Head

The pressure at any point in a liquid can be thought of as being caused by a vertical column of the liquid whose weight exerts a pressure equal to the pressure at the point in question. The height of this column is called the "static head" and is given in terms of feet of liquid.

The static head corresponding to any specific pressure depends on the weight of the liquid according to the formula:

Head in Feet = Pressure in psi x 2.31 / Specific Gravity

The energy imparted to a liquid as the Discpac pump impeller rotates is transformed into pressure energy as the liquid leaves the pump. Therefore, the head developed is approximately equal to the velocity energy at the periphery of the impeller. This relationship is expressed by the formula:

Head = v2/2g

Note: Head and pressure are interchangeable terms-provided they are expressed in the appropriate units. The pressure-head developed (in psi) will be directly proportional to the specific gravity of the fluid being pumped.

TOTAL HEAD: The total head-also called total dynamic head (TDH)-for a specific system is equal to the total discharge head (hd) minus the total suction head (hs). It is recommended that total head calculations for the suction side be listed separately from those for the discharge side to help avoid the possibility of overlooking a troublesome suction condition.

Fig 1.1 shows how pressure and head are related for identical pumps handling fluids with different specific gravities. Fig 1.2 shows how pressure and head are related for pumps giving the same pressure for pumping fluids with different specific gravities.

SUCTION HEAD: Suction head exists when the liquid supply level is above the pump centerline. The total suction head is equal to the static height or submergence in feet that the liquid supply level is above the pump centerline, less all suction line losses. Suction line losses include entrance loss plus any pressure existing at the suction supply source.

Note: Even when the liquid supply level is above the pump centerline, the equivalent of a lift will exist if the total suction line losses exceed the positive static suction head. This condition can cause problems particularly when handling volatile or viscous liquids.

For an existing installation, the total suction head would be the gauge reading at the suction flange converted to feet or meters of liquid, corrected to the pump centerline elevation, plus the velocity head at the point of gauge attachment.

SUCTION LIFT: Suction lift exists when the liquid supply level or suction source is below the pump centerline. Total suction lift is equal to the static lift in feet plus all friction losses in the suction line including entrance loss.

When the liquid supply level or suction source is above the pump centerline and under a vacuum, the equivalent of a suction lift will exist. Its value will be equal to the vacuum effect, less the net submergence.

On an existing installation, the total suction lift is the reading of a mercury column or vacuum gauge at the suction flange converted to feet of liquid and corrected to the pump centerline elevation minus the velocity head at the point of gauge attachment.

TOTAL DISCHARGE HEAD: Total discharge head is the sum of: static discharge head, all piping and friction losses on the discharge side, including straight runs of pipe, losses at all valves, fittings, strainers, etc., pressure in the discharge chamber (if a closed vessel), losses at sudden enlargements, exit loss at discharge velocity (usually assumed to be equal to one velocity head at discharge velocity), and any other loss factors that should ideally be taken into account.

On an existing installation, the total discharge head would be the reading of the pressure gauge at the discharge flange converted to feet or meters of liquid and corrected to the pump centerline elevation, plus the velocity head at the point of gauge attachment.

VELOCITY HEAD: Velocity head represents the kinetic energy in a moving liquid at a given point in the system. It is equal to the vertical distance the mass of liquid would have to fall (in a vacuum) to acquire the velocity V. The velocity head is used in system head calculations to help calculate entrance losses, L, in valves and fittings, at other sudden enlargements and exit losses. In system head calculations for high head pumps, the velocity head will be a small part of the total head and is not significant. But in low head pumps, it can be a substantial percentage and must be taken into account.

VAPOR PRESSURE: This pressure comes from the vapor formed by a liquid above its free surface. High values of vapor pressure can lead to serious reductions in NPSH, which can in turn lead to pump cavitation.

The following example calculation will allow you to determine if a liquid will vaporize before arriving at the pump inlet.

If this result is greater than the suction lift, the liquid will not vaporize before arriving at the pump inlet. To determine suction lift, add all pipe lengths plus any losses from pipe fittingselbows, tees, etc.-as equivalent length of straight pipe. For example, an elbow in the piping may be equivalent to 10 ft of straight pipe, depending on the curve and pipe diameter.

ENTRANCE LOSSES: Losses in head occur at the entrance to the pump under various conditions, for example when taking suction from the bottom of a tower or a side outlet (Fig 1.3). These losses should be taken into account to ensure sufficient static head to prevent vortices from entering the suction line. Sufficient static height must be provided to acccount for the entrance loss and velocity head at point A, to prevent the formation of vortices.