The Challenge

In a typical spray dryer, organic materials are sprayed into the top of the unit, heated by air and processed out through the coned-shaped bottom of the system. Although the material concentrations are above lower explosive limits throughout the spray dryer, the primary hazard area is in the lower cone section where high levels of dry combustible materials are contained in a turbulent state of air suspension.

Spontaneous combustion is largely to blame for touching off most spray dryer explosions. Materials accumulate on the walls of the dryer and when combustion occurs, sizeable pieces of smoldering powder detach from the walls, breaking apart as they fall and causing destructive explosions.

Explosion protection for spray dryers must be designed to prevent large scale explosions from destroying the entire process. This is done by protecting both the upper and lower sections of the dryer as well as material and air outlets. by doing so, the explosion will not be allowed to propagate to other areas of the plant. 

The solution

A Fenwal Explosion Protection System is custom-designed to the exact specifications of each spray dryer. Pressure detectors are placed in the lower unit where an explosion is likely to start.  Fast acting high-rate-discharge extinguishers are strategically located to deliver an explosion suppressant to the lower and upper sections of the dryer and also to the outlet duct. To the satisfaction of risk managers and insurance carriers, the Fenwal System suppresses explosions before they have a chance to gather destructive force.

protection system consideration

Protection systems for drying systems must accommodate the need to maintain smooth inside vessel walls.  This is to prevent product buildups and for efficient cyclone performance.  Fenwal's suppression systems achieve this by using flush spreader assemblies for suppressant dispersal.

protection system description

Drying systems are typically in two sections.  The first section dries the material.  The second section separates the dry material from the air stream.  Consequently the explosion suppression system used is typically a two-zone system.  The drying vessel and the dust separation vessel(s) have protection zones that operate independently of each other while providing isolation barriers to mitigate explosion propagation between interconnected vessels.   In such a system, between interconnected vessels.  In such a system, explosion pressure detectors mounted on the dryer and dust separator, such as a cyclone, detect the pressure excursion from an impending explosion.  The detectors transmit a signal to the control panel.  This triggers high rate discharge extinguishers in the effected zone which rapidly discharge suppressant in an effort to quench the fireball before maximum pressures are reached.  Isolation extinguishers mounted on the ducts mitigate explosion propagation to interconnected process equipment.  The control unit also shuts down the process automatically mitigating the transfer of burning materials, dust, and air.