OilPro was recently contacted by one of its clients working for a major oil company in Nigeria about firetube fires in treaters.
A formal operating procedure is not yet written by anyone we know of, but in the spirit of sharing know-how we would like to share the currently best-known way to minimize injuries to people and damage to the site.
First we will talk about how to put out a fire, then we’ll discuss methods to decrease the chance of being in such a situation in the first place.
The best way to effectively kill fires in fire tubes is to:
A Relieve the pressure from the vessel. This will slow down the rate
combustibles are leaking.
(This may be all you have to do if it is a pin hole leak)
B. Eliminate the fuel source. You accomplish this by pumping or recycling
water in to the treater, displacing the oil and submerging the firetube, or
leaking area with water.
C. The fire will either die out because of lack of combustibles, or can
easily be extinguished.
The best way to prevent firetube fires is with regular turnarounds during which firetubes are inspected with a Magnetic Particle Inspection (MPI) and an Ultrasonic Thickness (UT) Test are excellent prevention methods.
Firetubes can sometimes collapse due to a buildup of solids brought up with the emulsion or from salt present in the water. The buildup gets in the way of heat transfer which may cause the burner to continue to burn (due to no noted delta T inside the treater) while the flame is heating the firetube to the point where it starts to collapse under the external pressure. A non-stick coating, specifically designed for firetubes, such as Corchem 239, can help wash solids off during normal production. A dedicated “desand system” can aid in solids removal also.
Anodes, ideally insulated ones with measuring shunts, can provide an indicator of the galvanic corrosion protection your firetube still gets. As it’s typically bare metal inside a coated vessel, it can be particularly subject to corrosion.
Prevent liquids from forming in the fuel gas and pilot supply system.
One common cause of firetube fires can be a slug of liquid which may make its way into the firetube during non-fired operation. This can happen due to an outage in an on-site fuel gas system, or simply a scrubber which is not drained regularly. Liquids may also form in exposed fuel gas supply lines during colder weather.
What can happen is that liquids make their way into a firetube without getting ignited.The liquid may form in the firetube and if there’s enough of it, it may eventually leak out of the flame arrester. So far, no problem, however, when the next fire sequence starts it is possible for the liquid to melt part of the flame arrester and thus ignite the leaked liquid under the flame arrester. This fire in turn starts to melt the firetube gasket and eventually degrades the gasket to the point oil, under pressure, leaks out and ignites with ensuing miserable consequences.
To prevent such instances at your facility, do your inspections regularly,measure for current on your anodes, look inside your firetube and check for a collapsed section and finally: think about condensation in fuel supply lines.
Stay safe and be prepared!