# Unbalanced Forces: Answering Some FAQs

PIPENET Transient Module users often have questions about the following two aspects of calculating forces in PIPENET:

## 2.What are rigid and elastic joints?

### Part 1: Steady State Condition

We do not consider forces in steady state. We only make sure that the pressure is below the allowable pressure. What is the reason?

Let us consider an elbow pair in the horizontal plane.

Elbows 1 and 2 will individually experience forces even in steady state. This is because the pressure inside the pipe is higher than the pressure outside. The direction of the momentum also changes. Furthermore, there are frictional forces acting on the pipe wall. However, F1Y and F2Y will exactly cancel out each other. Please note that friction will act on the wall of the pipe. That will compensate for the difference in the pressure due to frictional loss. Why do we need elastic joints? Do we need to use them even in steady state conditions? There are circumstances in which one end of a force control volume must be defined as an elastic joint. It should be noted that an elastic joint does not necessarily mean it is a connection with a flexible pipe. PIPENET allows the user to choose between rigid and elastic joints depending on the purpose for which the user wants to generate the forces.

Let us consider the upper half of the system. There are a few reasons why we may define one end point as elastic.

1. Let us suppose that the upper half of the system is metallic and the lower half is flexible. The force on the metal section in the y-direction will be solely the force on the elbow on the metal section. The flexible pipe will not exert a restraining force on the metal section.
2. If there was a flexible connector without tie rods, then it must be defined as an elastic joint even if it has metal pipes on both sides. Or, if it is a flexible connector and our aim is to calculate the force on the tie rods, then it must be defined as an elastic joint.
3. Suppose the pipes are completely metal, are there circumstances in which I would use an elastic joint? Yes. If the force on the bolts of the flange needs to be calculated, then the flange end of the pipe must be defined as elastic.
4. Suppose the flexible pipe was not there, would I use an elastic joint? This depends on the exact configuration.  If the free standing end of the pipe has pressure = 0 specification and there is flow at that end, then it must be defined as elastic.   Examples are nozzles and open valves.  On the other hand, if there is no flow then it must be defined as rigid.  Examples are blank flanges and closed valves. Let us suppose we generate a pressure surge at the end of the system. Let us assume that this pressure surge is a very narrow spike. The unbalanced force will be 0 when the pressure surge is generated because it has not reached the elbow yet. When the pressure surge reaches elbow 2, F2Y will become a large value because the pressure will reach a high value. The net force F1Y- F2Y will be the net force on the pipe section between the elbow pair. When the pressure surge reaches elbow 1, F1Y will become a large value because the pressure will reach a high value. The net force F1Y- F2Y will be the net force on the pipe section between the elbow pair. When do we have to define rigid or elastic joints? The reasons are the same as what was described under steady state forces. For convenience they are reproduced here.

Let us consider the upper half of the system. There are a few reasons why we may define one end point as elastic. Please refer to the drawings below as appropriate.

1. Let us suppose that the upper half of the system is metallic and the lower half is flexible. The force on the metal section in the y-direction will be solely the force on the elbow on the metal section. The flexible pipe will not exert a restraining force on the metal section.
2. If there was a flexible connector without tie rods, then it must be defined as elastic joint even if it has metal pipes on both sides. Or, if it is a flexible connector and our aim is to calculate the force on the tie rods, then it must be defined as an elastic joint.
3. Suppose the pipes are completely metal, are there circumstances in which I would use an elastic joint? Yes. If the force on the bolts of the flange needs to be calculated, then the flange end of the pipe must be defined as elastic.
4. Suppose the flexible pipe was not there, would I use an elastic joint? This depends on the exact configuration.  If the free standing end of the pipe has pressure = 0 specification and there is flow at that end, then it must be defined as elastic.   Examples are nozzles and open valves.  On the other hand, if there is no flow then it must be defined as rigid.  Examples are blank flanges and closed valves.  