Valves and valve actuation – With non-fluid retarder
Reexamination Certificate
2000-06-26
2002-02-19
Fox, John (Department: 3753)
Valves and valve actuation
With non-fluid retarder
C137S56100R
Reexamination Certificate
active
06347783
ABSTRACT:
The invention relates to an arrangement for preventing undesirable pressures, which occur when fittings are subjected to rapid control processes, particularly during rapid shutting-off or throttling processes in pipelines conveying liquid, by activating rapidly-acting valves or gates (shut-off fittings), when the flow of the liquid column is decelerated shortly after the valve is actuated. Because the liquid is decelerated, the pressure increases rapidly upstream of the fitting. A pressure surge develops, which represents a strain for the pipeline, the support for the pipeline as well as for the fitting itself. Downstream, there is a drop in pressure. If at the same time the pressure drops below the vapor pressure of the liquid, a cavitation bubble is formed, which subsequently collapses once again which, in turn, leads to pressure surges, the so-called cavitation knocks. In some cases, for example, when certain pipeline materials are used, a lower limit is specified for the permissible pressure, below which the pressure may not the drop during shutting-off or throttling processes.
In many technical applications, it is necessary to shut off or throttle the flow of liquids in pipelines as quickly as possible. Examples are pipelines for conveying liquid materials in chemical plants from one step of the process to another, plants for filling liquids, which may contaminate the environment, into tanks, tank cars or tankers, or beverage filling plants. Rapidly acting shut-off fittings are also provided, in order to limit the amount of medium emerging from a leak in the case of a line break and to prevent the malfunction from spreading.
Pressure surges generally are reduced in magnitude by setting the shutting-off or throttling rate of the fitting at a lower value. Experience has shown that the pressure surge at the admission side can be controlled by these means only if the plant and operating conditions of the pipeline correspond to the parameters set for the fitting. Furthermore, it is difficult to prevent a cavitation knock downstream from the shut-off fitting by decreasing the shut-off or throttling rate, because a cavitation bubble can be formed behind the fitting even when the shutting-off or throttling times are longer, especially if the pipeline is very long (Thorley, A. R. D., “Fluid Transients in Pipeline Systems”, D & L George LTD, 1991, Hadley Wood, Barnet, GB). With this, there is in most cases, from the very start, an incompatibility between a fixed, reduced shutting-off and throttling rate, and the requirement that the pipeline be shut off in the shortest possible time even under the varying operating and plant conditions. The procedure can therefore frequently not be used to prevent pressure surges and cavitation knocks.
One possibility for preventing pressure surges upstream from the shutting-off or throttling fitting is to discharge the medium being transported rapidly into a different section of the pipeline, which is opened when a limiting pressure or flow rate is reached (DD patent 211615). A similar method is pursued by Gustavsson to avoid pressure surges in remote heating pipelines with forward and reverse flow. Upon shutting off, the forward and reverse flow is coupled, so that a pressure increase is prevented (WO patent 94/02775). The diversion of the medium being transported with a subsequent, slow closing of the main fixture, is also known (DD Offenlegungsschrift 23 46 754). It is a disadvantage of the last-mentioned inventions that, in addition to a large expenditure for measurements, additional pipeline sections and/or buffer containers must be integrated in the transporting pipeline system.
A further possibility consists of measuring and limiting the pipeline pressure or the rate of pressure increase by a high-speed pressure measuring and control system. When a maximum pressure or a maximum rate of pressure increase is exceeded, the high-speed controller interrupts the shutting-off or throttling process of the fitting (DD patent 201 041). This procedure makes the highest demands on the measurement and control technology. Moreover, the driving mechanism of the shutting-off or throttling fitting must be in a position to interrupt the shutting-off or throttling process within very narrow time limits of a few milliseconds. It must be assumed that the development and investment costs are high here.
One possibility of preventing particularly the cavitation knock consists of venting or of feeding other noncondensable gases into the pipeline behind the shut-off fitting (Raschke, E., Salla, M., Hültenschnmidt, W.: “Pressure Surge Dampened, Pipeline Protected—Condensation Knocks in Pipelines Under the Dampening Influence of Gases,” Verfahrenstechnik 7-8/97, pp. 45-49). For this purpose, a venting fitting is mounted close behind the shut-off fitting and, when a reduced pressure occurs behind the shut-off fitting, opens automatically and admits a certain amount of air into the transporting pipeline. In this case, it is assumed that the reduced pressure results from the presence of a cavitation bubble. The air flowing in leads to a partial increase in the reduced pressure of the cavitation bubble. In addition, due to the presence of the air, the condensation rate of the vapor during the reverse flow phase of the liquid is greatly decreased. Thirdly, after complete condensation of the vapor, an air bubble remains in the pipeline, which dampens the pressure surge by its compressibility. All three of the effects mentioned lead to a reduction in the pressure peaks during the cavitation knock. In practice, air frequently cannot be used, because it reacts with the liquid or contaminates it impermissibly. In these cases, other gases, which do not react with the liquid, are used. In every case, it is a disadvantage that the liquid, which is to be transported, is contaminated by the gas supplied.
Furthermore, the possibility exists of reducing the pressure surges by installing air chambers in front of or behind the shut-off fitting. In front of the fitting, the air chamber creates a volume for accommodating liquid, as a result of which the braking of the liquid colunmn proceeds more slowly and the pressure peak is minimized. An air chamber behind the shut-off fitting provides volume, so that the formation of the cavitation bubble is hindered (Raschke, E., Seelinger, A., Sperber, A., Stra&bgr;burger, A.: “Simulation of the Unsteady Hydraulic Behavior of Chemical Engineering Plants with Long Pipelines,” CIT 66 (1994) 5, pages 652-660). Under normal operating conditions, air chambers are partly filled in both cases with the liquid, which is to be transported and over which there is an air cushion. It is a disadvantage that the air chamber takes up additional space and that the solubility of the gas in the liquid in the air chamber leads to a contamination of the liquid that is to be transported in the pipeline. In addition, it is necessary to make up for gases lost because of solubility.
In order to prevent cavitation knocking, it has already been proposed to provide an auxiliary fitting in the pipeline behind the shut-off fitting. The driving mechanism of the auxiliary fitting is connected over a threshold switch and a flow sensor with the pipeline (German patent application, file No. 198 15 242.6). A check valve can also function as auxiliary fitting. The auxiliary fitting prevents collapse of the cavitation bubble. However, it does not prevent a reduction in the pressure, downstream from the shut-off fitting, to the level of the vapor pressure. In many cases, however, the drop in pressure below a certain limiting pressure above the vapor pressure must be prevented, for example, when pipelines of composite materials are used, the liner of which could become detached. In addition, the auxiliary fitting prevents the cavitation knock but not the pressure surge in front of the shut-off fitting.
It is an object of the invention, to provide an arrangement, with which the shutting-off or throttling process of the shutting-off fitting can be carried out in the shortest possible time
Dudlik Andreas
Prasser Horst-Michael
Schlueter Stefan
Forschungszentrum Rossendorf E.V.
Fox John
Jordan and Hamburg LLP
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