A valve which permits the release or intake of air in to or out of a tank only when the pressure or vacuum within the tank exceeds the preset limits of the valve. This can prevent tank explosion or implosion when loading or discharging a tank.
Related Terms 
DIRECTIVITY FACTOR

LIFTING DEVICE
A device to manually open a pressure relief valve by decreasing the spring loading in order to determine if the valve is in working order.

LEAK TEST PRESSURE
The inlet pressure used for a standard quantitative seat leakage test.

KNUDSEN VACUUM GAGE
Device to measure negative gas pressures; a rotatable vane is moved by the pressure of heated molecules, proportionately to the concentration of molecules in the system.

KIRCHHOFF'S EQUATIONS
Equations which state that the partial derivative of the change of enthalpy (or of internal energy) during a reaction, with respect to temperature, at constant pressure (or volume) equals the change in heat capacity at constant pressure (or volume).

KILOBAR
A unit of pressure equal to 1000 bars (100 megapascals). Abbreviated kb.

KELVIN EQUATION
An equation giving the increase in vapor pressure of a substance which accompanies an increase in curvature of its surface; the equation describes the greater rate of evaporation of a small liquid droplet as compared to that of a larger one, and the greater solubility of small solid particles as compared to that of larger particles.

JOULETHOMSON COEFFICIENT
The ratio of the temperature change to the pressure change of a gas undergoing isenthalpic expansion.

JOULE'S LAW
1. The law that when electricity flows through a substance, the rate of evolution of heat in watts equals the resistance of the substance in ohms times the square of the current in amperes. 2. The law that at constant temperature the internal energy of a gas tends to a finite limit, independent of volume, as the pressure tends to zero.

ISOTHERMAL CALORIMETER ISOTENISCOPE
An instrument for measuring the vapor pressure of a liquid, consisting of a U tube containing the liquid, one arm of which connects with a closed vessel containing the same liquid, while the other connects with a pressure gage where the pressure is adjusted until the levels in the arms of the U tube are equal.
