11/2014 - Assessment of SENTRY GS Excess Flow Valves in Underground Gas Service Lines
This article examines the effectiveness and reliability of SENTRY GS spring-loaded excess flow valves in underground gas service lines. Gas utilities use excess flow valves to minimize the risk of damage in the case of a break in the gas service line.
A 2004 study titled The Assessment of Excess Flow Valves for Impeding Manipulations (the German Technical & Scientific Association for Gas & Water [DVGW] project 4.1.04), examined excess flow valves that had been in service since 1992. The results of the study state that excess flow valves are effective and reliable. All the German public gas utilities that participated in the study preferred excess flow valve use because of added safety benefits and a positive image. The result was that in 2004 excess flow valves were made mandatory in Germany for both underground and in-house gas service lines.
A number of other countries have a history of using excess flow valves. For more than 35 years U.S.A. gas utilities have used excess flow valves, and since 2008, excess flow valves have been mandatory for underground service lines. France has used excess flow valves for underground service lines since 1997. In 2012, France, Germany, the Netherlands, Austria, Switzerland, and Italy combined had more than 5 million excess flow valves in service.
Excess flow valves are designed to close, shutting off the gas flow, when a predefined flow rate is reached. Mertik Maxitrol excess flow valves are calibrated at the factory, providing a precise and reliable closing flow rate. They are installed in service lines (25 mbar to 5 bar operating pressure) between the main gas line and the gas meter. The excess flow valve should be installed as close to the main gas line as possible. In the nominal flow range, an excess flow valve remains in a stable, open position. If equipped with a by-pass orifice, the SENTRY GS excess flow valve can be reopened after the downstream line has been repaired and re-pressurized.
Materials and Components of Excess Flow Valves
In practical, real-world applications excess flow valves will open and close only a few times during the production process and when the gas system is tested. When in service, excess flow valves remain in the open position. It is extremely unlikely that a drop in the closing flow rate will occur during long-term operation.
Mertik Maxitrol has used the materials that make up the SENTRY GS in various controls for more than 65 years. The housing and spring are made of brass and stainless steel, and the closing unit is either brass or aluminum. The pressure springs are made of corrosion resistant stainless steel and are sized for dynamic loads of > 107. The springs have been evaluated and undergone testing for a minimum of 100,000 opening and closing cycles.
Long-term Reliability of Excess Flow Valves
Gas utilities build underground gas networks with an estimated service life of 50 years. Although excess flow valves have not yet been in service for 50 years, performance testing has been done to quantify their longevity through laboratory and field tests. Mertik Maxitrol SENTRY GS excess flow valves have been examined under both static and dynamic conditions.
Many factors can affect the service life of an excess flow valve such as the natural gas used, moisture in the gas, temperature fluctuations, movement of the earth, or contaminants introduced into the system during installation. The type of service pipe also affects an excess flow valve's service life. Underground service pipes are usually made of polyethylene, cast iron, steel, or a combination of materials.
Laboratory tests studied the effects of contamination on Mertik Maxitrol's excess flow valves. 10 cm3 of contaminant was introduced into the pipe upstream of the excess flow valve. The contaminant consisted of pulverized coal, corundum (a mineral), and splints of a synthetic material. The flow was then slowly increased until the excess flow valve's closing flow rate was reached.
Before the contaminant was introduced into the pipe, leakage was less than 2 l/h. Corundum caused leakage at a rate of 25 l/h, and leakage exceeding 1 m3/h was measured when long or coiled splints of the synthetic material became lodged in the seat of the excess flow valve.
Despite the contaminant preventing the excess flow valve from closing completely, the amount of gas leakage was greatly reduced. (The SENTRY GS is designed for low-pressure drop, and its internal flow path allows contaminants to pass through.)
In addition, major German gas utilities' have performed their own SENTRY GS field tests in underground gas service lines since 1994, and National Grid (formerly Transco) the largest public gas utility in the U.K., has conducted SENTRY GS field testing since 2005.
The German utilities testing involved installing various types of excess flow valves into gas service lines located in various locations throughout Germany. Many of these areas had previously used town gas, which contained more moisture and impurities than the natural gas used today. The objective was to see how different types of contaminants in the service lines affected the SENTRY GS.
Five SENTRY GS25... excess flow valves were installed in a natural gas service line and removed 6 months later. On inspection it was found that aside from slight leakage in the closed position (2 l/h), the excess flow valves still functioned properly. Two of the SENTRY GS25s were reinstalled and left in the service line for 6 more years. Test results once again revealed slight leakage in the closed position (8 l/h).
Durability testing was performed at Mertik Maxitrol and at Gas und Warme Institut (an accredited third party test house) in Essen, Germany. The German Technical & Scientific Association for Gas & Water (DVGW) and GDF Suez (formerly Gaz de France) commissioned the testing. Mertik Maxitrol's excess flow valves were put through 10,000 to 400,000 opening and closing cycles. The test results showed no reduction in spring pressure and reliable reopening of the excess flow valves.
Mertik Maxitrol also performed a condensation/water climate test for electrochemical corrosion (the test was performed according to German Standard DIN 50017). SENTRY GS excess flow valves were exposed to 100 % humidity at 104 °F (40 °C) for an 8-hour period. The test pieces were then subjected to < 100 % humidity at room temperature for 16 hours. This process was repeated for 10 days.
Results showed that even with some corrosion on the surface of the brass and aluminum components, the closing flow rate remained within tolerances. Residual deposits on the O-ring did result in slightly higher leakage, but it was also within the tolerance of 3 l/h. GDF Suez conducted comparable climate tests on Mertik Maxitrol's excess flow valves with similar positive results.
For more information go to Products / SENTRY GS Excess Flow Valves and download the catalog SENTRY GS Excess Flow Valves for Gas Service Lines (literature code GSE-LT-EN-07.2013).