Shock wave absorber having apertured plate

The shock or energy absorber disclosed herein utilizes an apertured plate maintained under the normal level of liquid flowing in a piping system and disposed between the normal liquid flow path and a cavity pressurized with a compressible gas. The degree of openness (or porosity) of the plate is between 0.01 and 0.60. The energy level of a shock wave travelling down the piping system thus is dissipated by some of the liquid being jetted through the apertured plate toward the cavity. The cavity is large compared to the quantity of liquid jetted through the apertured plate, so there is little change in its volume. The porosity of the apertured plate influences the percentage of energy absorbed.

BACKGROUND OF THE INVENTION 
In many industrial applications, a liquid is conveyed through a piping 
system that can be extended over an elaborate three-dimensional network 
having bends, tees elbows, etc., held in place by braces or hangers. The 
possibility exists that a pressure pulse can be generated in the liquid, 
where the instantaneous liquid pressure might rapidly increase by a factor 
of 1.5-10 or even more at times. This pressure pulse is transmitted in the 
form of a shock wave through the liquid at sonic velocity (for example, 
4,000-8,000 ft. per sec.) through the piping system. 
The travelling pressure shock wave can damage equipment along the piping 
system and/or can damage the braces supporting the piping system. The 
effect this shock wave has on the piping system can also be amplified 
because of a resonant condition, occasioned, for example, when opposite 
ends of the same relatively straight section of piping are simultaneously 
exposed to a positive increase and a negative decrease in the wave 
pressure. Moreover, pressure intensities can be amplified where parallel 
flow paths reunite to combine the many waves in each path. 
This particular phenomenon has critical consequences in liquid metal fast 
breeder reactor designs where molten sodium is used as a first coolant 
conveyed through a piping system; and where water is used as a second 
coolant that at a heat exchanger interfaces with the sodium. In the event 
a large leak should occur at this interface allowing contact between the 
sodium and water, a rather violent almost explosive-like reaction can 
occur which would generate a large pressure shock wave. 
Because sodium is highly corrosive and is maintained at temperatures in the 
range of 400.degree.-600.degree. C., many conventional shock absorbing or 
energy-dissipating devices prove ineffective or are not usable. For 
example, a rupturable disc is frequently used in a T-connection off the 
sodium piping system to separate the system from a secondary or reaction 
products handling system. A reverse-buckling thin spherical shell is 
located in the T-connection with its convex side subjected to the fluid 
system, and a cutting-knife setup is placed immediately near the concave 
side of the disc. Upon the occurrence of a sufficiently intense pressure 
pulse in the sodium system, the disc is reverse flexed and ruptured, and 
the sodium can escape through the ruptured disc into the secondary system. 
This reduces the overall pressure of the sodium in the piping system, and 
also reduces the transmitted pulse, both in magnitude and duration. Even 
so, the transmitted pulse can cause significant damage to the piping 
system unless the system is strengthened and reinforced to take the 
increased loads. 
One major drawback to any diversion of sodium from the piping system (by 
rupture disc actuation) is the reduced capacity for cooling the reactor. 
Moreover, the rupturable disc system cannot distinguish between a shock 
wave generated by a sodium-water reaction and one generated by a severe 
seismic event. Consequently, the disc must be sized to withstand seismic 
events of probable intensity, which thereby limits the sensitivity of the 
system. Another drawback is that the ruptured disc must be replaced and 
the sodium that has been diverted into the secondary system must be pumped 
back into the piping system. 
Another commonly used shock absorber or pressure suppressing device is a 
surge tank connected by a tee off the main liquid line. The surge tank can 
be formed with a piston movable in a cylinder to expand and accept the 
diverted liquid, or the tank can have a pressurized gas overspace that is 
compressed in accepting the diverted liquid. Because of the reflection of 
the energy collected in the surge tank, the capacity to dissipate shock 
energy is limited as the energy basically is commonly returned back to the 
system after some delay. This system, however, can attenuate the intensity 
of the pressure, and moreover has appeal over the rupturable disc system 
in that it need not be replaced once it has been activated. Also, the 
accumulated liquid can be pumped or drained by gravity back into the main 
piping system after the pressure surge has been dissipated. 
Another type of shock or energy absorber commonly used in some liquid 
piping systems is an expandable rubber membrane formed off a tee in the 
piping system, which retains the liquid at one volume when the liquid is 
under stabilized pressure conditions but which increases in volume upon a 
surge of pressure to dissipate or absorb some of the shock wave energy. 
However, the energy absorbed by the rubber membrane is also stored in the 
membrane so that once the pressure wave has passed, the energy is released 
back into the piping system. Of even greater importance, the rubber 
membrane cannot function at the temperature and pressure limits 
(400.degree.-600.degree. C. and 100-200 psi) of molten sodium, and thus 
would be impractical in the reactor cooling conditions. 
This same deformable absorber concept is also employed in some applications 
as a sealed hollow collapsible rubber tube sized smaller than and located 
within the piping system. The sealed tube is thereby collapsed upon a 
pressure surge. However, this device, being internal, has limited capacity 
and impedes normal liquid flow through the piping system. Furthermore, as 
previously noted, rubber cannot be used in the high temperature and 
corrosive environments of molten sodium. 
SUMMARY OF INVENTION 
This invention relates to an absorber device for dissipating pressure 
pulses or shock waves generated in a piping system carrying liquid, the 
device being usable with liquids of a corrosive nature and at high 
temperatures and pressures. 
A basic object of this invention is to provide a shock wave absorber for 
incorporating into a piping system carrying liquid normally at a generally 
uniform pressure but potentially subject to pressure pulses of possibly 
many times this, the shock wave absorber being effective to dissipate or 
absorb large percentages (of the order up to and even exceeding 50%) of 
the energy level of the pressure surge. 
A specific object of this invention is to provide a pressure pulse shock 
wave absorber that is usable for repeated or sequential pressure shock 
waves automatically and without the need for any structural manipulation, 
adjustment or replacement. 
This invention provides a shock wave absorber to be located in series in a 
piping system line and thereby defining part of the normal liquid flow 
path, the absorber having a pressure confining housing larger than the 
normal flow path. An apertured plate is disposed between the flow path and 
a pressure cavity defined also in the housing, the plate extending a 
distance up to several diameters of the piping system line. Means 
adjustably pressurizes the cavity with a gas so as to maintain the liquid 
surface in the cavity with an overlying relatively large volume of the gas 
itself. Any pressure pulse in the absorber flow path, in the form 
specifically of a shock wave travelling through the piping system, forces 
coolant as small jets through the apertured plate, thereby attenuating the 
pressure intensity and also absorbing energy from the shock wave.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a typical three-dimensional piping system or network 10 
that might be used in conveying a liquid, the piping system having a 
plurality of runs 11, 13, 15, 17, 19 and 21 separated by bends 12, 14, 16, 
18 and 20 respectively. The illustrated three-dimensional network is 
symbolic only but it can be supposed that runs 11, 15, or 17 might extend 
parallel to the drawing sheet; runs 13 and 19 might extend normal to the 
drawing sheet whereby bend 12, 14 and 18 are formed over 90.degree.; and 
run 21 might be angled both from the horizontal and vertical and be angled 
relative to the drawing sheet also. The network 10 is also presupposed to 
have liquid flow as indicated by the arrow, so each pipe run would have an 
upstream end and a downstream end. Each pipe run also is of circular cross 
section although this is not material to the invention. Hangers 23, 24, 
25, 26, 27 and 28 are shown suspending the piping system 10 from 
indefinite structural anchors 30. The hangers must be capable of 
withstanding a seismic event, whereupon the piping network can be 
laterally loaded, and thus must be reinforced to provide for such strength 
and rigidity. 
As illustrated, it can be envisioned further that if a pressure pulse 
enters at the upstream end of the run 11, it would be transmitted as a 
shock wave axially along the run 11 to slam into the bend 12, thereby 
exerting a lateral load on the adjacent hangers 24 and 25. At least part 
of the shock wave further would be transmitted axially through pipe run 13 
to also slam against the bend 14, straining the hangers 25 and 26. These 
loads can induce movement of the piping system 10 relative to the fixed 
structural anchors 30, which could also build up in intensity should any 
harmonic frequencies of the hangers, etc., be encountered. In order to 
reduce such movement, elaborate snubbers (not shown) may be required to 
absorb the energy of impact. These snubbers can be quite expensive when 
they are sized to withstand the pressure shock waves, generated by a 
sodium-water interaction, or by a seismic event; but the former is 
generally of larger overall magnitude and is thus design limiting in a 
liquid metal cooled reactor. 
This invention thus becomes important in minimizing the added costs for any 
piping system against the adverse effects of high energy pressure pulses. 
In the illustrated example, piping runs 11 and 19 are interrupted and a 
shock wave absorber device 35, 36 is positioned serially in each. 
Referring now to FIG. 2, one embodiment of a typical shock wave absorber 
35-36 (identified in the singular as 35) is seen to include a T-shaped 
housing 38 defining a through flow section 39 and a stem or stub section 
40 vertically offset therefrom and extended normal thereto. Flanges 41 are 
appropriately located at the ends of the housing flow through section 39 
to allow connection respectively with complementary flanges 44 of the 
piping system itself. Flange 46 provided on the housing stub section 40 
allows for the connection thereto by flanges 47 of housing 48; and a 
pressure transducer or apertured separator plate 50 is interposed between 
the flanges 46 and 47. 
For sake of explanation, liquid flow in the piping system (at run 11, for 
example) is assumed to be in the direction indicated by the arrow 51, and 
the liquid would fill into the housing stub section 40 until the gas 
pressure in chamber 53 within the housing 48 balanced the pressure of the 
liquid in the piping system. In this regard, a level indicating control 54 
would be used to keep the liquid surface 56 at some specific predetermined 
height above the apertured plate 50 and within the housing 48, while yet 
having a relatively large overlying volume of gas, by regulation of the 
gas pressure within the chamber 53. A neutral gas (such as nitrogen or 
argon) would be used, admitted into or discharged from the chamber 53 via 
valve control 58 from a pressurized gas source 60. The height of the 
liquid surface 56 above the apertured separator plate 50 would normally be 
set anywhere between approximately 0.5" minimum and 6" maximum, depending 
on the size of the entire unit, the pressure of the liquid contained in 
the piping system, and the anticipated pressure and character of the shock 
wave that might occur in the piping system. 
Specifically referring to FIGS. 3 and 4, several potential hole arrays for 
the apertured plate are indicated; FIG. 3 for example, showing the plate 
50 having only seven holes 62 disposed in a somewhat hexagonal 
configuration, and FIG. 4 showing alternate plate 50a having a much larger 
plurality of holes 62a disposed over a somewhat squared configuration. The 
openings near the plate periphery for receipt of bolts (not shown) to hold 
the flanges together and the plate therebetween are not illustrated for 
clarity of disclosure. Also, gaskets or seal welds typically used at the 
interfaces of the flanges or plate are not shown. 
The intended range of open area in the apertured plate 50 (or porosity as 
the same will be termed hereinafter) is of the order between 0.05 and 0.60 
of the total area of the plate. The particular arrangement can vary; such 
as by having a large number of small openings (FIG. 4) or fewer openings 
each of relatively larger size (FIG. 3). In some applications, a single 
hole may be quite adequate. Within limits, the manner of how the porosity 
is obtained is not of too much importance to the basic invention. However 
the degree of porosity, as will be discussed later, can be adjusted to 
control reflections of the shock wave from the confinement in the pressure 
chamber 53 back through the plate, and thereby affect the performance of 
the device. 
In the shock wave absorber illustration in FIG. 2, the apertured plate 50 
is located at the terminal end of the transverse stub section 40, offset 
by possibly two or more diameters of the flow through section 39, from the 
walls of the piping system runs or the normal peripheral edge at line 64 
of the liquid flow through the shock absorber device 35. This general 
configuration is termed hereinafter as a "stub" shock absorber device. 
FIG. 5 on the other hand illustrates a shock wave absorber 135 in most 
respects identical to that illustrated in FIG. 2 except that the effective 
stem or stub section 140 of the housing 138 is quite small, even less than 
one diameter of the flow through section 139, without unduly disrupting 
the liquid flow in the through section. Accordingly, apertured plate 150 
with its holes 162 is offset very little from the walls of the piping 
system runs or the normal peripheral flow line 164 of the main through 
housing 139 of the shock wave absorber 135. 
The remaining components, including the level control 154, 158, 160 for 
maintaining the liquid surface 156 spaced above apertured separator plate 
150 and within the housing 148 by adjusting the gas pressure in space 153 
would in all respects be identical to the embodiment of FIG. 2. The stub 
sections 40 and 140 might typically be formed as tubular extensions 
disposed transverse to and the same size as the through flow section 39 
and 139 respectively so that each opens to the through flow section along 
approximately one diameter of axial length; although it would be possible 
also to flatten out the stub sections to open to the through flow section 
along an axial length of possibly several diameters of axial length. 
A further embodiment of a shock wave absorber 235 is illustrated in FIG. 6 
and 7. In this embodiment, the absorber housing is bifurcated including a 
lower half section 232 and an upper half section 233 which are connected 
together at matched flanges 234 and 236. The housing 235 is connected also 
by flanges 241 and 244 into the run 211 of the piping system 210, while 
the adjacent walls 245 and 250 of the lower and upper half sections 
respectively, match up to define a main flow through section 239 that 
complements and is axially in line with the upstream and downstream 
sections of the piping system itself. Note that the upper section wall 250 
has a plurality of openings 262 formed therein similar to the openings 
previously noted in the apertured plate 50. Further, the upper half 
section has imperforate outer walls 241 and 243 which define thereby a 
sealed chamber above the perforated wall 250 and open via conduit 251 to 
housing chamber 253. Thus, the upper half section 233 is gas tight and 
further the liquid surfaces 256 can be maintained therein above the 
apertured wall 250 by appropriate level sensing control 254, 258 and 260. 
Under normal operating conditions, the liquid level or surface (56 in FIG. 
2, 156 in FIG. 5 and 256 in FIG. 6) in the respective shock wave absorber 
device is at an elevation above or opposite the apertured plate (50, 150 
and 250, respectively) from the normal liquid flow through section of the 
shock wave absorber device. The gas space or attenuation region (53, 153 
and 253) maintained above the surface of the liquid likewise would be of 
sufficient volume, relative to any minor vertical fluctuations in the 
liquid level, that such fluctuations do not of themselves appreciably 
change the pressure in the gas space. The shock wave absorber device would 
normally be interposed, as noted, in a pipe run of a piping system where a 
shock wave might normally be generated in the piping system. This might 
occur, for example, in a nuclear reactor cooling system where sodium might 
under failure conditions interface with water, thereby generating a 
chemical reaction that would generate pressure shock waves travelling 
through the piping system at the sonic velocity. 
One basic characteristic of the travelling shock wave is that it produces a 
rapid increase in pressure compared to the pressure of the liquid under 
normal flow conditions and before the occurrence of the shock wave. The 
high pressures of the shock wave at the apertured plate, induce a liquid 
transfer through the apertured plate in the form of high velocity jets. 
The jets dissipate a large percentage of the energy of the shock wave, in 
the conversion between pressure and kinetic energies and ultimately in the 
generation of heat, and without appreciably transferring a large quantity 
of the liquid itself. Thus, the pressure in the gas space is not 
appreciably increased because of the influx of the liquid jets. The jets 
beyond the apertured plate are dissipated initially in the liquid 
proximate the apertured plate and further may be atomized into the gas 
space overlying the liquid surface. 
Specifically, the travelling wave is divided at the shock wave absorber 
device into an axial through flow portion and a lateral (vertical) 
portion. The pressure pulse diverted laterally is passed through the 
apertured plate and is either totally or at least partially absorbed 
behind the apertured plate. When the divided pulse is absorbed only 
partially, the remainder of the energy reflects back to the main flow 
section of the piping system. However, the returning pulse can negate the 
intensity of pressure in the primary shock wave if such sonic reflections 
are in opposite phase with the pressure pulse passing through the shock 
wave absorber device. In this regard, the offset of the apertured plate 
relative to the main flow of the piping system can be of some significance 
since a small offset assures prompt interaction by the reflective wave. 
In this regard, large negative wave reflections can occur in conventional 
surge tanks or accumulators or the like, such that although the initial 
effect may be to reduce the pressure of the travelling wave, additional 
pressure pulses may be reintroduced into the piping system at a later time 
or at a different location in the system. The apertured plate devices 
disclosed herein not only reduce the peaks of such wave generations but 
also most typically reduce the overall energy in the wave. 
In the stub tube type of shock wave absorber (FIG. 2) about half the shock 
wave energy passes through the main flow path and the remainder is 
diverted into the stub tube to the apertured plate. With the aperture 
plate porosity being in the low 3 to 5% range, nearly all of this diverted 
energy can be absorbed. With near total absorption, there will be no 
significant reflections (either positive or negative) to affect the rear 
portion of the primary pulse as it traverses through the device. If the 
porosity of the plate is too low, a positive reflection could occur to 
increase the pressure levels of the trailing portion of the primary pulse. 
If the porosity is too large, a negative reflection could occur to 
attenuate the trailing portion of the primary pulse and produce some 
negative pressure pulse waves in the piping system. Thus, the advantage of 
the absorber device of FIG. 2 is that with a low porosity nonreflective 
apertured plate the degree of energy absorption and pressure pulse 
attentuation is well defined, i.e., about half the energy will be absorbed 
and half transmitted. The overall effect is an approximate 33 % reduction 
in the amplitude of the transmitted pulse. 
In the reentrant type of shock wave absorber shown in FIG. 5, the offset 
between the through flow path and apertured plate is minimized to maximize 
the interaction of the reflected wave, and the porosity of the plate is 
increased such that the reflected negative wave will be just large enough 
to cancel out all or nearly all of the primary pulse traversing the 
device. This may require aperture plate porosities up to approximately 60% 
for the conditions encountered in liquid metal piping systems. The actual 
porosity selected depends on the size and duration of the primary pulses 
to be absorbed. The advantage of the reentrant high porosity FIG. 5 
absorber device is that it can produce a nearly 100%, instead of only a 
33% reduction in the transmitted pressure pulse. The main disadvantage is 
that the shock wave pulses of extended duration may require a larger gas 
space since the greater apertured plate porosity will pressurize the gas 
space much more rapidly than would be the case with a low porosity 
apertured plate more commonly used in the FIG. 2 stub tube absorber 
device. 
The shock wave absorber in FIG. 6 has further increased the effectiveness 
in attenuating the energy, because of the increased area of the apertured 
plate axially along the through flow section of the piping system. Under 
such a structural configuration, the apertured plate may extend several 
and possibly even up to 10 or 15 diameters of the through flow section. 
In a liquid metal cooled reactor application, the preferred range of plate 
porosity would be between perhaps 1% and 60%. The greater porosities tend 
to introduce negative reflections out of confinement of the sealed gas 
space and back through the apertured plate. This however may not be 
desirable depending on the strength and duration of the expected primary 
pulses. The disclosed shock wave absorbers are especially suited in piping 
systems where the operating pressures are relatively high where they can 
suppress or attenuate the wave pressures without producing undesirably 
large negative waves. 
The porosity of the apertured plate needed to properly control the negative 
wave reflection depends on the pressure of the liquid normally, the 
intensity of the shock wave itself, the viscosity of the liquid and the 
speed of the shock wave. Normally, there will be some reflection at the 
apertured plate, since the shock wave itself will not be fixed in pressure 
intensity but will vary depending upon its cause as well as its distance 
axially along the piping system from the cause. When the apertured plate 
has a relatively high porosity (greater than 35% or the like) the percent 
of energy reflected as a negative pulse will be high, reducing the 
intensity of the pulse transmitted down the line. However this reflected 
negative wave can under some circumstances be undesirable since negative 
pulses can also cause excitation of the piping system. 
Thus the portion of the shock wave impacting the apertured plate is subject 
to zero, positive, or negative reflection of energy depending on the 
porosity of the plate itself. In a zero reflection shock wave absorber, 
the portion of the pressure wave itself that is absorbed by the device 
approaches 1/3 of the total amplitude of the wave. This means that the 
shock wave absorber is effective to allow only 2/3 of the shock wave 
amplitude to pass through the device. If the reflection is positive, the 
latter portions of the transmitted pulses will be augmented by the 
reflective shock wave to create a minor pulsed surge or increase in such 
pressure. If the reflection is negative, the latter portions of the 
transmitted pulse will be reduced or negated by the reflective energy 
levels. In virtually all cases however, the lead edge of the travelling 
wave will have its pressure energy substantially reduced by the apertured 
plate shock wave absorber. Moreover, the duration of the reflective or 
precursor pulse can be reduced to zero or almost zero by reducing the 
offset distance to the orifice plate as is noted in the reentrant designs 
of FIGS. 5 and 6. Properly designed, shock wave absorber serves to spread 
out the duration and to reduce the peak pressure of any portion of the 
shock wave that is passed through the absorber. This mitigates the adverse 
effects of such shock waves. 
By way of example in a shock wave absorber device used in a sodium line in 
a nuclear reactor cooling system, the diameters of each aperture or 
orifice might be between 1/8 and 1/2 of an inch, depending upon the 
overall size of the shock wave absorber device, and the pressure levels 
and viscosity of the liquid in question. The jets may produce coolant 
velocities of between 200 and 2000 ft./sec., depending on the differential 
intensity of the shock wave and its duration.