There is no chance for the souls in the Purgatory to go down to Hell if no one prays for them when the soul is already in Purgatory they are already part of the Communion of Saints they have assurance of Salvation what are they doing in Purgatory is to purge for their sins (venial) because the scripture said “Nothing unclean will enter into Heaven (Revelation 21:22). We must be clean to enter Heaven. This is accomplished by fully surrendering to God’s grace, in this world or the next. “I tell you, on the Day of Judgment people will render an account for every careless word they speak.” (Matthew. 12:36). We will be Saved by Fire, “But if someone’s work is burned up that one will suffer loss; the person will be saved, but only as through fire.” (1 Corinthians 3:15) St. Paul tells us that sinners can be saved through fire (We call it Purgatory).