HYDRAULICS

"Pump Head"

Water and Wastewater

The following is from our text "MATH TEXT for WATER and
WASTEWATER TECHNOLOGY,
THIRD EDITION"
by GROVER WRIGHT

(In the text we start with the "basic level" topic such as this, show an example, and then have you do several just like it. You are able to check your answers against ours for each problem. We then move to the intermediate level, and then to the advanced level. We show an example at each level, and have several additional problems for you to do on your own at each level... all with answers!)

This is meant to be printed out... that is the only way you will be able to properly view the two math graphics.

Hydraulics is defined as the study of fluids at rest and in motion. In water and wastewater that almost invariably means water, and water containing solids.


Discharge Head: is the verticle distance between the pump datum point and the liquid surface in the receiving tank. The pump datum is at the center line for horizontal pumps and at the entrance eye of the impeller for vertical pumps.

Suction Head: if the water to be pumped has its surface ABOVE the center of the pump, then this relationship is called a "suction head". More technically, it is the positive verticle distance between the pump datum and the liquid surface in the suction well.

Static Head: "Static head is the distance that the water is to be lifted."

Therefore, if the liquid level is above the datum, then it is a "positive value", as the water does not need to be pumped to that elevation. In the calculation:
(Static Head, ft) = (Discharge Head, ft) - (Suction Head, ft)

"Once more for emphasis", the suction elevation is subtracted from the discharge head as the water is already at a positive, "not needed to be pumped elevation" ABOVE the pump. This resulting value is known as the static head.

 

Suction Lift: If the liquid level is BELOW the pump datum, then it is a negative value, as that is additional elevation that the water has to be lifted up to the pump zero level. This case is termed as having a suction lift. (Although it is rare for this case to happen, it does occur.) This is illustrated in the suction lift drawing:

In the calculation:

(Static Head, ft) = (Discharge head, ft) - (Suction Elev. ft)

Note that the suction elevation (lift) is below the pump datum. In this case the elevation is a negative number (minus) and therefore "a minus subtracting a minus is a positive value " in the equation. To illustrate a suction liquid level 5 feet BELOW the pump datum, with a Discharge head of 35 ft:

(40 ft) = (35 ft) - (-5 ft)

Friction Head: is the head necessary to overcome the friction in the pipes, fittings, valves, elbows, etc. This information is gathered empirically, and then recorded in tables so that we can estimate these values according to the flow, the pipe size, the pipes material it is constructed out of, pipe age and any deposits, the type of valve, etc. This additional resistance to flow must be compensated for, in order to deliver the desired flow rate. Please refer to the illustrations for suction head and suction lift, where you will notice that the friction head in feet, is added to the static head which results in a new value called the Total Head or Total Dynamic Head.

Total Head or Total Dynamic Head: The Total Head, also called the Total Dynamic Head (TDH), is the sum of the Static Head and the Friction Head. The Total Head, or TDH, is the value used in the horsepower calculations.

A pump can operate effectively only within the system for which it is designed. The system must take the above energy consumers into account, to deliver the fluid properly.


SUCTION HEAD & TDH PROBLEMS

EXAMPLE: The influent pump discharges into a channel where the liquid level is 14 feet above the pump datum line. The pump draws its suction from a wet well, whose water surface is 5 feet above the pump. The friction head is 5.6 ft.

Determine the Static Head, in feet.

Static Head, ft = (Discharge Elev, ft) - (Suction Elev., ft)

Static Head, ft = (14 ft) - (5 ft) = 9 ft Static Head

Calculate the Total Dynamic Head (TDH), in feet.

TDH = (Static Head, ft) + (Friction Head, ft)

TDH = (9 ft) + ( 5.6 ft) = 14.6 ft TDH

PROBLEM: The influent pump discharges into the grit chamber, where the liquid level is 8 feet above the pump datum line. The pump draws its suction from a wet well, whose water surface is 2 feet above the pump. The friction head is estimated at 2.5 ft.
Determine the Static Head, in feet. (Ans: 6 ft)
Calculate the Total Dynamic Head (TDH), in feet. (Ans: 8.5 ft)

 

PROBLEM: The polymer makeup pump discharges into the solution tank, where the liquid level is 8 feet above the pump datum line. The pump draws its suction from a sump, whose water surface is 2 feet above the pump. The friction head is 1.5 ft.
Determine the Static Head, in feet. (Ans: 6 ft)
Calculate the Total Dynamic Head (TDH), in feet. (Ans: 7.5 ft)

SUCTION LIFT & TDH PROBLEMS

EXAMPLE: The influent pump discharges into a channel where the liquid level is 14 feet above the pump datum line. The pump draws its suction from a wet well, whose water surface is 3 feet BELOW the pump. The friction head is 6 ft.

Determine the Static Head, in feet.

Static Head, ft = (Discharge Elev, ft) - (Suction Elev., ft)

Static Head, ft = (14 ft) - (-3 ft) = 17 ft Static Head

Calculate the Total Dynamic Head (TDH), in feet.

TDH = (Static Head, ft) + (Friction Head, ft)

TDH = (17 ft) + (6 ft) = 23 ft TDH

PROBLEM: The influent pump discharges into the grit chamber, where the liquid level is 8 feet above the pump datum line. The pump draws its suction from a wet well, whose water surface is 2 feet below the pump. The friction head is estimated at 2.5 ft.
Determine the Static Head, in feet. (Ans: 10 ft)
Calculate the Total Head (TDH), in feet. (Ans: 12.5 ft)

PROBLEM: The raw water pump discharges into the sand trap, where the liquid level is 18 feet above the pump datum line. The pump draws its suction from a sump in the reservoir, whose water surface is 2 feet below the pump. The friction head is estimated at 4 ft.
Determine the Static Head, in feet. (Ans: 20 ft)
Calculate the Total Dynamic Head (TDH), in feet. (Ans: 24 ft)

Next month: Horsepower!

 

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