To be able to select the correct check valve for your application, you should consider a number of selection criteria. In today’s guide, we’ll give you some pointers on how to choose the right check valve. Read on to learn more.
1. Different types of check valves
Choosing the correct check valve type, size and specification is important to ensure smooth, trouble-free, long-term operation and low energy consumption. In many cases, the rate of fluid reversal is not a concern and a standard check valve will perform fine. However, in pumping systems, selecting the correct check valve is critical.
1) Ball check valve
Generally speaking, ball check valves have simple structure and high cost performance. The advantage is that it is compact and has no external parts, which helps keep prices low and reliability high. A drawback for some apps might be that they don’t have an on/off indicator.
Ball check valves are commonly used in wastewater applications due to their clear bore design. A full, smooth bore ensures adequate flow throughout the valve and no accumulation of solid deposits on the bottom of the valve. The ball rotates during operation, eliminating the risk of impurities sticking to the ball.
2) Swing check valve
Swing check valve is the most common check valve. They are inexpensive and do not require any external power supply or control to operate, as flow and pressure determine the valve’s operation. Swing check valves are equipped with open/closed indicators, and typically these valves are equipped with a lever and weight or a lever and spring that allow for visual inspection, or a device that allows for digital feedback.
This valve design provides a full bore opening, making it efficient due to its low head loss characteristics. Swing check valves are typically used in systems where the risk of pressure surges is low. This valve is commonly used in water and wastewater applications due to its full bore opening.
3) Swash plate check valve
The appearance of the swash plate check valve is similar to that of the eccentric butterfly valve. The disc is held in place by an axis that is offset from the centerline of the body in both the horizontal and vertical axes.
The double eccentricity of the shaft causes the lower portion of the disc to occupy a larger area in the flow path. Therefore, the valve disc starts to open at a very low flow rate.
Therefore, swash plate check valves are often used in pumping systems with low flow rates and pulsating flows. Since the internal shaft is located within the water flow, they are used in water and treated wastewater applications. Swash plate check valves are ideal for reducing the risk of water hammer.
4) Inclined disc, inclined seat check valve
Inclined seat check valves also offer enhanced resistance to water hammer. The valve features a dual eccentric shaft position and increased seat angle. This creates a shorter valve stroke, which reduces the time it takes for the door to close.
Hydraulic dampers are recommended, especially when the valve is installed on a pumping station that requires frequent opening and closing of the valve. The valve disc closes rapidly during the first 85% of its angular travel before encountering the hydraulic damper. The damper then dissipates the disc’s kinetic energy and forces it to open slightly. The disc closes until it again contacts the damper, which cushions the disc until it returns to the fully closed position, sealing the valve. This feature greatly reduces the occurrence of water hammer due to the damping and control method of valve closing.
5) Nozzle check valve
Nozzle check valves are designed with the valve disc attached to a valve stem guided on a central horizontal axis. The spring is located between the disc and diffuser sleeve. When fluid enters the valve, hydraulic pressure applied to the front reacts on the spring, causing the spring to compress and allow the valve to open. When flow stops, the spring forces the disc back to the closed position.
Due to their spring-assisted closing and short linear valve travel, nozzle check valves are among the fastest-acting check valves available and are often used in pumping systems where water hammer may be an issue. Because the disc is always in the straight line of flow, this valve has higher head loss characteristics than a traditional swing check valve.
2. What should you consider when selecting a check valve?
There are several selection criteria to consider when choosing a check valve for your application. First, however, no one type of check valve is the best choice for all applications, and selection criteria may not be equally important for all situations. Some things you may want to consider include fluid compatibility, flow characteristics, head loss, non-impingement characteristics and total cost of ownership.
1) Body fluids
All check valves mentioned in this article are designed for use in water and treated wastewater applications, but using the valves in raw wastewater/sewage applications can cause some problems. When selecting a valve for these fluids, you should consider how the presence of solids may affect the operation of the valve.
2) Traffic characteristics
If a check valve closes very quickly, it may prevent it from hitting upstream equipment such as a pump. However, quick shutdown does not prevent surges caused by pump startup and shutdown. If the valve opens (and closes) quickly, the flow rate will change rapidly, increasing the likelihood of surge.
3) Head loss
Head loss is a function of fluid velocity, among other factors. The head loss through the valve is determined by the internal design and opening of the valve. When a valve is designed with a restricted (narrow) opening compared to the pipe, the velocity through the valve will increase, thereby increasing the head loss. There are several selection criteria to consider when choosing a check valve for your application. There are many values for head loss, including zeta, Kv and Kvs.
4) Total cost of ownership
The cost of a check valve includes more than just the purchase price. For some installations, the most significant costs may be procurement and installation, but in other cases, maintenance or energy costs may be equally or even more important. Consideration must also be given to protecting more valuable equipment, such as pumps, and looking at valve performance is critical. When considering cost as a check valve selection criterion, the total cost over the life of the valve should be considered. It is generally true that valve structures with simpler structures require less maintenance. The higher the Kv value, the lower the energy consumption. The better the performance, the better the protection.
5) Non-impact characteristics
A slamming check valve can negatively affect pressure fluctuations. The first step in the process is when the pump stops, the pressure surge begins. The second step is to slam the fully closed check valve when flow is reversed. If the check valve closes too quickly, kinetic energy is converted into high pressure, putting pressure on the pipe and producing high noise.
The pop sounds like the disc or ball on the check valve hitting the seat, and it can make a pretty loud noise. However, the sound is not caused by a physical closure, but by sound waves created by pressure spikes that stretch the tube walls.
To prevent check valve slamming from occurring, the valve should close in a controlled manner and slowly close as it approaches the closed position. In order for the check valve to close slowly, additional auxiliary equipment is required, such as a hydraulic damper, which acts as a cushion for the valve when it enters the seated position. This slower closing speed allows fluid to pass through the check valve until it closes, resulting in less kinetic energy being converted to high pressure and thus less energy to supply and sustain the surge. Consideration must be given to the upstream pump to ensure it is suitable for reverse rotation and flow.
Because the disc of the swing check valve is located in the liquid flow, which helps to close quickly, it has better non-shock characteristics. However, many pumps today feature variable frequency technology, which allows them to adjust start-up and shut-down times to avoid water hammer.
3. Frequently Asked Questions about Check Valve
Here are answers to frequently asked questions about check valves.
1) What are the advantages/disadvantages of check valves?
The main advantage of check valves is that they prevent reverse flow while generally requiring no external actuation. They also provide a quick and effective solution to prevent backflow. Some disadvantages include the inability to determine the condition of the valve and its status, limitations of its mounting configuration, and the possibility of becoming stuck in the open position (not typically an issue associated with ball and piston types).
2) How to choose a suitable check valve?
When deciding on a check valve that meets your needs, there are several factors to consider:
- Material Selection: The materials used in valve construction must be able to meet the piping system and media requirements associated with the specific type of gas or liquid flowing through the piping system.
- Performance Requirements: Performance considerations include the valve’s ability to handle a wide range of temperature and pressure conditions.
- Type and Size: Selecting the correct check valve includes control considerations often related to improving safety in hazardous applications and sizing the valve to properly accommodate the flow conditions of the piping system.
- Maintenance: In addition to considering the frequency of maintenance or replacement, operators must also consider the valve’s ability to resist clogging, sticking, or other malfunctions.
- Special Requirements: The selection of a check valve also includes some special requirements specific to its function, including response time, cracking pressure, normally closed vs. normally open, vertical vs. horizontal installation, flow direction (up or down) and fluid density.
3) How to prevent check valve failure?
One of the disadvantages of check valves is the difficulty in determining the condition of a closed valve. Therefore, it is necessary to take certain measures to identify its failure and prevent its occurrence. Several indicators that signal a malfunction or potential failure include valve vibration (chatter), measured backflow, excessive component wear, audible indication of leaks (water hammer), and leaks. You can prevent failure by:
- right size
- Regularly check cleanliness
- Install correctly
- Replace valves as needed
Check valves play an important role in controlling flow in high-performance piping systems. Choosing the correct check valve can improve piping system efficiency and increase operational safety.
