Valves are an integral part of any processing plant. From crude commodity valves used in meters to complex control valves. They all have a purpose, they all cost money, and they all need to be replaced at some point in the future. It is well known that unexpected failures can be a major cause of valve replacement. In this article, we will explore the top 10 causes of valve failure and some mitigation strategies.
It all starts here. Specifications can cause valve failure in two ways: bad data and missing data. Bad data refers to improper valve type and material selection due to incorrect information. Data sheets were filled in, but the assumptions used were not consistent with the actual conditions of the process. This is usually due to a lack of understanding of the process or valve operation.
Regarding the process, the designer may have made appropriate choices for the proper operation of the plant, ignoring conditions such as cleaning, downtime and aging that may require different valve characteristics. Misunderstanding how a valve works can also lead to failure.
For example, floating ball valves depend on upstream pressure of the system to press the ball against the seat for proper sealing. The higher the pressure, the greater the force of the ball against the seat. Conditions that result in a high seal are full constant pressure upstream and atmospheric pressure downstream (no back pressure trying to remove the ball from the seat). But opening the valve depends on size and pressure class, which can be a daunting task. Operators around the world may use cheat bars to solve the problem, which can damage the gearbox or valve. Even if the valve is open, high flow may cause damage to the seat or ball valve during the initial opening. High friction between the seat and ball valve can also cause damage. This can be avoided by connecting upstream and downstream balance lines so that the valve can be opened in less laborious conditions. This is not a practical solution for every situation, and the decision may be to stick with ball valves and adapt to their limitations. No matter what decision is made, it is essential to understand the sealing principle behind the valve properly. Analysis of every valve in an industrial plant (which may contain thousands of valves) is rarely feasible, but it is a welcome procedure, at least for the most critical. In addition, it is worthwhile to get feedback from a similar plant and see which valves are working and which are not performing well.
As we saw above, bad data is about insufficient information on the data sheet. The lack of data is exactly what it sounds like: some lines of the data sheet are left blank and it is up to the supplier or EPC company to decide what is suitable for application. Companies sometimes use this strategy to reduce the diversity of specifications in their factories. If both PEEK and Devlon seats are considered "soft seats" in specifications, the company now has only one model reference number instead of two. The cost of this simplification may or may not be paid in the event of failure. This depends on the complexity of the application and the quality of the vendor.
2. The supplier
The end user's assessment of supplier quality usually depends on the size of the company. Small businesses have few resources to properly vet potential suppliers, including compliance with quality standards such as ISO 9000, cyclic testing, design reviews, and access to sub-suppliers. They need to rely on field-proven solutions, or at least buy from reputable manufacturers. Large companies usually have strict procedures for maintaining AML (list of approved manufacturers). Vetting new suppliers is an expensive exercise that requires the involvement of different departments and can be time-consuming, often taking nearly a year. Even after manufacturers join AML, audit visits need to be simplified to ensure quality is maintained.
Unfortunately, there are times when projects need to be completed and time constraints require the purchase of valves from an unproven supplier. This is far from ideal, as there are no shortage of examples of valves that look good on paper but fail in the field. This is not always the case with poor quality. Sometimes valves of the same specification have very different performance in the same application due to slightly different design or manufacturing methods.
The inspection cost associated with the inspection is one of the expenses of careful inspection by the manager. A common temptation is to cut costs when the supplier has a record of valve performance. Examples of valve failures related to defects in the inspection process are not hard to find. Untrained professionals sent to review documentation and traceability, perform all visual and dimensional verification and witness stress tests are easily overwhelmed by the program. Suppliers have repeatedly applied vaseline to seal surfaces to pass seat tests or used measurement techniques that greatly reduce the chance of detecting leaks. Even when dealing with a reputable manufacturer, it is still possible to approve valves of the wrong size or material. It takes a lot of money and time to train inspectors. This is not a case of simply providing classroom training. Conducting mentoring and accompanying inspections of several suppliers in order to develop professional skills appropriately.
Transport risks are associated with impacts, falls and contamination of water and/or solid particles. They involve not only moving the valve from the manufacturer to the warehouse, but also moving the valve from the warehouse to the final location. Valves shall be preferentially transported in their original transport package and properly secured to the vehicle. Failure is possible when the valve is not transported according to standard. Figure 1 shows a jet engine that fell off a truck during transportation. It's hard to imagine securing a $45 million piece of equipment that weighs about 7,000 kilograms with 900 kilograms of tape. But that's what happened. Another common problem has to do with removing valves from their original packaging to save space when transporting multiple items. Sometimes even the protective plastic cap is removed, leaving the valve's internal components exposed and vulnerable to contamination. Transport risks also involve rigging and lifting. Always check the OEM (original Equipment Manufacturer) manual for appropriate procedures. One final warning: proper valve transportation is not just a matter of avoiding material loss. It should be primarily a security issue. Valves are usually heavy enough to cause fatal accidents, and even small valves can become projectiles when not secured in a moving vehicle.
Preservation risks are closely related to transport risks such as removing valves from original packaging, removing plastic covers and storing in sunlight, often in hazardous environments. Long-term storage at offshore facilities or in maritime transport may require special procedures to ensure adequate preservation.
Proper installation procedures are usually well explained in the OEM manual. It usually starts with rigging and lifting instructions, local cleaning and assembly into the pipe. For ring joint washers, cleaning is of Paramount importance. This type of metal-to-metal gasket relies on a smooth surface finish to seal, surfaces that can easily be damaged if not properly cleaned prior to installation. Special care should also be taken with drive valves with large actuators, as shown in Figure 2. They may require special supports or additional support to avoid fatigue fracture due to pipe vibration, especially in vertical or inclined installations.