12.2 Prohibited Traps, Double Trapping & Trap Seal Primer Mechanisms

12.2 Prohibited Traps, Double Trapping & Trap Seal Primer Mechanisms

Exam Focus: The longevity and safety of a plumbing installation depend not only on installing approved P-traps, but on rigorously eliminating obsolete or unvented configurations that inevitably lose their water seal. The Michigan Journeyman Plumber licensing examination frequently tests MPC Section 1002.3 (Prohibited Traps), the mechanical pathology of double trapping, the causes of trap seal depletion, and the selection, operation, and backflow protection of trap seal primers under MPC Section 1002.4 and associated ASSE standards (1018, 1044, 1072).

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1. Statutory Prohibitions: Outlawed Traps (MPC Section 1002.3)

Under MPC Section 1002.3, several trap configurations that were historically widespread are strictly prohibited in modern sanitary construction. Each of these prohibited traps possesses a specific mechanical or hydraulic flaw that leads to trap seal evacuation, chronic clogging, or undetected sewer gas leakage:

+-----------------------------------------------------------------------------+
|                   PROHIBITED TRAP CONFIGURATIONS (MPC 1002.3)               |
+-----------------------------------------------------------------------------+
|                                                                             |
|  [1] S-TRAPS:                                                               |
|      - Drop vertically immediately after crown weir without a horizontal arm|
|      - Discharge forms an unbroken liquid column that siphons out the seal. |
|                                                                             |
|  [2] BELL TRAPS:                                                            |
|      - Inverted cup sitting in a shallow water trough (< 1" seal).          |
|      - Removing strainer/bell removes the seal; rapid sludge accumulation.  |
|                                                                             |
|  [3] CROWN-VENTED TRAPS:                                                    |
|      - Vent connects at crown weir or within 2 pipe diameters of weir.      |
|      - Splashing wastewater deposits hair/grease, choking the vent orifice. |
|                                                                             |
|  [4] DRUM TRAPS:                                                            |
|      - Large cylindrical volume lacks scouring velocity; accumulates sludge.|
|      - Exception: Approved solids/acid interceptors with removable covers.  |
|                                                                             |
|  [5] INTERNAL PARTITION / MOVABLE PART TRAPS:                               |
|      - Traps relying on internal concealed baffles or mechanical flaps.     |
|      - Baffle corrosion allows sewer gas bypass with NO external leak visible|
+-----------------------------------------------------------------------------+

Detailed Analysis of Outlawed Traps

1. S-Traps:
   • Physical Configuration: In an S-trap, the piping turns 180 degrees horizontally and then immediately turns 90 degrees downward vertically without an intervening horizontal vented trap arm.
   • Failure Mode: When the fixture discharges, the vertical drop acts as a siphon siphon leg. Because there is no atmospheric vent at the weir, the falling liquid column pulls the entire water seal out of the trap dip, leaving an open pipe for sewer gas.
2. Bell Traps:
   • Physical Configuration: Historically installed in basements and garages, a bell trap consists of an inverted cast-iron cup (the bell) attached to the underside of the floor strainer grate, which dips into a shallow water reservoir around the central vertical drain pipe.
   • Failure Mode: The trap seal is typically less than 1 inch deep (violating the 2-inch minimum). More critically, whenever the strainer cover is removed for cleaning or rodding, the "bell" is lifted out, instantly eliminating the trap seal entirely.
3. Crown-Vented Traps:
   • Physical Configuration: A trap where the vent pipe connects directly to the top (crown) of the trap curve or within two pipe diameters of the crown weir.
   • Failure Mode: As waste surges over the crown weir, turbulent splashing throws grease, soap scum, hair, and particulate directly into the mouth of the vent pipe. Over time, these deposits solidify and completely obstruct the vent opening, converting the assembly into an unvented S-trap.
4. Drum Traps:
   • Physical Configuration: A cylindrical canister (usually 3 or 4 inches in diameter) with an inlet near the bottom and an outlet near the top, sealed by a threaded top access plate.
   • Failure Mode: The cross-sectional area of the drum is vastly larger than the inlet pipe. When wastewater enters, its velocity drops to near zero, preventing self-scouring and causing the drum to fill with putrefying sludge. Drum traps are prohibited on standard fixtures, though specialized drum-style acid-resistant interceptors are permitted for chemical laboratory sinks.
5. Traps Depending on Concealed Partitions or Moving Parts:
   • Any trap with an internal partition separating the sewer side from the fixture side is outlawed. If this internal partition corrodes, cracks, or punctures, sewer gas passes directly through into the fixture without any water leaking onto the floor to alert occupants.

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2. The Pathology of Double Trapping (MPC Section 1002.1)

Under MPC Section 1002.1, "No fixture shall be double trapped." Double trapping occurs when wastewater discharged from a fixture must pass through two independent traps before reaching the vented building drain (for example, installing a P-trap on a commercial prep sink whose waste line then connects into a trapped floor sink, or placing a secondary trap on a building sewer line).

                          THE DOUBLE TRAPPING AIR LOCK

         FIXTURE DRAIN
               |
           [TRAP 1]
             \___/
               |=======================\   <--- UNVENTED INTERMEDIATE PIPE
                                        \       (Trapped Pocket of Air)
                                         \ 
                                       [TRAP 2]
                                         \___/
                                           |==============> TO SEWER

  PROBLEM: Water entering Trap 1 compresses the air pocket in the intermediate pipe.
  The compressed air cannot escape, creating an AIR LOCK that halts drainage flow.
  When head pressure finally forces water through Trap 2, a vacuum is induced that
  siphons the water seal completely out of Trap 1.

Why Double Trapping Paralyzes Drainage

1. Pneumatic Air Lock: Water cannot enter a pipe unless the air inside that pipe has a pathway to escape. When water discharges into Trap 1, the volume of air trapped between the outlet of Trap 1 and the inlet of Trap 2 becomes sealed and compressed. The compressed air exerts backpressure against Trap 1, causing the fixture to drain agonizingly slowly with loud bubbling and gurgling noises.
2. Induced Self-Siphonage: When hydraulic head pressure in the fixture finally forces a slug of water through Trap 2, the rapid movement of that slug through the second trap creates a strong suction in the intermediate pipe, siphoning the water seal completely out of Trap 1.

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3. Mechanisms of Trap Seal Loss

Even code-compliant P-traps can lose their protective liquid seals if exposed to adverse hydraulic, pneumatic, or environmental conditions. Plumbers must diagnose the six primary mechanisms of trap seal depletion:

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4. Evaporation & Trap Seal Primers (MPC Section 1002.4)

In heated commercial facilities, boiler rooms, and residential mechanical spaces, floor drain traps are rarely used. Under typical ambient indoor conditions (70°F, 30% relative humidity), a standard 2-inch trap seal evaporates at a rate of approximately 0.05 to 0.10 inches per day, causing complete seal loss in 20 to 30 days.

To prevent toxic sewer gas intrusion from dried floor drains, MPC Section 1002.4 mandates:

"Building drain traps and all fixture traps that are subject to evaporation shall be protected by an approved trap seal primer valve complying with ASSE 1018 or ASSE 1044, or an approved barrier-type trap seal protection device complying with ASSE 1072."

+-----------------------------------------------------------------------------+
|                   TRAP SEAL PROTECTION TECHNOLOGIES                         |
+-----------------------------------------------------------------------------+
|                                                                             |
|  [1] ASSE 1044: PRESSURE-DROP PNEUMATIC PRIMER                              |
|      - Installed on cold potable water branch feeding a frequent fixture.   |
|      - A 1-3 psi dynamic pressure drop flexes a diaphragm/piston.           |
|      - Injects a metered burst (~2-4 oz) of water through an air gap.       |
|                                                                             |
|  [2] ASSE 1018: CONTINUOUS FLOW / FLOAT PRIMER                              |
|      - Connects to supply; cycles on volumetric flow or float action.       |
|      - Requires integral or downstream atmospheric vacuum breaker (AVB).    |
|                                                                             |
|  [3] ELECTRONIC PROGRAMMABLE PRIMERS                                        |
|      - Solenoid valve wired to digital timer/microprocessor.                |
|      - Automatically injects water at scheduled intervals (daily/weekly).   |
|      - Manifold serves multiple floor drains (up to 36 drains).             |
|                                                                             |
|  [4] ASSE 1072: BARRIER-TYPE FLOOR DRAIN SEAL VALVES                        |
|      - Elastomeric (silicone) one-way duckbill/flapper inserted in throat.  |
|      - Opens under water flow; snaps tight against sewer gas & evaporation. |
|      - Requires NO potable water supply, NO piping, NO valves.              |
+-----------------------------------------------------------------------------+

ASSE 1044 Pressure-Drop Primers: Operating Principles

• Installation Location: Must be teed into a cold water supply branch feeding a fixture that is used multiple times per day (e.g., a commercial flushometer water closet, a staff restroom lavatory, or a breakroom sink).
• Cycle Mechanics: When the downstream fixture valve opens, the rapid velocity creates a momentary dynamic pressure drop of 1 to 3 psi across the primer's internal chamber. This pressure differential shifts an internal elastomeric diaphragm or brass piston, opening a poppet valve that discharges approximately 2 to 4 fluid ounces (60 to 120 mL) of water into the primer distribution line.
• Air Gap Requirement: The primer must be equipped with an integral physical air gap or vacuum breaker conforming to ASSE 1044. Under no circumstances may a trap primer create a direct, unvented cross-connection between potable drinking water and sanitary drainage.

ASSE 1072 Barrier-Type Trap Seal Protection Devices

• Mechanical Function: An engineered elastomeric silicone membrane or weighted one-way mechanical valve inserted directly into the top throat or strainer barrel of the floor drain body.
• Performance: When liquid enters the drain, the weight of the water (as little as 2 to 4 ounces) pushes the elastomeric silicone lips or flapper open, allowing full gravitational discharge into the trap. Once flow ceases, the elastomeric memory snaps the valve closed.
• Benefits: Prevents 99% of trap seal evaporation, completely blocks sewer gas, prevents wastewater backflow up to several feet of head, and eliminates the need to run costly copper supply lines and primer valves through concrete slabs.

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5. Realistic Exam Application Scenarios

Scenario A: Odor Complaint in a School Boiler Room

Exam Scenario: A plumbing contractor in Ann Arbor is dispatched to investigate complaints of foul sewage odors in a high school boiler room. The room contains two 3-inch cast iron floor drains. Upon inspection, the journeyman finds that the floor drains have completely dry P-traps. A 1/2-inch copper line runs from an ASSE 1044 pressure-drop trap primer, but the primer is connected to a cold water line serving an emergency eye wash station that is only tested once a year.

What code violations exist, and how should the system be re-engineered?

Code Analysis:

1. Primer Malfunction / Misapplication: An ASSE 1044 pressure-drop primer requires frequent, daily dynamic pressure drops to cycle. Connecting the primer to an emergency eye wash that is operated once per year guarantees that the primer will never cycle, allowing the floor drain trap seals to evaporate completely within 30 days.
2. Code Mandate: Under MPC Section 1002.4, traps subject to evaporation must have active, functioning trap seal protection.
3. Re-Engineering Options:
   • Option 1: Re-pipe the ASSE 1044 primer to the cold water supply branch feeding a frequently used fixture, such as a nearby multi-stall student restroom flushometer group.
   • Option 2: Install an electronic timer-controlled primer programmed to cycle a solenoid valve for 10 seconds once every 24 hours.
   • Option 3 (Most Cost-Effective): Install an ASSE 1072 barrier-type floor drain trap seal protection device directly into the throat of each boiler room floor drain, sealing the trap against evaporation without modifying the water supply piping.

Scenario B: Unlawful Double Trapping in a Commercial Kitchen

Exam Scenario: A journeyman is inspecting the plumbing rough-in for a commercial kitchen in Kalamazoo. An apprentice has connected a 2-compartment vegetable prep sink with a 1-1/2 inch P-trap directly into the inlet hub of an indirect floor sink that has its own integral 3-inch P-trap below the floor.

Is this installation code-compliant, and what operational issues will result?

Code Analysis:

1. Violation of MPC Section 1002.1: The installation constitutes double trapping because waste must pass through the sink's 1-1/2" P-trap and subsequently through the floor sink's 3" P-trap.
2. Operational Consequence: Air trapped between the two traps will create an air lock, causing the prep sink to drain sluggishly and burp sewer gas into the kitchen. Furthermore, indirect waste receptors must receive discharge via an approved air gap (minimum 1 inch or twice the pipe diameter).
3. Correction: The sink drain must be converted to an indirect waste pipe that terminates with an approved physical air gap above the flood level rim of the floor sink, with no trap on the sink discharge pipe (or removing the secondary trap if direct-connected).