milk tank fittings

[whalems]

I have a zero vacuum tank and the valve on the drain leaks. I would like to change it to something with NPT threads so I can use a ball valve does anyone have a source for these fittings or a suggestion? the valve that is one it is a friction fit type design and after opening and closing it several times the nut on the bottom loosens and it starts to leak.

[PerryW]

I use a Fernco Fitting to get from my milk tank fitting to NPT Thread for 1-1/2" gate valve, but I'm not using vacuum.

http://www.grainger.com/Grainger/FERNCO-Shielded-Coupling-2ZJ84?gclid=CP3fr9b5obYCFQyg4AodwycA1Q&cm_mmc=PPC:GooglePLA-_-Plumbing-_-Fittings-_-2ZJ84&ci_src=17588969&ci_sku=2ZJ84&ef_id=UI8DkQAAG7IW0B@v:20130329124231:s