In the flow-control and fluid-process industries, ensuring proper sizing for key components like a pressure safety valve is a given. Despite that, this is not always observed when it comes to check valves, which often leads to failures. As a refresher, check valves serve the essential functions of preventing the reverse flow of liquid, steam, gases, and other media, as well as vacuum breaking, line isolation, low-pressure relief, media injection, and various other applications. By enabling flow solely into one direction, check valves ensure damaging backflow does not occur. Read on to learn what happens if sizing is not prioritised at the outset and tips to get it right.
The problem with oversizing and undersizing
Oversized valves tend to have significantly reduced service lives because they accumulate more premature wear, which gradually leads to complete failure. Upon breaking, they often cause damage to other components downstream or upstream or, worse, destroy them. A telltale sign of an oversized check valve is rattling or chattering in the check mechanism due to the component not opening and closing properly.
On the other hand, undersized valves lead to a choked flow condition that prevents a process from getting the pressure or flow of media it needs downstream of the check valve, interfering with its performance. And even if the valve is sized perfectly to match that of its corresponding line, the flow capacity will still not meet what is required for the application, causing choked flow. In short, it cannot deliver the flow of media required downstream to achieve the intended function. Additionally, when the check valve’s spring is too heavy, its checking mechanism may not work as expected. This obstacle that prevents effective operation also puts more strain and stress on the flow source, like the pump.
Why it pays to ensure accuracy
A good rule to keep in mind is that a valve’s final cost is much more important than its initial cost, i.e., an inexpensive check valve that requires frequent replacement will likely cost more in the long run compared to a high-quality check valve that meets all of an application’s needs. Hence, it is always better to pay a little more for a valve that is guaranteed to last much longer and experience fewer issues since its operational cost will be far lower over time.
Some of the benefits of investing in properly sized check valves include improved valve operation and, ultimately, a longer life cycle; guaranteed adequate flow and pressure, ensuring the system runs at optimum performance; and reduced frequency and costs of valve maintenance.
Tips on getting sizing right
Proper sizing largely depends on the type of valve to be used. For instance, most flow-control valves, such as ball, globe, and gate valves, simply need to match the size of the line. They are typically manually throttled and use an opening mechanism that operators can manipulate as much as necessary to achieve their desired flow coefficient (Cv) or pressure loss. This capability makes it preferable to use valve types with flow capacities that exceed the requirements of the intended application. Take, for example, a use case that requires a valve with a 100 Cv to achieve the desired media flow and pressure drop. Here, using a valve capable of 150 Cv is acceptable if turning its handle can throttle down its performance.
In contrast to the valves mentioned above, a check valve works automatically – flow conditions dictate its activation – with no other external controls involved. This is why accurate valve sizing is imperative for this type of valve, especially for spring-loaded check valves, to ensure the best performance. The media used in the process, be it liquid or gas, also matters for sizing. For gas, temperature and pressure considerations take priority. On the other hand, these are not usually a concern when working with a liquid medium.
Last but not least, keep in mind that the cracking pressure equates to the minimum pressure differential required to open the valve initially, with an extra amount of pressure differential and flow being necessary to reach a full opening. This cannot occur if the check valve’s spring is too heavy and, ultimately, causes the valve to rattle, chatter, and suffer premature wear.
With all that said, these are the basic steps to facilitate proper sizing for check valves:
- Verify the application’s line size, temperature, and medium used.
- Determine the pressure and flow of the system.
- Evaluate the application flow conditions to quantify the required flow coefficient.
- Select a valve that is equal to or in excess of said flow coefficient and features the right spring and valve style to guarantee flawless operation.
Last but not least, confirm that the check valve can be fully opened and closed by the media’s flow and pressure differential. Ideally, it should open completely at the minimum setting, yet take away as little pressure as possible. The exception to this applies to applications that, for instance, require check valves to retain some amount of pressure to achieve more effective pump operation.
Conclusion
There is much to gain from installing properly sized check valves, from making sure the overall system runs more reliably to effectively preventing avoidable problems such as premature wear and failure. In other words, such a simple step can significantly contribute to minimising the risk of disruptions and damage to other equipment and components, including choosing the right sight glass for your industrial equipment.
If your application ever needs quality check valves that are guaranteed to have the perfect fit, Techmatic Controls has you covered. Consult with our team today, and we will make sure you receive nothing less than high-quality check valves and other industrial products that match all of your requirements.
