How cola helps us describe complex phenomena

On the sizing of safety valves and bursting discs for foaming media

The protection of pressure vessels with mechanical safety devices can become a challenge when foaming media are used, regardless of whether they are typical foaming media such as surfactants or fluids stimulated to form foam by gases such as CO₂ or N₂. Foam formation itself can occur in various ways: by injecting gas, evaporation of liquid, decomposition of individual components with gas formation, or even by intense stirring of the liquid phase.

In the best case, the resulting foams are unstable, have a low surface tension and quickly decompose. Unstable foams are often unproblematic for plant safety. The situation is different with stable foams. In the event of a discharge, the foam can reach the inlet of the safety device and always carry a portion of liquids. Accordingly, the flow through the safety device can be two-phase. The cola bottle effect can be useful here as an analogy to explain the relationship between foam formation and two-phase flow when relief is provided by safety valves and bursting discs.

“Please don’t shake!”

Everyone is familiar with this effect: When a Coke bottle is shaken and the lid is subsequently opened, the discharged CO₂ pushes part of the liquid through the opening – a two-phase discharge as we often expect in pressure vessels and reactors. The experiment with the Coke bottle shows us the most important factors influencing the occurrence of two-phase flows in vessels:

1. Low filling levels: If the cola bottle is only slightly filled, shaking the bottle and opening the lid causes the cola to foam, but the foam does not reach the opening. All that can be observed is a single-phase outflow of CO₂.
2. Slow opening: If the cola bottle is shaken and the cap is then slowly loosened, the dissolved CO₂ can initially escape without strong foaming, after which the bottle can be opened without risk. But be careful: Of course, the opening must always be large enough to be able to discharge the required mass flow.
3. Gas formation rate: If the cola bottle is opened without shaking and left to stand for a while, the CO₂ content drops. If the shaking experiment is repeated, correspondingly less CO₂ will escape from the bottle, and foam formation will be lower.

In plant safety practice, the gas formation rate is often determined in reaction calorimetric experiments. In addition, foam formation can be determined in advance in open experiments during discharge from safety equipment.

Pressure increase despite open safety device!

The two-phase discharge is problematic in the case of foam formation. The lower flow velocity in contrast to pure gas can lead to the foam blocking the cross-section of the safety device to be released. As a result, not enough mass flow can be discharged from the system and the pressure increases despite the safety device being open.

An increase in the mass flow to be discharged in the case of two-phase flows with foaming fluids may also be necessary. Due to strong foam formation, a large part of the liquid in the reactor can be entrained when the safety device responds by sudden boiling of the liquid phase or release of intergases. In addition to foam formation, this phenomenon occurs in particular with highly viscous media (η > 100mPas) and is called level swell of the liquid phase. It is mandatory to take it into account when designing the safety equipment and containment systems. The reactor contents can be almost completely emptied during discharge and the mass flow to be discharged increases sharply. This requires a safety device with a larger cross-section [1].

In industry, for foaming or highly viscous media, the safety device is sized by default with the homogeneous relief model for two-phase outflow according to ISO 4126-10 [2] or DIERS [3]. The method presented here, however, often leads to extremely large cross-sections of the safety devices as well as large volumes of all downstream devices [4]. On the one hand, this can cause high expenses, and on the other hand, depending on the application, the required relief cross-section can exceed the largest safety devices available on the market. Protection with mechanical safety devices is then not possible.

Especially for low-viscosity media and unstable foams, more precise calculation methods or tests in open calorimeters can lead to smaller relief cross sections, depending on the application. In addition, alternative safeguarding options such as PCT safety devices, emergency stop systems or quenches can be considered in the design process. However, the evaluation of the methods to be used must be carried out by experts on a case-by-case basis, as a misjudgement can lead to inadequate vessel protection.

Looking for advice?

The experts at CSE are your competent partner for all questions concerning the protectionwith foaming media. You will benefit from our many years of expertise and research in the field of relief of foaming media. Contact us!

Literature:

[1] G. Wehmeier, F. Westphal, and L. Friedel, “Pressure Relief System Design for Vapour or Two-Phase Flow,” IChemeE Symp. Ser., vol. 134, pp. 491–503, 1998.

[2] DIN Deutsches Institut für Normung e. V., “Safety devices against excessive pressure – Part 10: Design of safety valves for two-phase flow (liquid/gas) (ISO 4126-10:2010); Text in German and English,” Berlin, 2019.

[3] H. G. Fisher et al., Emergency Relief System Design Using DIERS Technology. Wiley, 1993.

[4] J. Schecker and L. Friedel, “Submodel for surging of foaming mixtures during depressurization,” Forsch. in Engineering/Engineering Res., vol. 69, no. 1, pp. 44-56, 2004.