Getting to grips with noise (TIA)
A particular challenge when designing valves for compressible media and in particular steam is the noise calculation.
Arca Regler GmbH
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For many years, Arca Regler GmbH has been designing, building, testing and delivering all kinds of valves for the circulation of steam, condensate and water in industrial power plants and in particular in refuse incineration plants, or thermal waste recycling plants as they are called today.
In addition to the plant-specific operating data and performance features, growing environmental consciousness means that great importance is increasingly being attached to noise emissions. There are clear legal directives for this as well as arithmetic methods for performing a preliminary calculation. Arca makes contact with plant planners and plant manufacturers at a very early stage in order to harmonise the plant needs and technical requirements. This is very important for noise emissions in particular, because several factors are incorporated into the preliminary calculation here, such as the inlet and outlet sections, the dimensioning of the nominal sizes, the pipe wall thicknesses and the structure of the decoupled thermal and noise insulation. Only then can already very precise noise calculations be carried out, only then can the control valves that have to relax the fresh steam from a high primary pressure down to a vacuum be precisely designed and only then can the combination of regulated and unregulated reduction steps be defined. Apart from the standardised calculation principles, a great deal of experience is required in order to be able to design as safely as possible.
Nevertheless, it is occasionally the case that theory and empirical values do not suffice in practice. For instance, Arca measured considerable development of noise in the case of a generously sized, multi-stage, subcritical design in the turbine bypass station that relaxed down to vacuum. Since this was noticed during the commissioning phase, fast action was absolutely necessary. As the boiler had to be operated for planned tests and the turbine was not yet in operation, the fresh steam had to be discharged via the bypass station. Immediate measures had to be initiated for the employees who were still busy with the construction and Arca looked for a solution together with the plant manufacturer.
First of all a noise spectrum was recorded for various load cases and the load cases were compared with the design data. Deviations were found, but these lay within the tolerance field. As a side effect, it could clearly be seen how accurately Arca had performed the flow calculation for the very complex multi-stage valve. In parallel, the isometrics and the structural implementation of the thermal and noise insulation were checked. Here, too, no striking deviations were found that could have explained this noise phenomenon.
Hence, the only thing that remained was to check secondary measures. To this end the flow design with the various regulating and fixed stages was recalculated with the current operating data to find possibilities to reduce the noise. An additional throttle package was developed as a solution. The particular challenge here was that the throttle package had to be retrofitted into the existing DN 900 pipeline and calibrated with the existing regulating and fixed stages.
In close cooperation with the customer, the solution was presented and the decision was taken immediately to implement it. The exact positioning of the throttle package in the existing pipeline was also intensively discussed. Thus, only a few days remained before the next planned shutdown to precisely dimension and manufacture the interface for the throttle package in the pipeline. Accuracy took precedence over speed here. Thanks to special measures and particularly intensive chasing by the project team, the throttle package arrived at the building site in time for the planned plant shutdown, which lasted only two days.
The thermal part of the refuse incineration plant was started up via the turbine bypass stations and put back into operation. The result of the research and recalculation could clearly be heard – or rather, to put it better, could no longer be heard.
Afterwards an attempt was made to comprehend the noise phenomenon arithmetically with the retrofitted implementation and the operating cases concerned. These efforts and also the involvement of noise experts produced no conclusive explanation. This phenomenon could only be solved thanks to Arca’s experience and knowledge, without knowing the exact cause. All the richer for the experience, Arca has proven once again that it can react quickly and competently to such cases and is always there to help its customers.
This short case study originally appeared in PTQ's Technology In Action feature - Q3 2016 issue.
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