One of the most critical parameters monitored and controlled in a power plant is pressure.
Pressure is defined as force per unit area and is commonly measured in pounds per square inch (psi)
The force of one pound per square inch (1 psi, or 0.07 bar) against a large object can be several hundred pounds of actual force
The air in the atmosphere exerts pressure on all objects due to the weight of the air
Pressure can be measured using many different units. Psig stands for pounds per square inch gage and is often referred to as gage pressure. Gage pressure is pressure measured relative to atmospheric pressure. It uses atmospheric pressure as its zero reference.
Psia stands for pounds per square inch absolute. Psia measurements use zero psi as their zero reference. To calculate absolute pressure one must add atmospheric pressure to the gage pressure.
Barometric pressure is normally measured in inches of mercury ("Hg). Other pressure measurements may be reported in inches of water ("H2O). Any pressure measurements can be converted to other units by using the proper conversion factors.
Negative pressure, or vacuum, as it is more commonly called, refers to pressure that is less than atmospheric.
In the power plant steam/water cycle, the turbine condenser is kept under as low a pressure (high negative pressure) as possible to maximize overall cycle efficiency.
Back pressure is the absolute pressure (psia) at the point where steam leaves the turbine and enters the condenser. It may be expressed in psia or "Hg. As absolute pressure (psia) gets smaller, the negative pressure expressed in inches of mercury ("Hg) gets larger.
Manometers are U-shaped tubes filled with mercury or water. The following is true of manometers:
If pressure is applied at one end of a manometer, the liquid is forced to the other end.
The difference in the heights of the two columns is a measure of the pressure applied.
The greater the pressure, the larger the difference in the "legs" of the column.
Mercury is 13.6 times heavier than water. Therefore, the height of a mercury column is much less than the height of a water column that measures the same pressure.
A reading in "H2O can be converted to "Hg by dividing the number of inches of water by 13.6. Similarly, a reading in "Hg can be converted to "H2O by multiplying by 13.6.
"H2O ÷ 13.6 = "Hg
"Hg × 13.6 = "H2O
The discharge pressure produced by a pump is commonly referred to as the discharge head or head pressure and is the height that a pump would be able to force a column of water. Discharge head simply is another method of stating pressure.
The term draft is used in power plants to designate what type of boiler airflow system is used.
Balanced draft means the boiler uses an induced draft (ID) fan to draw the flue gas out of the furnace and a forced draft (FD) fan to force combustion air into the furnace. This is the most common arrangement used today. The typical balanced draft furnace runs at approximately -0.5" H2O (-12.7 mm H2O, negative pressure) in the boiler furnace.
Forced draft means that there is no induced draft fan and furnace pressure runs positive all the time.