At the bottom of a suction line riser, the recommended device is a:

• A. Check Valve
• B. P-Trap
• C. Service Port
• D. Balancing Valve

Answer: (B)

At the bottom of a suction line riser, you want a device that provides a water seal to block gases from coming back up the line. A P-trap does that by holding a small amount of water in its curved section, which acts as a barrier against sewer or drain gases while still allowing liquids to pass through when the system is running. This prevents odors and gases from entering the space or equipment when suction isn’t actively drawing fluid.

The other options don’t provide this gas-blocking barrier. A check valve stops backflow of liquid but doesn’t seal against gases and can fail or clog. A service port is just for access or diagnostics, not protection. A balancing valve controls flow, not gas safety. So the P-trap is the best choice here because it specifically maintains a gas-tight seal at the bottom of the suction line riser.