Fluid handling – Line condition change responsive valves – Pop valves
Patent
1990-07-25
1991-10-29
Hepperle, Stephen M.
Fluid handling
Line condition change responsive valves
Pop valves
F16K 1732
Patent
active
050606883
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a pressure relief valve for pressure equalization between a closed space and the surrounding atmosphere. A particularly important field of use of such pressure relief valves is for tankers for oil products and chemicals, and in the following explanation and description this field of use will be taken as point of departure, but it will be understood that the same principles will be equally applicable to other fields of use, e.g. for stationary storage tanks or for rolling transportation tanks for liquid petroleum products or chemicals.
When a certain volumetric quantity of product per time unit, e.g. measured in m.sup.3 /h, is loaded into a tank, the pressure relief valve is to be opened, when the pressure in the tank has risen to a pre-set opening pressure, and thereafter a quantity of gas is to be blown off per time unit corresponding to the quantity of product loaded per time unit with an addition, where applicable, for evaporation from the liquid surface in the tank. The blow-off quantity per time unit depends on the flow resistance of the valve--and thereby on the lifting height of the valve--and on the pressure in the tank. The maximum blow-off quantity per time unit--with deduction of any addition for evaporation--which is obtainable without the pressure in the tank exceeding a certain safety limit is referred to as the capacity of the pressure relief valve, which is thus a measure of the maximum permissible loading quantity per time unit (e.g. m.sup.3 /h).
When loading is stopped, and the pressure in the tank drops to a value, the closing pressure, which is slightly lower than the opening pressure, the pressure relief valve is closed. When the tank is unloaded, the gas pressure in the tank drops. To prevent a drop of pressure below a certain safety limit a vacuum valve may be used, which may be built together with the pressure relief valve or may be separately mounted. In certain cases an inert gas, such as nitrogen, is for safety reasons supplied to the free space above the liquid surface in the tank, such gas being automatically maintained at a predetermined pressure, that may be higher than the atmospheric pressure, but lower than the opening pressure of the pressure relief valve. In this case, too, a vacuum relief valve may be used as a reserve in the case of failure of the supply of inert gas.
When the temperature of the surroundings increases or decreases during storage or transport, the gas pressure in the tank will increase or decrease, and thereby the pressure relief valve is opened and closed in principle in the same manner as in the case of loading and unloading, though normally for substantially shorter periods and with smaller blow-off quantities per time unit. Similarly a vacuum relief valve and/or a system for the supply of inert gas operates in principle in the same manner as in the case of unloading.
Pressure relief valves for use in oil tankers have to fulfill strict safety requirements which are laid down in internationally recognized rules. The most important of these are:
1. Passage of flame from the surroundings through the pressure relief valve to the interior of the tank, generally referred to as flash-back, must be positively precluded.
2. If safety against flash-back in the open condition of the valve is obtained solely by means of a high blow-off velocity--so-called high velocity valves--the blow-off velocity must not at any time during the opening sequence of the valve fall below a prescribed minimum value, e.g. 30 m/sec.
3. The blow-off shall be in the form of a vertically upwards directed jet in order to keep any injurious or inflammable gases as far as possible away from places where people are present.
4. In the case of icing-up of the valve, it shall be possible to de-ice the valve quickly, simply and so efficiently as to restore the valve to normal operation.
The pressure relief valve here considered is of the kind comprising a valve housing having an upwardly facing blow-off opening in which a drop-shaped body is provided, which is suitable
REFERENCES:
patent: 253622 (1882-02-01), Pearson
patent: 2784737 (1957-03-01), Kelly
patent: 3294116 (1966-12-01), Tremeau
patent: 3444886 (1969-05-01), Bailey et al.
patent: 3999571 (1976-12-01), Pedersen
patent: 4181144 (1980-01-01), Lander
patent: 4474208 (1984-10-01), Looney
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