing the risk of becoming air-locked. Any gas in the suction piping, pump or discharge piping will naturally travel to the highest point possible – in this case, the elevated section of discharge piping rather than the pump volute. 4.2.5

Flow Meter Chamber

The Region requires magnetic flow meters to be installed in all of their sewage pumping stations. Most magnetic flow meter installations require a straight runs of pipe upstream and downstream of the flow tube. This additional pipe length would require an increase in the pump room footprint. To avoid the expensive of a larger footprint, the flow meter will be located in a dedicated chamber constructed adjacent to the substructure at an elevation above the top of rock. The chamber will also include a valve bypass connection terminating at grade. In the event of a force main break, the operator will have the option of using the station pumps to discharge to haulage trucks. 4.3

Electrical/Control Room

The electrical/control room is physically separated from all hazardous area. As such, the equipment installed in this room does not require any special hazardous area classifications. 4.4

Standby Power

Standby power to the station will be provided by a 600 kW radiator cooled diesel generator, located in a dedicated room in the above grade building. This room is designated as an unclassified area as it is physically separated from the pump room area. During generator operation, the intake, exhaust and recirculation dampers will modulate to use heat from the generator to maintain a WEAO 2013 Technical Conference, Toronto, Ontario

Page 6

constant room temperature. The double walled diesel fuel storage tank is located outside, adjacent to the generator room. 4.5

Force Main

4.5.1

Transient Analysis

A previous study undertaken by the Region had raised concerns regarding the condition and pressure rating of the existing thin walled force main, in particular with respect to transient conditions. Accordingly, a transient analysis was performed during the preliminary design stage to determine the effects of the increased station flows and associated pressures. A transient condition in a pipeline is the phenomenon of pressure variations in both time and location brought about by a change in flow at some point in the system. These rapid flow changes often occur during pump start and stop conditions (either routine or from power loss), or in the event of a broken force main. The transient analysis was performed using propriety software called Pipe2010. This software was developed by the University of Kentucky, and is used throughout the water and wastewater industry to simulate transient conditions in pipe systems. It was found that negative pressures generated during a transient condition would exceed the collapse pressure rating throughout the Series 100 and 80 sections of force main. The total length of pipe in question is approximately 2150 m. Combination air release/vacuum relief valves could be installed at regular intervals to mitigate the upsurges and downsurges generated during the transient conditions. However, any failure in this equipment would place the force main in danger of damage or failure. Based on the results of the transient analysis, it was decided to replace the Series 100 and 80 sections of force main. The new force main section will be constructed of 450 mm diameter PVC C905 DR 25 pipe. In order to reduce the impacts of a transient condition, the new section will also include combination air release/vacuum relief equipment at four (4) locations along the force main. In the event of failure of this equipment, the selected C905 pipe class is capable of withstanding a range of negative and positive transient pressures. The valves will be housed in a total of four (4) pre-cast concrete chambers located at approximately 900 m intervals along the alignment. The approximate chamber locations were confirmed by additional transient modeling, and the exact locations were selected based on existing site conditions. The chambers also provide a convenient location for isolation valves, so that sections of the force main can be isolated in the event of a pipe break.


Page 7

The new 450 mm diameter PVC pipe section has a significantly larger cross sectional area than the existing 400 mm HDPE. As a result of this larger cross sectional area, the system head loss developed at the new station rated capacity of 260 L/s will be reduced by approximately 45%. This allowed for the use of sewage pumps with smaller motors and associated electrical equipment, reducing the overall project construction cost and reducing the amount of energy the station will consume during its lifecycle. 4.5.2

Alignment

The majority of the existing force main is located within a railway easement, surrounded on both sides by dense vegetation. The existing thin walled HDPE pipe has a history of breaks, and the Region has had difficulties accessing the pipe for repairs within the rail alignment. For this reason, the replacement section of force main will be installed in an existing municipal right of way, providing easy access during future maintenance operations In order to completely avoid the railway easement, the amount of force main to be replaced increased from 2150 m to 3550 m. While the new alignment increased the overall force main length by approximately 300 m, the additional head loss from the longer pipe section was more than offset by the decrease in system head losses resulting from the larger diameter pipe. 4.6

CONSTRUCTABILITY CONSIDERATIONS

Several constructability issues were identified during the design phase of the project. 4.6.1

Geotechnical Investigation

A geotechnical investigation was completed during the design stage to establish the foundation requirements for the new pumping station. The presence of limestone bedrock at shallow elevations of five to six meters below grade was found to provide an adequate foundation. However, the bedrock was found to be quite hard and would require either heavy mechanical rock breaking equipment or blasting to remove. In order to minimize the amount of rock removal, efforts were made throughout design to minimize the footprint of the new structure. Additionally, identification of a relatively high ground water level in the area of the PS resulted in special design considerations for the deep structure to avoid uplift forces.


Page 8

4.6.2

Hydrogeological Investigation

A hydrogeological investigation was completed to assess issues relating to groundwater ingress during the construction. Due to existing hydrogeological conditions and the close proximity of Lake Erie, the dewatering rate in the excavation was calculated to exceed 50,000 L/day. As a result, the Region made application for and received a Permit to Take Water for the duration of construction. Special construction dewatering considerations were also included in the contract documents. Early identification of the dewatering requirements and the need for permits mitigates potential delays during construction. 4.6.3

Force Main

The selection of an alternate force main alignment and a special valve chamber at the connection to the existing force main will allow the new pipe to be constructed with little impact to the existing station operation. This will also reduce the duration and associated cost for bypass haulage from the station to the Anger Avenue WWTP. The construction of the new tie-in valve chamber will allow the new PS and force main to be fully tested and commissioned while keeping the existing system as a fully functional backup. 5.0

SUMMARY

R.V. Anderson Associates Limited was retained by the Niagara Region to design a new 260 L/s sewage pumping station located in the Town of Fort Erie. The new facility will replace an existing aging facility that has a number of ongoing maintenance and operational concerns, and has reached the end of its service life. The new facility is a dry/wet well design, featuring an 11m deep substructure and 5m tall control building. The pump room will contain three (3) 200 HP dry-pit submersible pumps, operating in a two (2) duty and one (1) standby configuration. The electrical/control room will be electrically unclassified, as defined by OESC. The project also includes the replacement of 3550 m of the existing force main with new larger diameter 450mm PVC pipe. The force main work includes four (4) new air/vacuum relief chambers designed to minimize the effects from transients in the system. The project is currently in the early stages of construction and is expected to be completed by the winter of 2014.


Page 9

dominion road sewage pumping station and force main replacement

Recommend Documents