State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
State of Technology Report for Force Main Rehabilitation, Final ...
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5.0 DESIGN AND QA/QC<br />
This section will review the existing design concepts that pertain to the renewal <strong>of</strong> a <strong>for</strong>ce main. The<br />
design methods currently employed are <strong>for</strong> either interactive or independent liners and depend upon the<br />
condition <strong>of</strong> the existing (host) pipe. A complete review <strong>of</strong> most applicable ASTM standards is included.<br />
These standards are broken down by material type and functionality including product/material, design,<br />
and installation practices. The section also covers the QA/QC aspects <strong>of</strong> renewal by looking at short-term<br />
factory and field requirements, as well as long-term qualification requirements. The latter are notably<br />
lacking <strong>for</strong> many products <strong>of</strong>fered <strong>for</strong> both a pressurized and corrosive environment.<br />
5.1 System Design<br />
5.1.1 Redundancy in System Design. <strong>Force</strong> mains are <strong>of</strong>ten located at critical points in a utility’s<br />
sewerage system. They typically carry a large percentage <strong>of</strong> a utility’s untreated wastewater and in a few<br />
cases all <strong>of</strong> the flow to the wastewater treatment plant is conveyed through <strong>for</strong>ce mains. Despite the<br />
critical nature <strong>of</strong> sewer <strong>for</strong>ce mains, less than 5% have any redundancy built into the system (WERF,<br />
2009). The use <strong>of</strong> redundancy in system design appears to be a more frequent practice in Europe than the<br />
US. If a line is taken out <strong>of</strong> service due to a failure, by-pass pumping or the use <strong>of</strong> honey trucks is<br />
necessary. Those that do have a redundant line can use it during a failure to maintain a minimal level <strong>of</strong><br />
flow, avoiding overflowing wetwells, and surcharging upstream sewers. The other advantage to having a<br />
redundant line is the ability to take a <strong>for</strong>ce main out <strong>of</strong> service <strong>for</strong> an extended period <strong>of</strong> time to allow <strong>for</strong><br />
either an intrusive (internal) inspection or <strong>for</strong> extended repairs or rehabilitation.<br />
With redundancy very limited, most utilities have not been in a position to carry out in-depth inspections<br />
and assessments <strong>of</strong> their <strong>for</strong>ce main systems. Consequently, many utilities in the past have merely reacted<br />
to problems, such as failures, when they occur rather than being proactive in attempting to assess the<br />
<strong>for</strong>ce main’s in situ condition. Little is known about the condition <strong>of</strong> sewer <strong>for</strong>ce mains in US utilities,<br />
but that is gradually changing, primarily as a result <strong>of</strong> EPA consent decrees requiring municipalities to<br />
carry out condition assessment <strong>of</strong> their <strong>for</strong>ce mains.<br />
Heret<strong>of</strong>ore, there has been very little rehabilitation <strong>of</strong> sewer <strong>for</strong>ce mains due to lack <strong>of</strong> redundancy and<br />
the availability <strong>of</strong> reconstruction products <strong>for</strong> pressure sewers. The redundancy issue isn’t going to<br />
change overnight, although more utilities are considering investing in redundancy with new projects. Any<br />
attempt to rehabilitate a sewer <strong>for</strong>ce main, aside from outright replacement while the main remains in<br />
service, is going to either require by-pass pumping or a renovation technology that can be installed very<br />
quickly during a limited outage. The latter doesn’t exist at the moment so by-pass pumping is the norm.<br />
As a result <strong>of</strong> vendor success in the gravity field with CIPP and re<strong>for</strong>mable thermoplastic pipe products,<br />
adaptations <strong>of</strong> these products are now starting to find their way into the pressure sewer field.<br />
5.1.2 Pig Launchers/Retrieval <strong>for</strong> Cleaning and Inspection. Like redundancy, pig launchers and<br />
retrieval systems are not commonly designed into sewer <strong>for</strong>ce mains. Some utilities, like St. Petersburg,<br />
FL, have installed pig launchers and retrievals on most <strong>of</strong> their sewer <strong>for</strong>ce mains and use them on a<br />
regularly scheduled basis to clean each main. Keeping the mains clean maintains capacity and reduces<br />
pumping (energy) requirements.<br />
It is only within the past few years that some leak detection tools have become available to allow <strong>for</strong><br />
limited assessment <strong>of</strong> a <strong>for</strong>ce main with little interruption in service. These tools can be introduced into<br />
the flow stream and are either tethered (Sahara ® ) or free swimming (SmartBall ® ) and will locate leaks and<br />
gas pockets in a <strong>for</strong>ce main. These tools can be launched through 2 inches (50 mm) (Sahara ® ) or 4 inches<br />
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