A Plant for producing a pretreated biomass by subjecting raw biomass to thermal hydrolysis, said plant comprising: - a thermal hydrolysis reactor for producing biomass under pressure in the form of moist material including abrasive material having a particle size > 150 micron and rated at 7 or above on Mohs scale of hardness, and having a dry solid content above 5% whereof above 20% is volatile solids, - a pressure relief vessel in fluid communication with said reactor for relief of pressure of said biomass being under pressure and - a device for relieving the pressure of biomass upon entering said pressure relief vessel, comprising one or more blowdown conduits for transporting said biomass in the form of moist material from said reactor to said pressure relief vessel and discharging said biomass in the form of moist material into said pressure relief vessel, wherein: - each one of said one or more blowdown conduits is provided at the outlet thereof with a valve having a variable cross-sectional area providing an adjustable open area for regulating the blowdown discharge rate of said biomass in the form of moist material into said pressure relief vessel, and the valve being constructed in such a way that it ensures that the smallest cross sectional/opening area of said one or more blowdown conduits is at the outlet of said one or more blowdown conduits, said plant being characterized in that said one or more blowdown conduits have one of the following designs: (i) no piping is provided downstream of the valve so that expansion/spray due to flashing takes place inside said pressure relief vessel, wherein either sacrificial material downstream of the minimum open cross-sectional area, where the steam explosion takes place is provided, or a deflection plate is provided, towards which the biomass discharged from the blowdown conduit valve is directed; (ii) the valve is fitted with a vessel-inlet-nozzle/device through which the discharged biomass is directed from the adjustable open area into said pressure relief vessel wherein expansion/spray due to flashing takes place inside said vessel-inlet-nozzle/device and said vessel-inlet-nozzle/device is made from a highly durable material with high erosion resistance, which is silicon carbide. - a thermal hydrolysis reactor for producing biomass under pressure in the form of moist material including abrasive material having a particle size > 150 micron and rated at 7 or above on Mohs scale of hardness, and having a dry solid content above 5% whereof above 20% is volatile solids, - a pressure relief vessel in fluid communication with said reactor for relief of pressure of said biomass being under pressure and - a device for relieving the pressure of biomass upon entering said pressure relief vessel, comprising one or more blowdown conduits for transporting said biomass in the form of moist material from said reactor to said pressure relief vessel and discharging said biomass in the form of moist material into said pressure relief vessel, - each one of said one or more blowdown conduits is provided at the outlet thereof with a valve having a variable cross-sectional area providing an adjustable open area for regulating the blowdown discharge rate of said biomass in the form of moist material into said pressure relief vessel, and the valve being constructed in such a way that it ensures that the smallest cross sectional/opening area of said one or more blowdown conduits is at the outlet of said one or more blowdown conduits, (i) no piping is provided downstream of the valve so that expansion/spray due to flashing takes place inside said pressure relief vessel, wherein either sacrificial material downstream of the minimum open cross-sectional area, where the steam explosion takes place is provided, or a deflection plate is provided, towards which the biomass discharged from the blowdown conduit valve is directed; (ii) the valve is fitted with a vessel-inlet-nozzle/device through which the discharged biomass is directed from the adjustable open area into said pressure relief vessel wherein expansion/spray due to flashing takes place inside said vessel-inlet-nozzle/device and said vessel-inlet-nozzle/device is made from a highly durable material with high erosion resistance, which is silicon carbide.