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CHEMICAL ENGINEERING TRANSACTIONS

VOL. 77, 2019

A publication of

The Italian Association

of Chemical Engineering
Online at www.cetjournal.it

Guest Editors: Genserik Reniers, Bruno Fabiano
Copyright © 2019, AIDIC Servizi S.r.l.
ISBN 978-88-95608-74-7; ISSN 2283-9216

Explosion Mitigation using Aqueous Solutions of Water and
Flame Inhibitors

Kees van Wingerdena,*, Morten Holmea, Kjetil Olsena, Dirk Roosendansb,Pol
Hoorelbekeb
aGexcon AS, Fantiftvegen 38, 5072 Bergen, Norway
bTotal SA, Tour Michelet, La Défense, A0819, Paris, France
kees@gexcon.com

The current paper describes results of tests performed with aqueous solutions of flame inhibitors at a larger-
scale. Two sets of experiments were performed: experiments using superheated aqueous solutions and
experiments where aqueous solutions were dispersed using a water deluge system. The main aim of these
experiments was to verify the effects seen on small-scale on a larger scale to verify the possibility of using this
mitigation technique on industrial scale. Both sets of experiments were performed in a congested 50m3
chamber provided with explosion venting. To generate water-mists directly at a larger-scale in sufficiently high
concentrations a dispersion of superheated aqueous solutions of potassium carbonate seems to be the only
viable solution. The application of a water deluge system implies relying on break-up of the large water
droplets by the accelerating explosion flame itself.

1. Introduction

Vapour cloud explosions are among those events causing the highest losses in the petrochemical industry.
Research in the past has especially been devoted to predicting the consequences of these events. Obviously,
preventive measures is given high attention to prevent these incidents from happening including correct
design of the facilities, its maintenance and inspection and choice of equipment approved for use in potentially
explosive atmospheres. Less attention has been given to mitigation of vapour cloud explosions, i.e. limitation
of consequences.
Especially for offshore applications water deluge has been shown to be effective as a means of explosion
mitigation. The water deluge system shall be activated before ignition. The positive impact of water deluge has
been documented extensively (Catlin et al., 1993, van Wingerden, 2000). It has been demonstrated that only
droplets smaller than 10-20 μm will affect flame propagation due to evaporation in the flame (in methane-air
mixtures). Larger droplets need to break-up before having an impact. Hydrodynamic forces acting on droplets
in an accelerating flow will allow droplets to break up if these droplets are not able to adapt to the flow
accelerations. Flow accelerations occurring during explosions in congested areas are able to break-up
droplets larger than typically 200 μm (Van Wingerden, 2000).
Hoorelbeke (2011) suggested to use solid flame inhibitors dispersed into the vapour cloud upon its release as
an alternative mitigation technology. This technology has shown to be effective and has even been tested on
full scale showing its potential (Davis et al., 2017). Relatively low concentrations (50-100 g/m3) of solid flame
inhibitors such as sodium bicarbonate, sodium chloride, potassium carbonate and potassium bicarbonate are
able to reduce flame speeds and overpressures in hydrocarbon-air mixture explosions considerably (Van
Wingerden et al, 2011).
Roosendans et al. (2017) suggested to investigate the possibility of combining the two: applying aqueous
solutions of flame inhibitors to mitigate vapour cloud explosion. Experimental investigations were performed
on small-scale using water mist-potassium carbonate solutions. Whereas water mist alone already had an
impact and caused a reduction of flame speeds and overpressure introducing potassium carbonate in the
water resulted in an additional reduction. The experimental results show that by increasing the amount of

DOI: 10.3303/CET1977162

Paper Received: 28 December 2018; Revised: 17 May 2019; Accepted: 29 June 2019

Please cite this article as: van Wingerden K., Holme M., Olsen K., Roosendans D., Hoorelbeke P., 2019, Explosion mitigation using aqueous
solutions of water and flame inhibitors, Chemical Engineering Transactions, 77, 967-972 DOI:10.3303/CET1977162

967

potassium carbonate an increase of the degree of mitigation is observed up to full quenching of the explosion
flame. An example of typical results obtained are summarised in Figure 1. Water concentrations between 30 –
80 g/m3 were used.

Figure 1: Effect of aqueous solutions of potassium carbonate on explosion overpressures in 4.05-4.09 %
propane-air mixtures in a 0.135 m3 channel with baffles. Overpressures are measured at 3 locations for
increasing amounts of the flame inhibitor dissolved in water (increasing from 0 to 60 g/litre). Water mist
concentration is determined by the injection duration varying from 8.6 to 11.1 s.

2. Description of the test set-up

Both series of tests were performed in a congested chamber 8.0 m long, 2.5 m high and 2.5 m wide, thus
having a volume of 50.0 m³. The chamber has a coarse steel grating forming a mezzanine deck approximately
1.25 m above the floor of the vessel. Three of the walls of the chamber can be used as vent openings. As
Figure 2 shows the chambers contains several objects (obstructions) which cause explosion flames to
accelerate due to turbulence generated at these obstructions. Ignition was effected using an exploding wire
located close to one of the short walls of the chamber (1 m from the end in the centre).
The tests to investigate the effect of superheated aqueous inhibitor solutions were performed having both
short ends of the chamber opened. The water deluge tests were performed having only 1 short end opened.
The ignition end was chosen to be the closed end of the chamber.
The superheated aqueous solutions were introduced into the explosion chamber from three 4 litre suppression
bottles supplied by IEP Technologies UK. Each bottle was connected to a nozzle and mounted along the
centre line of the roof of the explosion chamber, regularly divided. The bottles were either filled with 2 or 3.5
litres of water or aqueous solution. The system was activated 6 s before ignition to limit turbulence generated
during the injection.
The water deluge was provided from 7 to 11 nozzles of type MV12 125° with K-factor of 18.7. The nozzles
were mounted regularly divided against the roof of the chamber. The total water/aqueous solution application
rate was varied between 9.5 l/min/m2 to 22 l/min/m2.

3. Results

3.1 Superheated aqueous solutions

Initially tests were performed to investigate the effect of superheated water only on explosions in propane-air
mixtures. Results obtained for 4.15 % propane-air mixtures are summarised in Figure 3 showing a clear
reduction of overpressures when increasing the amount of superheated water released from 0 to 10.5 l. The

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Figure 2: 50 m3 explosion chamber used to investigate effect of aqueous of flame inhibitors

Figure 3: Effect of superheated water on overpressure generated by optimal propane-air explosions in a
vented 50 m3 obstructed chamber. The amount of water released was increased from 0 to 10.5 l.

results show that both the pressure spikes are delayed and reduced when introducing the fine water droplets
indicating a reduction of the combustion rates immediately after ignition. Similar effects were seen when
introducing superheated water into both lean and rich propane-air mixtures.

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Figure 4: Effect of superheated aqueous solutions of water and potassium carbonate on overpressures
generated by optimal propane-air explosions in a vented 50 m3 obstructed chamber. The amount of water
released was 10.5 l. The potassium concentration in the water was increased from 0 to 210 g/l.

Introducing potassium carbonate into the water in relative amounts exceeding those used in the small-scale
experiments did not result in a considerable additional reduction of the overpressures seen. In fact, pressures
seem to increase slightly the more potassium carbonate was added. The moment of occurrence of the
maximum overpressure was however delayed for increased amounts of potassium carbonate in the water
indicating a reduction of combustion rates during the initial phases. The reason for this behaviour is not fully
clear. According to Tam et al., 2003 approximately 25 % of droplets generated by a superheated water system
are smaller than 20 μm and will be able to affect the flame speeds directly. This implies an effective mist
concentration of 52 g/m3 which is in the same range as the 30-80 g/m3 used during the aqueous solution mist
tests reported by Roosendans et al., 2017. Roosendans, 2018 points in the direction of the higher viscosity of
the solution as a possible explanation. The higher viscosity may affect the droplet size distribution causing
lower concentrations of fine droplets.
Secondly, an industrial water deluge system was applied. Again, both pure water and inhibitor-solutions were
applied. In this case the explosion itself generates fine droplets due to water droplet break-up. At high water
loads explosion peak pressures were reduced considerably by the water deluge but a clear effect of adding
inhibitor to the water could not be differentiated between the deluge tests using pure water and those
performed with inhibitor-solutions (See Figure 5). This may have been caused by the effect of initial turbulence
generated by the water deluge system itself. This initial turbulence may be stronger when applying aqueous
solutions of water and potassium carbonate due to the higher density of the solution. At lower water loadings,
the mitigation effect of pure water was nearly negligible (considering the maximum explosion overpressure).
For these low water loadings, however inhibitor-solutions were seen to have an effect. This is demonstrated in
Figure 6 showing the maximum rate of pressure rise during the explosion tests for different water application
rates (using pure water and a test where for a low application rate an aqueous solution of potassium
carbonate and water was dispersed. The rate of pressure rise (an indication of the overall combustion rate)
was considerably affected applying a low application rate with an aqueous solution of water and potassium
carbonate compared to pure water.
The reason for this behaviour is not fully understood. An effect of higher viscosity due to higher solution
concentration cannot explain the behaviour since instead the number of nozzles was reduced. Roosendans,
2018 argues that a balance situation would be reached where flame speeds cannot be reduced any further
without losing the ability of breaking up droplets.

-0.1

-0.05

0.05

0.1

0.15

0.2

0.25

300 350 400 450 500 550 600 650 700

Pr
es

su
re

(b
ar

Time (ms)

Optimal Propane/Air Mixtures -
Water Mist and Inhibitor Solutions Mist

Reference - 4.12% - P5 Water mist - 10.5 litres - 4.15% - P5
K2CO3 - 30g/litres - 4.19% - P5 K2CO3 - 70g/litres - 4.19% - P5
K2CO3 - 210g/litres - 4.08% - P5

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Figure 5: Effect of water deluge on overpressures generated by optimal propane-air explosions in a vented 50
m3 obstructed chamber. The Figure shows a reference test (no water), two tests where water deluge was
applied (water application rate 22 l/min/m2) and a test where the same water application rate was used for a
100 g/l aqueous solution of potassium carbonate and water.

Figure 6: Effect of water deluge on the maximum rate of pressure observed during tests where optimal
propane-air mixtures were ignited in a vented 50 m3 obstructed chamber. The Figure shows a reference test
(no water), a test where a high water application rate was used (water application rate 22 l/min/m2), a test
where a low water application rate was sued (9.5 l/ min/m2) and a test where the same water application rate
was used (9.5 l/min/m2) for a 100 g/l aqueous solution of potassium carbonate and water.

4. Conclusions

Whereas direct injection of mists of aqueous solutions of water and potassium had considerable improved
mitigation effects on explosions in a small-scale congested channel compared to injection of water mist only a
similar effect could not be reproduced on a larger scale.

-0.5

0.5

1.5

2.5

350 400 450 500 550 600

Pr
es

su
re

(b
ar

Time (ms)

Deluge tests - Pressure developments P1

Reference test - 4.10% Water - 21.9 LPM - 4.15%
Water - 21.9 LPM - 4.17% K2CO3 100g/litre - 22.2 LPM - 4.16%

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Generation of water mist using a superheated water injection system resulted in a considerable reduction of
explosions overpressures in a congested 50 m3 chamber when using pure water. This effect was however not
improving any further when applying aqueous solutions of water and potassium carbonate. This might be
related to the increased viscosity of the solution compared to water changing its dispersion properties.
Using a water deluge system instead, aqueous solutions were shown to have an improved mitigation effect on
explosions compared to that of pure water when low application rates were used. Increasing the application
rate aqueous solutions had no improved mitigation effect anymore.
Additional experimental work would be needed to understand this behaviour.

Acknowledgments

The authors acknowledge the financial support obtained from Total SA for performing these experiments.

References

Catlin, C.A, Gregory, C.A.J., Johnson, D.M. and Walker, D.G., 1993, Explosion mitigation in offshore modules
by general area deluge. Trans. I.Chem.E. 71B: 101-111

Davis, S.G., Pagliaro, J.L., Botwinick, D., Engel, D., van Wingerden, K., Merilo, E., Ziemba, A. & Groethe, M.,
2017, Large scale testing confirms: A deflagration to detonation transition needs to be considered when
facility siting. American Institute of Chemical Engineers, Thirteenth Global Congress on Process Safety
(GCPS), San Antonio, Texas, USA

Hoorelbeke, L., 2011. Numerical and Experimental Study of Vapor Cloud Explosions, PhD Thesis. Univ. of
Brussels

Roosendans, D., van Wingerden, K., Holme, M.N. and Hoorelbeke, P., 2017, Experimental investigation of
explosion mitigating properties of aqueous potassium carbonate solutions, Journal of Loss Prevention in
the Process Industries, 46, 209-226

Roosendans, D., Mitigation of Vapor Cloud Explosions by Chemical Inhibition using Alkali Metal Compounds,
PhD Thesis Vrije Universiteit Brussel, 2018

Tam, V.H.Y., O’Connell, M., Pedersen, G. and Renwick, P., Testing of the Micromist device: an active soft
barrier for explosion control, Journal of Loss Prevention in the Process Industries, 16(1), 81-80, 2003

van Wingerden, K., 2000, Mitigation of gas explosions using water deluge. Process Safety Progress, 19 (3):
173-178

van Wingerden, K. and Hoorelbeke, P., 2011, On the potential of mitigating vapour cloud explosions using
flame inhibitors American Institute of Chemical Engineers, Seventh Global Congress on Process Safety
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