FortisBC manages more than 100 below-grade compressed-natural-gas (CNG) distribution installations in British Columbia, with a 25-to-30-year lifecycle, many of these are due for replacement over the next several years.
Working in collaboration with Spartan Controls and Emerson Automation Solutions, the Canadian power and gas distributor installed the first of a new buried-pit design, which is being modeled for subsequent below-grade replacements.
“No one knows the infrastructure is there,” explained Julie MacLennan, application specialist at Spartan Controls, who co-presented the details of the new station at the 2017 Emerson Global Users Exchange in Minneapolis. “We executed the project on time. Confined-space, ergonomic and safety goals all were achieved for FortisBC.”
The installation reduced the operational/financial risk of the facility by $1.3 million/outage, and the commissioning and startup were executed with on-site support, making the station operational in time for peak demand loads in the winter of 2016.
FortisBC conducts annual analyses of gas supply and demand for the 1.1 million customers it serves. More than 60% of the customer base is located in the southwest part of the British Columbia province, which comprises the greater Vancouver area and Vancouver Island. The distribution pressure-control station tagged for upgrade was located on the corner of a busy street with six lanes of traffic nearby—an environmental incident waiting to happen. The installation serves 123,000 residential customers and 30 industrial/commercial customers.
“The station had an issue with potential growth,” explained co-presenter Niko Boskovic, engineering supervisor—stations, FortisBC. “The old station was already at capacity at maximum demand.”
FortisBC’s wish list included moving the site from the traffic-heavy corner; updating the technology, regulators and design; increasing capacity of the system to supply customers; changing monitor inherent failure response; and improving safety and accessibility.
After identifying potentially underserved customers and a plan to address system growth, project objectives were set:
The project was justified by the identified capacity limitations and recommended capacity increase, as well as the existing system and station risks. Project risks were identified from a failure analysis, which accounted for a process hazard assessment and the critical customer, socio-economic and business impacts of system failure.
“Using a HAZOP technique, we analyzed scenarios,” said Boskovic. “We looked at no-flow or low-flow scenarios and at high-flow scenarios, and the scores were surprising low.”
FortisBC considered three different technology options before deciding on Emerson’s FL series 4-inch axial two-path regulators. Boot-style regulators also were considered. And the EZH-SOL spring-to-open two-path regulator wasn’t yet available.
The FL series two-path regulator was the obvious choice because of its high accuracy and response time, noise abatement capabilities, flow capacity of axial body design, reliability of two-path design, no bleed to atmosphere and customization and control of pilot tubing to allow above-grade adjustment, explained MacLennan.
Safety first
“When we moved into the technical solution, we wanted to include, first and foremost, safety,” said MacLennan. “It was decided to go with the monitor solution for its no-bleed, environmentally friendly design. Plus, what were we going to do for the technicians to protect their safety?”
The environmental concern—how to keep the gas in the pipeline—demanded a no-bleed, not low-bleed, solution. “There were station flood design considerations,” explained MacLennan. “We’re just north of Seattle, so we have a lot of rain. Floods are a real concern, and this is close to one of our major rivers, so it was important that we can operate underwater and keep the system safe and contained.”
The location also dictated an assessment of noise impacts to residential surroundings. “Gas utilities in every city have to take this into account,” said MacLennan. “The closer you are to people sleeping, the more impact you can have.”
From an operational perspective, Spartan Controls was looking for a set-it-and-forget-it solution. “We looked at the filters and the isolation valves,” explained MacLennan. “The double-block-and-bleed (DBB) isolation valves were giving them better shutoff. We wanted this to be high-accuracy and robust for high shifts in demand, and it needed a huge amount of capacity because this area is growing.”
The new buried-pit design reduced the space size. “The smaller the package we can fit this into, the better off economically,” explained MacLennan. “And the FL has lots of noise attenuation capability. We don’t want the neighbors complaining. Fisher has come out with a polyurethane seat design.”
A technician can access the monitor above-grade and change the set points. In the pit, piping height is adjusted to technician level. “A regulator is a self-contained control system,” explained MacLennan. “It needs to be in a stable spot. The instrumentation and piping lines are all in the P&ID and used as part of the technician training. They were happy with the design and said it was easy to follow.”
The pit includes a hydraulically lifted lid, so it doesn’t come down on technicians, and there are safety steps for easy entry and exit. There’s also an emergency lift to get them out of the vault in case of an accident.
“In the event of a flood, we capped the sense lines,” said MacLennan. “This station could flood and the overpressure protection would remain intact. The two controllers are above-grade, and all testing can be done without opening the pit.”