Reality Paper

Characteristics of a Pump System

A pump is only a single component of a process although an important and vital one. The pumps’ role is to provide sufficient pressure to move the fluid through the system at the desired flow rate.

Pressure, friction, and flow are three important Characteristics of a Pump System. The pressure is the driving force responsible for the movement of the fluid. Friction is the force that slows down fluid particles. Flow rate is the amount of volume that is displaced per unit of time. The unit of flow in North America, at least in the pump industry, is the US gallon per minute, USgpm. From now on I will just use gallons per minute or gpm. In the metric system, flow is in liters per second (L/s) or meters cube per hour (m3/h).

Pressure provides the driving force to overcome friction and elevation differences. It’s responsible for driving the fluid through the system, the pump provides the pressure. The pressure is increased when fluid particles are forced closer together. For example, in a fire extinguisher work or energy has been spent to pressurize the liquid chemical within, that energy can be stored and used later.

What is friction in a pump system? Friction is always present, even in fluids, it is the force that resists the movement of objects. When you move a solid on a hard surface, there is friction between the object and the surface. If you put wheels on it, there will be less friction. In the case of moving fluids such as water, there is even less friction but it can become significant for long pipes. Friction can also be high for short pipes that have a high flow rate and small diameter as in the syringe example.

The amount of energy required to overcome the total friction energy within the system has to be supplied by the pump if you want to achieve the required flow rate. In industrial systems, friction is not normally a large part of a pump’s energy output. For typical systems, it is around 25% of the total. If it becomes much higher then you should examine the system to see if the pipes are too small. However, all pump systems are different, in some systems the friction energy may represent 100% of the pump’s energy.

The pressure is produced at the bottom of the reservoir because the liquid fills up the container completely and its weight produces a force that is distributed over a surface which is pressure. This type of pressure is called static pressure. Pressure energy is the energy that builds up when liquid or gas particles are moved slightly closer to each other and as a result, they push outwards in their environment. A good example is a fire extinguisher, work was done to get the liquid into the container and then to pressurize it. Once the container is closed the pressure energy is available for later use.

How does a centrifugal pump produce pressure? Fluid particles enter the pump at the suction flange or connection. They then turn 90 degrees into the impeller and fill up the volume between each impeller vane. This animation shows what happens to the fluid particles from that point forward.

A centrifugal pump is a device whose primary purpose is to produce pressure by accelerating fluid particles to a high velocity providing them with velocity energy. What is the velocity energy? It’s a way to express how the velocity of objects can affect other objects, you for example. Have you ever been tackled in a football match? The velocity at which the other player comes at you determines how hard you are hit. The mass of the player is also an important factor. The combination of mass and velocity produces velocity (kinetic) energy. Another example is catching a hard baseball pitch, ouch, there can be a lot of velocity in a small fast-moving baseball. Fluid particles that move at high speed have velocity energy, just put your hand on the open end of a garden hose.

 

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