CSE-Engineering Center of Safety Excellence GmbH

Safety devices


Experts for emergency relief.

Rupture discs, safety valves or PLC safety devices? Every safety device has its individual advantage. Sizing can be very complex. As complex as modern emergency relief systems that are routed all across plant buildings into catch tanks, quenches or separators. There are many Standards available that are constantly changing – standards like ISO 4126, AD 2000, ISO 23521 etc. – and many additional publications.

CSE-Engineering is specialised in sizing of emergency relief devices, dispersion calculation and many more services around the safety of processes and plants. The experience from hundreds of projects and our leadership groups gives our customers security.

CSE-Engineering – Carsten Schmidt


Carsten Schmidt, M.Sc.
Process Safety Engineer

phone_in_talk +49 721 6699 4719
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Safety devices for devices and plants

Reactors, pressure vessels and catch tanks


Heat exchangers, condensators



Storage tanks


CSE-Managing Director Prof. Jürgen Schmidt is chairman of the standards committee for the ISO 4126 series of standards (sizing of safety valves and bursting discs). The sizing procedure for safety valves in ISO 4126 Part 10 was developed, extended and validated by the Process Safety Engineers of CSE-Engineering.

CSE-Engineering leads the industrial exchange of experience EURISG (European Industrial Sizing Group) with more than 15 well-known companies since 2015 and has been involved for many years both in the effective solution of simple pressure protection systems and in the most complex design scenarios.

The sizing program CSE PROSaR was developed by CSE-Engineering and represents the current state of technology in the design of safety devices.

Required documents:

The following information and documents are usually required for the design of pressure relief devices:

  • Process description
  • Operating conditions | -states
  • P&ID-Flow charts
  • Isometry of existing discharge lines
  • Safety considerations | Sizing scenarios


  • Existing safety concepts
  • Experimental investigations of the thermal tone of a reaction (see also reaction calorimetry)
  • Property data of the substances involved


the sizing of safety devices according to ISO 4126-7 is typicaly done in 6 steps (see figure).

The mass flow rate to be discharged is determined individually for every sizing scenario. It depends on the type of energy input, for example in the event of incorrect heating, a cooling failure, a fire or a chemical runaway reaction. Sometimes even a valve gets stuck or encrusted.

The size of the safety device – and thus to some extent also the costs – are determined by the mass flow rate to be discharged. Here it is worth being careful and precise. Contact us.

CSE-Engineering – Valve Sizing

Sizing of a saftey valve analogous to ISO 4126-7.

Result of the sizing calculation

For your individual solution you will receive a sizing report with the input data used and a recommendation for a suitable rupture disc or safety valve. The required size of the connected pipes is also described.

On request, all data can be summarized in a compact data sheet.

Depending on the task, the design scenario, the mass flow rate to be discharged, reaction forces on the pipes and further results are described optionally.

The scope and content of the report can be adapted according to your requirements. Contact us.

Duration of the project

A typical sizing for a standard case is completed in a very short time. Half a day can be enough. Most of the time, however, consulting is the decisive factor, the support in gathering all relevant information and data. This is the particular strength of CSE engineering.

Complex questions can be considerably more time-consuming. Basically, we always try to achieve simple solutions first and only if this is not successful, more precise and complex tools are used. Until we have the solution.

Indivdual questions

At CSE-Engineering we like to solve individual questions. For this purpose we use our experience in process and safety technology. Or we use special tools, e.g. CFD simulations in our CSE3D laboratory. Contact us.

Overview safety devices:

  • Classical pressure relief valves for pressure vessels
  • Bursting safety devices
  • heat exchanger pressure protection
  • High pressure valves
  • Ventilation valves for storage tanks
  • Fuel tank protection
  • Explosion protection
  • Excess flow valves, e.g. for compressors
  • Valves for thermal expansion
  • Safety devices in the cryogenic area
  • Laboratory valves
  • Restriction orifices
  • Burst flaps
  • etc.

Overview Media:

  • Gases | Vapours
  • Liquids
  • Potentially explosive gases
  • Pressure liquefied media, e.g. refrigerants such as ammonia, propane, etc.
  • Flashing liquids and two-phase flows
  • Mixtures with solids
  • Gas-dissolving liquids (e.g. at high pressures)
  • Multicomponent mixtures
  • High viscous polymers
  • Deep cold media (cryogenic applications)
  • Supercritical media
  • Dust and dust | air – mixtures

Standards, publications, manuals

CSE-Engineering sizes safety devices in accordance with the state of technology. For this purpose, we use national and international standards, publications and years of experience.

  • DIN EN ISO 4126 part 1 to 10: Safety devices for protection against excessive pressure
  • EN ISO 23251: petroleum, petrochemical and natural gas industries – pressure relieving and depressuring systems

  • AD 2000 data sheet 1 and 2: safety devices against excess pressure
  • API 520, 2014, Part I – Sizing and Selection of Pressure-relieving Devices
Publications | Manuals
  • VDI-heat atlas 2010: chapter L2.3 – Sizing of Safety Devices for Heat Exchangers
  • VDI-Wärmeatlas 2019: chapter L2.4 – Kritische Massenstroeme
  • CSE-Sizing Reports of the EURISG – European Industrial Sizing Group (confidential), 2016
  • J. Schmidt, S. Claramunt: Sizing of rupture Disks for two-phase gas/liquid flow according to HNE-CSE-model, Journal of Loss Prevention in the Process Industries
  • J. Schmidt: Sizing of nozzles, venturis, orifices, control and safety valves for initially sub-cooled gas/liquid two-phase flow – The HNE-DS method, Forsch. Ingenieurwes. (2007) 71, S.47-58
  • J. Schmidt et al.: Sizing of safety valves and connected inlet and outlet lines for gas/liquid two-phase flow, Loss Prevention Paper (2001)
  • J. Schmidt: Sizing of safety valves for multi-purpose plants according to ISO 4126-10, Journal of Loss Prevention in the Process Industries (2012) 25, S. 181-191

Are you looking for further information? Please also read our “Basic knowledge safety equipment”

Trainings & Coaching

The Applied Safety seminar program of the CSE Academy offers several modules for the protection of pressure vessels for you and your team – for example for the design of safety valves, or retention devices. Also available as in-house training.

CSE Acadeny

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Basic knowledge safety devices

Function of safety devices

Pressure relief devices such as safety valves and rupture discs protect pressure vessels, columns, heat exchangers and other equipment from unacceptable overpressure. They are effective when other safety measures fail.

The equipment is intended to protect persons and the environment from the hazards of pressure in apparatus and systems. When the set pressure is reached, teh safety devices open and release material to prevent the pressure from rising any further. Safety valves will re-close when the pressure decreases.

The function and performance of safety devices can be significantly influenced by connected pipes and downstream equipment. Equipment and pipes must be designed as a system. The safety device only works reliably when correctly arranged.

SSafety valves can be stimulated to oscillate when opening or closing, fluttering (high frequency) or pumping (low frequency). This can destroy the pipes or the valve and considerably reduce the mass flow rate to be discharged. Safe operation of the valve is no longer guaranteed.

Bursting discs can fail unintentionally due to alternating pressure loads. The normal operating pressure must not be set too close to the set pressure.

CSE-Engineering – Basic knowledge safety devices

Source: LESER GmbH & Co. KG, Hamburg

Reasons for overpressure in apparatuses

There are many reasons why the pressure in pressure vessels, columns, heat exchangers, storage tanks etc. can rise above a permissible upper limit. Every single reason must be identified to find the right scenario for the design of the safety device and to make the device work later – the worst case scenario. We have listed the most common reasons below:

  • Blocked pipes
  • Cooling failure
  • Thermal expansion of liquids
  • Fire in a plant
  • Incorrect heating
  • Stirrer failure
  • Failure in substance mixtures
  • Rupture of a heat exchanger tube
  • Inner leakage
  • Incorrect inflow from pumps or compressors
  • Runaway reaction
  • Accumulation of reactiv components
  • Operating errors

How is the sizing sceanrio defined?

Within the framework of safety assessments, all possible faults in an apparatus or plant are systematically identified. Using this, the safety copncept is defined. The sizing scenario for the safety device is also defined using the safety assessment.

When to use which safety device?

Safety valves can be used if…

  • the pressure rise velocity is significantly lower than the opening velocity of the safety valve
  • the temperature rise velocity of reacting systems does not exceed a permissible limit
  • the materials of the valve are suitable to resist the properties of the process (e.g. corrosion)
  • the media to be discharged do not tend to stick or encrust

Rupture discs can be used if…

  • the pressure oscillation load is not too high
  • the materials of the rupture disc are suitable to resist the properties of the process (e.g. corrosion)
  • safety valves are not longer suitable
  • the combination of a safety valve and a rupture disc in the system appears advantageous
  • pressure surges must be protected

What must be taken into account during the sizing process?

Safety valves and rupture discs have to be sized individually and in conjunction with the inlet and outlet lines. Among others, the following data have to be taken into account:

  • Maximum allowable working pressure of the plant
  • Place of installation (maintenance / condensates / suitability for the environment)
  • Reaction forces during opening procedure / Reaktionskräfte beim Öffnen / Thermal expansions
  • Mounting of valve and outlet pipe / dynamic loads
  • Drainage of pipes
  • Changeover valves for continuous operation requirements
  • Back pressure at the valve outlet (external | internal back pressure)
  • Lift limitations of safety valves in case of low load
  • Pressure swing load for rupture discs | Durability
  • Leakage for flammabale | toxic media
  • Media compatibility, e.g. corrosion-resistant materials
  • Heating jacket for solidifying media
  • Pressure shocks in apparatuses
  • Noise emissions
  • Material combinations valve seat/plug or holder/rupture disc
  • Interior linings

May substances be released into the environment?

In general: if people, material goods or the environment cannot be harmed, substances may be released into the environment. However, the recognised reference values must be observed. It is common that liquids are separated with a retention system. These are usually separators, but also washers. For gases and vapours, a dispersion calculation has to be done to show arithmetically, that the concentration of the substances in the environment does not exceed the limits. With the right countermeasures, concentrations of substances can be reduced or emissions avoided altogether. CSE-Engineering will advise you on these topics. We will find the right solution.