For a combustor to perform well, it must have a high combustion efficiency, it must have a low pressure loss, and it must have a stable flame. A high combustion efficiency is necessary for a high overall thermal efficiency. When the combustion efficiency is low, unburned fuel could be exiting the combustor and burning in the turbine and exhaust. Fuel burning in the turbine is detrimental to the turbine and exhaust. Flame stability means a steady constant flame. A flame that has poor stability will pulsate and could blow out. Pressure and the velocity of the gases in the combustor have a large effect on the range of a stable flame. The lower the pressure is, the narrower the range of stable flame operation. If the pressure is continually lowered the flame will become unstable and will not be able to burn. Higher exhaust gas velocities will also decrease the stable flame operating range. If the velocity is increased to its critical velocity, the flame will blow out. In addition, requirements for good combustion include: low amounts of deposits formed in the combustor, turbine and regenerator (if there is one); and the equipment must be serviceable and have a reasonable life.