The Temperature and Pressure (T&P) relief valve is a safety
device that serves two functions. It is designed to open and relieve water should the pressure in the water heater exceed the rated working pressure of the tank, usually 150 psi in shore-side household systems; but (apparently) 75 psi in yours.
It will also open in the event that the temperature in the tank exceeds 210°F (standard).
If the T&P valve opens due to excess pressure, there will be either dripping or a small continuous flow of water that relieves the pressure.
If, on the other hand, it opens due to a high temperature condition, the valve will open completely and allow the whole tank to empty until cool water resets the thermal probe on the T&P.
Both excessive pressure and high temperatures in water heaters are extremely hazardous.
Elevated water pressure can be caused by thermal expansion*, a backflow preventer in the supply side, and/or high incoming water pressure (shoreside supply). These conditions can be managed by installing a thermal expansion tank and/or a pressure reducing valve in the system.
High water temperature can be remedied by replacing the thermostat.
In addition, the T&P valve may discharge (usually a drip, rather than a free-flow gush) because calcium (lime) or sediment has accumulated in the valve, preventing it from seating properly.
The T&P valve pressure setting must not be greater than the working pressure of the water heater, and must be higher than input of the water heater respectively. Accordingly, check your water heater pressure rating (75 psi?) - and do not exceed this psi rating with a hardware
store replacement (150 psi).
If your tank is rated at 150 psi, then you csan substitue any "standard" 150 psi/210°F T&P Relief Valve, that fits.