Refrigeration (C-38) · Lesson 3 of 8

Relief Valve Vent Piping: Termination, Back Pressure and Length (CMC 1112)

"Relief Valve Vent Piping: Termination, Back Pressure and Length (CMC 1112)" is lesson 3 of 8 in the Refrigeration (C-38) Learning Hub. It opens with "Where the Vent Must End" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Where the Vent Must End
  2. Rules for the Vent Pipe and Valve
  3. Allowable Back Pressure P0
  4. Maximum Equivalent Length of the Vent
  5. Rating Rupture Members and Fusible Plugs

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Atmospheric pressure" through "Comparison".

What you'll be able to do

  • Explain the main idea of "Relief Valve Vent Piping: Termination, Back Pressure and Length (CMC 1112)" in plain language you can use on the exam.
  • Apply this step with confidence: "Allowable Back Pressure P0".
  • Apply this step with confidence: "Maximum Equivalent Length of the Vent".
  • Apply this step with confidence: "Rating Rupture Members and Fusible Plugs".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Relief Vent: Termination and Back PressureVENT TERMINATION (1112.11.2)BUILDINGWindowGRADEReceiverRelief valve20 ft minfrom window15 ftminVertical-up vent: 24 in drip pocket (1112.11.2)BACK PRESSURE AND LENGTH (1112.12.4)Conventional valve, set P = 250 psig3,000 ft: Pa = 13.2 psia (Table 1112.12.4)P0 = 0.15 x 250 + 13.2 = 50.7 psiad = 1.5 in, Cr = 50 lb/minf = 0.02 (given), P2 = Pa = 13.2Term 1 = 78.10 ftTerm 2 = 16.82 ftL = 78.10 - 16.82 = 61.3 ft maxBalanced valve: P0 = 75.7, L = 159.3 ftRupture or fusible: 50% of set pressureVent run may be up to 61.3 ft equivalent lengthPlanned 52 ft + 3 ft raise = 55 ft passes; raise the end from 12 ft to 15 ft or moreVENT PIPE RULESNot smaller than valve outlet (1112.12.2); no stop valve between device and vessel (1112.7)Rupture member C = 0.64 x P1 x d² (1112.14): 300 psig, d 0.75 in, P1 344.7 = 124.1 lb/minSource: 2025 California Mechanical Code 1112.7, 1112.11.2, 1112.12.2, 1112.12.4, 1112.14, Table 1112.12.4CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Where the Vent Must End

Relief devices that discharge outside the refrigeration system must be arranged to discharge outside the building (2025 CMC Section 1112.11.2). The point of discharge must be not less than 15 feet above the adjoining ground level, except that outdoor Group A1 systems may discharge at any elevation if the point is in an access-controlled area for authorized personnel. It must also be not less than 20 feet from windows, building ventilation openings, pedestrian walkways or building exits. The end must not spray onto people or let debris into the pipe, and a vertical-up vent subject to moisture needs a drip pocket at least 24 inches long, sized as the vent pipe, with a valve or drain plug.

Step 2: Rules for the Vent Pipe and Valve

The discharge pipe may not be smaller than the outlet of the relief device or fusible plug (Section 1112.12.2). No stop valve may sit between a relief device and the parts it protects (Section 1112.7), and the device connects directly to the vessel above the liquid level, accessible for inspection (Section 1112.8). Pipe on a fusible plug or rupture member must be arranged so it cannot plug when the device operates (Section 1112.12.1). Vessels of 10 cubic feet or more discharging to the atmosphere use rupture members or dual relief valves with a three-way valve (Section 1113.6).

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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