Vent pipe termination with flapper and side vent

A vent termination assembly for a vertical pipe, stack or tube including a hinged flapper to normally close the end of said vent termination assembly and at least one downwardly sloping normally open to the atmosphere side pipe bevelled to discharge generally upward. The flapper contains energy adsorbing side beams to reduce velocity of the flapper during opening.

FIELD OF THE INVENTION 
This invention pertains to vent pipes or stacks used to direct gases 
venting from a storage vessel away from contact with the areas where 
persons normally would be present around the vessel. 
BACKGROUND OF THE INVENTION 
Chemical storage vessels such as liquid hydrogen storage tanks are 
protected from overpressurization by relief devices. The relief device is 
normally a control valve that is set to open when internal pressure in the 
storage vessel reaches a predetermined level. When the valve is opened, 
material venting from the tank is normally conducted through a vent pipe 
which is arranged to discharge the venting gas into the air or ambient 
atmosphere at levels between 12 and 60 feet above grade in the case of a 
hydrogen storage tank. In the case of storage tanks for normally liquefied 
gases such as liquid hydrogen, the storage vessels range in size where the 
inner vessel can contain between 1000 and 25,000 gallons of liquid 
hydrogen. Normally relief valves on these tanks are set to discharge at 
150 psig at 700.degree. F. In addition to the relief valves there are 
primary rupture discs which are set to relieve at 180 psi g at 70.degree. 
F. and secondary rupture discs set to relieve at 225 psig at 70.degree. F. 
The primary rupture discs may discharge into the same vent pipe that the 
primary relief valves direct the venting gas toward. The secondary discs 
discharge into a separate vent pipe, directed vertically upward and 
normally covered by a sealed cap. Normally the vent pipe or stack 
terminates substantially vertically upward at a distance of 12 to 60 feet 
above grade on which the tank or storage device is installed so the 
venting gas is directed substantially vertically upward. In the case of 
hydrogen, a flammable gas, if ignited, could constitute a hazard to 
personnel and equipment in the immediate vicinity of the storage vessel. 
An upwardly directed vent pipe left uncovered in ambient atmosphere and 
exposed to everyday weather conditions could fill with rain water, which 
in turn could freeze, thus plugging the vent pipe. Since vent pipes 
associated with secondary rupture discs do not have valves which could 
leak and because the burst pressure is greater than the operating 
pressure, these discs seldom rupture prematurely and thus the vent pipe 
associated with these discs may be protected from the elements with a 
sealed cap. 
In the past, primary vent pipe closures have used hinged flappers which 
have had the problem of shearing the hinge pin when the vent was operated 
or tested thus exposing the vent stack to atmospheric conditions. Branch 
conduits with downwardly projecting openings have been used, but these 
would direct the venting gas downwardly instead of upwardly into the 
ambient atmosphere. Various other devices have been proposed, including 
internal baffles on vertical uncapped pipes to drain rain or snow while 
discharging gas vertically upward, and redirecting downwardly exhausted 
gas from any side pipes upwardly. 
SUMMARY OF THE INVENTION 
The apparatus of the present invention is a device to terminate a vertical 
vent pipe, stack, or tube and is particularly useful with a pipe, stack or 
tube used for venting hydrogen to the atmosphere. The device of the 
present invention is adapted for mounting on the end of the vent pipe 
stack or tube and consists of a hinged flapper with extended beams to 
absorb the opening energy as the flapper contacts a pair of shear pins. In 
addition, the termination device includes at least one and preferably a 
pair of downward sloping side pipes terminating in upward facing beveled 
ends to maintain a continuous path to the atmosphere in the unlikely event 
that the flapper becomes inoperative and there is a need to vent through 
the pipe or stack.

Referring to FIG. 1, the vent pipe termination according to the present 
invention is shown generally at 10 and includes a base or flange 11 which 
is adapted by means of mating flange 12 and a series of bolts 14 and nuts 
16 (one pair only being shown for clarity) to fix the apparatus 10 to a 
vent pipe, stack, or tube from a storage vessel, the vent stack not shown. 
The vent pipe termination 10 includes a body 18 having a straight first 
portion 20 and a diverging second portion 22 connected to the main stack 
24. 
Intersecting and communicating with the interior of the main stack 24 is at 
least one and preferably two side pipes 26, 28 which are generally 
cylindrical tubes which are inclined at a downward angle, the angle being 
downward relative to the vertical axis of the main stack portion 24 of the 
vent pipe termination 10 so that when the vent pipe termination 10 is 
installed on a vent stack which is normally disposed vertically, the pipes 
26, 28 slope toward the grade upon which the device containing the vent 
stack is placed. Side pipes 26, 28 communicate with the interior of the 
main stack 24 and are continuously opened to the atmosphere. As shown in 
FIG. 2, the side pipes 26, 28 terminate in a upward facing bevel 30 which 
is closed by a pair of bars 32, 34 normally used to prevent birds or other 
large flying animals from nesting in the side pipes 26, 28. 
Disposed on the top of the main stack 24 is a closure assembly 36 shown in 
greater detail in FIGS. 3 and 4. Closure assembly 36 includes a flapper 
lid 38 fixed to a pair of side bars 40, 42. Fixed to flapper lid 38 and 
side bars 40, 42 are a pair of side flaps or skirt portions 46, 48. 
Flapper lid 38 has a rear skirt or flap 50 which serves in cooperation 
with flaps 46, 48 and front lip 52 to prevent water from entering main 
stack portion 24. The entire assembly of the lid 38, side bars 40, 42 and 
skirts 46, 48, 50 define a lid which is hingeably mounted on a mounting 
frame 54, frame 54 being generally rectangular in shape and adapted to fit 
around the main stack portion 24 as shown in the drawing. Frame 54 can be 
fixed to main stack 24 by welding, brazing or the like. The flapper 
assembly including the top 38 is mounted to the frame 54 by bolts or pins 
shown as 56, 58. Bolts or pins 56, 58 can be a single rod or any other 
convenient type of device that will permit the lid 38 to pivot from a 
closed position shown in FIG. 4 to an open position shown in FIG. 3. 
Disposed on the mounting frame 54 are a pair of shear pins 60, 62 which 
are positioned to intercept and contact the side bars 40, 42 respectively 
when the flapper is moved away from the main stack 24 by the presence of 
venting gas inside of the vent pipe termination. 
A device according to the invention can be fabricated from 3" schedule 10 
pipe. The downward sloping side pipes 26, 28 can be fabricated from 11/4" 
schedule 5 pipe and positioned at a downward sloping angle of 8 degrees to 
the vertical axis of the main stack 24. The combination of the 3" vent 
pipe terminator mounted to a 2" schedule 5 or schedule 10 pipe and the 
side pipes as disclosed produce a minimal side pipe flow when the flapper 
38 is in the open position as shown in FIG. 3. The side pipes 26, 28 are 
joined to the main stack 24 in a manner to create a 0.155" ledge or weir 
64. This weir further discourages the entrance of moisture into the 
vertical run during high lateral winds. 
The hinged flapper represents an unsymmetrical obstruction to radial 
expansion of the sonic flowstream exiting the stack. This may deflect the 
existing jet somewhat away from the vertical. The angle cannot exceed 
45.degree. and the lateral force cannot exceed 207 lbs. The stack must be 
braced within 21 inches of the lid to absorb this force and prevents 
longitudinal bending stresses in the pipe from exceeding 16,000 psi. 
In normal operation, loading on the flapper assembly including the lid 38 
is greatest following bursting of a rupture disc in the pipe associated 
with the vent stack termination 10. Since the flapper is closed as the 
compression wave reaches the top of the vent pipe, dynamic pressures at 
the flapper may approach the vessel pressure. The piping preceding the 
rupture disc is relatively long on most tanks used to store normally 
liquefied gases such as hydrogen. Pressure is expected to drop local to 
the rupture disc immediately after disc rupture until reestablished at 
sonic speed from the tank. This process would normally take about 10 
milliseconds. The flapper cover 38 accelerates during this period and is 
expected to be fully open in 15 milliseconds and may experience an opening 
energy input on the order of 866.5 inch-pounds. This energy estimate is 
based on the center of pressure of the flapper traveling in an arc under a 
dynamic pressure (steady state) of approximately 33.55 psig. The actual 
process is considerably more complex. 
The flapper accelerates to its open position, but must be brought to rest 
in a position which will allow it to reclose, in this case by gravity. The 
sidebars 44, 42 comprise more than half the polar mass moment of the 
flapper, and reduce the velocity during opening. The bars 40, 42 are 
intended to flex plastically during impact as they decelerate the flapper 
assembly 36. The construction of the remainder of the flapper assembly 36 
is intended to reduce the possibility that any water may enter the 
vertical run of the vent, or the hinge assembly. This is accomplished with 
the side skirts 46, 48 and rear flap 50 and the front ledge 52 which is 
bent downwardly. The position of the sidebars 40, 42 shifts the flapper 
center of gravity so that the closing moment is much larger than previous 
versions of rear hinge flapper designs. 
The hinge can be a single piece bar with push-on retaining rings so the 
hinge pivots in both the frame 54 and the flapper assembly 36 by being 
disposed in oversize holes to minimize the possibility that moisture will 
freeze the linkage during discharge. 
The sidebars 40, 42 impact on the shear pins 60, 62 mounted in the frame 54 
so that they stop the motion of the flapper assembly 36. Two pins are used 
so that they can be readily replaced after repeated impact. 
The device according to the present invention consists of a flapper lid 
capable of absorbing relatively large opening energy in the sidebars 40, 
42 which are intended to bend plastically in combination with the 
replaceable shear pins 60, 62 to absorb energy by bending plastically 
thereby reducing the forces at the hinge. In addition, the combination 
includes side pipes installed at a slightly downward angle to the main 
stack 24 terminating in a weir to reduce the likelihood that snow or 
rainwater will be driven into the main pipe. Side pipes 26, 28 terminate 
in upward facing bevels to direct flow out of the side pipes 26, 28 in a 
substantially upward direction. 
The device according to the present invention prevents snow or rainwater 
from entering the main vent pipe when the flapper is closed. When venting 
of gas occurs, the flapper lid 38 swings to the open position, and vents 
substantially upward to the atmosphere. The impact of sudden stopping of 
the flapper lid is absorbed in elastic and plastic deformation of the 
flapper assembly, thus positioning the flapper for closure when venting 
terminates. 
Having thus described my invention, what is desired to be secured by 
letters patent of the United States is set forth in the pending claims.