December 2011, Vol. 238 No. 12
Features
Take A BIG First Gas Pressure Cut To Protect Delivery System
Despite the higher gas pipeline pressures (up to 1,000 psi) and varying inlet pressures on peak days in recent years, Wisconsin Public Service Corporation believes the most effective pressure-reduction practice is to take a first cut from the supply pipeline in one big step while sizing for minimum contract pressure.
This operating philosophy makes sense because single-step pressure reduction is less complicated and allows us to use smaller valves and get more capacity than with multiple pressure cuts requiring larger valves. In some cases, building another regulation run/cut may be unnecessary. The company expects lower installation costs and lower overall cost of ownership over the long term.
To obtain this financial benefit, we must be able to handle a large pressure differential in such a way as to safely maintain a reliable gas supply for our customers. We are finding this possible with new, all-metal gas regulators that are designed to withstand high pressures and any contaminants that may be present.
Wisconsin Public Service, a subsidiary of Intergys Energy Group and headquartered in Green Bay, has a 125-year history of providing safe, reliable energy. The company serves about 315,000 natural gas customers throughout central and northeast Wisconsin and a portion of Michigan’s Upper Peninsula.
Wisconsin Public Service is planning to continue retrofitting gate stations in the coming years. The upgrade last year of our North Manitowoc Gas Gate, which is where we take custody of natural gas from the ANR Pipeline Company (a subsidiary of TransCanada Corporation) for distribution in the cities of Two Rivers and Manitowoc, is a model where high pressure drops may be encountered.
Since the 1950s, this station has taken a two-stage cut, first reducing pipeline pressure with a regulator and relief valve combination set at 350 psig, followed by another regulator and relief valve controlling the pressure at 100 psig for one of the two local distribution systems. Still another regulator and relief valve brought the pressure down further to 60 psig for the other existing distribution system.
Boot-style regulators have served well in this scenario, but in recent years they have required more maintenance with replacement parts becoming scarce. When it was decided to replace the regulators in the station, there was no question the company would upgrade to state-of-the-art regulators reflecting our single-cut philosophy. This meant dealing with a high differential pressure.
Our basic single-cut design involves an operator/monitor setup for heated pipeline gas following custody metering to prevent pressure-drop freeze-ups by maintaining a 40oF outlet temperature. In this configuration, the operator accommodates the entire pressure drop to below 100 psig. The upstream inline monitor is set at a slightly higher pressure to maintain constant over-pressure safety on the downstream supply. A redundant, or parallel, operator/monitor pair is located adjacent to the primary regulators. This identical backup configuration will take over in case of an operating failure. It is also used during maintenance periods.
Differential pressures higher than 800 psig caused us to consider the Fisher® EZH Series regulator, rated for inlet pressures up to 1,500 psig. Contamination such as oil, dirt, or debris may also be present in the gas supply under those higher pipeline pressures. The rugged, spring-close EZH has hardened metal trim with a soft seat for reliable bubble-tight shutoff. It is built for severe service, able to withstand contaminants as well as pressures and velocities that are detrimental to rubber components.
Because of their favorable design and maintenance characteristics, EZH Series regulators are generally specified by Wisconsin Public Service as monitors in gate stations where the single-cut differential pressure is greater than 800 psig.
In the North Manitowoc Gas Gate, an EZH regulator was installed as the monitor and the recently introduced Fisher EZHSO Series was chosen for use as the operator due to the potential 900 psig differential. This model is designed to “fail open” with a positive spring action. If it should cease operating for any reason, there is a high probability it will remain wide open, allowing the monitor to continue controlling the downstream pressure.
This allows Operations time to respond to an alarm by sending a repair crew to the station. Generally, the crew can switch to the backup system with minimal impact on the gas supply to our customers. In the worst case, if the monitor also fails, there is a high probability it will fail-closed, shutting off the flow entirely to protect the distribution system from over-pressurizing. This setup is depicted in Figure 1.
This upgrade was implemented at the North Manitowoc Gas Gate in July 2010 with the support of Novaspect, Inc., Emerson’s local business partner headquartered in Schaumburg, IL.
After about a month of operation, the gas flow was switched to the backup pair so that the operating regulators could be opened up and inspected. This was done once more following the first cold snap last winter, and they haven’t been touched since then. This system operated perfectly throughout the unusually long winter. However, an annual lock-up and orifice/seat inspection will still be done. The top-entry design of the EZH Series regulators reduces the amount of time required for preventive maintenance – an additional cost-saving feature.
The regulator buildings at the North Manitowoc Station are well-insulated and noise attenuation was not employed with the new setup. However, Emerson’s Noise Attenuation Technology is available on EZH Series regulators to reduce aerodynamic noise generated by internal turbulence created by high pressure drop ratios.
A new gate station for the City of Manitowoc constructed during the summer of 2011 is employing the same method of pressure reduction in recognition of the effectiveness of this approach for high pressure drop situations. The EZH and EZHSO Series regulators will be installed for pressure reduction to replace existing equipment in many instances, depending on the service conditions at each regulation station.
Either way, Wisconsin Public Service is committed to a retrofit program that will provide long-term reliability and security for our commercial and residential customers while reducing maintenance costs.
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