Check Valve Education
Introduction to Check valves
A check valve, clack valve, non-return valve, reflux valve, retention valve or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.
Swing Check Valve click to shop
Check valves are automatic valves that open with forward flow and close with reverse flow.
The pressure of the fluid passing through a system opens the valve, while any reversal of flow will close the valve. Exact operation will vary depending on the type of Check valve mechanism. Most common types of Check valves are swing, lift (piston and ball), butterfly, stop and tilting-disk.
Types of Check valves
SWING CHECK VALVE
A basic swing Check valve consists of a valve body, a bonnet, and a disk that is connected to a hinge. The disk swings away from the valve-seat to allow flow in the forward direction, and returns to valve-seat when upstream flow is stopped, to prevent backflow.
The disc in a swing type Check valve is unguided as it fully opens or closes. There are many disk and seat designs available, in order to meet the requirements of different applications. The valve allows full, unobstructed flow and automatically closes as pressure decreases. These valves are fully closed when flow reaches zero, in order to prevent backflow. Turbulence and pressure drop in the valve are very low.
LIFT CHECK VALVE
The seat design of a lift-Check valve is similar to a Globe valve. The disc is usually in the form of a piston or a ball.
Lift Check valves are particularly suitable for high-pressure service where velocity of flow is high. In lift Check valves, the disc is precisely guided and fits perfectly into the dashpot. Lift Check valves are suitable for installation in horizontal or vertical pipe-lines with upward flow.
Flow to lift Check valves must always enter below the seat. As the flow enters, the piston or ball is raised within guides from the seat by the pressure of the upward flow. When the flow stops or reverses, the piston or ball is forced onto the seat of the valve by both the backflow and gravity.
DUAL PLATE CHECK VALVE
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.
NON SLAM CHECK VALVE
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).
Figure 2. A nozzle-type check valveThese 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.
STANDARDS
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.
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