Two commonly used low temperature tests today measure the degrees for liquid nitrogen and liquid natural gas.BY GREG JOHNSON
Valves today face more challenging conditions from a wider range of applications. As a result, users are asking for more and better testing.
Filling a valve up with water, adding pressure and looking for leaks might work for some valve specifications, but many of today's demanding valve requirements call for much more stringent testing and evaluation. Special service applications such as hazardous fluids, nuclear power plants, high-pressure pipelines and more dictate a much broader testing and inspection regimen than traditional simple tests.
Many users are requesting valve manufacturers prove their products will operate satisfactorily at the higher and lower temperatures and more extreme pressures that their valves are advertised to reach. These may be the lowest cryogenic temperatures or elevated temperatures close to 1000° F (538° C). Such tests call for specialized equipment and test procedures.
Cryogenic testing is generally performed at temperatures ranging between -50° F (-46° C) and -320° F (-196° C).The most common of these more extreme tests is cryogenic testing. Such testing is generally performed at temperatures ranging between -50° F (-46° C) and -320° F (-196° C)—most often at -320° F (-196° C), which is the temperature of liquid nitrogen (LN). Standard practice is for the valve to be immersed in the LN up to the packing gland area, if the valve is equipped that way. The packing must be kept out of the LN or it could freeze the packing, seizing the stem and causing the valve to lock up and fail to operate. Because polymer seals do not function well at cryogenic temperatures, valve end connections must be the type that makes a solid mechanical connection. These include threaded, flanged or caps welded onto buttweld-end ends. Socketweld-end and buttweld-end valves without welded-on caps are very difficult to test at the lowest cryogenic temperatures.
One of the most popular low temperature services today is liquid natural gas (LNG). Valves for LNG are sometimes tested at -320° F (-196° C), but a more accurate test is performed at the actual LNG temperature of -260° F (-162° C).
Cryogenic testing is costly and hazardous and should only be performed by experienced, trained personnel. The test procedures for cryogenics are available from several standards-making-organizations, as well as end users. The most significant differences in testing procedure documents are allowable leakage rates.
PIPELINE VALVES
Pipeline safety has come to the forefront lately because of catastrophic pipeline failures. These failures have occurred primarily on older pipelines because quality requirements for new pipeline construction are very stringent. Valves for pipeline service are also scrutinized very closely. While all pipeline valves are hydrostatically tested at the factory, usually in accordance with API 6D, additional tests are almost always performed. The most common extreme test for pipeline valves is a long duration shell test, which is carefully monitored by a recording device tracking the pressure and the temperature of the valve as it is tested.
During these enhanced duration shell integrity tests, the pressure on the valve must be maintained, or the pressure drop must coincide with a proportional drop in temperature to avoid valve failure. It is not uncommon for test durations to run several hours long.
FUGITIVE EMISSIONS TESTING
A valve is prepared to undergo fugitive emissions tests.The desire to keep our nation's air clean is manifested in the valve industry through the Clean Air Act and various state and local regulations. For manufacturers to meet today's low emissions requirements, valves must be tested to determine their ability to contain these fugitive emissions (FE). FE testing is now a requirement by most refiners and chemical companies that must contain hazardous fluids as part of their everyday processes.
FE testing requires the valve be pressured up with an easily measurable gas such as methane or helium, and then checking the body and seals, particularly the packing, for leakage. An alternative method is to create a vacuum drawn on the valve through a closed piping system and introducing a tracer gas into the areas of the valve exterior susceptible to FE leakage.
Two distinct schools of thought exist on what gas should be used to FE test a valve—schools separated by the Atlantic Ocean. In Europe, it is deemed unsafe to test with methane, so all testing must be performed with helium; in the U.S., the preferred test media is methane, which more closely resembles the molecular structure of the volatile organic compounds (VOCs) that both industry and government are working hard to control.
The procedures for FE testing generally require the test valve be mounted in a device that can firmly hold both the valve and its actuation mechanism. The valve is then cycled from dozens to hundreds of times while pressured up with methane or helium. As an added service simulation, the valve is heated at least twice to around 500° F (260° C) during the test sequence. The heating cycles simulate the temperature variances that might be acting on an installed valve. The temperature spikes also provide prototypical movement in the packing due to the expansion and contraction of the materials of construction. It is usually after a thermal cycle that a valve under test will show leakage.
The two schools of thought on the test media also relate into two schools of thought on the test procedures themselves. The International Organization for Standardization (ISO) has FE testing standards as do the American Petroleum Institute (API) and the International Society of Automation (ISA). The ISO standard 15848-1 is currently under revision to make it more user friendly. API is updating its FE packing qualification standard (API RP622) as well creating a new valve FE test standard (API RP 624).
FIRE TESTING
Fire testing involves pressuring the valve with water and then focusing several jets of flame onto the valve.The most visually exciting of the extreme valve tests is the fire test—when you mix high pressure, water and several gas-fueled "flame throwers" together, the result is certainly not boring. Fire testing is used to simulate a fire in a plant or refinery and determine how well specific valves and components will function during that fire and after it has been extinguished. The procedure involves pressuring the valve with water and then focusing several jets of flame onto the valve. After the test piece has attained the prescribed temperature, the valve is doused with jets of water, simulating the firefighting aspect of the process. The leakage rate of the valve or component is then measured and compared with acceptable leakage rate for the test standard.
Fire testing is performed regularly on soft-seated ball valves to determine if they are fire-safe, meaning that, after the initial polymer seal burns away, a secondary metal-to-metal backup seal can still prevent major flow through. Upstream valve manufacturers use a variety of fire-testing standards to confirm the efficacy of the seals in their products to withstand fires at the wellhead.
RADIOGRAPHY 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. 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.
In today's marketplace, castings are especially vulnerable to poor workmanship.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.
NUCLEAR TESTING
CONCLUSION
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