Today’s newer, low-cost manufacturing sources, combined with a loss in U.S. manufacturing expertise, are creating increased scrutiny on valves and other piping components. Castings are especially vulnerable to poor quality and workmanship. Because of the nature of their potential defects, unaided visual examination is not enough to instill a sense of security with many valve users. For this reason, users often call for additional nondestructive evaluation (NDE).
Some exterior NDE methods such as dye penetrant examination (PT) and magnetic particle examination (MT) are helpful. But what is often specified is a look inside the walls of the valve, or as this look is sometimes called, a volumetric examination. The best way to accomplish this for castings is by radiography. Radiographic examinations (RT) can provide a useful look inside the pressure-containing wall of the valve and help to determine the overall soundness of the casting, as well as determine if potentially hazardous defects are present.
A layman reviewing radiographs of a valve casting would be hard pressed to determine whether or not the valve metal is of good quality and fit for service. The subjective nature of comparing reference films and their “shades of gray” mean that even some experienced radiographers have a hard time evaluating what they are seeing. Radiographers that shoot valve castings every day and diligently scan their reference radiographs like the latest issue of Sports Illustrated have a better chance of providing correct and repeatable interpretations.
Casting defects are divided into various types and classes. These include shrinkage, porosity, gas and inclusions. The categorization of these defects is generally viewed as a good way to judge the overall soundness of the casting and the workmanship of the foundry.
Defects such as hot tears, which are areas of shrinkage open to the surface, or cracks are viewed as performance-affecting defects. Because of this, none of these defects are allowed in most casting evaluation standards. These types of defects can reduce the effective wall thickness of a casting, causing its pressure-retaining capability to be compromised. Another issue with cracks and hot tears is that they are “stress-raisers,” which means they can act to initiate further cracking of the metal to the point that catastrophic failure could occur.
Despite its subjectivity and limitations, radiography is still the de facto method of valve casting evaluation, especially for critical refinery and power plant applications.
The most critical valve applications today are in the nuclear industry. Valves for use in nuclear power plants, including nuclear-powered navy ships, are subjected to very rigorous examination under a wide range of tests that would make for a separate article. One of the special tests required for on-shore nuclear power plant service, for example, is seismic testing, which simulates the stresses caused by earthquake activity. The joke in the valve industry used to be that the stack of paperwork certifying a nuclear valve was often bigger than the valve itself though today those test reports and traceability documents are stored on digital media.
With the widespread concern on safety these days, both valve manufacturers and end users have responded by creating appropriate tests for valve integrity, especially for critical flow control applications. Many of these extreme tests were randomly called out 50 years ago; but today, they are commonplace. And, as even more critical valve applications are developed, undoubtedly, there will be newer and tougher extreme valve tests to ensure product integrity for those yet-to-be designed products.
- << Prev