Multiple air chamber foam pump

Abstract

An improved foam piston pump in which a multiple of air pumps are disposed coaxially about a piston member and spaced axially along the piston member, preferably coaxially with a liquid pump for discharging air and liquid as foam. Each air pump is preferably provided by a modular piston within a modular casing assembly such that arrangements with one, two or more identical air pumps can be assembled from a plurality of the same modular components.

We claim: 
     
       1. A piston pump comprising:
 a piston chamber-forming member ( 20 ) extending longitudinally about an axis ( 23 ) from an inner end ( 24 ) to an outer end ( 25 ); 
 the piston chamber-forming member ( 20 ) defining a central chamber ( 26 ) therein coaxially about the axis within an annular chamber wall ( 27 ); 
 the piston chamber-forming member having a liquid inlet ( 28 ) at the inner end (24) in communication with a liquid ( 14 ) in a reservoir ( 13 ); 
 a piston-forming element ( 22 ) coaxially slidably received within the chamber ( 26 ) in the piston chamber-forming member ( 20 ); 
 the piston-forming element ( 22 ) comprising an elongate tubular stem ( 90 ) with a central passageway ( 63 ) longitudinally therethrough, the passageway ( 63 ) extending from an inner end ( 134 ) of the passageway (63) to an outer end of the passageway (63); 
 the piston-forming element ( 22 ) coaxially slidable within the piston chamber-forming member ( 20 ) between an extended position and a retracted position in a cycle of operation comprising a withdrawal stroke and a retraction stroke to draw the liquid ( 14 ) from the reservoir ( 13 ) via the liquid inlet ( 28 ) and discharge the liquid mixed with air through the outer end of the passageway ( 63 ); 
 a liquid pump ( 70 ) formed between the piston chamber-forming member ( 20 ) and the piston-forming element ( 22 ) proximate the inner end  134  (24) of the piston chamber-forming member  22  (20), 
 the liquid pump ( 70 ) operative in the cycle of operation to draw the liquid from the reservoir ( 13 ) via the liquid inlet ( 28 ) and discharge the liquid into the passageway ( 63 ) proximate the inner end ( 134 ) of the passageway ( 63 ); 
 an inner air pump ( 170 ) formed between the piston chamber-forming member ( 20 ) and the piston-forming element ( 22 ) axially outwardly of the liquid pump ( 70 ) operative in the cycle of operation in a the withdrawal stroke to draw air from the atmosphere and in a the retraction stroke to discharge air into the passageway ( 63 ) through an inner air port ( 142 ) which extends radially inwardly through the stem ( 90 ) into the passageway ( 63 ); 
 a first outer air pump ( 270 ) formed between the piston chamber-forming member ( 20 ) and the piston-forming element ( 22 ) axially outwardly of the inner air pump ( 170 ), 
 the first outer air pump ( 270 ) operative in the cycle of operation in the withdrawal stroke to draw air from the atmosphere and in the retraction stroke to discharge air into the passageway ( 63 ) through a first outer air port ( 162 ) which extends radially inwardly through the stem ( 90 ) into the passageway  91  (63) at an axial location on the stem ( 90 ) spaced axially outwardly of the inner air port ( 142 ); 
 a first outer air sealing annular flange  203  (231) on the piston chamber-forming member ( 20 ) axially outwardly of the inner air pump ( 170 ), 
 the first outer air sealing annular flange  203  (231) extending from the chamber wall ( 47 ) radially inwardly to an annular distal edge ( 232 ) in engagement with a radially outwardly directed first outer cylindrical wall ( 152 ) on the stem  91  (90) axially inwardly of the first outer air port ( 162 ); 
 the annular distal edge ( 232 ) of the first outer air sealing annular flange  203  (231) engaging the first outer cylindrical wall ( 152 ) of the stem  91  (90) to prevent fluid flow axially inwardly therepast, 
 a first outer air sealing disc ( 256 ) on the stem  91  (90) axially outwardly of the first outer air port ( 162 ) and axially outwardly of the first outer air sealing annular flange  203  (231) on the piston chamber-forming member ( 20 ); 
 the first outer air sealing disc ( 256 ) extending radially outwardly from the stem  91  (90) to an annular distal edge ( 257 ) in engagement with a first cylindrical outer portion ( 210 ) of the chamber wall  47  (27) on the piston chamber-forming member  22  (20) axially outwardly of the first outer air sealing annular flange  203  (231); 
 the annular distal edge ( 257 ) of the first outer air sealing disc ( 256 ) engaging the first cylindrical outer portion ( 210 ) of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) to prevent fluid flow axially upwardly outwardly therepast; 
 the first outer air pump ( 270 ) having a first outer air compartment ( 271 ) open to the first outer air port ( 162 ) and defined (a) annularly between the stem ( 90 ) of the piston-forming element ( 22 ) and the first cylindrical outer portion ( 210 ) of the chamber wall  47  (27) of the piston chamber-forming member ( 20 ), and (b) axially between the first outer air sealing annular flange  203  (231) and the first outer air sealing disc ( 256 ); 
 in a the cycle of operation in the withdrawal stroke, an axial distance between the first outer air sealing annular flange  203  (231) and the first outer air sealing disc ( 256 ) increases thereby increasing a volume of the first outer air compartment ( 271 ) and drawing air into the first outer air compartment ( 271 ) and, in the refraction retraction stroke, the axial distance between the first outer air sealing annular flange  203  (231) and the first outer air sealing disc ( 256 ) decreases thereby decreasing the volume of the first outer air compartment ( 271 ) and discharging air from the first outer air compartment ( 271 ) through the first outer air port ( 162 ) into the passageway ( 63 ). 
 
     
     
       2. A piston pump as claimed in  claim 1  wherein:
 the liquid pump ( 70 ) operative in a the cycle of operation in the withdrawal stroke to draw the liquid from the reservoir ( 13 ) via the liquid inlet ( 28 ) and, in the retraction stroke, to discharge the liquid into the passageway ( 63 ) proximate the inner end ( 134 ) of the passageway ( 63 ). 
 
     
     
       3. A piston pump as claimed in  claim 1  including:
 a second outer air pump ( 370 ) formed between the piston chamber-forming member ( 20 ) and the piston-forming element ( 22 ) axially outwardly of the first outer air pump ( 270 ), 
 the second outer air pump ( 370 ) operative in the cycle of operation in the withdrawal stroke to draw air from the atmosphere and, in the retraction stroke, to discharge air into the passageway ( 63 ) through a second outer air port ( 262 ) which extends radially inwardly through the stem ( 90 ) into the passageway ( 63 ) at an axial location on the stem ( 90 ) spaced axially outwardly of the first outer air port ( 162 ); 
 a second outer air sealing annular flange  303  (331) on the piston chamber-forming member  20  above (20) axially outwardly of the first outer air pump ( 270 ), the second outer air sealing annular flange  203  (331) extending from the chamber wall  47  (27) radially inwardly to an annular distal edge ( 332 ) in engagement with a radially outwardly directed second outer cylindrical wall ( 252 ) on the stem ( 90 ) axially outwardly of the second outer air port ( 262 ); 
 the annular distal edge ( 332 ) of the second outer air sealing annular flange  303  (331) engaging the second outer cylindrical wall ( 252 ) of the stem ( 90 ) to prevent fluid flow axially inwardly therepast, 
 a second outer air sealing disc ( 356 ) on the stem ( 90 ) axially outwardly of the second outer air sealing annular flange  303  (331) on the piston chamber-forming member ( 20 ); 
 the second outer air sealing disc ( 356 ) extending radially outwardly from the stem ( 90 ) to an annular distal edge ( 357 ) in engagement with a second cylindrical outer portion ( 310 ) of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) axially outwardly of the second outer air sealing annular flange  303  (331); 
 the annular distal edge ( 357 ) of the second outer air sealing disc  256  (356) engaging the second cylindrical outer portion ( 310 ) of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) to prevent fluid flow axially outwardly therepast; 
 the second outer air pump ( 370 ) having a second outer air compartment ( 371 ) open to the second outer air port ( 262 ) and defined (a) annularly between the stem ( 90 ) of the piston-forming element ( 22 ) and the second cylindrical outer portion ( 310 ) of the chamber wall  47  (27) of the piston chamber-forming member ( 20 ), and (b) axially between the second outer air sealing annular flange  303  (331) and the second outer air sealing disc ( 356 ); 
 in a the cycle of operation in the withdrawal stroke, an axial distance between the second outer air sealing annular flange  303  (331) and the second outer air sealing disc ( 356 ) increases thereby increasing a volume of the second outer air compartment ( 371 ) and drawing air into the second outer air compartment ( 371 ) and, in the retraction stroke, the axial distance between the second outer air sealing annular flange  303  (331) and the second outer air sealing disc ( 356 ) decreases thereby decreasing the volume of the second outer air compartment ( 371 ) and discharging air from the second outer air compartment ( 371 ) through the second outer air port ( 262 ) into the passageway ( 63 ). 
 
     
     
       4. A piston pump as claimed in  claim 3  wherein:
 the first cylindrical outer portion ( 210 ) of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) extends axially between the first outer air sealing annular flange  203  (231) and the second outer air sealing annular flange  303  (331); and 
 the second cylindrical outer portion ( 310 ) of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) extends axially outwardly from the second outer air sealing annular flange  303  (331). 
 
     
     
       5. A piston pump as claimed in  claim 4  wherein:
 the first outer air sealing annular flange  203  engages the second cylindrical outer portion  210  of the chamber wall  47  on the piston chamber-forming member  20  axially outwardly of the first outer air sealing disc  256  on the stem  90  and axially inwardly of the second outer air sealing disc  356  on the stem  90 . 
 
     
     
       6. A piston pump as claimed in  claim 5  wherein:
 a cylindrical first inner portion  110  of the chamber wall  47  on the piston chamber-forming member  20  extends axially inwardly from the first outer air sealing annular flange  203 ; 
 a first inner air sealing disc  156  on the stem  90  axially inwardly of the first outwardly air sealing annular flange  203  on the piston chamber-forming member  20 ; 
 the first inner air sealing disc  156  extending radially outwardly from the stem  90  to an annular distal edge  157  in engagement with the cylindrical first inner portion  110  of the chamber wall  47  on the piston chamber-forming member  20  axially inwardly of the first outer air sealing annular flange  203 ; 
 the annular distal edge  157  of the first inner air sealing disc  156  engaging the first cylindrical inner portion  110  of the chamber wall  47  on the piston chamber-forming member  20  to prevent fluid flow axially upwardly therepast; 
 the inner air pump  170  having an inner air compartment  171  open to the inner air port  142  and defined (a) annularly between the stem  90  of the piston-forming element  22  and the first cylindrical inner portion  110  of the chamber wall  47  of the piston chamber-forming member  22 , and (b) axially inwardly of the first inner air sealing disc  156 ; 
 in a cycle of operation in the withdrawal stroke, a volume of the inner air compartment  171  increases drawing air into the inner air compartment  171  and, in the retraction stroke, the volume of the inner air compartment  171  decreases discharging air from the inner air compartment  171  through the inner air port  142  into the passageway  63 . 
 
     
     
       7. A piston pump as claimed in  claim 6  wherein:
 a cylindrical second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  extends axially inwardly from the cylindrical first inner portion  110  of the chamber wall  47 , the cylindrical second inner portion  109  of the chamber wall  47  having a diameter less than a diameter of the cylindrical first inner portion  110  of the chamber wall  47 ; 
 a second inner air sealing disc  140  on the stem  90  axially inwardly of the first outer air sealing annular flange  156  on the stem  90 , 
 the second inner air sealing disc  140  extending radially outwardly from the stem  90  to an annular distal edge  144  in engagement with the cylindrical second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20 ; 
 the annular distal edge  144  of the second inner air sealing disc  140  engaging the second cylindrical inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  to prevent fluid flow axially inwardly therepast; 
 an inner air compartment  142  defined (a) annularly between the stem  90  of the piston-forming element  22  and the first cylindrical inner portion  110  and the second cylindrical inner portion  109  of the chamber wall  47  of the piston chamber-forming member  20 , and (b) axially between the first inner air sealing annular flange  156  and the second inner air sealing disc  140 . 
 
     
     
       8. A piston pump as claimed in  claim 7  wherein:
 a liquid sealing disc  139  on the stem  90  axially inwardly of the first inner air sealing disc  140 ; 
 the liquid sealing disc  139  extending radially outwardly from the stem  90  to an annular distal edge  145  in engagement with the cylindrical second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  axially inwardly of the second inner air sealing disc  140 ; 
 the annular distal edge  145  of the liquid sealing disc  139  engaging the second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  to prevent fluid flow axially inwardly therepast; 
 the liquid sealing disc  139  being resilient and having an inherent bias biasing the annular distal edge  145  into engagement with the second inner portion  109  of the chamber wall  47  and deflectable against the bias from engagement with the second inner portion  109  of the chamber wall  47  to permit liquid fluid flow axially outwardly therepast when a pressure differential between a pressure on an inner axial side of the liquid sealing disc  139  is sufficiently greater than a pressure on an inner axial side of the liquid sealing disc  139 ; 
 a one-way valve  102  across the liquid inlet  28  permitting fluid flow therepast from the reservoir  13  into the chamber  26  and preventing fluid flow therepast from the chamber  26  into the reservoir  13 , 
 a liquid port  141  on the stem  90  axially between the liquid sealing disc  139  and the second inner air sealing disc  140 ; 
 the liquid port  141  extending radially inwardly through the stem  90  into the passageway  63 ; 
 the liquid pump  70  having a liquid compartment  106  open to the liquid port  141  and defined (a) annularly between the stem  90  of the piston-forming element  22  and the second inner portion  109  of the chamber wall  47  of the piston chamber-forming member  20 , and (b) axially inwardly of the liquid sealing disc  139  between the liquid sealing disc  139  and the one-way valve  102 ; 
 in the cycle of operation in the withdrawal stroke, a volume of the liquid compartment  106  increases drawing liquid past the one-way valve  102  from the reservoir  13  into the liquid compartment  106  and, in the retraction stroke, the volume of the liquid compartment  106  decreases discharging liquid from the liquid compartment  106  past the liquid sealing disc  139  to between the liquid sealing disc  139  and the second inner air sealing disc  140  and through the liquid port  141  into the passageway  63 . 
 
     
     
       9. A piston pump as claimed in  claim 8  wherein:
 the inner air pump  170  having a cylindrical inner air pump wall  110  provided by the piston chamber-forming member  20 , 
 a first inner air sealing disc  156  extending radially outwardly on the stem  90  axially outwardly of the inner air port  142  for sealing engagement with the inner air pump wall  110 ; 
 the first outer air sealing disc  256  extending radially outwardly on the stem  90  axially outwardly of the first inner air sealing disc  156  of the inner air pump  170 ; 
 the first outer air sealing annular flange  203  on the piston chamber-forming member  20  being axially outwardly of the first inner air sealing disc  156  of the inner air pump  170 , 
 the first outer air sealing annular flange  203  extending radially inwardly for sealing engagement with the radially outwardly directed stem wall  152  of the stem  90  axially outwardly of the first inner air sealing disc  156  of the inner air pump  170 ; 
 the first inner air sealing disc  156  extending radially outwardly on the stem  90  above the second inner air sealing disc  140  of the inner air pump  170  into sealing engagement with the cylindrical outer air pump wall  110  provided by the piston chamber-forming member  20  axially inwardly of the outer air sealing annular flange  203 . 
 
     
     
       10. A piston pump as claimed in  claim 9  wherein:
 the piston-forming element  22  comprises:
 a liquid piston  130  having a hollow tubular stem  131  with the inner end  134  and an outer end  135  and a central passage  133  longitudinally therethrough about the axis  23 , 
 an inner air piston  150  having a hollow tubular stem  151  with an inner end  154  and an outer end  155  and a central passage  153  longitudinally therethrough about the axis  23 , and 
 a first outer air piston  250  having a hollow tubular stem  251  with an inner end  254  and the outer end  255  and a central passage  253  longitudinally therethrough about the axis  23 , 
 
 the liquid piston  130 , the inner air piston  150  and the first outer air piston  250  coaxially disposed about the axis  23  with:
 (a) the outer end  135  of the liquid piston  130  coupled to the inner end  154  of the inner air piston  150  and the outer end  155  of the inner air piston  150  coupled to the inner end  254  of the first outer air piston  250 , and 
 (b) the central passage  133  of the liquid piston  130  opening axially into the central passage  153  of the inner air piston  150  and the central passage  153  of the inner air piston  150  opening axially into the central passage  253  of the first outer air piston  250  forming the central passageway  63  through the liquid piston  130 , the inner air piston  150  and the first outer air piston  250 , 
 
 the first outer air sealing disc  256  carried on the inner end  254  of the first outer air piston  250  extending radially outwardly from the stem  251 ; 
 the first outer air port  162  on the inner air piston  150  proximate but axially inwardly of the first outer air sealing disc  256 , 
 the piston chamber-forming member  20  comprising:
 a hollow tubular inner casing  101  having the end  24  and an outer end  105 , and 
 a hollow tubular first outer casing  201  having an inner end  204  and the outer end  205 ; 
 
 the inner casing  101  defining a liquid chamber  106  and an inner air chamber  107  therein, the liquid chamber  106  axially open to the inner end  24  of the inner casing  101  and opening axially inwardly into the inner air chamber  107  which is open to the outer end  105  of the inner casing  101 ; 
 the first outer casing  201  defining a first outer air chamber  207  therein, 
 the inner casing  101  and the first outer casing  210  coaxially disposed about the axis  23  with:
 (a) the outer end  105  of the inner casing  101  coupled to the inner end  204  of the first outer casing  201 , and 
 (b) the inner air chamber  107  opening axially into the first outer air chamber  207 ; 
 
 the first annular air sealing flange  203  coaxially carried by the first outer casing  201  fixed to the first outer casing  201  at the inner end  204  of the first outer casing  201 , 
 the stem  151  of the inner air piston  150  passing coaxially through the first annular air sealing flange  203  axially inwardly of the first outer air sealing disc  256 , 
 the first annular air sealing flange  203  extending radially inwardly from sealed engagement with the first outer casing  201  into engagement with the stem  151  of the inner air piston  150  axially inwardly of the first outer air port  162 , 
 the first outer air chamber  207  defined in an annular space radially inside the first outer casing  201  between the first outer casing  201  and the stem  151  of the inner air piston  150  and axially between first annular air sealing flange  203  and the first outer air sealing disc  256 , 
 the first outer air chamber  207  open via the first outer air port  162  into the passageway  153 . 
 
     
     
       11. A piston pump as claimed in  claim 4  wherein:
 the piston-forming element ( 22 ) comprises: 
 a liquid piston ( 130 ) having a hollow tubular stem ( 131 ) with the inner end ( 134 ) and an outer end ( 135 ) and a central passage ( 133 ) longitudinally therethrough about the axis ( 23 ), 
 an inner air piston ( 150 ) having a hollow tubular stem ( 151 ) with an inner end ( 154 ) and an outer end ( 155 ) and a central passage ( 153 ) longitudinally therethrough about the axis ( 23 ), and 
 a first outer air piston ( 250 ) having a hollow tubular stem ( 251 ) with an inner end ( 254 ) and the an outer end ( 255 ) and a central passage ( 253 ) longitudinally therethrough about the axis ( 23 ), 
 a second outer air piston ( 350 ) having a hollow tubular stem ( 351 ) with an inner end ( 354 ) and an outer end ( 355 ) and a central passage ( 353 ) longitudinally therethrough about the axis ( 23 ), 
 the liquid piston ( 130 ), the inner air piston ( 150 ), the first outer air piston ( 250 ) and the second outer air piston ( 350 ) coaxially disposed about the axis ( 23 ) with: 
 (a) the outer end ( 135 ) of the liquid piston ( 130 ) coupled to the inner end ( 154 ) of the inner air piston ( 150 ), the outer end ( 155 ) of the inner air piston ( 150 ) coupled to the inner end ( 254 ) of the first outer air piston ( 250 ), and the outer end of the first outer air piston ( 250 ) coupled to the inner end (354) of the second outer air piston ( 350 ), and 
 (b) the central passage ( 133 ) of the liquid piston ( 130 ) opening axially into the central passage ( 153 ) of the inner air piston ( 150 ), 
 the central passage ( 153 ) of the inner air piston ( 150 ) opening axially into the central passage ( 253 ) of the first outer air piston ( 250 ) and the central passage of the first outer air piston opening axially into the central passage of the second outer air piston forming the central passageway ( 63 ) through the liquid piston ( 130 ), the inner air piston ( 150 ), the first outer air piston ( 250 ) and the second outer air piston ( 350 ), 
 the first outer air sealing disc ( 256 ) carried on the inner end ( 254 ) of the first outer air piston ( 250 ) extending radially outwardly from the stem ( 251 ), 
 the second outer air sealing disc ( 356 ) carried on the inner end ( 354 ) of the second outer air piston ( 350 ) extending radially outwardly from the stem ( 351 ), 
 the first outer air port ( 162 ) on the inner air piston ( 150 ) proximate but axially inwardly of the first outer air sealing disc ( 256 ), 
 the second outer air port ( 262 ) on the first outer air piston ( 250 ) proximate but axially inwardly of the second outer air sealing disc ( 356 ), 
 the piston chamber-forming member ( 20 ) comprising: 
 a hollow tubular inner casing ( 101 ) having the inner end ( 24 ) and an outer end ( 105 ), a hollow tubular first outer casing ( 201 ) having an inner end ( 204 ) and the an outer end ( 205 ); and a hollow tubular second outer casing ( 301 ) having an inner end ( 304 ) and an outer end ( 305 ); 
 the inner casing ( 101 ) defining a liquid chamber ( 106 ) and an inner air chamber ( 107 ) therein, the liquid chamber ( 106 ) axially open to the inner end ( 24 ) of the inner casing ( 101 ) and opening axially inwardly into the inner air chamber ( 107 ) which is open to the outer end ( 105 ) of the inner casing ( 101 ); 
 the first outer casing ( 201 ) defining a first outer air chamber ( 207 ) therein, the second outer casing ( 301 ) defining a second outer air chamber ( 307 ) therein; 
 the inner casing ( 101 ), the first outer casing ( 201 ) and the second outer casing ( 301 ) coaxially disposed about the axis ( 23 ) with: 
 (a) the outer end ( 105 ) of the inner casing ( 101 ) coupled to the inner end ( 204 ) of the first outer casing ( 201 ), 
 (b) the inner air chamber ( 107 ) opening axially into the first outer air chamber ( 207 ), and 
 (c) the first outer air chamber ( 207 ) opening axially into the second outer air chamber ( 307 ); 
 the first annular outer air sealing annular flange  203  (231) coaxially carried by the first outer casing ( 201 ) fixed to the first outer casing ( 201 ) at the inner end ( 204 ) of the first outer casing ( 201 ), 
 the stem ( 151 ) of the inner air piston ( 150 ) passing coaxially through the first annular outer air sealing annular flange  203  (231) axially inwardly of the first outer air sealing disc ( 256 ), 
 the first annular outer air sealing annular flange  203  (231) extending radially inwardly from sealed engagement with the first outer casing ( 201 ) into engagement with the stem ( 151 ) of the inner air piston ( 150 ) axially inwardly of the first outer air port ( 162 ), 
 the first outer air chamber ( 207 ) defined in an annular space radially inside the first outer casing ( 201 ) between the first outer casing ( 201 ) and the stem ( 151 ) of the inner air piston ( 150 ) and axially between first annular outer air sealing annular flange  203  (231) and the first outer air sealing disc ( 256 ), 
 the first outer air chamber ( 207 ) open via the first outer air port ( 162 ) into the passageway ( 153 ), 
 the second annular outer air sealing annular flange  303  (331) coaxially carried by the second outer casing ( 301 ) fixed to the second outer casing ( 301 ) at the inner end ( 304 ) of the second first outer casing ( 301 ), 
 the stem ( 251 ) of the first outer air piston ( 250 ) passing coaxially through the second annular outer air sealing annular flange  303  (331) axially inwardly of the second outer air sealing disc ( 356 ), 
 the second annular outer air sealing annular flange  303  (331) extending radially inwardly from sealed engagement with the second outer casing ( 301 ) into engagement with the stem ( 251 ) of the first outer air piston ( 250 ) axially inwardly of the second outer air port ( 262 ), 
 the second outer air chamber ( 307 ) defined in an annular space radially inside the second outer casing ( 301 ) between the second first outer casing ( 301 ) and the stem ( 251 ) of the first outer air piston ( 250 ) and axially between the second annular outer air sealing annular flange  303  (331) and the second outer air sealing disc ( 356 ), the second outer air chamber ( 307 ) open via the second outer air port ( 262 ) into the passageway ( 253 ). 
 
     
     
       12. A piston pump as claimed in  claim 11  wherein the inner air piston ( 150 ) and the first outer air piston ( 250 ) are identical modular elements. 
     
     
       13. A piston pump as claimed in  claim 11  wherein the first outer casing ( 201 ) and the second outer casing ( 301 ) are identical modular elements. 
     
     
       14. A piston pump as claimed in  claim 1  wherein: a cylindrical first inner portion ( 110 ) of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) extends axially inwardly from the first outer air sealing annular flange  203  (231);
 a first inner air sealing disc ( 156 ) on the stem ( 90 ) axially inwardly of the first outwardly outer air sealing annular flange  203  (231) on the piston chamber-forming member ( 20 ); 
 the first inner air sealing disc ( 156 ) extending radially outwardly from the stem ( 90 ) to an annular distal edge ( 157 ) in engagement with the cylindrical first inner portion ( 110 ) of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) axially inwardly of the first outer air sealing annular flange  203  (231); 
 the annular distal edge ( 157 ) of the first inner air sealing disc ( 156 ) engaging the first cylindrical first inner portion  110  of the chamber wall  47  (27) on the piston chamber-forming member ( 20 ) to prevent fluid flow axially outwardly therepast; 
 the inner air pump ( 170 ) having an inner air compartment ( 171 ) open to the inner air port  142  and defined (a) annularly between the stem ( 90 ) of the piston-forming element ( 22 ) and the first cylindrical inner portion ( 110 ) of the chamber wall  47  (27) of the piston chamber-forming member  22  (20), and (b) axially inwardly of the first inner air sealing disc ( 156 ); in a the cycle of operation in the withdrawal stroke, a volume of the inner air compartment ( 171 ) increases drawing air into the inner air compartment ( 171 ) and, in the retraction stroke, the volume of the inner air compartment ( 171 ) decreases discharging air from the inner air compartment ( 171 ) through the inner air port ( 142 ) into the passageway ( 63 ). 
 
     
     
       15. A piston pump as claimed in  claim 14  wherein:
 a cylindrical second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  extends axially inwardly from the cylindrical first inner portion  110  of the chamber wall  47 , the cylindrical second inner portion  109  of the chamber wall  47  having a diameter less than a diameter of the cylindrical first inner portion  110  of the chamber wall  47 ; 
 a second inner air sealing disc  140  on the stem  90  axially inwardly of the first outer air sealing annular flange  156  on the stem  90 , 
 the second inner air sealing disc  140  extending radially outwardly from the stem  90  to an annular distal edge  144  in engagement with the cylindrical second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20 ; 
 the annular distal edge  144  of the second inner air sealing disc  140  engaging the second cylindrical inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  to prevent fluid flow axially inwardly therepast; 
 an inner air compartment  142  defined (a) annularly between the stem  90  of the piston-forming element  22  and the first cylindrical inner portion  110  and the second cylindrical inner portion  109  of the chamber wall  47  of the piston chamber-forming member  20 , and (b) axially between the first inner air sealing annular flange  156  and the second inner air sealing disc  140 . 
 
     
     
       16. A piston pump as claimed in  claim 15  wherein:
 a liquid sealing disc  139  on the stem  90  axially inwardly of the first inner air sealing disc  140 ; 
 the liquid sealing disc  139  extending radially outwardly from the stem  90  to an annular distal edge  145  in engagement with the cylindrical second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  axially inwardly of the second inner air sealing disc  140 ; 
 the annular distal edge  145  of the liquid sealing disc  139  engaging the second inner portion  109  of the chamber wall  47  on the piston chamber-forming member  20  to prevent fluid flow axially inwardly therepast; 
 the liquid sealing disc  139  being resilient and having an inherent bias biasing the annular distal edge  145  into engagement with the second inner portion  109  of the chamber wall  47  and deflectable against the bias from engagement with the second inner portion  109  of the chamber wall  47  to permit liquid fluid flow axially outwardly therepast when a pressure differential between a pressure on an inner axial side of the liquid sealing disc  139  is sufficiently greater than a pressure on an inner axial side of the liquid sealing disc  139 ; 
 a one-way valve  102  across the liquid inlet  28  permitting fluid flow therepast from the reservoir  13  into the chamber  26  and preventing fluid flow therepast from the chamber  26  into the reservoir  13 , 
 a liquid port  141  on the stem  90  axially between the liquid sealing disc  139  and the second inner air sealing disc  140 ; 
 the liquid port  141  extending radially inwardly through the stem  90  into the passageway  63 ; 
 the liquid pump  70  having a liquid compartment  106  open to the liquid port  141  and defined (a) annularly between the stem  90  of the piston-forming element  22  and the second inner portion  109  of the chamber wall  47  of the piston chamber-forming member  20 , and (b) axially inwardly of the liquid sealing disc  139  between the liquid sealing disc  139  and the one-way valve  102 ; 
 in the cycle of operation in the withdrawal stroke, a volume of the liquid compartment  106  increases drawing liquid past the one-way valve  102  from the reservoir  13  into the liquid compartment  106  and, in the retraction stroke, the volume of the liquid compartment  106  decreases discharging liquid from the liquid compartment  106  past the liquid sealing disc  139  to between the liquid sealing disc  139  and the second inner air sealing disc  140  and through the liquid port  141  into the passageway  63 . 
 
     
     
       17. A piston pump as claimed in  claim 1  wherein:
 the inner air pump  170  having a cylindrical inner air pump wall  110  provided by the piston chamber-forming member  20 , 
 a first inner air sealing disc  156  extending radially outwardly on the stem  90  axially outwardly of the inner air port  142  for sealing engagement with the inner air pump wall  110 ; 
 the first outer air sealing disc  256  extending radially outwardly on the stem  90  axially outwardly of the first inner air sealing disc  156  of the inner air pump  170 ; 
 the first outer air sealing annular flange  203  on the piston chamber-forming member  20  being axially outwardly of the first inner air sealing disc  156  of the inner air pump  170 , 
 the first outer air sealing annular flange  203  extending radially inwardly for sealing engagement with the radially outwardly directed stem wall  152  of the stem  90  axially outwardly of the first inner air sealing disc  156  of the inner air pump  170 ; 
 the first inner air sealing disc  156  extending radially outwardly on the stem  90  axially outwardly of the second inner air sealing disc  140  of the inner air pump  170  into sealing engagement with the cylindrical outer air pump wall  110  provided by the piston chamber-forming member  20  axially inwardly of the outer air sealing annular flange  203 . 
 
     
     
       18. A piston pump as claimed in  claim 1  wherein:
 the piston-forming element ( 22 ) comprises: 
 a liquid piston ( 130 ) having a hollow tubular stem ( 131 ) with the inner end ( 134 ) and an outer end ( 135 ) and a central passage ( 133 ) longitudinally therethrough about the axis ( 23 ), 
 an inner air piston ( 150 ) having a hollow tubular stem ( 151 ) with an inner end ( 154 ) and an outer end ( 155 ) and a central passage ( 153 ) longitudinally therethrough about the axis ( 23 ), and a first outer air piston ( 250 ) having a hollow tubular stem ( 251 ) with an inner end ( 254 ) and the an outer end ( 255 ) and a central passage ( 253 ) longitudinally therethrough about the axis ( 23 ), 
 the liquid piston ( 130 ), the inner air piston ( 150 ) and the first outer air piston ( 250 ) coaxially disposed about the axis ( 23 ) with: 
 (a) the outer end ( 135 ) of the liquid piston ( 130 ) coupled to the inner end ( 154 ) of the inner air piston ( 150 ) and the outer end ( 155 ) of the inner air piston ( 150 ) coupled to the inner end ( 254 ) of the first outer air piston ( 250 ), and 
 (b) the central passage ( 133 ) of the liquid piston ( 130 ) opening axially into the central passage ( 153 ) of the inner air piston ( 150 ) and the central passage ( 153 ) of the inner air piston ( 150 ) opening axially into the central passage ( 253 ) of the first outer air piston ( 250 ) forming the central passageway ( 63 ) through the liquid piston ( 130 ), the inner air piston ( 150 ) and the first outer air piston ( 250 ), the first outer air sealing disc ( 256 ) carried on the inner end ( 254 ) of the first outer air piston ( 250 ) extending radially outwardly from the stem ( 251 ); 
 the first outer air port ( 162 ) on the inner air piston ( 150 ) proximate but axially inwardly of the first outer air sealing disc ( 256 ), 
 the piston chamber-forming member ( 20 ) comprising: 
 a hollow tubular inner casing ( 101 ) having the inner end ( 24 ) and an outer end ( 105 ), and a hollow tubular first outer casing ( 201 ) having an inner end ( 204 ) and the an outer end ( 205 ); 
 the inner casing ( 101 ) defining a liquid chamber ( 106 ) and an inner air chamber ( 107 ) therein, 
 the liquid chamber ( 106 ) axially open to the inner end ( 24 ) of the inner casing ( 101 ) and opening axially inwardly into the inner air chamber ( 107 ) which is open to the outer end ( 105 ) of the inner casing ( 101 ); 
 the first outer casing ( 201 ) defining a first outer air chamber ( 207 ) therein, the inner casing ( 101 ) and the first outer casing ( 210 ) coaxially disposed about the axis ( 23 ) with: 
 (a) the outer end ( 105 ) of the inner casing ( 101 ) coupled to the inner end ( 204 ) of the first outer casing ( 201 ), and 
 (b) the inner air chamber ( 107 ) opening axially into the first outer air chamber ( 207 ), 
 the first annular outer air sealing annular flange  203  (231) coaxially carried by the first outer casing ( 201 ) fixed to the first outer casing ( 201 ) at the inner end ( 204 ) of the first outer casing ( 201 ), 
 the stem ( 151 ) of the inner air piston ( 150 ) passing coaxially through the first annular outer air sealing annular flange  203  (231) axially inwardly of the first outer air sealing disc ( 256 ), 
 the first annular outer air sealing annular flange  203  (231) extending radially inwardly from sealed engagement with the first outer casing ( 201 ) into engagement with the stem ( 151 ) of the inner air piston ( 150 ) axially inwardly of the first outer air port ( 162 ), 
 the first outer air chamber ( 207 ) defined in an annular space radially inside the first outer casing ( 201 ) between the first outer casing ( 201 ) and the stem ( 151 ) of the inner air piston ( 150 ) and axially between first annular outer air sealing annular flange  203  (231) and the first outer air sealing disc ( 256 ), 
 the first outer air chamber ( 207 ) open via the first outer air port ( 162 ) into the passageway ( 153 ). 
 
     
     
       19. A piston pump as claimed in  claim 18  wherein a cross-sectional area of the passageway ( 63 ) normal the axis varies along the axis. 
     
     
       20. A piston pump as claimed in  claim 1  including a foam generator in the passageway ( 63 ) to mix the air and the liquid passing outwardly therethrough to produce foam.