Beginning at the front-end of the plant, the Acid Gas Knockout Drum is the first vessel in the SRU. The purpose of the Knockout Drum is to protect the Reaction Furnace from liquid carryover from the upstream Amine Unit. Without the Knockout Drum in place, these liquids would carry over to the Reaction Furnace and cause severe thermal shock to the refractory, which may lead to a complete failure of this vessel. It is therefore important to ensure that this vessel is operating properly. Testing of the pumps, level controller, and the high-high shutdown should be conducted regularly. Heat tracing for the lines to and from the Knockout Drum should also be checked regularly.

SRE is pleased to offer SRU Design Consultation.
Sulfur Recovery Engineering SRE


Sulfur Recovery Engineering SRE
The SRU’s Combustion Air Blowers can be very sophisticated steam turbines or electric motors, or a combination of the two. The lubrication system is extremely important to minimize wear on the bearings and therefore should be fully understood by Operations and monitored closely. Low lube oil level and/or low pressure should result in an immediate shutdown of this unit. Inlet air filters should be kept clean to ensure that dirt and dust do not enter the system and that a high differential pressure shutdown can be avoided. The vent valve or ‘blow-off’ valve must be operational and set to control any possible situations that would cause the Combustion Air Blowers to surge.

Learn more about an SRU Capacity Study.


The Main Burner and the Reaction Furnace are extremely important pieces of equipment and therefore must be protected from potential damaging circumstances emanating from: Liquid carryover, which can cause plugging of the vanes/jets and/or refractory failure (followed shortly by a carbon steel shell failure); Burn back/flame impingement due to operation of the burner below the recommended minimum acid gas and/or fuel gas flow rates; An explosive shock wave due to improper lighting procedures; and Refractory thermal shock from heating or cooling rates greater than 50 degrees Centigrade per hour during SRU start-ups and shutdowns.

Does your Main Burner / Reaction Furnace destroy all potential Claus catalyst contaminants present in your SRU feed streams?

Learn more about an SRU Performance Evaluation. Burner Vendors
Sulfur Recovery Engineering SRE


Sulfur Recovery Engineering SRE
The Wasteheat Boiler (WHB) is a critical piece of equipment in a severe duty and therefore must be operated with great care. This includes proper protection of the tubesheet (exposed to the Reaction Furnace), which must have either the traditional ceramic ferrules/plastic refractory or the hex-head ferrules that are self-sealing. Failure of the tubesheet is a serious problem and requires an immediate, unscheduled SRU shutdown. The level controller and low-low level shutdown should be checked and verified regularly.

Has your facility experienced WHB tubesheet failures?

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Dry out procedures for refractory must be completed as per the vendor’s instruction, which will include a temperature ramping and holding program. Any deviation from this program may result in the refractory cracking or even failure (falling down). The worst case scenario is having to bring the SRU down shortly after acid gas has already been introduced. A third party SRU testing company should be used for these procedures to help control the fuel gas stoichiometry, while slowly adjusting the heating rates. Conducting the dry out properly can and will save you significant repair costs and plant downtime.

Contact us with any procedural questions.

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Sulfur Recovery Engineering SRE


Sulfur Recovery Engineering SRE
The Condenser level controllers and low-low shutdowns should be checked regularly to ensure that the tubes are never exposed to the process gas without a heat sink (boiler feed water, glycol, etc.). This can result in sagging of the tubes and tubesheet failure. The coalescing mesh pad in the outlet section must be intact and in good condition to minimize liquid sulfur entrainment to the downstream Reactor, TGCU, or Incinerator. Minimizing thermal shock during start-ups and shutdowns is also important to ensure that the tube seal welds on the tubesheet are not weakened. A serious tube or tubesheet leak will result in an unscheduled plant shutdown.

Is the process gas mass velocity through each of your Condensers Aligned with Best Practices?


Condenser Rundowns and Seal Legs must be jacketed (i.e. steam, glycol, etc.) to ensure that sulfur solidification does not occur. For steam jacketing, it is important to ensure that all steam traps are hot by taking a piece of solid sulfur, touching the surface of the trap, and observing that it melts. Catalyst beads, finings, corrosion products, and sulcrete can plug up the seal leg and therefore frequent observation into the ‘look-box’ should be performed to ensure steady flow of the produced liquid sulfur. Any abnormalities in the sulfur flow may be due to partial plugging of the seal leg, failure in part of the jacketing, and/or a tubesheet leak in the upstream Condenser due to a tube or tube sheet leak, which causes sulfur to solidify in the outlet section.

Seal Legs are often overlooked in the SRU – Are yours Properly Sized?

Sulfur Seal Vendors

Sulfur Recovery Engineering SRE


Sulfur Recovery Engineering SRE
Direct-fired acid gas or fuel gas Reheaters have the potential to cause Claus catalyst deactivation. Direct-fired acid gas Reheaters should be operated with a 60 to 75 percent burn stoichiometry (not 33 percent as with the Reaction Furnace) and direct-fired fuel gas Reheaters should be operated with a 95 to 99 percent burn stoichiometry. As with the Main Burner, it is very important to ensure that the burners are not operated below minimum flow rates, in order to protect the burner tip from flame impingement. Indirect Reheaters (steam) are the preferred reheat method in the SRU and must be protected from thermal shock to ensure the integrity of the tubes and tubesheet. The sulfur recovery potential of your SRU is a function of the specific reheat methods employed.

Contact us now to create a ‘What If’ Model of your SRU.

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Claus Reactors contain Alumina and/or Titania catalysts that must be protected from Claus contaminants/poisons, thermal shock (sulfur fires), amine, condensed water, liquid sulfur, and soot. Claus catalyst can be permanently poisoned by BTEX, heavy hydrocarbons, amine, and methanol. The thermal shock from sulfur fires, as well as condensed water and amine, can destroy the integrity of the catalyst, while liquid sulfur and soot only plug the catalyst pores (reversible).

An annual SRU Performance Evaluation

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Free Testing Quotation

Claus Catalyst Vendors

catalytic reactors


Instrumentation plays a critical role in the reliability of the SRU. The most important instrument in the SRU is the Tail Gas Analyzer or the ‘Air Demand Analyzer’. This instrument measures the concentration of H2S and SO2 in the tail gas stream and relays this information (i.e. ‘feedback’) to the combustion air control system, normally the trim air valve. This feedback signal allows the process chemistry to be maintained with a H2S:SO2 ratio of 2:1 for optimal Claus conversion. Any significant deviation from this ratio will result in an immediate loss in the overall recovery efficiency of the SRU. Regular evaluation of this instrument must be conducted, including calibration. Plugging of the sample line can be a result of inadequate heat tracing of the sample line and/or liquid sulfur entrainment from the final (or upstream) condenser. Other instrumentation that should be maintained on a regular basis include the acid gas and combustion air flow meters, Reaction Furnace optical pyrometers and ceramic thermocouples, igniter operation, flame scanners, direct-fired acid gas or fuel gas Reheater flow meters, level controllers, and high-high level shutdowns.

A comprehensive assessment of all SRU-related instrumentation is included in an SRU Performance Evaluation.

Contact us for a Free Testing Quotation. Analyzer Vendors


Sulfur Recovery Engineering SRE
Sulfur pit degassing is critical from both a safety and liquid sulfur product quality perspective. When an engineered degassing process is in place, the industry standard is for no more than 10 ppm H2S to remain in the liquid sulfur product. While the H2S content should be very low, concentrated pockets can indeed develop and pose a serious risk to those handling the final product. More specifically, it should not be assumed that these individuals are experts in terms of mitigating the risks associated with H2S. (i.e. knowledge of wind direction, body position, etc.)

An On-Site Liquid Sulfur Product Analysis provides invaluable information in terms of your degassing unit’s performance.

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The Thermal Incinerator or Thermal Oxidizer (TOX) serves to convert all remaining sulfur bearing species to SO2 prior to atmosphere release via plume dispersion from the stack. From an environmental perspective, it is imperative to minimize both sulfur and greenhouse gas (GHG) emissions. From experience, most Thermal Incinerator’s are not operated under optimized conditions. Here, operations that introduce excessive air or call for an elevated stack top temperature are associated with increased fuel gas requirements (and CO2 production). Due to increased combustion gas production, reduced residence time in the combustion chamber can negatively impact the destruction of the remaining sulfur bearing species from the SRU.

A Thermal Incinerator Optimization Program can reduce fuel gas requirements by upwards of 50 percent!

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Burner Vendors
thermal incinerator



SRE has partnered with Virtual Materials Group (VMG A Schlumberger Technology), allowing SRE to use the powerful VMG Sulfur Plant simulation software in-field (Symmetry Process Software Platform). Using years of accumulated SRE sulfur unit data, our team is able to accurately model any given SRU. In conjunction with our field results, the models provide important process values such as Reaction Furnace adiabatic temperature, Reactor bed dewpoints, and catalyst activity.

Contact us now to learn more about the creation of both Steady-State and Dynamic models of your SRU!