The goal of the analysis is to avoid mechanical resonance in the compressor/piping system. In many situations, and especially at higher order frequencies, it may be impossible to avoid resonance.
In this case, the engineer must “manage resonance”. Vibration will exist, but through detailed mechanical modeling and Forced Response Analysis techniques, the vibration consultant can predict whether a certain mode or location will have acceptable vibration and stress amplitudes.
In this case, the engineer must “manage resonance”. Vibration will exist, but through detailed mechanical modeling and Forced Response Analysis techniques, the vibration consultant can predict whether a certain mode or location will have acceptable vibration and stress amplitudes.
The mechanical analysis starts by developing an accurate computer model of the mechanical system. The model starts at the compressor (or cylinder); includes the pulsation control devices, scrubber, and main piping, and ends, as a minimum, at the second clamp away from the discharge/suction.
this model includes a (super element) frame FE model to accurately model the boundary conditions of cylinder/piping interface. This superelement frame model is an option for our standard Mechanical Vibration Analysis.
Pig launchers and pig receivers are installed on a pipeline to launch and receive pipeline pigs and pipeline inspection tools.
Piping system must be designed in the way that connecting pipes will not over-stress launcher or receiver. Let’s start from pig launcher supports. The first support has holes that fit bolts, but the second support has slotted holes that will provide free thermal expansion for the launcher itself. Since the first support is fixed, pipeline thermal expansion will imposed thermal force on the pig launcher. The pig launcher and receiver must be arranged in relation to a pipeline in such way that thermal expansion force from pipeline is minimal. Please see the the figure below.
In the arrangement shown above, the underground pipe will be anchored by the virtual anchor that is result of soil friction. Pipeline flexibility is achieved by having elbow, allowing free thermal expansion in the direction perpendicular and parallel to launcher. If this arrangement is not suitable for your application, pig launcher can be set in direction of pipeline. Please see the figure below.
Usually, this type of arrangement will require a concrete stop block to limit pipeline thermal expansion. Only stress analysis conducted on underground pipeline can determine requirement for the stop block. In the case that stop block is used, it must be sized in such way that resisting soil pressure does not exceed allowable soil pressure. Please see the figure below.
Normally, comprehensive stress analysis using Caesar II is required, since high stresses are expected. Pig launcher stress can be obtained by Caesar II and it can be easily modelled, since the pig launcher is just big pipe having reducer.
The steps involved in modeling a sway brace in Caesar are as follows:
Select the sway brace from catalogue depending on given pipe nominal diameter or depending on the force calculated to restrain the pipe work. (SB45 as per C&P catalogue, reproduced in Fig. 1)
Mark a node (Node 10-Fig.2) at the location in the piping system where sway brace will be installed. Run Caesar analysis and note down the displacement of the point in specified direction from cold to operating condition. For the sake of example, let’s assume that CAESAR II calculated displacement from cold to operating position is 0.5 inch in +X direction.
Now in CAESAR II input spread sheet (See Fig. 2) check the restraints box and define bi-linear restraint (X2 for the assumed case) at Node 10 with CNode at 101. Here, K1 is the initial stiffness of a bi-linear restraint. Do not enter anything on this cell as the restraint is assumed to be rigid. The value of K2and Fy to be obtained from catalogue. Where, K2= Post yield stiffness of a bilinear restraint. When the load on the support restraint exceeds Fy then the stiffness on the support restraint changes from K1 to K2. Fy = Yield Load. If the load on the support restraint is less than “Fy” then the initial stiffness K1 is used. If the load on the support restraint is greater than “Fy” then the second stiffness ” K2″ is used.
Define restraint X at node 10 with CNode at 101. Provide a gap of 3 inch (=distance the sway brace is able to move in both positive and negative direction before it gets locked/ become fully rigid depending on manufacturer= 3 inch as per C&P catalogue)