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SpaPartsNet UNION & PIPE CONNECTION REFERENCE
The Buttress Thread pattern is the most commonly used in the industry for pump and union connections.
It is illustrated here and can be described as slanted on the side facing the compression areas and flat on the side
away from the compression area. Usually a union size is stated as the size of the pipe it accepts such as 1", 1-1/2",
or 2". The actual diameter measurements of the Buttress Thread would have to be larger than the pipe itself and is
approximately 1" larger than the stated flow rate size of the piping. The Buttress Thread pattern is (was) used com-
monly by such manufacturers as Aqu-Flo, Brett Aqualine, some C&S Plastics, Grid, Magic Plastics, Premier, Sonfarrel
(Martec), Waterway and WMC. Many pump wet end manufacturers also used the Buttress Thread pattern.
NOTE: Although the Buttress Thread pattern
is commonly used in the spa industry, the
pilot depth (and sometimes the design of the
nut) may effect the compatability of mating
parts between manufacturers.
A complete union * is an assembly of four basic components:
1.
Nut (aka collar)
A female threaded component that fits over the flanged tailpiece. It's threads must mate
those of the threaded tailpiece in order to compress the two tailpieces together against a
sealing component (either an o-ring or gasket). The stated size of the complete union is
determined by the equivalent water flow size of the union compared to PVC pipe
(i.e. 1", 1-1/2", 2", etc.)
2.
Flanged Tailpiece
The end of this component, that butts up to the sealing component, is always unthreaded.
The neck (that portion that accepts the pipe or fiting) "flanges"or enlarges at this end so
that the nut can slip over the neck but not over the flange.
3.
Sealing Component
This is either an o-ring or gasket. An o-ring generally provides a better seal if it is
properly sized to the union and if it retains its original elastic memory. A gasket, due to
its larger surface area, will fit a wider range of applications as long as it matches the flo
size of the union.
4.
Threaded Tailpiece
The neck (that portion that accepts the pipe or fitting) may be smooth or threaded but the
name of this component stems from the fact that the compression end must mate the
threads of the union nut. Proper mating of the nut and threaded tailpiece result in com-
pression of the tailpieces against the o-ring or gasket.
* Note: A "pump union" is an assembly of parts (1 thru 4 above). In this case the thread matches to the pump threads.
A union's stated size is it's liquid flow rate as compared to PVC pipe. Generally measuring the inside diameter of the compres-
s
inside diameter of the union nut threads (or outside diameter of the threaded tailpiece threads since they must mate) then sub-
tract approximately 1" will also give the stated "flow size" of the union component.
ID
ID minus approx. 1" =
Flow Size =
A approx.
Buttress OD minus approx. 1" =
Flow Size =
A approx.
OD
Nom. Flow
1-15/16" 1"
2-7/16"
1-1/2"
3-1/16"
2"
3-11/16"
2-1/2"
OD
NOTE: Threads MUST mate properly.
Neck
1. Nut
2. Flanged
Tailpiece
3. O-Ring
or Gasket
4. Threaded
Tailpiece
Flange
Buttress
Threads
Neck
A
Unions
