- Published on Monday, 23 June 2014 10:54
- Written by Paul van Oudenaren and Kelly Watson
Metal-seated ball valves are industrial process valves commonly specified for use in mining, chemical plants and non-refinery settings. These valves are used in applications that demand critical isolation, high temperature, high pressure, corrosion, erosion/abrasion and solids (slurry or otherwise). It is these processes that drive the need for metal-seated quarter-turn valves. End users who choose metal-seated ball valves require quick actuation and repairability. The ball and seat set can be repaired in several types of ball valves including: floating ball valves, trunnion ball valves and spring-loaded floating ball valves.
Repair or Replace?
Some factors to consider when making a decision to repair or replace a valve include: economics, plant maintenance planning and corrosion/erosion failures.
The first step is defining objectives and defining what success means when considering the technical aspects of the valve’s design. Valve repair is a lengthy process, so you want to make sure it is the right solution and a user will not consider repair unless it’s an economical option. So, before performing a repair, consider other valve styles that could be used. Also, determine if the valve is the right style for its particular location? Is it a severe service application? These and other factors should be considered before starting the repair process.
Valve repair is helpful for plant maintenance/shutdown planning and allows for preventative maintenance programs. In many cases repairs can be turned around in 2¬ to 6 weeks.
The Repair Process: Evaluation
There are several steps involved in metal-seated ball valve repair. This article will consider the evaluation of scrapped, replaced and repairable components, and then look at parts manufacturing and repair including thermal spray coating, grinding and lapping processes.
Scrapped components: Typical scrapped components include one-time-use soft goods such as packing, body gasket, thrust washer, seat gaskets, any severely damaged components and seats. What you can and can’t re-use heavily depends on your valve concept.
Repairable components: Typically these include the ball, stem, body and end piece, and metallic parts with minor damage. That minor damage can be repaired by welding/machining or undercut/coating to rebuild them back to size.
After valve cleaning, a dimensional check is performed to evaluate its actual repairability. Some might see a severely damaged valve and think, “Why would one ever want to repair this part?” If that ball is made out of titanium grade 12 or Hastelloy C276, it’s a high price for the user to bear. In terms of repair economics, a lot of what’s driving the decision-making process is the expensive materials that are needed and used in these services.
In most severe service design concepts, there is a straight-thru flow port. Normally the amount of damage to the body and end piece is not severe, unless there is a high-velocity leakage situation occurring. Any leakage found around the OD (outside diameter) of the seat can cause other erosive phenomenon to occur. If this is the case, find out what damage it can bear and if it can be welded. Massive welding can cause distortion. With large weld depths you will need to consider the applicable valve standards and pressure vessel codes.
The Repair Process: Thermal Spray Coatings
Thermal spray coatings are processes in which melted or heated materials are sprayed onto a surface. Even if there is only minor wear, in order to put back into the field a product that is going to seal again, it is necessary to strip and re-coat the ball, and remanufacture and coat the seats. It’s rare that a damaged valve can be simply cleaned and lapped. Usually the coating will be stripped off and re-applied. There are two thermal spray coating processes that are utilized: high-velocity oxygen fuel (HVOF) and air plasma spray (APS).
HVOF coatings are applied by combining oxygen and fuel gas at high pressures and accelerating them to supersonic speeds while powder is being injected into the flame to apply coating. HVOF produces dense coatings with low porosity and high bond strength. These coatings rely on creation of molten splats that hit the part, and then deformation and mechanical bonding of those splats as they build up. HVOF is used to apply carbides and metal/alloy coatings. They have a typical speed of 2,051 feet per second, a plume temperature of 3,800°F, and the inter-pass temperature of the part is less than 300°F.
One type of coating applied via HVOF is tungsten carbide (WC). HVOF carbide coatings feature a mechanical bond, in contrast to spray and fuse, or weld overlays, which achieve a metallurgical bond. Tungsten carbide coatings can handle high temperatures and provide toughness; they are useful in combatting against abrasion, erosion and corrosion. Depending on the properties of the valve’s service, different coatings will work. More than likely, you will not start with the hardest tungsten carbide coating matrix, but you may realize that the corrosion chemistry of the service requires another type of coating. The corrosion chemistry can be changed by adjusting the matrix without sacrificing any of the abrasion resistance or hardness because the tungsten carbide is still present.
A second type of thermal spray coating is air plasma spraying. In air plasma spraying, an arc is formed between an electrode and spray nozzle, and pressurized inert gas is heated to high temperatures to form a plasma gas. Powder is then injected and melts as it is propelled onto substrate at high velocity. Plasma spray is used to apply ceramics and materials that melt at high temperatures. The typical speed is 727 feet per second. The arc temperature is greater than 20,000°F, the plume temperature is approximately 4,000°F, and the inter-pass temperature of the part is less than 300°F.
Plasma-applied ceramic coatings feature a mechanical bond and are used to combat corrosion and abrasion. Typical materials include titanium dioxide and chrome oxide. Titanium dioxide provides toughness and extreme corrosion resistance. Chrome oxide provides abrasion and extreme corrosion resistance. While it may not seem as if chrome oxide is needed, this may be the type of coating to employ if a previously selected carbide coating is not enough.
The Repair Process: Thermal Spray, Grinding and Lapping Processes
Grit blast is used to remove thick scale on parts. A post-grit blast evaluation is performed to determine whether the part needs weld repair. To prepare for coating re-application, the coating is then removed to the base metal. The goal is to essentially start from scratch. Once that coating is removed, there is a better view of the underlying part. At this stage the roundness of the ball is verified and a final evaluation determines if anything more needs to be done to the part before the new coating is applied.
A finish size is then established (driven by a dimensional drawing of the part), and the part is prepared for thermal spray. Spray parameters are set, the part is coated, and then it is finished down to tolerance by grinding or lapping. Ceramics require finish lapping to bring to tolerance. Carbides are finished by grinding and lapping. Finished thickness will vary depending on the number of repairs previously done. Typically, customer specifications will outline maximum allowable thickness.
Grinding is the use of an abrasive to wear away at the surface of a work piece to change its shape. Spherical grinding is used to finish ball valve balls. Although the grinding process is used as an initial step to remove the old coating, it is also used as a finishing process. Grinding brings the ball to the correct finished size. It is important to have a good surface finish on the ball in order to achieve a good metal-to-metal seal. When the grinding process is used to finish, a lapping process will follow.
Lapping is a precision abrasion process used to bring a surface to a desired finish or dimensional tolerance by removing an extremely small amount of material. This is done to achieve a sealing surface. There are two types of lapping: rough lapping, which is a machine-driven process used to remove a lot of material quickly (similar to grinding), and finish lapping, which removes an extremely small amount of material to achieve a sealing surface.
After the repair process is completed, the assembly and testing phase begins. This includes: assembly… stack fitting, bolting, labeling, pressure testing and in some cases O2 cleaning.
The Repair Cycle
Since valve repair is cyclical, the same valve may be seen repeatedly, so repair decisions must be made using a long-term thought process. If a particular valve is failing prematurely, tell-tale signs will be revealed each time. That would initiate failure analysis and even a potential re-design. During valve repair, there is always an opportunity to modify and improve the valve for the end user.