In field vibration assessment of the piping in a reciprocating compressor plant

Dr. R.J. Fawcett, Dynaflow Research Group, 3rd International Rotating Equipment Conference (IREC); Pumps, Compressors and Vacuum Technology, Düsseldorf, 14 – 15 September 2016

Abstract
For a number of years visible vibrations were noticeable in the process piping connected to a reciprocating compressor at a refinery, this was despite a pulsation analysis having been conducted at the design stage. The effects of these vibrations were also visible in the small-bore instrumentation pipes, even though they were braced back to the main run pipe. The operator of the plant was worried that fatigue cracks could occur, especially in the small bore lines, and therefore a study was conducted to determine how the vibration levels could be reduced and whether they were leading to stress levels exceeding the endurance limit.

To calculate the stress magnitudes arising in the piping, with a special emphasis on the small bore connections, a forced mechanical response analysis was performed using a numerical computer model. As well as using the as-built technical drawings the behaviour of the model was tuned to replicate the findings of in-field vibration measurements taken upon both the piping and the bracing. Tuning a piping model to replicate the dynamic behaviour of an operating piping system is not a trivial undertaking. Within this paper the effect of various factors that were given special attention in tuning (matching) the computational model will be discussed.

Special attention was given on how to ensure that the correct mechanical mode shapes were present in the model and that they were excited to the same level as in the field. These mode shapes were identified from the vibration measurements taken using a three-axis accelerometer. Factors such as equipment weights within the piping, and gaps and stiffnesses in the supporting deviate to varying degrees from those envisaged at the design stage in any piping system. Consequently the mechanical resonance modes predicted by the numerical model, initially based on the as-built technical drawings, exhibited some differences from those measured in the field. This was in terms of their shapes but also their response at a given excitation frequency.

In tuning the model the stiffness of the spring loaded guide supports, both laterally and axially had to be varied, as well as the stiffness of the bracing of the small bore branches. Only by modifying these values was it possible to match the vibration amplitudes seen in the field with the computational simulation of the piping system. It is impossible to include these factors at the design stage and they are adressed by the requirement that all mechanical resonance modes should be above 2.4 times the compressor rotational speed. However unintensional installation factors could result in this margin not being met in the field, and thus this additional modelling step with a tuned model is required for determining the stress level and the margin of safety.

The output of the study was a robust set of conclusions to the operator of what changes should be made to ensure there was sufficient margin to prevent cracking in the line. The vibrations in the header lines were reduced using rigid supports where possible, given thermal expansion of the system, which have far fewer unknowns in their installation in the field than supports with pre-loaded springs. Additionally recommendations were given for the bracing and gussets on the small bore instrumentation lines so they were less sensitive to vibrations in the header.

In sharing this study though the intention is to increase the awareness of the factors that need to be considered when tuning a numerical piping model to replicate the field experience under a dynamic loading such as pressure pulsations. Thus increasing the accuracy of numerical simulations used for assessing potentially critical situations in the field.