HELP FOR THE HEAD TO PRESSURE CONVERSION AND FLOW TO VELOCITY APPLET PROGRAM
This document will cover two topics, one a general discussion of this subject and how the equations were developped. The other some specific comments on how the applet functions.
Head is a term used in the pump industry and it represents the amount of energy per unit weight of fluid displaced at a certain flow rate. The unit of head is feet in the Imperial system and meters in the Metric system. The maximum head that a centrifugal pump can deliver will be at zero flow and this will correspond to the maximum height that a fluid can be displaced to. This height is measured from the fluid surface of the discharge tank to the pipe end of the discharge line if the pipe end is open to atmosphere or if it is submerged then to the fluid surface of the discharge tank.
In the normal operating situation the amount of head, also known as total head, that a pump can deliver is greater than the height that the fluid must be raised to because there is also friction energy that is lost in the system. Therefore the total head of the pump must be equal to the height that the liquid must be raised to, also know as static head, and the friction energy loss.
The total head or energy per unit weight of fluid that the pump can provide can be measured by measuring the difference in pressure across the pump.
For the Imperial system the relationship is:
where HP is the total head in feet, p2 and p1 are respectively the pressure in psi at the pump outlet and inlet and SG is the specific gravity of the fluid, for water SG = 1.0.
In the metric system:
Technically one needs to also take into account the difference in velocity energy from one side of the pump to the other but since this difference is usually small I will neglect in the interest of simplicity.
Flow rate and velocity
Flow rate (q) is the amount of volume that is displaced per unit time. It can be determined with the pipe inside diameter (D) and the average fluid velocity (v).
In the Imperial system:
And in the metric system:
You can find out more about total head in this tutorial and you can see an experiment involving total head in this video.
Applets are programs based on the java language that are designed to run on your computer using the Java Run Time environment.
This program offers two simple calculations that are easy to do if you can remember in which book to find the formulas. They are intended to help you save time.
The first one allows you to convert pressure to pressure head or vice versa. This may be useful if you are taking pressure measurements on a pump and need to know the pressure head to compare with the performance curve. You must know the fluid's specific gravity to do these calculations. By default it is set at 1.0, the same as water.
The second part of this applet will allow you to convert flow to velocity or vice versa if you know the pipe internal diameter. This can be useful when you are trying to estimate if your pipes are too small or too big. Typical fluid velocity range for the discharge side of the pump is 7-13 ft/s and on the suction side of the pump 2-6 ft/s.
The calculations can be done in US units or metric and in either French or English.
ENJOY!