HYDRAULICS

"Pump Head"

**Water
and Wastewater**

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

**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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