Fire Control in Sulfur Pit (NFPA655)

[Paul R OrlowskiKeymaster]

As per NFPA-655 current edition:2012.
Protection of covered liquid sulfur storage tanks, pits and trenches shall be by one of the following means:
-Inert gas system, or
-Steam extinguishing system capable of delivering a minimum of 2.5 pounds per minute (lb/min) (1.13 kilograms per minute [kg/min]) of steam per 100 cubic feet (ft3) (2.83 cubic meters [m3]) of volume.

Pronab Mistry Chemical Engineer, Expert in Gas Processing & Sulfur Recovery Plant, Ammonia & Methanol plant operation.
United Arab Emirates asks:
In case the operating liquid level in the sulfur pit reduces from 70% to 55%, should the existing snuffing steam line be enough to deliver the said amount of steam or should the line be modified?

[Paul R OrlowskiKeymaster]

I would like to contribute by one example applied in one of the companies I worked for. They based their calculations using the minimum operating level of the pit.

Edward Vera
Sr. Process Engineer Al Hosn Gas
United Arab Emirates

[Paul R OrlowskiKeymaster]

In case of operating level changed from higher to lower, the free space volume will increase. Does it full fill the requirements of NFPA655, since it is already calculated.

Pronab Mistry

[Paul R OrlowskiKeymaster]

Reducing the operating liquid level in a sulphur pit or a tank should NOT affect the already designed snuffing steam system. Because in most cases, the amount of snuffing steam would have been calculated based on entire enclosure volume (empty tank or pit). This would be resulting in a higher steam flow. The idea for considering such a high steam flow is only to develop a positive pressure (lesser than 2.5 psi-g in case of API 650 tanks) in the enclosure in order to prevent the outside air entering inside and extinguishing the fire. If in case, snuffing steam flow was calculated based on HLL or HHLL (wrong approach!!), then reducing the operating liquid level – or increasing vapor space – can reduce the degree of pressure which is to be developed in case of fire. And if the inside pressure drops and equals the atmospheric pressure, then the purpose of having snuffing steam would be defeated.

Huzefa Dhinda
Process Consultant at UniverSUL Consulting
United Arab Emirates

[Paul R OrlowskiKeymaster]

I have to agree with Huzefa – use the larger number. In re-reading the code, I would go even farther. The code refers to “volume” without making any clarification on vapor volume or total pit volume. Without that clarification, the only reasonable approach is to use the volume of the pit, not the volume of the vapor space.

Martin Taylor
Sulfur Technologist at Bechtel Hydrocarbon Technology Solutions, Inc.
Houston, Texas

[Paul R OrlowskiKeymaster]

The code specifies an inert gas system. Nitrogen sounds like the obvious choice because it eliminates the oxidizing agent – oxygen.

But iron sulfide (FeS) is created in the absence of oxygen – anaerobic conditions – which would be found in unvented tanks or tanks swept with nitrogen. The iron sulfide is fine in the “dormant” state when covered, but when tanks are opened to atmosphere it can spontaneously catch on fire (it’s pyrophoric). Since sulfur is also combustible, there is potential risk.

So you weigh the risk factor when using nitrogen. Do the benefits offset the safety issues caused by the formation of the pyrophoric iron sulfide.

Would anyone care to share experience when using nitrogen or other gas?

-Paul

[shimmyshakes183@gmail.comMember]

We have an operating plant running with nitrogen blanketed sulfur tank for 20 years with no issue.

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