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Michael Chuba (Sunoco)

Sunoco typically looks at the use of isolation equipment on the reactor charge heaters on a case by case basis. The need for these devices is driven by the design configuration of the unit, the process stream being charged to the heater, the emergency depressuring capabilities installed on the unit, and LOPA/HAZOP analysis.

Sunoco’s general heater safety practice is to install a check valve in the combined process outlet of heaters to prevent backflow in the event of tube rupture if there are no downstream vapor depressurizing facilities or liquid blowdown facilities and if the heater operates at 150 psi or higher, and/or if there is a downstream liquid holdup of 350 cu ft or greater. Thus, as previously stated the requirement for a check valve is dependent on the application.

Typically on units where feed to the heater is H2 Treat gas only, these heaters generally have discharge check valves just upstream of where the H2 mixes with the oil as standard practice. On oil only heaters this same practices applies. It is primarily in mixed phase heaters where there is a tendency to find some units with no isolation device and others with discharge check valves. The need for an outlet devise is typically evaluated on a case by case basis. For instance a LOPA analysis on high pressure units with large downstream volume might show the need for additional layers of protection. In this case, a check valve might be added to the design and identified as “safety critical devices”. Being identified as a safety critical device would generally require the valve to be placed on a periodic inspection and test program. Valves of this nature are allowed an IPL credit of “1” during the LOPA analysis. Actuated ESD valves are generally not recommended due to the potential of inadvertent closure and the resulting dead head situation. In most units this would result in the need for additional relief valves for preheat exchanger protection. In addition, if designed with a SIL rating of “1” the ESD receives the same LOPA credit as a discharge check valve.

Vern Mallett (UOP)

UOP’s practice is to install a check valve at the outlet of a recycle gas only heater. UOP does not install a check valve at the outlet of a combined feed heater. The philosophy is that we want to try to keep liquid out of the heater if the recycle compressor stops. Some customers/contractors seem to feel that a check valve at the outlet of the heater will help 3 in some way to prevent a release in the event of a tube rupture at the heater. This ignores the rather large volume of equipment and piping upstream of the heater. In any event the whole unit will eventually depressure through the ruptured heater tube, so the presence of a check valve will not mitigate that. We believe that at least for a combined feed heater, it is better to put the money that would have been spent on a check valve into a more conservative heater design that won’t be as likely to sustain a tube rupture.

If an actuated shutoff valve were placed in this service, it would create a blocked outlet scenario for equipment upstream of the shutoff valve. That scenario might require that upstream equipment be designed for a much higher mechanical design pressure than would be required in the absence of an actuated shutoff valve. UOP does not specify an actuated shutoff valve downstream of a charge heater.

Brian Slemp (CITGO)

The three CITGO refineries have different design heritages therefore different configurations on the furnace outlet backflow prevention. Most of our hydrotreaters do not have backflow prevention on the reactor charge furnace outlets. One of our high pressure hydrotreaters with a recycle gas only furnace has a check valve without an actuated isolation valve. We have never had a tube failure in this system or any other indication that the check valve was ever utilized. The newest hydrotreaters in our system were evaluated for the installation of furnace outlet backflow prevention and the study indicated that check valves were not appropriate. The new unit design incorporated a rapid depressurizing system to minimize the duration and volume of any loss of containment. One of CITGO’s hydrocracking units did have a furnace tube failure and the back flow prevention on the outlet did help mitigate the release inside the firebox.

Vern Mallett (UOP)

UOP considers that an ESD valve between the cold separator and product stripper is not required and not recommended. The design philosophy behind this practice is to prevent liquid from filling the cold high pressure separator and carrying over into the auto depressuring system and filling the relief header. Once the liquid has filled the depressuring line to the relief header, and if the auto depressuring system were actuated, there is a possibility of damaging the depressuring system and relief lines.

However, some UOP customers require such an ESD valve, which UOP will incorporate into the design of the particular unit. When an ESD valve is specified instrumentation is designed for the ESD valve that will allow the valve close on low level as intended, and then to be reset once the level of the cold high pressure separator reaches approximately 10% of scale above the low level trip set point.

UOP’s philosophy is that the code requires that the downstream flash drum be provided with a properly sized relief valve to handle the loss of level in the upstream drum. Since we have the relief valve at the downstream vessel sized for the loss of level, a cutoff valve in the hydrocarbon line from the high pressure vessel is not going to provide any more protection. In addition, it introduces an additional failure mode that is undesirable. Often such cutoff valve systems are set up with manual reset. Therefore if a low level (or just a system failure) causes the valve to shut, then it is likely that the separator will overflow because the operator may not have time to go out and reset the solenoid valve (or find out what failed and get the valve open again) before the separator overflows. When that happens, the separator and recycle gas line will fill up with liquid and eventually shutdown the recycle gas compressor. At this point the unit is not safe. If it is desired to depressure the unit it will not be possible because the separator and piping to the depressuring system will be full of liquid and therefore the depressurizing capability is lost. The downstream vessel s already protected according to the code. Therefore UOP does not recommend a cutoff valve at the separator

The downstream stripper should have a relief valve that will be sized for the loss of level case per the requirements of the code. The only issue is the location of the relief valve. In order to avoid tray damage on loss of level, the relief valve could be located at the feed tray.

Michael Chuba (Sunoco) 

Although this needs to be evaluated on a case by case basis, general practice has been to properly size the relief valve protecting downstream equipment based on the blow through scenario. Doing this eliminates the need for a separate emergency shutdown (ESD) valve between the high pressure separator and the downstream lower pressure rated equipment. If the relief valve is not adequately sized, at a minimum a high pressure protection system or HIPPS system would have to be installed. The HIPPS system is a safety instrumented system (SIS) that would have to be designed with a reliability equal to or greater than that of a pressure relief valve. Thus, the safety integrity level (SIL) of this system could be as high as three or greater. Because the control is on level and the potential fouling nature of the material in these vessels, designing a SIL 3 type interlock is very difficult. Therefore, the preferred approach is to ensure that the CV of the letdown control valve and its bypass valve match the RV of the downstream equipment.

To assist in operation and control of the level in the high pressure separators most recently designed or modified separators are equipped with independent transmitters for High and Low level alarms. In addition LOPA analysis typically finds that an additional independent high level interlock switch is required to trip the recycle compressor if a separate KO drum is not present.

Alan Leute (El Paso Refinery)

For typical Hydrotreaters (up to nominal 1000 psi Separator pressure), we design the Stripper overhead relief valve to handle blow through of hydrogen/vapor from total loss of level in the Separator(s).

There may be additional concerns with this for high pressure Hydrotreaters (greater than 1500 psi Separator pressure) and hydrocrackers, but Western Refining does not have any of these units.

Praveen Gunaseelan (Vantage Point Energy Consulting)

The question is general; however, there are a variety of steam reformer designs with different operating and control strategies used industrially so a general answer is provided. Operators are urged to consult with their technology providers or qualified engineering contractors for specific guidance on this issue. For simplicity, it is assumed that natural gas is the hydrocarbon feed being reformed with steam. Steam reforming of methane (natural gas) is an endothermic reaction which requires heat input that is typically provided by the combustion of process off-gas and make-up natural gas. A number of scenarios can result in furnace overheating, including:

•Loss of feed flow (resulting in reduced transfer of heat from the furnace)

•Over-firing

•Hot spots in the reformer tubes (e.g. due to catalyst maldistribution, poisoning, deactivation, etc.)

•Flame impingement on reformer tubes, resulting in stress failures

•Inadequate heat removal during start-up

Accordingly, there are a number of strategies to prevent overheating of SMR furnaces that address potential overheating scenarios, and which need to be used in combination to minimize the possibility of an occurrence. Typical strategies are listed below:

•Increased surveillance, both visual and with devices such as pyrometers, during start-up as well as during normal operation

•Minimize the possibility of hotspots (e.g. closer monitoring of catalyst loading, etc.)

•Reduce the occurrence of poor flame patterns

•Implement advanced control schemes to reduce the possibility of or recover from overheating situations

Brian Moyse (Haldor Topsoe)

First of all regular measurements of reformer tube temperatures and inspection of the radiant side of the reformer is very important.

 New hydrogen plants will often be designed with software that allows continuous control of the firing duty with proper adjustments made as function of the hydrogen capacity. All the PSA off-gas is used as primary fuel and the firing control is handled by the secondary fuel.

Obviously a proper catalyst loading with only a minor variation of tube pressure drops is mandatory for proper flow distribution through the tubes. Maldistribution may cause individual tubes to overheat.

Proper catalyst selection and operating conditions should be adopted to avoid carbon formation on the process side, which will lead to hot-banding and tube overheating.

Burner operation should be checked regularly to avoid flame impingement on the tubes.

The Haldor Topsoe reformer furnace is designed to allow for profiling of heat input from burner rows, resulting in catalyst tubes operating at a higher average temperature and heat flux at lower peak tube temperature. This profiling of heat input in itself reduces the risk of overheating of the furnace, but also the measurements of the tube skin temperature made on a regular basis will prevent from operating the reformer with too high tube skin temperatures.

However, the biggest threat to the tubes is a sudden over-heating, which will damage the tubes very rapidly. This could in principal be due to:

•Loss of cooling (feed)

•Too high fuel input (overfiring). Fuel is controlled with a duty controller, summarizing all fuel duty input.

To prevent from such sudden over-heating, the reformer is protected by low feed and steam flow trips, and the fuel is controlled with a duty controller, summarizing all fuel duty input, to prevent overfiring.

Further, as secondary protection, the reformer is protected by flue gas temperature measurement. This is the fastest indicator of over-firing, as process gas in a cold collector can increase in 15-30 seconds if the firing is increased rapidly and a high temperature trip is initiated.

Brian Slemp (CITGO)

The best strategies to prevent overheating steam methane reformer tube:

1)Ensure the catalyst tubes are pressure drop balanced. Reformer furnaces have an inlet manifold to evenly distribute the feed. An increase in pressure drop from an improperly loaded catalyst tube will reduce the flow of the reactants in that tube increasing the tube temperature and potentially leading to coking and overheating.

2)Routinely monitor the skin temperatures and balance the fire box heat distribution to prevent localized hot spots.

3)Prevent direct flame impingement.

4)Routinely monitor catalyst activity with your vendor to prevent operating in the carbon deposition region and generating internal thermal insulation thereby overheating a tube.

5)Maintain proper steam to carbon ratio.

6)Monitor feed composition to ensure potential coke precursors are not introduced above allowable limits.

7)Ensure high purity steam is used to prevent catalyst poisoning and coke generation.

8)Closely monitor tube temperatures and nitrogen flow during startup.

Tina Moss (Johnson Matthey Catalysts)

If not monitored properly, a variety of operating issues can lead to high steam reformer furnace tube wall temperatures. High tube wall temperatures are most often due to poor catalyst loading, problems during start-up operations, or carbon formation due to poisoning. Therefore, the best strategies to prevent overheating of steam reforming furnaces are to address the operating issues that can cause high tube wall temperatures.

The first step in avoiding high tube wall temperatures is to ensure a good catalyst loading that avoids flow imbalances and large catalyst voids. The general tolerance on flow variations between tubes is ± 2.5% for a good catalyst loading, which corresponds to a pressure drop variation of ± 5%. The most common loading technique used by North American refiners is the trickle loading method. There are several trickle loading techniques available in the market and these can typically achieve between 2 to 5% pressure drop variation.

Problems that result in furnace overheating often occur during plant start-ups. Feed flow imbalances on the process side of the reformer tubes often occur during start-ups when rates are significantly less than design. To minimize the impact of the feed flow imbalances, nitrogen flow needs to be sufficient to provide a heat sink as the burners are going through a sequenced start-up. Once steam flow is introduced it provides a significant heat sink but needs to be at least 30% of, and preferably 40 to 50% of the design rate as soon as possible to allow even firing of the furnace. In addition, the operating pressure during start-up should be kept low to maximize process gas velocity and hence improve gas flow distribution as well as minimize the pressure differential across the tube walls reducing the stresses on the steam reformer tubes. While these concerns are most prevalent during start-ups, these conditions can also be present when there are significant rate fluctuations.

 Another source of high tube wall temperatures is the result of catalyst deactivation. High tube wall temperatures can occur due to carbon formation caused by insufficient steam rates or poisoning of the catalyst. Insufficient steam to carbon ratios can result if steam flows are not appropriately modified following feedstock changes. Low steam flow can result in carbon deposition on the catalyst and overheating of furnace tubes. Poisoning of the catalyst that results in carbon formation is typically due to sulfur. The best strategy to prevent catalyst deactivation by poisoning is to ensure good purification operation and monitoring during operation. In the event that carbon formation has occurred, one potential strategy is to steam the catalyst to remove the carbon formed. This is most effective if the catalyst loaded in the top is alkali promoted

. Steam reforming furnaces are very expensive, complex units. Overheating of these units can result in reduction in hydrogen production and equipment failure. Creating a strategy to address proper catalyst loading and start-up procedures, as well as, transient conditions during daily operation will help to avoid steam furnace overheating.