Friction head, Pressure head,Total dynamic suction head,Total dynamic discharge head,Total dynamic suction lift,Total dynamic head

Friction head is the head needed to overcome resistance to liquid flowing in a system. This resistance can come from pipe friction, valves, and fittings. Values in feet of liquid can be found in the Hydraulic Institute Pipe Friction
Manual.

Pressure head is the pressure in feet of liquid in a tank or vessel on the suction or discharge side of a pump. It is important to convert this  pressure into feet of liquid when analyzing systems so that all units are the same. If a vacuum exists and the value is known in inches of mercury, the equivalent feet of liquid can be calculated using the following formula:

vacuum in feet =in. of Hg x 1.13/specific gravity

When discussing how a pump performs in service, we use terms describing dynamic head. In other words, when a pump is running it is dynamic. Pumping systems are also dynamic when liquid is flowing through them, and they must be analyzed as such. To do this, the following four dynamic terms are used.

Total dynamic suction head is the static suction head plus the velocity head at the suction flange minus the total friction head in the suction line. Total dynamic suction head is calculated by taking suction pressure at a pump suction flange, converting it to head and correcting to the pump centerline, then adding the velocity head at the point of the gauge.

Total dynamic discharge head is the static discharge head plus the velocity head at the pump discharge flange plus the total friction head in the discharge system. This can be determined in the field by taking the discharge pressure reading, converting it to head, and correcting it to the pump centerline, then adding the velocity
head.

Total dynamic suction lift is the static suction lift minus the velocity head at the suction flange plus the total friction head in the suction line. To calculate total dynamic suction lift, take suction pressure at the pump suction
flange, convert it to head and correct it to the pump centerline, then subtract the velocity head at the point of the gauge.

Total dynamic head in a system is the total dynamic discharge head minus the total dynamic suction head when the suction supply is above the pump. When the suction supply is below the pump, the total dynamic head is the total dynamic discharge head plus the total dynamic suction lift.

Centrifugal pumps are dynamic machines that impart energy to liquids. This energy is imparted by changing the velocity of the liquid as it passes through the impeller. Most of this velocity energy is then converted into pressure energy (total dynamic head) as the liquid passes through the casing or diffuser. To predict the approximate total
dynamic head of any centrifugal pump, we must go through two steps. First, the velocity at the outside
diameter(o.d.) of the impeller is calculated using the following formula:
 
v = (rpm x D)/229
 where v = velocity at the periphery of the impeller in ft per second,
D = o.d. of the impeller in inches,
rpm = revolutions per minute of the impeller,
and 229 = a constant.

Second, because the velocity energy at the o.d. or periphery of the impeller is approximately equal to the total dynamic head developed by the pump, we continue by substituting v from above into the following equation:

H = v2/2g

where H = total dynamic head developed in ft,
v = velocity at the o.d. of the impeller in ft/sec, and
g = 32.2 ft/sec2.
A centrifugal pump operating at a given speed and impeller diameter will raise liquid of any specific gravity or weight to a given height. Therefore, we always think in terms of feet of liquid rather than pressure when analyzing centrifugal pumps and their systems.