Understanding a Valve’s Cycle Life (and Why You Should Care)

A singular valve cycle is defined as the opening and closing of the valve in an installed process environment. The total expected number of times the valve can cycle open/close under ideal conditions is called the valve’s cycle life. A valve’s cycle life is not fixed, however, and varies greatly depending on temperature, pressure, cleanliness of media, chemical compatibility with media, lubrication, installation method, cycle rate, quality of materials of construction, and valve design.

Manufacturers design valve components to endure a wide range of conditions. Applications can vary from room temperature clean water to high-temperature abrasive media or aggressive chemicals. The cycle life of identical valves in those very different conditions may not be equal, meaning that different types of valves may be more suitable than others for a specific application. When designing a system, engineers will assess valves based on the application or process environment. The frequency of valve operation (cycle rate) within this process environment will help engineers determine the best type of valve to achieve a reasonable cycle life goal.

Initial Design Test Considerations

Asahi/America performs cycle testing in our lab to validate designs or design changes, and these tests are conducted under varying conditions.

• Pulsation testing is designed to validate the durability of a valve, like a butterfly valve, under the effects of dynamic pressure conditions. The test simulates real-world applications like ‘pulsations of a pump,’ and can help determine the durability of a valve’s design or materials of construction. A target value of an acceptable number of cycles is typically determined before the test begins.
• Heat cycle testing involves subjecting a valve to repeated heating and cooling cycles, and is intended to evaluate performance and endurance. This test simulates real-world conditions and validates performance under extreme conditions. A target degree of seal performance or degradation is determined before the test begins.

Both of these tests are often a key component of design review meetings, ensuring the materials of construction are suitable for the wide range of applications a valve may see.

A common question is, “How long will my valve last?” but the answer isn’t quite that straightforward. While we can pull some presumptive information from laboratory development test data, it doesn’t paint an entirely accurate picture for the real-world use case. The exact working conditions of a valve in a given application can often provide better insight into longevity and help determine the type of valve that gets chosen. Unfortunately, this is not an exact science, and while Asahi/America will recommend a valve type based on application criteria and our support capabilities, ultimately, it is the customer’s decision on what they want to use.

Let’s dive a little deeper into what affects a valve’s cycle life:

• Temperature: Temperature extremes will have a significant impact on valve cycle life. A valve in an environment with unstable or extreme temperatures will be stressed based on the different temperature limits of the various thermoplastic and elastomeric materials used. A good example is a PVC butterfly valve with an FKM seat/seal. FKM becomes hard at temperatures below 32° F, which will cause the open/close cycling of the valve to become more difficult and stressful due to its rigidity. Naturally, working temperatures within the provided range will result in better cycle life.
• Pressure: Changes or spikes in pressure, even within a rated pressure range, can cause seals to wear. For example, damage can happen to the FKM seats/seals in butterfly valves during pressure spikes. Stable pressure environments within the given temperature range will result in better cycle life.
• Lubrication: Valves require lubrication, and while process fluid can sometimes serve as a lubricant, a separate lubricant is typically applied during assembly to ensure smooth operation of the valve. There are specific environments where lubricant cannot be used, as it will contaminate the process; for example, dry chlorine gas. These valves are typically labeled as “lubricant-free” and, as a rule of thumb, should not be actuated prior to process installation and media presence.
• Cycle rate: The cycle rate is the frequency at which the valve is actuated. Neither a higher nor a lower cycle rate equates to a longer cycle life; both process conditions have their respective influences. A valve that is only cycled once per year may require periodic “exercising” to ensure smooth operation when a cycle becomes necessary. Alternatively, a valve that is cycled repeatedly can create heat and stress the material properties of the sealing components.
• Cleanliness of media and chemical compatibility: Depending on the type and design of the valve, increased media cleanliness will always equate to improved cycle life. Choosing properly rated materials of construction based on the process media the valve will operate in will also result in better cycle life. Media that has contamination or media not intended for use with a valve’s materials will dramatically shorten the expected valve cycle life.
• Materials of construction: Using quality resins for thermoplastic molded valves is a key factor in longevity. Plasticizers and fillers used in the molding process can affect the overall performance of the material, particularly in more extreme conditions.

Ball Valve Design and Cycle Life

Ball valves are typically used as shut-off valves, intended for use in clean fluid applications. Quarter-turn open/close operation (via lever handle or pneumatic or electric actuator) rotates a stem attached to the ball. A smooth, polished ball with Teflon® seats, backed by elastomeric O-ring cushions, creates a tight seal and positive shut-off. Repeated cycling of a ball valve creates wear by shearing microparticles off the Teflon® seats. In time, this wear can result in a leak path. The number of cycles it can handle before it begins to leak depends mainly on the cleanliness of the media; abrasive particles suspended in the fluid will not only accelerate the wear of the Teflon® seats, but can also become embedded in the Teflon® itself. When this happens, those particles are scraped across the face of the ball during open/close cycling, creating score marks in the ball and resulting in leakage. For such types of applications, Asahi/America may recommend a different type of valve, like a butterfly valve.

Butterfly Valves Design and Cycle Life

Butterfly valves are typically used as shut-off valves as well, but butterfly valves are better equipped to handle particulate or suspended solids due to their more robust design. These quarter-turn valves feature a stainless steel stem connected to a thermoplastic disc. The open/close cycling drives the disc into or out of an elastomeric seat or liner, creating wear of the valve seat over repeated cycles. The shearing of micro particles from the seat by the force of the disc as the disc cycles into or out of the seat during the open/close cycle creates leak paths over time. In an application involving clean, particle-free water, a ball valve would be expected to outlast a butterfly valve due to design differences. However, once you introduce suspended solids or particulate matter, the butterfly valve becomes the clear preference.

Electric or pneumatic actuation can also accelerate wear based on the frequency of actuation. For example, in a bottle-filling application where rapid repeated open/close cycles are performed over an hours-long shift, the preferred choice would be the ball valve. The self-lubricating properties of the Teflon® seats against the thermoplastic ball will provide for minimal strain on the actuator and valve. The result is smoother operation and longer life. The same bottle-filling application for a butterfly valve would be far more strenuous. The way the disc is fitted into the seat and breaks away, the seating force required to cycle the valve open and closed would accelerate wear on both the actuator and the butterfly valve.

Stay tuned for part two, where we will cover the cycle life and performance tests conducted for actuators.

EDITOR’S NOTICE: Please note, the information in this article is for educational purposes only and does not supersede any Asahi/America technical information or product specifications. Please consult Asahi/America’s technical department at 1-800-343-3618 or [email protected] on all product applications in regards to material selection based on the pressure, temperature, environmental factors, chemical, media, application, and more.