Introduction to type of flow in pipeline.

In water flow system, mainly two types of flow exist, transient flow and steady flow. Here I will elaborate both types. Transient flow can be study in two cases, sudden change of momentum and gradually change of momentum. Sudden change causes water hammer and it is hazardous/dangerous for the pipelines. Gradual change does not create water hammer, but it is difficult to analyse mathematically whereas Steady state flow is very simple and easy to analyse. So, let us discuss in detail.

1.    Transient flow (type – 1)

To understand the transient flow, I would like to take the following example. The following figure represents a simple water network with one source and a delivery reservoir. Let us assume that network having no flow. If there is no flow in the network, water will stagnant in the lower portion of the pipe.

Once water will enter the first reservoir, it will push the accumulated water in the forward direction. The force required to push the water is change of momentum of accumulated water plus frictional head. Let us say force “F_m” required to compensate the change of momentum, and F_f required for frictional force. This force will give the required water head to push the water. We can calculate required head  by formula.

By this head water will starts moving out to the delivery reservoir. From the following figure if can be observe that velocity at t =0 will be zero, and then it will increase gradually to attain the steady state velocity. Figure 2 is showing the situation at t=0 and figure 3 showing situation at steady state.

i.e we can write as follows

Actually ha will be the correct notation instead hm because water will be the accelerating from t =0 and then it will become zero at steady state. The above figure represents the situation at t =0 and below at steady state.

Hence, we must write like  

As we know that time is very important factor in the transition stage. If time increases in change of momentum the required force will reduce. Here is the same situation, larger area of reservoir will give more time in changing the momentum of accumulated water.  The optimum area can be calculated by following some guidelines. Many guidelines are available but no one software dealing this type of problem as per my knowledge. I request to the readers if you know any software please share with me. The guidelines available is limited to the single delivery reservoir. If you have more delivery reservoir, you must convert to the single delivery reservoir.

2.    Transient flow (type – 2)

Second type of transient flow stats with steady flow. In this flow sudden change of velocity occurs. Sudden change of velocity may have many causes like sudden close of valve, cause of column separation, single pump failure and power failure. This is also known as water hammer or surge. There are many software/applications which deals with surge like

1.      SAP (Surge Analysis Program).

2.      Water Hammer

3.      Hytran http://www.hytran.net/

4.      Kypipe http://kypipe.com/surge/

Surge is a very big chapter, and can be discuss for a semester.

3.    Steady state flow

Steady state flow is simplest flow in the pipe network. It is also known as pressure developed flow. These flows do not have change of momentum term. Its governing equation will have mass balance and energy balance equation but change of momentum will not be there.