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From the 2015 UPC Illustrated Training Manual
1004.1 Prohibited. No form of trap that depends for its seal upon the action of movable parts shall be used. No trap that has concealed interior partitions, except those of plastic, glass, or similar corrosion-resisting material, shall be used. "S" traps, bell traps, and crown-vented traps shall be prohibited. No fixture shall be double trapped. Drum and bottle traps shall be installed for special conditions. No trap shall be installed without a vent, except as otherwise provided in this code.
Manufacturers have produced various kinds of traps in the past and continue to do so with innovative designs. However, not all designs have been found favorable for a safe plumbing system. Certain anti-siphon traps have been designed with movable parts or mechanical means to maintain its trap seal (see Figure 1004.1a). The problem with these types of traps is that they may foul and malfunction over time since they are not self-cleansing. Traps with interior partitions are permitted only if the partition is of noncorrosive material (see Figure 1004.1b). Interior partitions are not visible and if the partition corrodes, the trap seal will be lost without notice.
The unsuitability of crown vented traps to be used in the drainage system has already been discussed in Section 1002.2, and the S trap is ineffective in protecting the trap seal because the arrangement of the S trap makes the trap seal too susceptible to siphonage (see Figure 1004.1c). The bell trap (see Figure 1004.1d) consists of a cup with a standpipe in the middle over which is a bell that dips into the water contained in the cup to form a seal. Also, this trap is not self-cleansing, the seal depth is very shallow (often less than one-half inch), and it is known to be very sluggish in draining.
Drum and bottle traps (see Figures 1004.1e and 1004.1f) may be installed only when permitted by the Authority Having Jurisdiction (AHJ) for use under special conditions. Drum traps are used as sediment traps, separating out small solids before they enter the drainage system. The bottle trap is similar to the P trap, but with a slightly different configuration and is somewhat more prone to siphonage than the P trap. It is smaller than the P trap and used where space saving is necessary. These traps may still be used upon approval but there should be a valid reason.
When a fixture is double trapped, the piping between the two traps could become air bound, resulting in a slow drain and increasing the possibility of a stoppage. There is also the possibility of one or both of the traps being siphoned by the flow through them; therefore, no fixture may be double trapped.
The 2015 Uniform Plumbing Code Illustrated Training Manual eBook is available for purchase here.
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