Valves in action -
Pressure reducing valve DM 620
Careful material selection for the use of valves in acid gas environment
If left untreated, natural gas or biogas contains hydrogen sulphide (H2S) and water vapour. When the H2S percentage in the natural gas exceeds 1 %, it is commonly referred to as acid gas.
The gas humidity, i. e. the water vapour percentage, brings about hydrate formation and corrosion. To feed the gas once again into the transport network, limit values for the content of water and other fluid constituents are specified. Therefore, withdrawn gas must be dried. The drying process comprises mechanical and thermodynamic process steps. During the final drying process stage the gas is conducted through adsorption towers, in which the residual humidity is collected with the help of highly hygroscopic substances such as glycols. Similarly to the extraction of natural gas, every field station of a natural gas reservoir is equipped with a drying plant for the withdrawn gas. The gas drying process takes place in three stages: mechanical separation of free water, then pressure reduction and lastly the use of glycol.
In many cases medium-operated high pressure reducers of the type DM 620 according to NACE are employed. All the valves in the process plant are required to comply with the NACE standards as described in MR0175, for example. In accordance with NACE International (National Association of Corrosion Engineers), the material for natural gas valves has to be carefully selected, which means that steels with significantly reduced hardness are taken into consideration. Otherwise there will be the danger of excessive embrittlement of the steel. For the above mentioned application, the question of corrosion resistance and, consequently, the question of material selection became more vital inasmuch as the gas terminal was built in close proximity to salt water, that is to say sea water atmosphere.
