March 13, 2020

A small portable see-through pump system as an educational tool... Download more details

This compact pump system allows you to experiment with the transfer of liquids. You can modify the pressure at key points in the system by adjusting the flow rate with a variable speed pump or the location of these points or the tank levels. The micro-lab will help you deepen your understanding of how liquids are moved, you will see a centrifugal pump react to changes due to modifications to the connected equipment.

The system will allow users to measure the:

  - static head at the pump suction;
  - static head at the pump discharge;
  - static head at the system high point and move that high point up or down;
  - shut-off head;
  - friction head loss;
  - the static head and therefore pressure at various point in the system;
  - pump suction submergence;
  - flow rate;
  - and all of this at different pump speeds.

As the famous Lord Kelvin (William Thompson) in the late 1800's put it:

"To measure is to know." "If you cannot measure it, you cannot improve it. When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind."

Static head and pressure are terms often used interchangeably because they make the same point in the context they are used. These terms are related but they have different meaning and usage. Pressure is the liquid equivalent of a force applied to a solid object. A liquid is moved by applying pressure, a solid by applying a force. If there is a difference in pressure at one end of the pipe vs. another, the liquid will move in the direction of lower pressure.

Head is the notion that a body of liquid at an elevation has potential energy with respect to a lower point, for example an elevated lake will drive a turbine at a lower elevation because of its potential energy. A cyclist has potential energy at the top of a hill, this energy corresponds to the weight of the cyclist and her bike times the vertical distance to the level road below. Energy is called work when we need to move an object with our own muscles such as the energy it takes to lift a weight up vertically or with a device purpose built such as a pump in a pump system. Energy or work is expressed in ft-lbf in the Imperial system of units or m-N in the metric system. How we describe the energy of a liquid is different to the energy of a solid. The elevated liquid does not move in its entirety but one unit of volume at a time, therefore we divide the potential energy by unit weight of liquid displaced, the unit is then ft-lbf/lbf or simply ft (feet). This term which is specific energy is called head and is expressed in feet or meters which is very convenient. Why is it convenient? Because pumps are rated in terms of head and flow rate, for example in feet and gals/min or meter and meter cube/h.

This next figure shows a comparison of a solid in movement compared to a liquid. A ball is going down a hill, then being acted on by a force and pushed up a higher hill on the other side. This is similar to a liquid in a suction tank connected to a pump which then pressurizes the liquid transferring it to a higher tank.

Head is useful in all aspects of analysis of a pump system, i.e. flow rate, friction, pump performance. Pressure is useful when determining the resistance of equipment, or maximum pressure ratings, vapor pressure or atmospheric pressure.The micro-lab allows measurement of the static head at various parts of the system.

The system has 3 measurement tubes that will allow you to determine the static head at 3 points within the system: the pump suction, the pump discharge and the high point. From the static head we can easily determine the pressure, the pressure equals the density of the liquid times the static head (p = dens. x h).

Check out this app which resides on my other web site to do calculations for head to pressure or vice versa using Imperial or metric units: head to pressure

How can we ensure that we are choosing the right pump for a given task since a pump can provide a range of flow rates at different heads? After all the pump does not know which flow rate to give you. The pump will react to how much pressure is present at its outlet (discharge) and inlet (suction). The pressure available at the suction and discharge depends on what is connected (i.e. suction tank level, discharge tank level, piping size and length). A pump with a 4" suction and 3" discharge diameter is built to move water within a range of 400 and 700 gpm. When the pump starts it is trying to attain these flow rates but it encounters resistance to flow in terms of friction, perhaps due to a small pipe or the energy required to get the liquid to the surface level elevation of the discharge tank. This will limit its ability to establish the flow rate that you require unless you have planned carefully. This is where the pump performance curve comes into play which can be investigated with the micro-lab. The complete system has to be considered when selecting a pump and this will become evident by experimenting with the micro-lab.

Because we use flexible tubes and variable speed pumps, the micro-lab offers 3 distinct pump systems in one or a combination of them:

1. A typical pump system with positive pressure at the pump suction and a discharge tank fed from above;
2. A system with a high point to demonstrate how pressure can vary drastically depending on the pipe run;
3. A system with low pressure (below atmospheric) at the pump suction.

contact: Jacques Chaurette (Send mail)

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