The WOG rating stands for water, oil and gas and indicates the maximum pressure per square inch that the valve can take. Each valve is given a WOG rating, which should not be exceeded.
Each different material that the valve is mad of has different limits in temperature and pressure. The stronger the material is, the higher the WOG rating is likely to be. If the pressure or temperature values stated on the valve are exceeded, the valve will be unable to work properly and there is a chance that it may be damaged. The WOG rating has largely been replaced with the class system that rate the temperature and pressure ratings of valves.