PRESSURE HEAD P033
Any pump head calculation is made from a total of three types of head:
- Static head
- Friction head
- Pressure head
Pressure head looks at any pressurized vessels that might be located on the suction or discharge side of the pump. We use gage pressure for our calculations most of the time and use the conversion:
Head = psig x 2.31 / specific gravity (sg) or Head = bar x 10,2 / specific gravity
Please look at the above drawing Figure #2.
Since both the suction and discharge tanks are open to atmosphere there is no pressure head on the system.
|In this next example #3:
The discharge is to an open tank so there is no discharge pressure head.
The suction is under a twenty inch vacuum that must be converted to head if we want to make our calculations.
To do the numbers we will assume the fluid has a specific gravity of 0.98.
You can use any of the following formulas to convert to head units
Using the first formula:
Suction pressure head = – 20″ hg. x 1.133 / 0.98 = – 23.12 feet
There is a five foot static head that will be deducted from this number when you calculate the total head, but for this discussion the pressure head is still -23.12 feet
|For our next example we will look at the gages mounted on the piping. Please look at the diagram Fig #5.
We are going to complicate the problem by having the suction gage show absolute pressure while the discharge gage shows standard gage pressure
- Specific gravity = 1.3
- Atmospheric pressure = 14.7 psi.
To make our calculations we will divide the heads into two sections:
The discharge gauge head corrected to the centerline of the pump, in feet of liquid absolute, is found by adding the atmospheric pressure to the gauge reading to get absolute pressure, and then converting to absolute head:
= 257.1 feet, absolute
Note the 4 foot head correction to the pump centerline.
The suction gauge reading is in absolute terms so it needs only to be converted to feet of liquid, absolute.
= 73.08 feet absolute
Note the 2 foot head correction to the pump centerline.
The total pressure head developed by the pump is:
Total pressure head = Discharge pressure head – suction pressure head
= 257.1 – 73.08 = 184.02 feet