Fluid handling – Line condition change responsive valves – Direct response valves
Patent
1989-10-23
1993-07-06
Rivell, John
Fluid handling
Line condition change responsive valves
Direct response valves
137514, F16K 2110
Patent
active
052245110
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention relates to a spring-loaded safety valve.
Such a safety valve is described in EP-A1-0 032 346. In the latter, the movable part of the damping device connected to the valve spindle is a piston. The latter displaces hydraulic oil by way of a restrictor between two cylinder working chambers, which it defines together with the cup-shaped stationary damper part receiving it.
However, hydraulic dampers operating with restrictors are sensitive to impurities and on account of the friction between the piston and cylinder do not work in a satisfactory reproducible manner, in particular at the time of transition between static friction to sliding friction.
In the known safety valve, the starting point of damping and the magnitude of damping also cannot be adjusted. However, this would indeed be desirable in safety valves, in which the forces acting on the valve spindle vary very quickly with the opening displacement in the vicinity of the closing position.
A further spring-loaded safety valve is described in the Journal VGB Kraftwerkstechnik (VGB Power Station Technology), volume 8, August 1984, pages 746 onwards. In the latter, the damping device operating on the spindle comprises brake shoes with a spherical outer surface engaging on the outer surface of the spindle, which brake shoes are biased against the outer surface of the spindle by a spring by way of conical pressure members. The damping device thus operates according to the principle of solid friction, and with this damping device one can partly eliminate the harmful effects of valve vibrations. Valve vibrations of this type occur in particular due to the reactions of pressure waves. The latter occur for example at the time of each unstable movement of the valve spindle and of the closure member supported thereby.
The system, which is formed by the safety valve, the valve supply pipe leading to it and the blow-off pipe leading away from it, is capable of the selfexcitation of vibrations, the initiation being able to take place due to small disturbances, which occur in the pipes or are transmitted by way of the latter. The self-damping of this system is low in comparison with the energy which is inherent in the flow medium to be protected by the safety valve.
If the system formed by the pipes and the safety valve begins to vibrate, then the safety valve acts as an unstable oscillator. With these vibrations, pressure amplitudes may occur in the pipes, which far exceed the limit of stability or the design limit of the pipes.
In this known safety valve, the damping device operates in a manner which can promote the occurrence of vibrations, since in the case of large disturbances, due to which the static friction between the brake shoes and the outer surface of the spindle is overcome, a surplus of force is produced, which cannot be absorbed by the constant sliding friction which is less in comparison with the static friction. This surplus of force leads to a jerky movement of the closure member, which once more leads to the production of compressional vibrations in the pipe system. One thus obtains an unfavourable feedback to the safety valve. This transition between static friction and sliding friction takes place for each reversal of movement of the valve spindle. The damping is furthermore independent of the speed of movement of the valve spindle, so that with respect to a vibration cycle, independent of the effective spindle speed, approximately a constant portion of the kinetic energy present as a whole is dissipated. This means that high frequencies can be less well damped.
On account of the unavoidable static friction determined by the damping principle of this known safety valve, even the static force management of the safety valve is influenced unfavourably by the biasing of the brake shoes. The response point of the safety valve shifts in a manner which cannot be controlled accurately. Also the friction surfaces must be protected against corrosion, which means additional maintenance expenditure. In the case of a forced vibrat
REFERENCES:
patent: 957311 (1910-05-01), Davis et al.
patent: 2392009 (1946-01-01), Stern
patent: 3048188 (1962-08-01), Hunter
patent: 3789873 (1974-02-01), Westwood
patent: 3850405 (1974-11-01), White
patent: 4018248 (1977-04-01), Carr
patent: 4756334 (1988-07-01), Panet et al.
Babcock Sempell AG
Rivell John
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