A Grant is a small wort collection vessel, placed between the lautering vessel and the wort kettle. The traditional purpose of a grant was threefold: (a) to avoid a potential vacuum in the lauter or mash/lauter tun during wort pumping for recirculation or filling the kettle, which could seal the mash to the false bottom, thus causing turbid worts or stuck mashes; (b) to allow the brewer to asses wort clarity and wort flow; and (c) in larger systems with multiple lauter tun outlets, to determine whether all parts of the grain bed flow sufficiently well or require raking or other measures to improve flow-through. The grant thus serves as a flow buffer.
Why do we need wort grant?
In a pump-driven system, It can be easy to pull wort from the lauter tun faster than the grain bed wants it to flow, this will compact the grain bed and cause a stuck runoff. The grant provides a buffer between the outflow from the lauter tun and the inflow to the boil kettle. Gravity gently pulls the wort from the lauter tun mitigating the potential for a Vacuum. The brewer is able to take wort samples from the grant at any time to conveniently measure the pH and gravity of the runoff and avoid over-sparging. Also, a brewer can observe wort entering the grant and pump it back to the top of the mash (Vorlauf) until it has become sufficiently clear to be redirected to the boil kettle.