A pipeline full of data—on a single wire
11 March 2009
Facing tightening environmental and regulatory requirements, a municipal water jurisdiction designed a first-case leak detection and monitoring system. HART® technology is key to cost-effectiveness.
Wiring for the magnetic flowmeter
Historically most Canadian cities were built alongside waterways, and each has multiple sewer pipe crossings through large pipes laid in tunnels beneath the river. ‘Probably 99 percent of those crossings are not monitored or looked at other than on a yearly basis,’ says Kent McKean, senior automation designer with AECOM, the firm hired by the city of Winnipeg to resolve the problem.
Of those crossings, a certain percentage will have inevitable leaks. These may not be catastrophic but can add up to significant cumulative contamination.
‘To correct all of those issues in some cities in one fell swoop would mean spending billions of dollars,’ says Mr. McKean, and it's not going to happen anytime soon. So when we started this project, we looked at what everyone else was doing. And everyone else was doing nothing in terms of technology. There really is no monitoring of this type of sewage crossing under a river.’
Unfortunately, most leaks are only detected when a waterway's colour changes significantly. Weeks can pass before a winter-time leak permeates surface ice and snow, by which time it is too late to prevent contamination from spreading from one waterway to the next.
The particular challenge was to monitor force-main sewer lines at river crossings, where gravity cannot lift material through lines so low-pressure pumping is used. ‘This type of flow-monitoring for a force-main leak detection had never been done,’ Mr. McKean explains.
The basic architecture involved instrumentation connected to a remote terminal unit, in this case the Rosemount Remote Operations Computer (ROC) from Emerson Process Management. The ROC was equipped with an Ethernet data radio at each site to remotely access site data and a HART communications card.
‘Because we were limited somewhat in what knowledge we had up front, we decided to specify a system that would give us flexibility, perhaps more flexibility than what we actually needed.’ One of the advantages of HART is that it can be applied to extract data from multiple variables on an analogue wire.
In mid-2007, the first test installation came online using Rosemount magnetic flowmeters sending data to the ROC to be totalised and trended and compared. This was initially based on a time period of 8 to 10 hours.
‘It was working, but it wasn't giving us the resolution we were hoping for,’ Mr. McKean says, citing lower accuracy than desired (10 to 15 litres/second) and the averaging period's length, which could allow a leak to go unnoticed for up to ten hours.
The costs were also too high to allow widespread replication and adoption. The flowmeters required insulated above-grade housings or alternately below-grade enclosures with manhole access that could drive costs above $500,000 per installation. They also required additional monitoring for the presence of dangerous gas.
CHANGE IN PHILOSOPHY
Mr. McKean's team decided on a ‘change in philosophy’ that would require reconfiguring the system to provide instantaneous, real-time data in addition to the existing totalisation strategy. Instead of having to rewire and reconfigure physical equipment in the field, he says, ‘HART and the system's software put all of the variables we needed at our fingertips.’
After implementing the new strategy, resolution of data increased from 10 to 15 litres per second, to one or two litres per second.
Late in the summer of 2008, the AECOM team got the opportunity to install a completely new pipeline at a crossing. Instead of flowmeters, this installation used double-walled pipe and Rosemount pressure transmitters that were much smaller and easier to install. The control strategy involved measuring pressure inside the interior pipe as well as the interstitial space between the inner and outer pipes. As a result, engineers and technicians could pinpoint the location of a leak as well as its failure mode, such as whether the exterior or interior pipe has failed.
This solution was less intrusive and provided easier, safer access and reduced a typical trip to the field by hours. In addition, McKean estimates that it reduced costs from roughly $500,000 to between $30,000 and $50,000 per crossing.
The system has greatly reduced leaks and provides an early warning system to mitigate risk, eliminate fines and help justify the cost of new technology. Additionally, the HART Communication interface provides a full data pipeline to reduce the cost of future upgrades by minimising or eliminating the need to re-wire or run new wires in the field.
‘If we had gone with a non-HART solution or a non-data bus solution, we would have been very limited in the data that we could have gotten,’ he says. ‘We would have had to use a separate radio or data channel for each variable that we may need, or we would have had to change things on the fly as we were developing. And we didn't really want to have to add in all the extra communication channels for analogue values if we could handle everything through a single communication bus.’
HART technology's role is likely to become even more beneficial now that the WirelessHART protocol eliminates the need for a physical connection to traverse river crossings.
‘If we were to implement this system today, we would utilise WirelessHART,’ Mr. McKean says.
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