EN 13136: sizing relief valves and discharge lines in refrigerating systems
How relief valves and discharge lines are sized to EN 13136: fire, compressor, heat source, Kdr and Kb, 3 % inlet loss, back pressure, CO2 and dry ice, and the successor EN ISO 24664.
Key points
- EN 13136:2013+A1:2018 describes how to calculate the required discharge capacity, the capacity of the relief valve and the pressure loss in the inlet and discharge lines of refrigerating systems and heat pumps [1].
- The standard has been replaced by EN ISO 24664:2024, which has been harmonised under the Pressure Equipment Directive since 2 October 2026. EN 13136 gives presumption of conformity only until 2 April 2028 [1][2].
- The required capacity is calculated for external fire, a compressor against a closed valve, internal heat sources and liquid expanding in an isolated line [1].
- The valve capacity is calculated with Qm = 0.2883 · C · Kb · A · Kdr · √(p0/v0), where Kdr = 0.9 · Kd is the certified, derated coefficient of discharge [5].
- The pressure drop in the inlet line may be at most 3 % of the set pressure. For standard relief valves the back pressure may be at most 10 % [4][6].
- CO2 has high pressures, dry ice forms when the pressure falls below the triple point (5.2 bar), and liquid or dense gas gives two-phase flow in the valve [1][7].
What the standard covers
EN 13136, Refrigerating systems and heat pumps – Pressure relief devices and their associated piping – Methods for calculation, covers three things: the mass flow a relief valve or bursting disc must handle, the device's discharge capacity when relieving to atmosphere or to a part of the system at lower pressure, and the pressure loss in the lines before and after the device [1]. The latest edition is EN 13136:2013 with amendment A1 from 2018 [1][2].
The link to EN 378-2, the PED and ISO 4126
- EN 378-2 covers the whole system: design, PS, pressure relief and testing. EN 13136 (now EN ISO 24664) is used to calculate the relief valves and their lines [1][4].
- PED 2014/68/EU: relief valves are safety accessories and as a rule are classified in category IV [3][5]. A harmonised standard gives a presumption of conformity with the essential requirements of the directive [2].
- ISO 4126: part 1 covers safety valves and part 2 bursting discs. The capacity formula and the certified coefficient of discharge follow ISO 4126-1 [1][5].
When is pressure relief needed?
The standard deals with four causes of overpressure: external heat (fire), internal heat sources, compressors and thermal expansion of trapped liquid [1]. Every section that can be isolated from the existing safety device, for example by a stop valve, should have its own relief valve or bursting disc [7]. A typical relief valve starts to open at the set pressure and is fully open before the pressure has risen 10 % above it [5].
Required discharge capacity Qmd
| Cause | How Qmd is calculated | Points to note |
|---|---|---|
| External fire | The heat flux (10 kW/m² in EN 13136) times the vessel's outer surface, divided by the latent heat at relieving pressure [1] | Surface in m², not volume. |
| Compressor against a closed valve | Swept volume times suction gas density times volumetric efficiency [1] | Use the highest possible suction temperature. |
| Internal heat source | Heat input divided by the latent heat [1] | E.g. an electric heater. |
| Trapped liquid | Thermal expansion of liquid in a line or vessel that can be shut in [1] | Small flow, very fast pressure rise. |
Valve flow area and capacity
For gas and vapour the formula gives the valve capacity Qm in kg/h: Qm = 0.2883 · C · Kb · A · Kdr · √(p0/v0). A is the flow area in mm², p0 the relieving pressure in bar absolute (set pressure plus 10 % overpressure) and v0 the specific volume at p0 in m³/kg [5]. Solving the formula for A with Qm = Qmd gives the smallest flow area needed.
- C depends on the refrigerant's isentropic exponent κ and applies to critical (choked) flow [5].
- Kdr is the certified, derated coefficient of discharge: Kdr = Kd · 0.9. The value is in the valve maker's data sheet [5].
- Kb accounts for subcritical flow. Kb = 1 as long as the back pressure is below about half the relieving pressure; at a higher back pressure the capacity falls [5].
- The discharge line is sized for what the valve can actually discharge, not only for Qmd.
Limits for the inlet and discharge lines
- Inlet: the pressure drop from the vessel to the valve may be at most 3 % of the set pressure at full flow [4][6].
- Back pressure: for standard (conventional) relief valves at most 10 % of the set pressure [4]. Back pressure also raises the opening pressure of a conventional valve [5]. Balanced valves accept more, as stated by the maker.
- Discharge line: calculated with compressible flow. Check that the gas velocity stays below the speed of sound, or that choked flow at the outlet is included in the calculation [1][4]. The line must never be smaller than the valve outlet.
- Common line: with several valves, each valve's back pressure is calculated with all the valves that can relieve at the same time, for example in a fire [1].
The special case of CO2 (R744)
- High pressures: the critical point is at 31.0 °C and 73.8 bar. In transcritical systems the high-side pressure is often between 80 and 120 bar, and the standstill pressure on the low side can be much higher than the operating pressure [7]. Relief valves, changeover valves and pipes must be rated for that pressure.
- Triple point: below −56.6 °C and 5.2 bar CO2 cannot exist as a liquid [7]. When the pressure falls to atmospheric in the discharge, dry ice forms, which can briefly block narrow sections and raise the back pressure.
- Flashing and two-phase flow: if liquid or dense supercritical gas reaches the valve, it flashes inside the valve and the gas formula does not apply. EN ISO 24664 has its own annex for flashing liquids [1]; use the valve maker's two-phase method.
- Above the critical pressure there is no latent heat, so the fire and heat cases cannot be calculated the usual way. Calculate with gas at a temperature instead and give the mass flow.
Changeover valve for two relief valves
A changeover (three-way) valve connects the vessel to one of two relief valves at a time, so that the other can be inspected or replaced while the vessel is still protected [6]. Each relief valve must therefore handle the full Qmd on its own. The pressure drop of the changeover valve counts towards the 3 % inlet limit; makers state which combinations meet the limit [6]. Check the changeover valve's maximum pressure: a common model for industrial refrigeration is rated for 65 bar, which is not enough on the high-pressure side of a CO2 system [6].
Practical checklist
- List the parts that can be isolated and their causes of overpressure.
- Calculate Qmd for each cause and size for the largest.
- Choose a valve from the data sheet (Kdr, area) with Qm ≥ Qmd.
- Check the inlet pressure drop (at most 3 %), including through the changeover valve.
- Check the back pressure with all valves that can relieve together.
- CO2: check pressures, dry ice, two-phase flow and material temperature.
- Document the calculation for the PED assessment.
Calculate it yourself
Terms in this article
Sources
- CEN (iTeh Standards catalogue) – EN ISO 24664:2024 Refrigerating systems and heat pumps – Pressure relief devices and their associated piping – Methods for calculation (replaces EN 13136:2013+A1:2018)
- Commission Implementing Decision (EU) 2026/2211 of 1 October 2026 on harmonised standards for refrigerating systems and heat pumps (and others) under Directive 2014/68/EU – EUR-Lex
- Pressure Equipment Directive 2014/68/EU – EUR-Lex
- Danfoss – How to select the best safety valve for your system (EN 378 / EN 13136, inlet and outlet pressure drop)
- Danfoss – Safety relief valve SFA, data sheet (discharge capacity to ISO 4126-1 / EN 13136, Kdr, Kb, PED category)
- Danfoss – Change-over valve DSV 10, DSV 1 and DSV 2, data sheet
- Danfoss – A guide to designing transcritical refrigeration systems with carbon dioxide (CO2)