Plumber's Licensing Exam: Traps, Interceptors, And Separators

All fixtures need to be trapped, but there are many fixtures that require special traps or have special provisions concerning their traps. Restaurants, manufacturing facilities, vehicle service buildings, medical offices, and dental offices have fixtures with uses that are not found in typical residential plumbing. This guide explores the varieties of traps outside of the home, as well as their uses and applicable code requirements.

Objectives
- Identify trap design details
- State the purpose and design requirements of interceptors
- Discuss separator theory and application
- Explain the need for and application of backwater valves

Key Term:
trap seal

Trap Requirements
By the time you get to this point in your work experience and your preparation for the licensing test, you know that every fixture needs a trap to keep out sewer gas. Still, there are some fine points and some exceptions you need to know about. (Refer to IPC 1002.1–1002.10; UPC 1001.0–1008.0)
The vertical distance between a fixture outlet to the trap weir cannot be greater than 24 inches. In other words, waste cannot fall straight down farther than 2 feet before it lands in the trap. This protects the trap from the possibility of self-siphoning. The accelerated velocity of drainage falling from a drop higher than 24 inches can sometimes cause siphon through a trap. The trap cannot be more than 30 inches horizontally away from the fixture outlet. This keeps the length of the inlet side of the trap to a minimum because bacteria will grow in this piping and can create odors.
The standpipe from an automatic clothes washer would appear to be an exception to the 24-inch vertical distance rule; however, the standpipe is a receptor for an indirect waste. The washing machine is not directly connected to the standpipe.
Combination fixtures, including three-compartment sinks, can be trapped with a single trap. A fixture trap is not needed if waste from the fixture drops vertically no more than 30 inches and horizontally no more than 60 inches and then flows into a grease interceptor. The grease interceptor functions as the trap for the fixture, whether it is a single-compartment fixture or a multiple-compartment fixture of no more than three compartments.

Real-Life Situation: The steel grease interceptor at Burger Bonanza has completely rusted out. Julio, from Ace Plumbing Company, has been dispatched to size up the situation. The owner reported problems with the interceptor long before it rusted out. He complained that it had to be cleaned out too often. Julio suspects that it is undersized. What information must Julio gather so that the replacement grease interceptor can be properly sized?

Fact: The first known use of a trap was in 1775. Alexander Cummings created an S-trap for use under a water closet. It used a sliding valve to initiate a flush. The function of that trap more than 200 years ago was no different than that of modern traps—keeping sewer gas out of the building.

Trap Configurations
(Refer to IPC 102.3–102.10; UPC 1003.0–1006.0)
- Any trap that depends on a moving part, whether it is a floating ball, a flapper, or any other moving device, is illegal.
- Bell traps are illegal traps found most commonly in floor drains. The drainpipe extends up through the connection point at the bottom of the floor drain body, which creates a moat within the floor drain body. Then, a bell-shaped casting is hung from the strainer by a bolt. When the cover is dropped into its place on the body, the edge of the bell gets its feet wet in the moat. This creates a trap, which prevents sewer gas from entering the building. In a perfect world, this would be fine, except most of the world does not have an address called “Utopia.” The trap seal depth of the bell trap is generally only about 1/2 inch, far short of the 2-inch depth requirement. The large exposed water surface area of the bell trap promotes evaporation of the trap seal, further exacerbating the small depth of trap seal. The bell trap also fills with debris in the bottom of the trap causing a failure that defeats the whole purpose of having a floor drain (Figure 1 and Figure 2 below).

Figure: Bell Trap Cutaway

Figure: Bell Trap

- Crown-vented traps are illegal because waste tends to sling up into the crown vent, then accumulates, hardens, and plugs the vent
(Figure below).
- Any trap that is not an integral part of a fixture and has a partition to create a seal is illegal, unless it is built of an approved material that is highly resistant to corrosion.
- All S-traps are illegal. This trap, which was the mainstay of earlier plumbing systems, has a strong tendency to self-siphon, breaking the trap seal.

Figure: Crown-Vented Trap

Figure: Trap

- Drum traps, except for the special ones used as interceptors of solid waste such as hair, precious metals, or plaster, are not allowed (
Figure below). Sometimes a drum trap configuration is used as an acid neutralizer. Illegal drum traps are found mostly on old bathtub installations.
- The seal of all traps must be a minimum depth of 2 inches with a common maximum depth of 4 inches. In certain installations deeper trap seals are allowed.
- If a trap seal is likely to be lost due to evaporation, such as in a little-used floor drain, a trap primer must be installed.

Caution: It’s not uncommon on many large commercial projects to find that the architect has specified a building trap when one is not necessary. Be sure to check with the architect and the Authority Having Jurisdiction before going ahead with installing a building trap. The inspector may look at an unnecessary building trap as a second trap, which would place every fixture in violation of the provision prohibiting any fixture to be double-trapped.


Figure: Drum Trap

- Building traps, otherwise known as house traps, are prohibited unless the local jurisdiction requires it because of conditions in the municipal sewer system. If local conditions require a building trap, the trap must have a cleanout and must have a vent on the inlet side of the trap.

The vent must terminate above grade on the outside of the building.
- All traps must be installed level. A trap that is not level cannot maintain the trap seal depth for which the trap was designed.
- Traps must be protected from freezing.
- On slab construction, the recess created to make room for the bathtub or shower trap must be waterproof, rat-proof, and termite-proof.
- If a trap constructed of brittle material is installed underground, it must be encased in cement that extends 6 inches below and around the trap. This type of installation could occur if a vitrified clay trap is installed for chemical waste.
- Traps and pipes in any mental health center cannot be exposed. This provision is designed to protect patients and their caregivers from possible injury by preventing plumbing parts from being used as weapons.

Tips
When testing the integrity of the seals on a slip joint trap, run hot water through it. The heat tends to soften the seals and will encourage leaking, if it is going to happen at all. Also, before leaving the fixture, fill the bowl at least halfway and then let the water go out. Sometimes the head pressure of a partially filled bowl will root out a leak before it becomes a callback.

Interceptors and Separators
When there are materials entering the plumbing system that can be harmful to the system, an interceptor must be installed. Interceptors and separators can be located downstream of the building drain. You know the kind of “junk” I’m talking about—grease, oil, sand, chemicals, and solids. (Refer to IPC 1003.1–1003.10; UPC 1014.1–1017.2) Plumbing appurtenances must be installed in the sanitary drainage system to remove this “junk,” and it is called a grease interceptor.
A grease interceptor is now called a hydromechanical grease interceptor by IPC.

Design
Separators must be installed following the instructions of the manufacturer. You must ensure that any type of discharge that does not need to pass through the interceptor does not get into it. That would only serve to overload the interceptor and possibly allow waste that is supposed to stay in the interceptor to pass through it.
Grease interceptors are the most common type in use and justify some special attention (Figure below). Standards for the sizing and installation of grease interceptors are not consistent among manufacturers. Therefore, follow the manufacturer’s instructions for the particular grease interceptor you are installing.
It is always best to keep the grease interceptor as close to the source as possible; this will ensure that the grease is warm enough to allow for the best separation in the interceptor. All the fixtures in a commercial kitchen that handle food grease must flow to the grease interceptor. If a food waste grinder drains to a grease interceptor, a solids interceptor must be installed before the grease interceptor to keep it clear of food waste solids. Grease interceptors are not required for private homes.

Figure: Typical Grease Interceptor

Sizing
Most manufacturers offer sizing assistance when you purchase a new grease interceptor. Manufacturers understand how their specific products will perform in the field. Never undersize a grease interceptor to bid lower than your competition. A selection like that will haunt you in maintenance until you install the correct size.

As a general guide, the table below provides information from the IPC’s Table 1003.3.4.1 for sizing grease interceptors. If due to renovation an existing hydromechanical grease interceptor has become too small to carry the new load, additional interceptors can be added upstream of the existing interceptors.

Other Interceptor Requirements

- Oil separators must function by slowing the flow of drainage enough to allow oils to separate from other liquids so they can be skimmed at the top into a receiver. An oil separator’s depth cannot be any less than 2 feet below the bottom (invert) of the discharge pipe, with no less than an 18-inch water seal. An oil separator is sized according to the square feet of the service area of a garage area.

A minimum of 6 cubic feet of capacity for the first 100 square feet is required, plus another cubic foot of capacity for every extra 100 square feet of building area.
- Sand interceptors must be accessible for cleaning and servicing.
- Any commercial laundry must have a separator that catches all solids larger than 1/2 inch in diameter. The filter basket must be readily accessible for cleaning.
- Bottling plants need a separator to catch any broken glass that may find its way into the drainage system.
- Slaughterhouses need separators to prevent a variety of products from entering the drainage systems that could cause stoppages in sewage systems.
- Separators must be vented as their design demands. Follow the manufacturer’s instructions. When an interceptor cover is tightly sealed, venting must allow for changes in liquid levels in the separator without causing pressure changes.
- Interceptors and separators must remain accessible for servicing.
 

Materials
The design of interceptors and separators must not cause reductions in flow, other than the flow restrictor that may be provided with the interceptor. Any ledges or shoulders within the construction of the interceptor or separator can obstruct flow. The materials used must follow the guidelines for drainage piping. (Refer to IPC 1004)