How to avoid chattering in pressure relief valves

In the design of industrial piping systems, relief or excess pressure valves are the guardians of structural integrity.

However, there is an acoustic and mechanical phenomenon that can transform these safety devices into a threat to the plant: chattering. chattering.

This term, usually translated as vibration or “rattling”, describes an extremely rapid and violent opening and closing cycle that occurs when the valve enters a state of dynamic instability.

Models such as the S1, S2 and S3 series from Valfonta have been perfected to provide stable response, but even the best engineering can succumb to chattering if the root causes are not understood.

Avoiding this phenomenon is not just a matter of acoustic comfort; it is a safety imperative to prevent catastrophic failures, product leakage and costly unscheduled downtime.

Table of contents

On chattering relief valves is not an ordinary vibration. It is technically defined as the rapid oscillation of the plug on the valve seat. Unlike a smooth regulation, where the plug is positioned proportionally to the detected overpressure, in chattering the moving component repeatedly strikes the metal seat at frequencies that can reach hundreds of Hertz.

This behavior is usually the result of a mismatch between the spring force (trying to close the valve) and the force generated by the fluid pressure (trying to open the valve). When these forces do not find a stable equilibrium point, the valve enters into an infinite loop of opening-pressure-drop-closing-pressure rise loopgenerating a severe mechanical impact at each cycle.

The physics behind chattering is explained by the interaction between the valve and the fluid dynamics of the installation. The cycle begins when the line pressure reaches the set pressure. set pressure (set point).. At that instant, the poppet is lifted to relieve the excess. However, if there is an excessive excessive pressure drop in the inlet pipingHowever, if there is an excessive pressure drop in the inlet pipeline, the pressure detected by the valve sensor suddenly drops below its blowdown value.

As a consequence, the spring forces the plug to close immediately. Once closed, the flow stops, the pressure recovers instantaneously at the inlet and the cycle repeats itself violently. The main causes of this imbalance are usually:

Damage from chattering is cumulative and often irreversible. In high-precision models such as the Valfonta S1which uses membranes and bellows for fine adjustment, high-frequency vibrations can fatigue elastic materials in a matter of minutes. The most critical risks include:

It is essential that the plant engineer does not confuse chattering and chattering with other sound phenomena. Often “vibrations” are reported that are in reality cavitation. Cavitation occurs when the liquid pressure drops below its vapor pressure, forming bubbles that collapse violently; it sounds like “pebbles” passing through the pipe, but does not necessarily involve the plug hitting.

On the other hand, the regulation instability (or “hunting”) is a slow, oscillatory movement of the shutter seeking its position, common when the control loop is not properly adjusted. The chattering, on the other hand, is is violent, sharp and generates a physical vibration that can be that can be felt when touching the valve body.

While hunting is a problem of precision, chattering is a problem of mechanical survival of the equipment. Correctly identifying this difference makes it possible to apply the appropriate technical solution, either by adjusting the control takeoff on a S2 or by revising the shutter guidance on a S3.

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To mitigate the risk of instability, Valfonta has developed different excess pressure valve architectures that adapt to the severity of the process and the nature of the fluid.

There is no single solution for chattering, but a suitable valve for each pressure and flow scenario. While the S1 series stands out for its sensitivity, the S2 and S3 offer solutions based on mechanical robustness and remote impulse control.

Below is a technical table summarizing the capabilities of these three series in relation to operational stability:

The series S1 series is Valfonta’s answer for applications requiring surgical precision, especially in saturated steam and gas services. The chattering relief valves in this model is prevented by a combination of large sensing surface and an internal balancing system.

The use of a large diameter diaphragm The use of a large diameter diaphragm allows the valve to detect minimal pressure variations. However, in valves of this type, the downstream pressure (back pressure) could interfere with the movement of the plug, causing oscillations. To avoid this, the S1 incorporates a stainless steel bellows which acts as a balancing element.

This bellows cancels the upward forces caused by the discharge pressure, ensuring that the plug moves only in response to the inlet pressure we want to relieve. In addition, the bellows drastically reduces mechanical friction, eliminating the hysteresis that often initiates low-frequency vibrations before they lead to violent chattering.

The series S2 series is known for its versatility and its ability to work in conditions where the fluid can be more turbulent. One of the major advances to prevent chattering in this series is the possibility of using an external external control tapping (or impulse tapping).

When a relief valve draws pressure directly from its own body (internal tapping), it is exposed to turbulence and velocity fluctuations that occur right at the valve seat. This turbulence can “fool” the actuator, causing the poppet to vibrate. When connecting the control plug of the Valfonta S2 to a point in the pipeline at a safe distance (minimum 10 times the nominal diameter), the actuator receives a stable, laminar pressure signal. stable, laminar pressure signal.

This design is vital in installations with positive displacement pumps or where the relief flow is very close to the valve’s critical stability point.

For smaller diameters and more straightforward applications, model S3 offers a solution based on mechanical stiffness. Chattering occurs not only due to unstable pressure signals, but also due to lateral displacement of the stem when the flow pushes it asymmetrically.

The S3 series mitigates this risk by means of a guided shutter system. Unlike other low-cost valves where the plug “floats” slightly, in the S3 the moving assembly is guided with tight tolerances within the valve body.

This guide ensures that the contact between the plug and seat is always perfectly concentric. By eliminating side play, the possibility of the plug mechanically resonating with the fluid, one of the most common causes of high-pitched noise and premature wear in small relief valves, is eliminated.

Its construction in stainless steel or carbon steel (WCB) ensures that, even in the event of small oscillations, the material withstands mechanical stress without immediate plastic deformation.

Even the best manufactured valve, such as Valfonta’s S1, S2, or S3 models S1, S2 or S3 models from Valfontacan be unstable if the design of the surrounding piping does not respect the basic principles of fluid dynamics.

On chattering relief valves is, in most cases, a system problem and not a product defect. The key to quiet and long-lasting operation lies in the harmony between the valve capacity and the volume of fluid that the line can deliver without critical pressure drops.

A professional installation should consider the relief valve as part of a dynamic control loop. If the inlet piping is too long or has too many fittings (elbows, tees, dirty strainers), the accumulated head loss will prevent the valve sensor from receiving a true plant pressure signal, causing the fateful rapid opening and closing cycle.

The most common mistake in process engineering is to oversize the valve “for safety”. An excess pressure valve with a Kvs (flow coefficient) will open only a fraction of its travel to relieve the excess, emptying the pipeline so fast that the pressure will drop below the closing point in milliseconds.

This initiates chattering immediately. The valve should be sized for the actual relief flow rate, not the existing pipe diameter.

Regarding the control or impulse line, especially critical inespecially critical in the S2 seriesseries, its location determines the stability of the entire system. It should be connected in a straight section of pipe, away from turbulent areas such as pumps or control valves. A distance of 10 times the nominal diameter (DN) upstream of the relief point is recommended. of the relief point is recommended.

In addition, the impulse line must have an adequate slope to avoid the accumulation of condensate or air bubbles that dampen or distort the pressure signal, which could induce erratic vibration in the membrane.