At some locations within the various processes that take place inside a power plant, the pressure is negative. Negative pressure, also referred to as a vacuum
, is any pressure that is less than the atmospheric pressure.
The condenser vacuum system is an example of a system that operates at a negative pressure
A common everyday example of negative pressure is someone using a drinking straw. Sucking on the top of the straw reduces the pressure on the small surface of the water inside the straw. Atmospheric pressure on the top of the water in the glass then pushes water up the straw. It is the difference between atmospheric pressure and the pressure inside the straw that causes the flow. This is known as pressure differential.
In the power plant steam/water cycle, the turbine condenser is kept at a very low pressure, or high negative pressure. This maximizes the overall plant cycle efficiency.
Because of the negative pressure in the condenser, improper valving may result in air entering the condenser, thereby increasing the pressure, or reducing the negative pressure, and causing a drop in the cycle efficiency.
Back pressure
can be expressed in psia or inches of mercury. The most common way of expressing back pressure is inches of mercury ("Hg). As absolute pressure (psia) decreases, the vacuum becomes larger.
Maintaining the proper level of negative pressure in the condenser is very important and requires constant awareness on the part of the operator.
Another critical concern affecting cycle efficiency is the introduction of non-condensible gases to the condenser from the turbine steam system. This, in turn, reduces vacuum and increases turbine back pressure.