Because natural gas is currently considered a good source of energy for both environmental and economic reasons, it’s growing in use. This trend promises to continue worldwide for many years to come.
As a result, the liquified natural gas (LNG) market also will see growth, which means more use of cryogenic valves. In the LNG industry, as well as some other cryogenic fields, the importance of check valves is indisputable.
HOW AND WHY THEY ARE USED
Cryogenic check valves are a critical component of the LNG supply chain. They have the sole purpose of directing fluids or gases in a single direction and preventing their reversal. One reason that’s vital is that while LNG is a comparatively safe fuel, its vapors can pose a hazard if not properly contained.
Check valves are used throughout the process from conversion to storage to transport. All of these can be costly and inefficient if the right check valves are not used. Using the right engineered check valves, on the other hand, can help to ensure equipment is protected, productivity is maximized and operational safety is ensured.
Backflow prevention is needed in almost every piping system. Most of these pipelines use pumps or compressors to generate the required pressure for movement of fluids or gases. When this equipment stops, flow reversal or backflow can occur (depending on downstream pressures). One of the most significant elements of the cryogenic production system design is integrating a means to prevent this backflow.
Not only does this minimize damage equipment, it also ensures plant infrastructure is protected and emission leakage is kept to a minimum. For these reasons, engineered check valves are paramount across the value chain of LNG whether that’s in the gas field, liquefaction plant, storage tank, tanker or gasification terminal. These valves not only offer solutions for mitigating the destructive effects of flow reversal, they also provide reliability in the severe conditions presented in cryogenic applications.
HOW THEY WORK
Check valves are totally different from shut-off and control valves. While those valves can stop flow completely, check valves allow the flow of fluid in a single direction. They are intended primarily to protect pumps, compressors, piping systems and other critical components from dangerous conditions such as system deceleration and water hammer. Additionally, unlike on/off valves, check valves are flow sensitive. They rely on the line fluid to open and close them, which means they are one of the few self-automated valves.
As a result of constant exposure to fluid mediums, check valves are highly susceptible to wear, sticking, jamming and wedging. Traditional check valve solutions also can suffer from heavy slamming caused by turbulence, which in turn can compromise the ability to prevent flow reversal. Traditional solutions such as swing and tilted disc check valves do not close quickly, which can exacerbate the effects of dynamic pressure surges or water hammer. They also have significant pressure loss that results from the flutter or rapid movement of internal metal valve components susceptible to low-flow conditions that can cause wear and premature failure.
To address these challenges, the industry has sought out alternative check valve solutions that incorporate features and benefits that can directly address and counter problems. For example, a highly engineered dual-plate wafer valve design is a stronger, lighter, smaller and more efficient check valve in common use in this field (Figure 1). Using springs to increase the valve reaction and provide more efficient response time (faster closing), this type of dual-plate check valve can better protect equipment already in place in the liquefaction and regasification terminals within an LNG processing plant. The lesser closing time can reduce the dynamic surge effect in the pipeline for an improved non-slam performance as well.
To address the extreme cryogenic demands of the liquification process, certain dual plate check valves have been approved for sub-atmospheric to cryogenic temperatures ranging from -58°F (-50°C) through -321°F (-161°C). These valves are wafer non-slam check valves that can be modified and sized for the specific application. Another alternative check valve is the non-slam nozzle-style, which can be specifically designed and sized for fast-reversing systems, again to protect against backflow concerns (Figure 2).
These highly engineered check valves not only minimize the effects of water damage, they also eliminate the chatter associated with conventional valves, protect rotating equipment from flow reversal damage, minimize pressure loss in piping systems and provide fast dynamic response to reduce reverse velocity.
Also, for critical applications such as LNG, the internal geometry of check valves can be modified to suit the service conditions. A benefit of both the dual-plate and nozzle style check valves, for example, is the lack of a leak path to the atmosphere. Without this path, zero fugitive emissions escape for the life of the valve, and adjustments are not required.
Check valves typically have higher allowable leakage rates than isolation valves, but in cases where valuable equipment, pipes and other aspects of the plant’s infrastructure need to be protected, a check valve that restricts backflow more efficiently will provide additional value in terms of protection for equipment, pipes and other aspects of the plant’s infrastructure.
SIZING AND SELECTIONS
A concept not often understand about check valves is that the most critical time for them is during fluid deceleration. All valves need to close quickly to prevent reverse flow. Check valves are automatic in this process, and therefore susceptible to fluctuations in fluid flow. They require no outside stimulation and rely instead completely on the basic forces inside the valve, as well as external factors such as fluid type, temperature and more to determine when they will close. The actuator for these valve types, then, is the fluid system inside the pipe. The valves close only when the forward flow begins to decelerate below critical velocity. Because of this, varying internal system pressures can cause problems that influence check valve selection and functionality.
To eliminate backflow, ensuring the valves in place are also sized appropriately for the application is critical. Generally, valve sizing is done by selecting a valve that matches the pipe size. However, this process can leave much to chance, which can result in impractical situations as far as investment and valve performance. The flow the valve can provide is just as important as the pipe or line size. In most cases, this flow can be determined by the principles of energy conservation in the flow calculations. For example, a valve that is too small will not pass the required flow rate and could develop high-pressure loss. A valve that is too large will be expensive and create instability in the system. A valve that is too large also can wear out prematurely, ultimately leading to valve failure.
Knowledge of the process conditions for a particular application is also vital in determining proper valve size. Users must understand the process to select valves that can withstand those conditions. They include factors such as energy loss, pressure loss from friction and turbulence, flow coefficient and the Cv provided by the valve manufacturer. How a valve performs and holds up under these conditions and what is the listed service life, are key factors that must be considered before selecting a check valve for cryogenic applications. The total process system and its design must be part of the decision. Understanding what is needed, for how long, and under what conditions is paramount for safe and trouble-free performance.
In short, valve sizing and selection should consider valve material compatibility with the fluid medium, the valve rating (ASME pressure class), application flow, design and operating conditions, the installation requirements of a particular facility, line size, end connections, system modifications and proper leakage regulations.
Most valves are not geometric models of each other. Because of this, to predict the performance of various valve sizes requires a thorough and comprehensive modeling program. Manufacturers should test various sizes of valves and then apply the modeling criteria to accurately predict the performance of other sizes.
The direct method to determine valve performance is a laboratory test. However, that might not be practical when the valve is too large, a multiplicity of variables might cause the test to take too long or the facilities are not available to do the right type of testing.
For example, wind tunnels have been used to predict performance using fixed and small objects. However, valves that have moving internal components could complicate such a modeling process. Predicting performance of large valves such as 48 or 72 inches is not practical unless a geometric model can be tested.
Valve characteristics in the modeling process may include resistance to flow, flow areas, component thickness and inertia, spring forces and loop flow.
Many world organizations such as the International Organization for Standardization (ISO), British Standards (BS), the Manufacturers Standardization Society (MSS) and individual end users such as Shell, have published standards for the cryogenic industry as well as for LNG applications. These standards govern the performance of valves, and some have existed for many years.
The specifications that require special testing of valves for particular applications will vary in methods and in acceptable valve seat leak rates. Many of these standards specify leak rates for check valves that are close to isolation valve rates. However, with the currently available sealing solutions, the manufacturers of cryogenic valves can struggle to meet the global seat leakage performance specified. Some of the standards are shown in Table 1.
Some standards recognize the big difference between metal-seated valves for isolation and metal-seated check valves for flow reversal control and allow a higher seat leak rate on metal-seated check valves. This type of leakage approach would make the valves less costly and still provide important back flow prevention.
Check valves play an important role in the protection of the expensive and critical equipment in liquefaction and regasification facilities. Their sole purpose of preventing the damaging effects reverse flow can have on equipment and on the safety of plant personnel make check valves a critical addition to any operation.
As cryogenic applications and the LNG industry continue to grow, so, too, will the prevalence of check valves. It is important to ensure that highly engineered solutions are in place. Dual-plate and nozzle-style valves offer some benefits over more traditional check valves so they have emerged as essential components within the value chain.