## 1 Introduction  
Despite the progress made in recent decades, dust explosions still occur in industry [1; 2; 3; 4; 5; 6]. One of the most dangerous aspects of dust explosions is its propagation through ducts to other industrial equipment, the consequences of which can be catastrophic. Nowadays, research is still needed to understand the propagation process of explosions, to analyse the different influencing factors and assess risks, so that when there is an explosion, the blast does not affect other process areas.  
Explosions depend on multiple factors [7], such as the volume of the enclosure where it occurs, location of the ignition source, type of dust, concentration, etc. [6; 8], but most of all, turbulence is the factor that still needs better understanding [9]. There are several options for protecting against dust explosions. The most widely used technical solution for protecting equipment or facilities, by reducing the potential overpressure, is venting. This is probably due to its simple and effective design and apparently relatively low cost [10; 11; 12]. However, if isolation from the explosion is needed, then flap valves are one of the most widely used protection systems (Figure 1).  
These valves are designed to prevent the propagation of a dust explosion towards other areas of the industrial plant through ducts that are connected to the equipment or installation where the explosion occurs [13]. They remain open when the industrial plant is operating normally; only in the event of an explosion does the pressure wave generated cause it to close (see Figure 1). In Europe, these protective systems need to be certified according to the standard EN16447 to ensure that they meet the minimum requirements regarding its intended use. If these are met, then they may be distributed in the European market [13]. Nowadays, there is a dearth of data on how bends in ducts influence the propagation of dust explosions and the functions that these protective systems have. The presence of bends could introduce more turbulence, accelerate the flames and affect the functionality of the protective system against the dust explosion. In the literature, there are a few studies in this regard, and considerable debate on the influence bends have. For instance, [PERSON] et al., studied the effect of flow obstacles on how the blast from dust explosions spread, and observed that they accelerated the flame and increased explosion pressure [14]. [PERSON] et al. also found that the flame propagation speed rose four-fold when bends were used [15]. However, the opposite effect was found by [PERSON] et al., who carried out experiments using different angles, and detected that the flame speed decreased significantly as it went around the bends [16].  
However, it should be stressed that plenty of experimental research conducted on dust explosions are being carried out on a laboratory scale, where the operating conditions and variables are assumed to remain constant and are thus easier to control. However, and although economic and technical complexities are usually introduced, in order to scale up these laboratory results, more large-scale experimental studies are still needed. In this research, dust explosions have been developed on a large-scale using a DN500 flap valve, to help make decisions when designing these passive explosion isolation valves. This information could also be taken into consideration for the discussions established in the working group in charge of the revision of the standard EN16447.