Patent Application
20080149201
This invention relates to the mitigation of acoustic resonances in deep cavities, found, for example, in power and process piping.
Deep cavity resonances are a common problem in power and process plant piping systems. Typically, these resonances result in premature wear of, for example, Safety and Relief Valves (SRVs) in a main steam line (MSL) or similar conduit. In the SRV example, the typical installation configuration has the SRV mounted in a standpipe fitted to the MSL, forming a deep cavity in the standpipe which is susceptible to aero-acoustic resonances. In recent years some boiling water reactor (BWR) steam dryers have experienced significant degradation as a result of acoustic loading caused by deep cavity resonances initiated in the SRVs in the MSLs. Existence of these cavity resonances has affected the ability of some BWRs to operate at optimum levels. In other cases, these resonances have caused the utility to spend significant money conducting preemptive maintenance on the affected valves during every outage.
From a theoretical perspective, the self-excited resonance in the standpipe occurs when the vortex shedding frequency of the shear layer across the branch line cavity approaches the ¼ wave organ pipe acoustic resonance of the SRV standpipe. Once the two frequencies align then the two can exhibit a “lock-on” phenomenon and create a high amplitude acoustic resonance in the standpipe. This resonance has been mitigated in practice by taking the following actions: 1. Separate the vortex shedding frequency from the organ pipe frequency at the power level of operation; and 2. Disrupt the shear layer such that the vortex shed from the upstream edge cannot remain coherent and/or cannot impinge upon the downstream edge of the cavity. Commonly used mitigation concepts are described in an article entitled “Review Self-Sustaining Oscillations of Flow Past Cavities,” by D. Rockwell and E. Naudascher; Journal of Fluids Engineering, Vol. 100, June 1978.
The concepts described in this disclosure appears to be fundamentally different than those used in the past and has not been described in the literature.
The exemplary but non-limiting embodiments described herein mitigate the existence of acoustic resonances in deep cavities such as those commonly encountered in SRVs used in power and process piping industries. In one exemplary embodiment, a sleeve is inserted into the SRV standpipe, with the remote or distal end of the sleeve projecting into the flowpath in the MSL. The sleeve accomplishes two significant changes to the combined SRV and standpipe cavity which will be hereafter described as the deep cavity. The first change is a reduction in the deep cavity diameter by the insertion of the sleeve. The second change is a disruption of the flow path of the free shear layer at the mouth of the deep cavity by having the distal end of the sleeve project into the main flowpath. The first change reduces the Strouhal number associated with the configuration which can shift the system away from resonances, if sized properly. The second change disrupts the boundary or free shear layer, which in turn disrupts the fluid resonant excitation mechanism responsible for exciting the aero-acoustic resonance in deep cavities.
The sleeve can be installed in an existing SRV configuration with minimal modifications to the valve assembly. This is desirable because the valves and standpipes are contaminated in a nuclear plant. For a non-nuclear application this ease of installation reduces critical path times for plant maintenance.
In another exemplary embodiment, the distal end of the sleeve may be cut at an angle (this is referred to herein as a “sleeve ramp”), such that the upstream side of the distal end projects into the main flowpath, while the downstream side of the distal end terminates at the inner wall surface of the MSL.
In still another exemplary embodiment, the distal end of the sleeve is formed or otherwise provided with one or more vanes or grid members extending across the distal end of the sleeve (this is referred to herein as a “grid sleeve”). In this instance, the distal end of the sleeve may be flush with the inner wall surface of the sleeve because the grid members will cause the desired disruption in boundary or free shear layer of flow in the MSL.
Accordingly, in one aspect, the invention relates to a main flow line and relief valve assembly comprising: a main line fitted with a standpipe; a relief valve fitted within the standpipe; and a sleeve inserted in the standpipe below the relief valve, with a distal end of the sleeve configured to disrupt boundary layer flow in the main line.
In another aspect, the invention relates to a method of mitigating acoustic cavity resonances in a standpipe fixed to a main line and supporting a relief valve comprising: a) inserting a sleeve in the standpipe to separate vortex shedding frequency from organ pipe frequency at a power level of operation; and b) locating the sleeve such that a distal end thereof disrupts a boundary or free shear layer of flow in the main line.
The exemplary embodiment will now be described in greater detail in connection with the drawings identified below.
Referring to
With particular reference to
In the above example, the SRV valve body may be attached to the standpipe by means of a bolted flange. The standpipe, in turn, may be connected to the MSL through a sweepolet fitting. In an illustrative but non-limiting example, the MSL may be a 20 inch Sch. 80 pipe and the standpipe may be a six inch Sch. 80 pipe, but the invention is applicable to a wide variety of pipe and standpipe sizes.
The above-described sleeve is effective in that it separates the vortex shedding frequency from the organ pipe frequency at the power level of operation, and also disrupts the shear layer such that the vortex shed from the upstream edge cannot remain coherent and/or cannot impinge upon the downstream edge of the cavity. In other words, effectively reducing the diameter of the standpipe separates the vortex shedding frequency from the organ pipe frequency at the power level of operation by shifting the power level at which a resonance is expected to a lower level. At the same time, having the distal end of the sleeve project into the main flow shear layer disrupts the shear layer, preventing the feedback necessary to create a self-sustained oscillation.
It is desirable to minimize the clearance between the sleeve OD and the standpipe ID so that Flow Induced Vibration (FIV) displacements are minimal.
Referring now to
It will be further appreciated that other constructions are contemplated so long as they meet the two criteria discusses above, i.e., separating the vortex shedding frequency from the organ pipe frequency at the power level of operation, and disrupting the shear or boundary layer of the main flow. In this regard, it will also be appreciated that the distal end of the sleeve could be retracted into the standpipe and still cause the desired boundary layer disruption. In any case the projection or retraction dimensions should be calculated to provide the desired boundary or free shear layer disruption while also minimizing mean pressure loss to bulk flow across the standpipe cavity.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
