Retention systems to protect humans and the environment

When is retention required?

In case of actuation of a safety valve or a rupture disc, gas, vapor, liquid, or hybrid mixtures can be discharged. Technical guidelines (e.g. TRB 600 in Germany) govern the safe disposal of chemical substances. It depends mainly on the amounts and hazard characteristics of possible discharged substances whether treatment or retention is necessary. The German guideline TAA-GS-06 recommends quantity thresholds for the contained substances.

An individual case study of plant-specific discharge conditions has to prove that the concentration of substances in the event of a release would be below a legally established limit (e.g. ERPG 2) at certain emission points. According to the German TAA guideline, a retention system is required if this cannot be proved.

Technical solutions

Retention systems can be divided into normal in-plant disposal systems (scrubber, flare) and collection in special catch tanks. Usually these plant retention systems are not satisfactorily sized to receive the mass flow rates that occur when safety valve or rupture disc actuate.

Separators

Cyclone or gravity separators prevent emission of the liquid phase. This is sufficient, unless complete retention is necessary, and vapor / gas fractions can either be released into the environment or specially treated. Satisfactory separating performance can often be achieved with a relatively small cyclone separator.

Direct condensation

If complete retention is necessary, a very large containment volume is usually required. The very large volume can be efficiently decreased by condensation of the vapor flow. In this case the reaction mixture is passed through a gas distributor into a liquid receiver. The vapor fraction condenses into the cold fluid.

Safebag^©

A novelty in retention systems is the Safebag^©. This fluid- and gas-tight containment system consists of a flexible vessel made of Trevira^© high tenacity fabric with a chemical-resistant coating. The system, developed by A&D AS Solutions Process Industries, takes up a minimal amount of space and can be installed directly by the plant to be protected or at any other suitable location outside the building or on the roof. It unfolds automatically when the safety valve opens.

Choice and design of the retention system

Before starting to select the optimum disposal system, a failure scenario has to be established. Determination of the mass flow to be discharged or the size of the vent area often requires measurement of reaction kinetics to ascertain the possible gas/vapor production rate, especially if the safety devices are to be sized for a chemical reactor. The size of the disposal system depends on the total discharge, which can usually only be estimated in a dynamic simulation. Another important and crucial factor for the size of the disposal system is whether two-phase flow can occur when a safety device opens. If this is the case, the composition of the mixture has to be considered.

How can we assist you?

We offer a disposal system that is tailor-made to your requirements from concept to construction. We also give you advice on describing the failure scenario and selecting the optimum disposal system for your plant. The required safety characteristics will be determined in our laboratories.

If requested, we can undertake detailed design of your retention system down to the final drawing. We can even carry out planning and construction on-site. Here we can call on our many years' experience of in-house engineering.