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High pressure reduction and metering station for Gigawatt power plant

[Johannesburg, 28 October 2015] Energas Technologies, a leading supplier of high‐end and specialized equipment to the oil and gas industries in Southern Africa since 2001, has played a significant role in bringing a Mozambique power plant to life.

Located in the border town of Ressano Garcia in the Moamba District of Maputo Province, the power plant covers approximately 5.4 hectares of greenfield site and, once complete, it will consist of an engine hall housing 13 natural gas‐fired internal combustion engines manufactured by Rolls Royce in Bergen, Norway and associated generators from ABB.

The electrical energy produced by Gigawatt Mozambique, who in 2010 were awarded a 25‐year power generation concession for up to 350 MW by the Mozambican Government, will be exported to the Electricidade de Mozambique (EDM) transmission grid via a 275 kV bus bar connected to the recently built Ressano Garcia substation.

The plant is being constructed by WBHO Construction in partnership with Parsons Brinckerhoff (PB), the contractor responsible for the plant engineering, procurement and construction. The high pressure reduction and metering station (HPCMS) for the power station has recently been completed and the engineering, procurement and construction (EPC) contract for its design, supply and construction was awarded to Energas Technologies in June 2014.

Station design

Gas for the power station is supplied from the Temane Gas field via a 500 km high pressure pipeline (design pressure 125 Bar). Gas is required at a pressure of 6 Bar and a temperature above the dew‐ point. For billing purposes, a custody transfer meter is incorporated in the pressure reduction station.

Comments Hendrik van Huyssteen, MD of Energas Technologies, “Pressure reduction and metering stations are often situated in remote locations with no or unreliable electricity supply or communication infrastructure. Equipment used in such stations is therefore required to be highly reliable and safe, with minimum maintenance requirements.” It was for this reason that pilot operated regulators and slam‐shut valves from Gorter by Honeywell are predominantly used. These regulators can accurately regulate pressure from 125 Bar down to 6 Bar in one stage and with low noise levels.

High safety integrity levels (SIL) are required for pressure reduction stations due to the risks of explosions should the regulating system fail. The design standard for gas stations in South Africa is ASME B31.8, which has specific mandatory requirements to protect users. Two regulators are used in series to comply with the standards, one of which is a controlling regulator and the other a monitoring regulator in a fully open position. “The operating regulator is designed to open if it fails, at which point the monitoring regulator will instantly assume control. A second level of over‐pressure protection is provided in the form of Class 900 slam‐shut valves,” remarks Van Huyssteen.

Once the gas pressure is reduced, the temperature also decreases due to the Joule‐Thompson effect. If the outlet temperature drops below the dew point, the gas has to be heated prior to pressure reduction for which various options, including gas‐fired water heaters or electric heaters, are available. Both water bath gas heaters and skid‐mounted heaters with in‐line heat exchangers have been supplied to local industry for this purpose before. In this instance, HPCMS skid‐mounted gas‐fired hot water heaters were supplied to Gigawatt Mozambique. These heaters, with a capacity of more than 1 MW each, were built on skids in South Africa and then transported to site.

Isolation & Filtration

For isolation of the station, fully welded ball valves from RMA Pipeline Equipment in Germany were selected. Such high integrity valves are essential for pipelines, as the valves are welded into the lines. Debris, which often results in leaking soft seated valves, is often found in newly constructed pipelines as well as after pigging of the lines at 3 to 5 year intervals. Van Huyssteen comments, “RMA valves are also used in the transmission lines due to their triple sealing system and double‐block and bleed features. We are happy to report that no valve leakages have been experienced in the more than 13 years that RMA valves have been supplied to local industry.”

In order to protect down‐stream equipment from pipe debris or condensate, filter separators are used on all station inlets and are designed for filtration of solids and liquid particles down to 1 micron particle size. Filter vessels are required to comply with the ASME VIII, division 1 pressure vessel code.

Metering & monitoring

Meters for billing are incorporated into the pressure reduction station. Ultrasonic meters are used for the main line billing, while positive displacement meters are used for lower pressures. “Besides being extremely accurate, ultrasonic meters have the advantage of having no moving parts and so they require no maintenance. As gas pressure and temperatures vary all the time, volume correctors are used to calculate the gas volume to a standard base pressure and temperature for billing purposes,” says Van Huyssteen, adding,“Metering standards are mostly based on the American Gas Association standard AGA‐ 8. Where stations are in remote locations, clients usually require remote metering systems to read the meters from a control or supervisory station. Technologies available are cell phone networks, satellite communications or fiber optic lines.”

In addition to taking meter readings, clients also need to monitor the status of the station and control the emergency shut down valve on the inlet of the station. In and outlet gas pressure and temperature, and filter differential pressures are typically monitored.

“At a time of local energy crisis, it is imperative that we actively seek out feasible power supply options. Natural gas is both feasible and viable in answering to this need and is increasingly used for power generation and industrial plants – but constant supply is crucial. With the expertise to serve all spheres of the oil and gas industries, Energas Technologies is well‐equipped to provide advanced monitoring and control technologies for a smooth and consistent energy supply – with no interruptions,” Van Huyssteen concludes.

Equipment for the stations and pipelines was sourced mainly from world‐leading suppliers in Europe, such as Gorter and RMG by Honeywell, RMA Pipeline Equipment, GTS Thielmann, Galli & Cassina and Tai Milano. Construction started in June 2014 and the power generation plant is expected to produce its first power towards the end of October 2015.