Traps, Interceptors & Separators — Plumber Practice Questions

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• The water standing in a fixture trap is the only barrier between the room and the drainage system, and the code fixes how deep it must be. Under the IPC, what liquid seal depth is required for a fixture trap of ordinary design?
• Trap seals are lost to several distinct mechanisms, and identifying the right one determines the repair. In a properly vented building, which mechanism most likely emptied the seal of a floor drain that is almost never used?
• The S-trap is prohibited in modern codes even though it holds a proper seal when it is full. What is the reason the IPC bars that arrangement?
• The IPC bars several trap designs outright because each one fails to hold a reliable seal or fouls in service. Which trap may you install on a standard fixture?
• A vent taken too close to the trap becomes a crown vent, which the code prohibits. Under the IPC, how far from the trap weir must a vent connection be kept?
• A long drop between the fixture outlet and the trap lets the discharge gain enough velocity to siphon the seal. Under the IPC, what is the greatest vertical distance permitted from the fixture outlet down to the trap weir?
• Trap size is bounded at both ends: too small and the fixture drains slowly, too large and the trap loses its scouring velocity. Under the IPC, what governs the size of the trap you may install?
• A building trap places a second seal in series with every fixture trap in the building, which is why modern codes take a position on them. Under the IPC, what is the status of building traps, also called house traps?
• Where a water supply connection for a primer valve would be awkward, the code recognizes an arrangement that uses an existing fixture to keep a floor drain sealed. Under the IPC, what is that permitted arrangement?
• A grease interceptor separates grease by letting it float in a quiet body of water, and solids from a disposer would defeat that. Under the IPC, how must the discharge from a food waste disposer be handled?
• A gravity grease interceptor works by holding the flow long enough for grease to rise and separate. The measured peak drainage flow into the interceptor is 40 gpm, and the code sizes a gravity interceptor by multiplying peak flow by a 30-minute retention time. What is the minimum required capacity?
• A hydromechanical interceptor is rated by the flow it can handle and by the weight of grease it can hold before it needs servicing. A unit with a flow-through rating of 35 gpm is selected, and the capacity table you are given pairs each flow-through rating with a grease retention capacity equal to twice that rating expressed in pounds. What grease retention capacity must the unit have?
• An oil separator at a repair garage is sized from the floor area that drains to it rather than from a fixture count. The area is 700 square feet, and the code sizes the separator at 6 cubic feet of capacity for the first 100 square feet of area plus 1 cubic foot for each additional 100 square feet. What minimum capacity is required?
• Commercial laundry waste carries lint, string and hardware that will foul a drainage system if it is allowed through. Under the IPC, what must a commercial laundry discharge pass through?
• A lavatory in a daily-use guest bathroom smells of sewer gas. The trap is properly vented, the vent is proven clear, and every other fixture in the house behaves normally. You pull the trap and find a rope of hair and dental floss draped over the outlet leg and hanging down into the trap arm. What has been emptying the seal?
• A floor drain in an unoccupied storage room has a 2-inch trap seal and receives no water at all. Take as given that the seal in this space loses about 1/4 inch of depth per week to evaporation. About how long can the drain sit before the seal is gone and sewer gas has a path into the room?
• Four floor drains in a mechanical room all need trap seal protection, and there is only one practical location to make a potable water connection. What arrangement does the code permit?
• A floor drain is required in a room where there is no water supply available for a primer valve and no practical route to run one. What alternative does the code recognize for keeping sewer gas out of that room?
• The IPC bars traps whose seal depends on movable parts, and traps that form their seal with an interior partition. What is the reason behind that prohibition?
• A hydromechanical grease interceptor is listed for a specific rated flow, and a flow control fitting is used to keep the incoming flow at or below that rating. How must that fitting be installed?
• A sand interceptor is being detailed to serve a vehicle wash bay that will send grit, mud and stone into the drainage system. What does the code require so that the device can actually be maintained?
• A beverage bottling plant will wash and fill bottles over floor drains, sending broken glass, crown caps and fruit pulp toward the drainage system. What does the IPC require for that waste?
• A floor sink in a commercial kitchen receives indirect waste through air gaps from a walk-in cooler and an ice machine, and it connects to the sanitary drainage system. Does that receptor require a trap?
• Fixture trap seals are required to fall between 2 inches and 4 inches deep, unless the code official approves a deeper seal. Between which two points is that seal depth measured?
• An under-sink hydromechanical grease interceptor is listed with an integral trap and is installed directly on the outlet of a two-compartment sink. The plumber then adds a P-trap on the interceptor outlet before connecting to the branch drain. What is wrong with that arrangement?
• A gravity grease interceptor measures 8 feet long by 4 feet wide and holds a liquid depth of 4 feet. Take 7.48 gallons per cubic foot as given. The design requires a minimum liquid capacity of 1,000 gallons. Does the tank on the truck comply?
• A 4-inch drain leaves a restaurant with its invert 30 inches below the finished floor and runs 60 feet at a slope of 1/8 inch per foot to the inlet of an outdoor grease interceptor. Taking the finished floor and the finished grade as the same elevation, how far below that elevation is the invert at the interceptor inlet?
• An oil separator is being installed to serve the floor drains in a vehicle repair shop, where the waste can carry gasoline and solvent vapors. How does the plumbing code treat the venting of that separator?
• Two garages are being reviewed: an enclosed residential parking garage where cars are only parked, and a commercial repair garage where vehicles are serviced. How does the plumbing code treat floor drains in each?
• A three-compartment pot sink drains into a hydromechanical grease interceptor. Take these values as given: each compartment holds 25 gallons of water, all three compartments are emptied together, and the drain-down takes 2 minutes. What flow rate must the interceptor and its flow control be able to handle?
• A wall-hung fixture's outlet sits 43 inches above the floor, and the only place the trap will fit puts its weir at 17 inches above the floor. Take as given the code limit of 24 inches for the vertical distance between a fixture outlet and its trap weir. What is the status of the installation?
• Two lavatories in adjacent restrooms are set back to back in a common wall, about 20 inches apart. The contractor proposes a single trap in the wall cavity serving both fixtures. What does the plumbing code require?
• An in-ground grease interceptor serving a restaurant kitchen has been installed in the parking lot. Where must the effluent leaving that interceptor be discharged?
• During a remodel you find a lavatory served by an S-trap that drops straight through the floor into the drainage below. The homeowner wants the fixture reconnected. What is the compliant repair?
• A vehicle wash bay will discharge grit, mud and stone along with oil and fuel residue. Both a sand interceptor and an oil separator are being installed in series. How should they be arranged?
• A small food production operation generates drainage carrying grease and food solids, but its occupancy is not one the code lists as an example of a use requiring an interceptor. What authority applies?
• A commercial laundry has six washing machines. Take these values as given: each machine discharges 35 gallons in 90 seconds, and no more than three machines discharge at the same time. What peak flow must the lint interceptor be able to handle?
• A building's plumbing work is complete and the final inspection has been scheduled. What does the code require regarding fixture traps at that point?
• A gravity grease interceptor is being installed below grade in a parking area outside a restaurant. What does the plumbing code require for access to it?

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