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Tips Jul - Sept 1998
Certain Helpful Tips for using CAEFLOW-Steady
CAEFLOW-Steady, our new program that you can download, is for steady state fluid flow analysis of piping networks. Given below are a few tips that will help you make better use of CAEFLOW.
1) How to get better convergence during computation:
Ideally, fastest convergence can be obtained for a piping network with equal flow resistance for all its elements. Convergence deteriorates as the variation of flow resistance of the elements increases. Worst case will be when an element has negligible flow resistance compared to its neighboring elements. In this case, we either have to avoid this element completely or merge it with the connecting pipe and specify an additional pressure drop coefficient in the geometry input.
The following is a sample layout.
10 Origin 0 0 0
20 Pipe PIPE1 0 0 565 S1 0
30 Bend BEND1 1 0 1 B1 0
40 Pipe PIPE2 40 0 0 S1 0
The layout shown above will have better convergence during computation if it were modified as shown below.
10 Origin 0 0 0
20 Pipe PIPE1 0 0 565 S1 0.224
40 Pipe PIPE2 40 0 0 S1 0
2) Use Right Mouse Button to Access Pipe Size Databases, Bend/Reducer Pre-processors and Pipe Fittings:
A] Pipe Size Databases (Standard Pipings)
ANSI, DIN and JIS Pipe Size Databases can be accessed by clicking the right mouse button on a row of "pipe sections" dialog box. If the row is empty it shows the default values, otherwise it shows the pipe section's details for that row.
B] Bend Pre-processor
The pressure drop coefficient for a) 90 deg. bend b) miter bend c) std. 45 deg. elbow, or d) std. 90 deg elbow can be calculated by CAEFLOW. Click the right mouse button on a valid row of "bend types" dialog box to invoke "bend pre-processor" dialog box.
C] Reducer Pre-processor
The pressure drop coefficient for a reducer or expander depends on the geometry of the reducer (i.e., inlet diameter, outlet diameter, and length). If we have the pressure drop coefficient readily available, we can directly enter this value while defining the reducer types. Otherwise CAEFLOW can calculate this coefficient for us. Click the right mouse button on a valid row of "reducer/expander types" dialog box to invoke the dialog box.
D] Pipe Fittings
Users can enter the pipe fittings like Valves, Bends etc. in two ways. One is by adding the Kp factor of the fitting directly to the Pipe to which it is connected or by treating it as a separate element. The "pipe fittings" dialog box can be opened by clicking the right mouse button on the last column ("Add. Coef.") of the spreadsheet for any pipe element.
More Helpful Tips for using CAEFLOW-Steady
There are several ways of improving the convergence in CAEFLOW-Steady. For small networks, normally we do not need to worry about the computation times. However, for large networks we need to accelerate the convergence so that analysis can take less time. A few tips are given below to accelerate the convergence:
For a Given Geometry
CAEFLOW makes use of the Nodal Iterative Technique (NIT) Theory which has good control over the convergence via the relaxation parameters for flow and pressure. An optimum combination of these two parameters can improve the convergence by orders of magnitudes.
Once the geometry has been input and the user does not want to change the geometry, the only way to accelerate convergence is by arriving at an optimum combination of the relaxation parameters. This can be made within a couple of trial runs.
Network with elements of negligible flow resistance
Ideally, fastest convergence can be obtained for a network with equal flow resistance for all its elements. Convergence deteriorates as the variation of flow resistance of the elements increases. Worst case will be when an element has negligible flow resistance compared to its neighboring elements. In this case, we either have to avoid this element completely or merge with the connecting pipe and give as an additional pressure drop coefficient in the geometry input.
Optimizing at the Geometry Level
Following the previous discussion, much optimization can be done by avoiding several elements and merging with the pipes as mentioned. In this way, we reduce the number of elements in the network thus reducing the computation time.
Network with Pumps or Filters
We have to be very careful when we have pumps or filters in the network. Many of the checks for validity of the pumps or filters are done by CAEFLOW while defining the types. However, if the pump does not produce enough head to make any flow to take place, the solution does not converge. Similarly, if the filter offers negligible pressure drop also we encounter convergence problems. We have to make sure that we have placed proper pumps and/or filters in the network.
Modeling Expansion Joints in CAEPIPE
The file EXPJOINT.EXE contains CAEPIPE MOD(el) files. Please download it into a separate directory and type EXPJOINT (Enter) to expand it. These models can be read into CAEPIPE v4.0 and later (the results and the text output files are part of this file).
CAEPIPE provides the following types of expansion joints:
1. Ball joint,
3. Hinge joint, and
4. Slip joint
One can also model tied bellows, gimbal, a dual gimbal, pressure balanced elbows and tees, a slotted hinge joint, and a universal joint by using the joint types available in CAEPIPE.
This tip will show how to model the following types of joints: Tied Bellows, Universal, gimbal, dual gimbal, pressure balanced joints and slotted hinge joint.
Tied Bellows: Tie rods allow lateral deflection and absorb the full pressure thrust force in the event of an anchor failure. During normal operation, the adjacent equipment will see the pressure thrust force. Pressure thrust area must be input. One way of modeling the tie rods is to lump all four tie rods into a single limit stop (with stiffness = 4 x stiffness of tie rod = 4 x EA/L) at the center of the bellows. The reason for modeling the tie rods as a limit stop is that these rods act only in tension and not in compression. See bellow2.mod in EXPJOINT.EXE.
An elaborate method of modeling the above is to create four limit stops (instead of one) placed at their actual locations with rigid elements. See bellow3.mod in EXPJOINT.EXE.
Universal joint: Two single bellows with a centerspool, with tie rods across the whole assembly (end to end) is called a Universal joint. This is commonly used in multi-plane Z bend configurations to absorb lateral deflections. Axial growth in the vertical leg (that contains the joint) will introduce bending in the horizontal legs which may require other supports. See universl.mod in EXPJOINT.EXE.
Gimbal/Dual Gimbal joint: Allows angular motion in all planes. These are modeled in CAEPIPE as two back to back hinge joints with perpendicular axes. See gimbal.mod and dualgimb.mod in EXPJOINT.EXE.
Pressure Balanced Elbows and Tees: These joints are used to absorb axial and lateral movement at a change in direction while restraining the pressure thrust by means of tie rods. It is often used to reduce the pressure thrust forces coming on nearby equipment (rotating) whose allowable nozzle loads necessitate their elimination. See pbtee.mod and pbelbow.mod in EXPJOINT.EXE.
Slotted Hinge joint: In CAEPIPE, this joint is modeled using an elastic element, a limit stop and a hinge joint. The limit stop models the slot (+ and – 1 inch, for example). The elastic element has very low axial stiffness and all other stiffnesses rigid to model the slot. The hinge provides rotational stiffness about the Z axis. See slotted.mod in EXPJOINT.EXE.