Within an industrial plant it is frequently necessary to increase the pressure of a flow. To achieve such an increase in pressure, compressors and pumps are used, with compressors being used for gaseous flows and pumps being used for liquids.
Compressors are available in a number of forms, namely reciprocating, axial and centrifugal. Within a reciprocating compressor the pressure rise is generated by compressing a volume of gas using a piston. Whilst in a centrifugal or axial compressor the pressure rise is achieved by doing work on the gas by means of rotating aerofoils or blades. Both of these methods will cause pressure oscillations.
Design Codes: API 618 and VDI 3842
The industry standard design code API 618 provides guidelines for the maximum pressure vibrations due to reciprocating compressors. Likewise the VDI 3842 code also has guidelines for the maximum pressure fluctuation amplitudes. These experienced based limits can be used to identify excessive, potentially damaging, pressure pulsations within the piping, due to a compressor.
Reciprocating and centrifugal pumps are used to raise the pressure of a liquid medium. As with compressors, the oscillations from a reciprocating pump or the blade passing of a centrifugal pump will lead to pulsations in the flow upstream and downstream of the flow. The vibrations and dynamic loading due to these pulsations, can lead to excessive loading and fatigue issues in the piping.
Dynaflow Research Group (DRG) is able to offer a complete range of solutions to assess how critical a pump is for the operation of your system. Our assessments can be used to assess a concept at the design stage or to solve a pulsation issue observed during operation.
The API 674 Design Code for Reciprocating Pumps
Typically reciprocating pumps will lead to significantly larger amplitude pressure pulsations than centrifugal pumps. Within the industry standard API 674 design code, it is stated that the piping around a reciprocating pump should be analysed to ensure that the piping is capable of withstanding the pulsations. For bespoke systems, where an identical version is not already in operation, this assessment should take the form of a calculation.
The API 674 design code provides guidelines for acceptable pulsation levels in the piping connected to a reciprocating pump.
Mechanical Response Analysis
Any structure or piping system will have a number of mechanical resonance (natural) frequencies. If these frequencies coincide with those of external excitations, for example those due to compressors, pumps or the fluid flow within a pipe, then any small pipe deflection caused by the excitation mechanism at these frequencies, could be amplified and result in vibrations in the mechanical structure. These mechanical vibrations, if persistent, could result in problems due to Low Cycle Fatigue and/or High Cycle Fatigue.
DRG is able to identify the mechanical resonance frequencies of a piping system. DRG is then able to assess which of these resonance modes are likely to be excited by a pump or compressor. Combining this with our expertise in piping design means we at DRG can propose solutions to remove the critical resonance modes and ensure that the solution compiles with the relevant piping design codes.
Reciprocating and centrifugal pumps are used to raise the pressure of a liquid medium. As with compressors, the oscillations from a reciprocating
pump or the blade passing of a centrifugal pump will lead to pulsations in the flow upstream and downstream of the flow. The vibrations and dynamic loading due to these pulsations, can lead to excessive loading and fatigue issues in the piping.
DRG is able to offer a complete range of solutions to assess how critical a pump is for the operation of your system. Our assessments can be used to assess a concept at the design stage or to solve a pulsation issue observed during operation.
Removing Vibrations in a Flow Station
DRG was asked by one of our clients to reduce the excessive, potentially fatigue inducing, vibrations they were observing at one of their oil flow stations. These were seen in the suction and discharge headers for the reciprocating pumps, and the pump recirculation line.
In order to find the cause of the observed vibrations and to determine what measures were required to rectify the problem, DRG performed a pulsation analysis combined with a mechanical response analysis.
DRG worked with the client to obtain the information necessary to build a model of the system piping using the CAESAR II piping stress software. This model was used first to perform a scoping static stress assessment to verify if the existing pipe supporting design was adequate.
In order to perform the pulsation analysis, a model of the fluid path within the flow station was then generated in BOSpulse. Here pulsations, according to the maximum amplitudes given in the API 674 design code, were applied at the location of the pumps. Using BOSpulse it was possible to calculate the pressure amplitudes within the downstream piping and determine if any acoustic resonance modes were being excited.
The pulsations due to the reciprocating pumps led to unbalanced forces within the piping system. These forces were used as an input for the mechanical response study. As a result of the analysis it was found that the observed vibrations were likely to cause a fatigue problem.