Control apparatus for pressurized gas/liquid systems

Control apparatus for purging the pressure in conduits or hoses of pressurized gas/liquid systems such as hot melt foam generating equipment. The control includes a selectively actuable "purge valve" which upon actuation applies an output pressure to reverse the direction of rotation of the motor driving the gas/liquid mixing pump, or alternatively to operate a diverter valve that connects the pump outlet side back to the liquid supply line. Simultaneously, the purge valve cuts off the supply of gas to the mixing pump. The control desirably also includes a selectively actuable "start-up valve" for supplying gas to the gas/liquid mixing pump at higher than normal operating pressure, in order to overcome any internal blockage and prime the pump for starting operation.

This invention relates to means for controlling pressurized gas/liquid 
systems. More particularly, it relates to control apparatus for purging 
the pressure in pressurized conduits containing gas/liquid mixtures, as in 
hot melt adhesive foam generating equipment, and for providing increased 
pressures for start up of such systems. 
BACKGROUND OF THE INVENTION 
As taught in Scholl et al U.S. Pat. No. 4,059,714, issued Nov. 22, 1977, 
entitled "Hot Melt Thermoplastic Adhesive Foam System", in many instances 
it is useful to apply hot melt adhesives in foam condition. Such foams can 
be generated by dispersing a gas into liquid hot melt, for example by a 
gear pump, to form a gas/hot melt mixture in which the gas may be in true 
solution in the liquid, or it may be dispersed as tiny microbubbles. The 
gas/liquid mixture is conveyed under pressure from the mixing apparatus 
through a pressurized conduit, line or hose to a valved dispenser, which 
may be a gun or a foam dispensing head. Upon dispensing to atmosphere the 
pressure is released and the gas expands within the liquid to create the 
foam. A pressurized return or recycle line is usually provided to 
recirculate the mixture back through the pump, when the dispenser is not 
dispensing it. In such systems the gas/liquid mixture may be quite hot, 
and under substantial pressure in the lines. For example, in a hot melt 
foam generating system the temperature may be on the order of 175.degree. 
F. to 350.degree. F. for most hot melt adhesives, and the pressure is 
often on the order of 200 to 500 psi and may be as high as 1500 psi. 
It is occasionally necessary to disconnect the delivery and/or return 
conduits between the pump and the valved foam dispensing head, for example 
to change or clean the head, replace a worn hose, or for cleaning or 
servicing and the like. In the past, disconnection of the pressurized 
conduits in systems containing such gas/liquid mixtures has been a slow, 
difficult and to some extent even dangerous operation, while the 
gas/liquid mixture in the conduits is under substantial pressure. The 
conduits act as accumulators because of the pressurized gas they contain. 
If that pressure is released abruptly, as when a threaded hose coupling is 
opened, it expels the mixture rapidly, even violently when the pressure is 
high, and the mixture blows out of the conduit. (In this respect 
pressurized gas/liquid systems differ from hydraulic systems, in which the 
liquid pressure drops very rapidly upon opening a conduit, with little 
discharge of viscous contents when the pumping means is deenergized.) 
Because of the heat content, uncontrolled discharge of hot melt adhesive 
is a safety hazard and unacceptable in an industrial environment. 
Moreover, because of its high viscosity, the upstream pressure in the 
conduit drops slowly, and thereby prolongs discharge of the material. 
Prior to this invention, it was difficult to rapidly and safely release or 
"purge" the pressure in pressurized gas/liquid conduits, especially if the 
liquid contains a large quantity of gas or if the gas has a tendency to 
remain in admixture with the liquid, as is the case with hot melt 
adhesives. If a valved outlet is placed in the line and is opened to 
atmosphere to release the pressure, the material will eject through that 
valve as a foam around it, which is difficult to handle; moreover, even 
though the pump is not running, high viscosity of the liquid results in 
only a slow pressure drop so that material continues to exude for some 
time. One end of a long hose may still be at substantially higher pressure 
even though the other end is open. In any event, the "blow out" of 
material from the hose is wasteful and messy. In practice, this 
persistence of pressure in a hose, even after the pump has been stopped, 
has made disconnection of pressure conduits slow, difficult and dangerous. 
Thus, substantial need has existed for a means of purging the pressure in 
a controlled, rapid, and safe manner, so that the conduit may be 
disconnected. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention is directed to a selectively operable control 
apparatus whereby pressure in the system may be purged quickly, safely and 
without waste of material. The control apparatus includes a selectively 
actuable "purge valve" which, when actuated, operates valve means in the 
system to purge pressure. At the same time, operation of the purge valve 
cuts off the supply of gas from the gas supply to the mixing pump and 
thereby terminates further generation of gas/liquid mixture. In the 
preferred embodiment, actuation of the purge valve applies a pressure 
signal that operates a reversing valve to reverse the direction of flow 
through the system. The control apparatus preferably also includes a 
pressure reducing valve and a selectively actuable start-up valve for 
bypassing the pressure reducing valve in order to supply gas from the gas 
source to the mixing pump without the reduction in pressure that would 
otherwise be established by the pressure reducing valve. This provides 
super-charging or priming of the mixing pump with higher density gas in 
order to overcome any initial blockage and to start the flow of gas into 
the system. 
In the preferred embodiment, the pressure signal from the purge valve is 
applied to actuate a four-way reversing valve which reverses the flow of 
operating fluid through an air motor that drives the gas/liquid mixing 
pump. Reversal of operation of the air motor reverses the direction of 
fluid flow in the system, and fluid is drawn from the pressurized conduits 
and pumped back to the liquid source where it is exposed to atmospheric 
pressure, so that the foam dissipates. In an alternative embodiment of the 
invention, the pressure signal from the purge valve is applied to operate 
a diverter valve in the system which releases pressure in the lines 
directly back to tank, without reverse pumping.

In the drawings and in this explanation, the invention is described with 
particular reference to use with hot melt foam systems. However, it is 
noted more generally that the control apparatus of this invention is 
useful with other systems wherein gas/liquid mixtures are pumped under 
pressure and wherein it is desirable to purge the pressure, including for 
example systems for applying foam coatings. 
In FIG. 1 the preferred form of control apparatus is designated generally 
by 7. The control apparatus includes a gas supply line 10 which in use is 
connected to a gas source that may comprise a gas cylinder, a compressed 
air system, or the like. The gas supply usually will include a pressure 
regulator valve 11 such as the pilot operated spring biased valve shown. 
This valve is set to maintain a desired gas pressure from the supply. 
Gas line 10 of control 7 includes an off-on valve 12 which may be solenoid 
operated. This valve is desirably interconnected with the electrical 
control system for the hot melt liquid supply, so that it is opened only 
when the hot melt supply pump is energized. 
Downstream in the direction of gas flow from off-on valve 12 the gas line 
10 is connected to a selectively operable pressure purge valve 13. For 
normal foam generating operation valve 13 is biased to connect line 10 to 
a second pressure reducing or regulating valve 14 which controls the 
pressure of the gas being delivered to the pump which in turn determines 
the density of the foam. Valve 13 may be spring biased to this position, 
as shown. Upon selective actuation, as indicated by the arrow shown at the 
right of valve 13, that valve disconnects the gas source from valve 14 and 
connects the gas source to a pilot pressure line 20. 
In the preferred mode of use shown in FIG. 1, pilot pressure line 20 is 
connected to operate valve means 21 for reversing the direction of 
rotation of the pump 23 which pressurizes the gas/hot melt or other liquid 
mixture. In the embodiment shown, actuation of purge valve 13 from its 
normal position reverses the direction of pump 23 by reversing the 
direction of rotation of the air motor 24 which drives pump 23. Pump 23 is 
driven by air motor 24 which is supplied with air under pressure from a 
source indicated at "air in", through a reversing valve 21 which is 
responsive to the actuation of purge valve 13. The reversing valve 21 is 
desirably a four-way valve which upon actuation reverses the application 
of pressure between the motor inlet and exhaust lines 25 and 26. Biasing 
means such as the spring illustrated urges reversing valve 21 to a first 
position such that air from the pressure source is supplied through line 
25 to air motor 24, to cause it to rotate in a first direction of 
rotation. The air motor exhaust passes through line 26 and valve 21. When 
purge valve 13 is selectively actuated, the gas pressure from line 10 of 
control apparatus 7 is applied as a pilot pressure through line 20 to 
actuate reversing valve 21 against the biasing spring, so that the 
operating pressure is supplied to motor 24 through line 26 and exhaust air 
is relieved through line 25. This reversal of the connections to the air 
motor causes it to reverse its direction of rotation, and in turn drives 
the pump 23 in its opposite direction. 
Returning to the description of control apparatus 7, the foam density 
control valve 14 comprises an adjustable pressure regulating valve which 
may be a pilot pressure operated, spring biased pressure relief valve. A 
gauge 27 indicates this pressure on the downstream side of valve 14. In 
use, valve 14 is set to establish a desired gas pressure which is lower 
than valve 11 at the source. This valve controls pressure of the gas going 
to pump 23, and thus controls the density of the gas which in turn 
determines the density of the foam which is generated when the gas/liquid 
mixture is dispensed. 
Downstream of valve 14, gas conduit 10 is connected to a start-up valve 29 
which on actuation functions to bypass purge valve 13 and density 
regulator 14. In the normal operating position shown, start-up valve 29 
connects the outlet of valve 14 to pump 23; when actuated, for example 
manually or by a solenoid, start-up valve 29 establishes a bypass around 
density control valve 14, through a bypass line 30 that leads from conduit 
10 upstream of density regulator 14 and also preferably upstream of 
pressure purge valve 13 as shown. This bypass in effect avoids the 
reduction in gas pressure that would otherwise be established at valve 14; 
it delivers gas at the higher pressure established by gas source pressure 
regulator 11. A brief application to pump 23 of such higher pressure is 
desirable to start flow of gas into the liquid stream into which it is to 
be mixed by the pump. An indicator 33 measures the gas flow in line 10, 
downstream of an adjustable restrictor 31. The gas is supplied to pump 23 
through a check valve 34, which permits flow of gas in conduit 10 toward, 
but not hot melt flow from, the pump. 
The control 7 is useful with single stage pumps as well as multiple stage 
pumps. In the embodiment illustrated, pump 23 is a two-stage pump of the 
specific type disclosed in the copending application of Akers and Scholl, 
Ser. No. 874,333, filed Feb. 1, 1978 titled "Hot Melt Adhesive Foam 
System," now U.S. Pat. No. 4,200,207 assigned to the same assignee as this 
application. It should be understood, however, that the invention is not 
limited to that particular pump, or to two-stage pumps in general, and 
that it may be used with single stage pumps. The invention can also be 
used with electric motor driven pumps. 
Briefly, pump 23 as shown includes a first stage gear metering pump 40 
which meters the hot melt or other liquid to a second stage gear mixing 
pump 41. The gas is introduced into the liquid stream between the two 
stages, as described more fully in the Akers and Scholl application, the 
disclosure of which is incorporated herein by reference. Second stage pump 
41 has larger capacity than the first stage pump, in order to accommodate 
the additional volume of the gas. Air motor 24 drives the first stage 
through suitable drive means, as indicated schematically by the dashed 
line 42. The gears of the two stages are preferably coupled to rotate in 
synchronism, as indicated by the dashed lines at 43. 
The hot melt or other liquid to be mixed with the gas is supplied from a 
source 45 which may comprise a conventional hot melt supply or the like, 
for example as shown in U.S. Pat. No. 4,059,714, previously referred to. 
The hot melt is delivered from source 45 through a line 46, to the gear 
chamber of first stage pump 40, through an inlet port 47. For normal pump 
operation, i.e., to pump the liquid to the second stage 41, the gears of 
the first stage are driven by the motor 24 in the direction of rotation 
indicated by the arrows 51. In such operation the gear teeth are just 
coming out of engagement at liquid inlet 47. The rotating gears convey the 
liquid to the first stage pump outlet 52, from which it flows to the inlet 
53 of second stage pump 41. Gas from line 10 is introduced through a gas 
inlet port 54. In the second stage 41, the gas and liquid are mixed and 
the gas may actually be dissolved in the liquid. The second stage outlet 
side 58 may include a filter as at 59 for removing any entrained solid 
particles from the mixed, pressurized gas/liquid stream. (The filter may 
be as shown in U.S. Pat. No. 3,224,590, issued Dec. 21, 1964.) A heated 
pressure hose 67 connects the second stage pump outlet to valved dispenser 
50 from a manifold 63. The valved dispenser 50 may be a conventional fixed 
dispensing head, for example as taught in Baker et al reissue patent No. 
Re 27,865, reissued Jan. 1, 1974, titled "Applicator Having A Fixed Module 
With Static Parts And A Removable Module With Moving Parts," or it may be 
a hand-held foam gun. This hose or conduit 67 is connected at one end to 
the pump outlet and at the other end to the dispenser by releasable 
couplings, as at 68 and 69. 
A recirculation passage 60 including a circulation valve in the form of an 
adjustable orifice 61 leads from valved dispenser 50 back to the first 
stage pump inlet 47. The pump output stream is recirculated from dispenser 
50 back to the pump when the dispenser is closed and is not dispensing the 
mixture. A suitable construction for recirculation valve 61 is disclosed 
in Akers et al application Ser. No. 874,333, previously referred to. A 
bypass line 64 is connected internally in the manifold 63 between second 
stage pump outlet 58 and passage 60, and includes a check valve 65. 
Passage 60 will usually include a pressure recirculation hose 62 similar 
to supply hose 67, with disconnectable couplings as at 71 and 72. 
While it is operating, the pump 23 maintains the gas/liquid mixture under 
pressure in the hoses 67 and 62; and even when the pump is not operating, 
the pressure in the hoses decays only very slowly. If any hose or the 
valved dispenser is to be uncoupled, the hose or pipes coupled to it must 
be opened and, even though the pump has been stopped, pressure may persist 
for an undesirably long time. Because of the gas content, the entire 
pressurized system acts as an accumulator and, when a coupling 68, 69, 71 
or 72 is opened, this pressure causes the fluid to spew out. 
The control apparatus 7 of this invention enables the pressure in the 
connecting hoses or conduits to be purged quickly and safely. In the 
preferred mode of utilizing the control 7, this is accomplished by cutting 
off the supply of gas to the pump gas inlet 54 and simultaneously 
reversing the direction of rotation of the air motor 24 and thereby 
reversing the direction of operation of pump 23. Such reversal causes 
movement of the gas/liquid mixture through the system in the direction 
opposite to the normal direction of flow; tests have established that this 
procedure rapidly relieves the pressure, for example, in 2-5 minutes in a 
hot melt foam system of the type described. 
More specifically, when it is desired to purge the pressure, purge valve 13 
is actuated, manually or mechanically. This cuts off the admission of gas 
through line 10 to the hot melt at the pump and instead diverts the gas 
pressure from the source as a pilot pressure signal through pilot line 20 
to reversing valve 21, which it shifts. This actuation of the reversing 
valve reverses the direction of rotation of air motor 24 and drives pump 
23 in the opposite direction from normal. 
The normal outlet sides of both the first and the second stage pumps now 
become the respective inlet sides. Pump 41 delivers the gas/liquid mixture 
from port 58 to port 53 and back through pump 40. This fluid already 
contains the gas and the liquid, and apparently the gas is not dissociated 
from the liquid by the reverse flow through the pump, but is allowed to 
expand. Since the dispenser 50 is closed, it is believed that the 
pre-existing pressure in hoses 62 and 67 causes the mixture to be drawn 
into pump 41. The fluid delivered to port 47 by reverse operation of pump 
40 returns to tank through line 46, and the hoses are thereby 
substantially drained so that pressure is then reduced to atmospheric 
pressure or even below. In the embodiment shown, wherein pump 23 is that 
shown in the previously mentioned Akers et al patent application Ser. No. 
874,333, pump 40 preferentially directs the fluid to the hot melt source 
45 rather than recirculating it reversely through line 60, since the 
opening to the recirculation line (illustrated at 86 in FIG. 2 of the 
Akers et al application) is relatively restricted as compared to the 
opening to the source. In actual practice with gas/hot melt mixtures, it 
is found that recirculation line 60 is in fact substantially drained by 
the purging procedure, indicating that the flow is directed toward the 
source rather than being drawn back through line 60. 
After a short period of time, which can readily be established for a given 
system and material and which may be of the order of roughly about 
two-five minutes for a typical hot melt, the conduit pressure has been 
purged to a tolerably low level. The purge valve may then be released or 
reset to normal "run" position. Thus, in comparison to the previous 
practice which has provided no safe and easy way of releasing system 
pressures, this invention enables the system and particularly the conduits 
or hoses to be depressurized simply, safely, and quickly, merely by 
operating the purge valve 13. 
Moreover, by reason of the reverse flow established in hose 67 during 
purging, fluid flows reversely through filter 59, thereby back flushing it 
and dislodging any solid particles collected on it. Once removed from the 
filter, it is desirable to drain such particles through the conventional 
manifold drain valve so that they are not thereafter recirculated in the 
system. 
As already indicated, apart from its use to purge pressure, control 7 can 
also be used to provide a higher-than-normal start-up pressure, to prime 
pump 41 with the gas. This is desirable where a viscous or solidified 
material in pump 41 blocks intake of gas through port 54, so that no gas 
can be injected into the liquid. To overcome such blockage, the control 
provides for selectively bypassing pressure reducing valve 14, to supply 
gas at the (higher) upstream source pressure. For this operation, at start 
up the liquid supply is energized to melt the adhesive. Solenoid valve 12 
is energized so that pressure is supplied from the gas source at regulated 
pressure into line 10 of the control 7. With pump 23 operating, actuation 
of start up or bypass valve 29 bypasses reducing valve 14 and supplies gas 
at the higher (unreduced) pressure from gas source 11. This higher 
pressure (which for example may be of the order of 40 psi for a hot melt 
system of the type shown) helps to overcome any internal blockage. Once 
the flow of gas through pump 41 has started (as manifested by delivery of 
foam through the gun), start-up valve 29 can be released to normal run 
position. Gas thereafter flows from the source at the normal operating 
reduced pressure established by the density regulator valve 14. 
FIG. 2 illustrates a second mode of connecting control 7 to the gas/liquid 
system. The gas/liquid system may be otherwise similar to that shown in 
FIG. 1, but the control pilot line 20 operates a diverter valve 80 in the 
system that directs the pump output from manifold 63 back to the liquid 
source 45 where the gas escapes as the liquid is recirculated. 
For this purpose a normally closed air pressure operated diverter valve 80 
is connected to return the flow from line 62 directly back to tank 
(source) via a line 81. This valve 80 may be similar to the valve 
designated generally at 50 in Baker et al Re. No. 27,865, to which 
reference may be had for a more complete description. Normally it is 
biased closed by a spring; when purge valve 13 is operated, its output 
pressure signal in line 20 is applied to shift the movable valve element 
of valve 80 and open a path for flow from manifold 63 directly to the 
return passage 81. Actuation of purge valve 13 simultaneously cuts off 
supply of gas to the mixing pump. The already mixed fluid is thus returned 
to the tank, where the gas dissipates. In this system the purging 
operation does not require reversal of the pump and no reversing valve is 
necessary; as can be seen, rather than operating a reversing valve, the 
pilot pressure signal from the pump valve operates the diverter valve 80, 
to release line pressures. 
Yet another mode of utilizing the invention requires neither pump reversal 
nor operation of a diverter valve. In this mode, actuation of the purge 
valve merely cuts off the admission of gas to the mixing pump. The 
adjustable circulation valve 61 is opened to present minimal pressure drop 
and the air motor is adjusted so that pump 23 is run at a slow rate. With 
no gas supplied to second stage pump 41 and gun 50 closed, the 
gas-containing mixture in the hoses 67 and 62 is displaced slowly by the 
incoming liquid which contains no gas. The displaced gas/liquid mixture 
from hose 67 is returned to tank via return recirculation line and supply 
line 46. In this case the system gradually changes from a gas/liquid 
mixture-containing system, to a hydraulic system which, as already noted, 
does not present the problem of persistent accumulated gas pressure. 
When the gas/liquid mixture is returned to tank, the gas separates from the 
liquid at a rate depending on the nature of the liquid. The foam tends to 
rise to the surface of the tank, and the liquid supplied to the pump 
through line 46 contains a diminishing quantity of gas. 
The foregoing disclosure illustrates various modes of using the control of 
this invention to dissipate or purge system pressures in gas/liquid 
systems, and to facilitate system start up. From the description it will 
be understood that the invention is not limited to the specific 
embodiments disclosed, but that the invention can be incorporated in other 
embodiments within the scope of the following claims.