A noun in the Qenya Lexicon of the 1910s glossed “boil” under the early root ᴱ√PUŘU [PUÐU] “consume by fire” (QL/75). Its stem form was pust-, so presumably ancient ðt became st.
Early Quenya
wele-
verb. to boil, bubble
pus
noun. boil
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
A verb appearing as ᴱQ. ’wele- “it boils, bubbles” in the Qenya Lexicon of the 1910s under the early root {ᴱ√WELE >>} ᴱ√GWELE (QL/103).
Neo-Quenya: For purpose of Neo-Quenya, I would adapt this word as ᴺQ. vel- with the usual sound change initial w to v.