In the oil and gas industries, valves are key components involved in the control of the flow of corrosive, abrasive and erosive gases and fluids. However, resistance to wear and galling in valves poses a material problem to oil and gas companies across the globe.
Hard materials are generally used in extreme environments. However, the brittleness of hard materials affects their performance. In addition, such materials can be highly expensive.
Some valve components tend to deform with the application of high pressures and shock loads, resulting in chipping, fracture and even catastrophic malfunction of equipment. Pipelines, pumps and valves are generally corroded by acidic fluids, sour oil and gas containing aggressive H2S and different grades of crude containing CO2. Even under normal operating conditions, these valves may undergo microcracking and fatigue erosion that lead to premature failure.
Hard chromium electroplating was considered a reliable solution to prolong the service life valves. However, this technique is restricted under REACH and OSHA environmental and health and safety guidelines, owing to the use of carcinogenic hexavalent chromium salts.
Unlike other alternatives, CVD coatings can be precision-applied to complex geometries and internal surfaces. For instance, HVOF is a type of thermal spray coating technology that deposits tungsten carbide grains contained in a softer metal matrix. This line-of-sight technique is not suitable for coating undercuts. The resultant coatings have a porous structure and rough surface, and require machining post-coating, which is not always possible on intricate shapes.
For cages and plug trim in choke valves, the conventional line-of-sight methods may clad the component surface without coating the inner diameters of the through holes that pass along the sleeve. The choke valves are subjected to abrasive media and high velocities owing to their pressure reducing role which can result in additional wear and erosion issues.
Ball valves can also be subjected to severe abrasion by sand or stone chippings in fluids, and from erosion by accelerating flow while being opened or closed. The CVD coatings make the valve parts resistant to wear and scratches. The metal to metal seals in ball valves can suffer hard wearing, and there is often need for flex in the material. Traditional carbides, however, cannot meet this requirement as they are conducive to cracking due to their rigid nature.
Ball valve spray coating needs to be performed at a 90º angle to ensure the generation of a mechanical bond to avoid any impact on the adhesion of the coating. Choosing an inappropriate approach angle can result in edge chipping.
The CVD coating process avoids this potential pitfall, and is suitable for more intricate and smaller valves. Other advantages of CVD include cost savings achieved by avoiding the need for machining after coating, the strength of the chemical bond and shorter lead times through batch processing.
Although there are a number of choices to resolve valve wear issues, some of them may not meet the heavy duty requirement of oil and gas pipelines.
Coatings not only add value to the components by providing corrosion and wear resistance combined with toughness and ductility, but they also reduce operational costs by saving downtime, improving performance and increasing productivity.
The CVD process paves the way for the development of design of valves capable of operating in extreme oil and gas pipeline conditions, providing greater engineering flexibility not possible with other technologies.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)