Caution: Always consult the local code authority to verify discharge pipe sizing requirements for the local jurisdiction. While these requirements are typically based on ASHRAE Standard 15, local jurisdictions sometimes make modifications.
Background
ANSI/ASHRAE Standard 15, Safety Standard for Refrigeration Systems, requires that refrigeration systems “be protected by a pressure relief device or other approved means to safely relieve pressure due to fire or other abnormal conditions” (Section 9.4.1). Manufacturers of packaged refrigeration systems (such as water chillers) or components for “built-up” refrigeration systems usually engineer pressure relief devices into their equipment designs.
When a refrigeration system is being installed indoors, each pressure relief device must be piped to a safe location (typically outside for larger systems like water chillers) in the event of a discharge.
Section 9.7.9 in Standard 15 includes an equation and data for sizing this pressure relief discharge piping. The equation accounts for the relationship between pipe diameter, equivalent pipe length, and the pressure difference between the outlet of the pressure relief device and the eventual discharge location (typically outdoors), to ensure that the pressure relief system provides sufficient flow capacity. Standard 15 allows two or more relief devices to be connected to a common discharge pipe (manifold or header pipe), as long as this common pipe is sized large enough (Section 9.7.9.3.3), but prohibits different refrigerants from venting into a common relief piping system (Section 9.7.8).
The Trane Refrigeration Systems and Machinery Rooms application manual (APP-APM001*-EN) includes further explanation of these sizing requirements, as well as several examples of how to use this equation to properly size discharge piping.
Sizing spreadsheet for 2022 (or 2019 or 2016) version of Standard 15
Attached below is a spreadsheet that can be used to size a discharge line from a single pressure relief devices, or a common manifold to vent from multiple relief devices, using the equation found in Section 9.7.9 of Standard 15 and methods detailed in the User’s Manual for ANSI/ASHRAE Standard 15. Refer to the equipment manufacturer’s IOM for specifics on the type (and number) of relief device(s) used, including discharge capacity (Cr), set pressure (P), and outlet diameter (d).
This spreadsheet contains three worksheets:
- Single Relief Device: Used for sizing discharge piping from a single pressure relief device
- Headered Identical Reliefs: Used for sizing discharge piping from multiple, identical pressure relief devices that are manifolded together into a common discharge pipe (header) ... from two identical centrifugal water chillers, for example
- Headered Different Reliefs: Used for sizing discharge piping from multiple, non-identical pressure relief devices that are manifolded together into a common discharge pipe (header) ... from separate relief devices used on the various components of a helical-rotary water chiller, for example
Note: Appendix E in Standard 15-2022 (or Table 9.7.8.5 in the 2013 version) lists the capacities of various discharge pipe sizes and lengths. However, this data only applies to a single, conventional pressure relief valve, and NOT to balanced relief valves, rupture members, fusible plugs, or pilot-operated valves.
Sizing spreadsheet for 2013 (or 2010) version of Standard 15
The requirements for sizing press relief discharge piping were revised for the 2016 (and subsequent) version of the standard. Attached below is another version of the pipe sizing spreadsheet that is based on the requirements of Standard 15-2013 and -2010.
In the 2013 (and prior versions), Section 9.7.8.4 of the standard required that when two or more relief devices are connected to a common pipe (or header), the cross-sectional area of that common header must be larger than the sum of their outlet areas of the relief devices connected to it, if they are expected to operate simultaneously. This area-based sizing restriction was removed in the 2016 (and subsequent) versions of the standard.
Other Question/Problem/Cause Statements:
- Can you help me size refrigerant vent lines?
- Can I manifold refrigerant pressure relief vent lines?
- Does Standard 15 allow multiple relief devices to be connected to a common discharge pipe?
- Do all relief valves have to be piped outdoors?
- What are the requirements for running a vent line?
- How do I size a relief line?
- Relief valve pipe size questions?
- Can all relief valves be piped to a common header?
- ASHRAE 15
- Relief valve venting calculation
- Do all refrigerant relief vents have to piped outside?
Document Identifier: DOC-81554
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.