Take A BIG First Gas Pressure Cut To Protect Delivery System

Retrofitted Gate Station Handles Large Pressure Drop Safely And Reliably Using Gas Regulators Designed For Severe High-Pressure Service
By Jerry Peot, PE, Senior Gas Engineer, Wisconsin Public Service Corporation, Green Bay, WI | December 2011, Vol. 238 No. 12

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.