Sulfur Recovery Unit (SRU) Storage Tank Replacement

A used LPG storage vessel was modified and put into service as the sulfur tank for SRU#1 at the Cheyenne, WY, refinery, currently owned by HollyFrontier. Over the years, several repairs were made to that tank’s shell and internal steam piping. HollyFrontier determined that the tank should be replaced, due to concerns with tank integrity and safety if the tank remained in service for the longer-term. The following short-term challenges for a replacement plan for the sulfur recovery unit (SRU) were presented by 1) a desire to minimize downtime, 2) the tank being located in a very congested area and 3) the possibility that removal of the tank could destabilize soil around foundations for an adjacent pipe rack.

To address the concern with minimizing downtime, a new, free-draining, jacketed line was built to route sulfur from the sulfur recovery unit #1 seal legs under a road to the sulfur recovery unit #2 sulfur pit. With the old tank bypassed, installation of the new tank could proceed while sulfur recovery unit (SRU) #1 was running.

The other two challenges were addressed in the replacement plan for the sulfur tank. Previous inspections indicated that the bottom half of the tank shell had experienced little corrosion. It was therefore decided to leave that bottom half in place, to serve as a support for the new tank. In order to ease installation, the new tank was built in five U-shaped steel plate sections, field welded together after placement. The roof of the new tank was made of concrete poured in place. Care was taken to design durable and well heated roof penetrations by our engineering team. The sweep air system is designed to provide more than adequate flow, in order to prevent any combustible mixture. The sweep air entering the tank is also heated well above ambient in order to minimize corrosion. The new tank was successfully put into service.

Background – A Corroded, 38 Year-old Sulfur Tank

In 1978, a used 40 ft S-S x 10 ft O.D. LPG storage vessel had its saddles and some nozzles removed. A few new nozzles were installed along with internal steam coil piping. The tank was then buried even to grade, in order to serve as the sulfur tank for a new 2 bed, hot gas bypass sulfur plant.

In 1984, a new, 30 LTPD indirectly reheated 3 bed Ford Bacon & Davis sulfur plant was installed, which re-used the same sulfur tank. Over the years, many corrosion related repairs were made to this sulfur tank, including multiple internal steam pipe repairs, and a 2003 replacement of a portion of the shell that included the two pump openings.

After years of patching the tank roof and other repairs, HollyFrontier decided to replace the tank, concerned that its continued use could risk leaks causing odor and subsequent H2S exposure. Another concern was possible down time due to tank failures.

sulfur recovery unit

A concrete slab at plant grade covered part of the old tank, but left a 4'-6" wide x 40 ft long section open for connections. This section was covered by insulation. A 5 ft wide structural steel frame supported grating above that insulation. As seen in the picture below, that insulation was in a low area, the edges of which were below grade. This made that insulation susceptible to periodic wetting, likely contributing to corrosion underneath.

sulfur recovery unit troubleshooting

Indeed, after the tank was taken out of service, and insulation and covering was removed, corrosion underneath is seen in the picture below.

Phase 1 – Bypass the Old Tank: New Line to another Sulfur Pit

Another sulfur plant, sulfur recovery unit #2, has its sulfur pit is across a road from SRU#1. Further, the natural grade resulted in the sulfur recovery unit #2 pit roof being 23" lower than the entrance to the sulfur recovery unit #1 tank. Thus, a free draining line from SRU#1 to SRU#2 was possible.

SRU

Old SRU#1 steam jacketed 6"x8" header receiving sulfur from seal legs.

sulfur recovery unit

Steam jacketed cross at end of SRU#1 header, going down into the old sulfur tank

Current and planned work by HollyFrontier and the Job Industrial engineering and modeling team for SRU#1 and the SCOT Unit could increase sulfur recovery unit #1 sulfur production up to 49 LTPD. Although this rate could be handled by a free draining 4"x6" jacketed line, it was decided to continue use of 6"x8" jacketed piping for the new sulfur header and the new line to the SRU#2 pit.

Keeping the same line size simplified the new SRU#1 header design to simply adding a branch and two jacketed plug valves. That allows routing SRU#1 sulfur to either the SRU#1 tank or the SRU#2 pit.

The plan view at right shows the old sulfur header in place, taking sulfur from the SRU#1 seal legs. The new sulfur header (shown in red) is lined up to replace the old header. The new sulfur header has two jacketed plug valves and a branch to new piping.

An overall plan view shows this new header and piping to the SRU#2 pit. A total of about 83 ft of new 6”x8” jacketed pipe was installed.

 

sulfur recovery unit engineering

SRU engineering

The minimum slope of the new piping is 3/16" per foot for the 55 foot west to east run that crosses the road (in 2 spools).  The second road-crossing spool (Spool 5) is surrounded by an externally coated 16” casing when going under the road.  That jacketed spool was insulated prior to insertion into the casing.  The bottom of that casing is approximately even with the old top of the road.  Concrete retaining walls were built at the edge of the road, in order to contain enough fill to cover the casing about 6" deep, and slope back to the existing road surface at a 6% grade.

sulfur engineering design

At the sulfur recovery unit (SRU) #2 Pit, an 11" hole was bored into a clear section of the sulfur pit roof between seal legs #1 and #2, to serve as the entrance for piping from SRU#1.  The edge of this hole is about 1" inside the pit side wall.  Boring this hole was done by workers under fresh air, and a shutdown of SRU#2 was not needed.  A 316L SS plate and sleeve assembly supports the final spool from SRU#1.

sulfur recovery unit design

sulfur recovery unit storage

Final 8"x6" jacketed pipe and cross entering the SRU#2 pit.

Phase 2 – Replacing the old SRU#1 Sulfur Tank

There was economic incentive for continued SRU#1 operation during replacement of its sulfur tank.  With a line in place to bypass the tank, the remaining goal was to streamline and minimize the impact of tank replacement.

Phase 1a – Civil Work

Previous inspections found little corrosion in the bottom half of the old SRU#1 tank.  Therefore, early in the project, It was decided that a new tank could be supported inside the bottom half of the old tank.  This decision shortened the time to remove the old tank, and minimized any excavation. Since a pipe rack is next to the tank, it was desirable to minimize disturbance of existing soil around the pipe rack footings.

The sulfur tank is under considerable overhead piping and other interference.  West side access was effectively blocked by the sulfur condenser and its seal legs.  Some access was available through the pipe rack support steel on the east, and from the south.  The picture below shows several obstructions above the old tank.  The old tank here has insulation and grating removed.

SRU project experience

It was decided that the best way to fit the new tank in would be to build it in U-shaped sections, welded together when in place.  The sketch below shows that concept.

sulfur engineering project experience

The five individual sections are built of 1/2" A-516-70 steel, with 3x3x3/8" A36 angle on one end, along both top sides, and spanning across the top on each end.  The plates are beveled on each end for welding from the tank inside, when in place.

A cross section of the central tank section is shown below, with supports for the internal 4" steam heating pipes.

SRU tank installation

The diagram below shows civil details of the new tank installation.

tank engineering

The diagram above shows a 9" thick concrete tank roof and 12" thick end and side walls.   After tank placement and welding, the grout was poured to fill space between the old tank and the new.  The side wall rebar was placed, and the side walls poured.  Ribbed 16 GA galvanized decking panels were placed across the top, to a width spanning to the middle of each side wall.  Decking edge pour stops were installed, and needed deck openings were cut.  Then rebar and roof opening sleeves were placed.  Finally, asphalt-based water-stop stripping was placed around the side walls.  The roof was then poured.

Although the galvanized decking could last a considerable time in the sulfur tank atmosphere, if kept hot, no credit was taken for roof support by that decking.  The roof contains ample rebar for long term strength and support of the concrete roof, its connections, and personnel on the roof.

Edges of the galvanized decking sections were overlapped, and tack welded together.  Openings in the roof are lined with sleeves made of 1/2" thick steel pipe or plate.  To these sleeves, angle or channel was welded to support the sleeves and tie into the rebar.  Nelson concrete anchors were also welded to the sleeves.  The rectangular pump sleeves were seal welded to the decking.  Some other round sleeves were tack welded to decking, and edges of most openings in the decking were sealed with silicone prior to pouring the concrete roof.

Tank in place, welded together, steam heating pipes installed, and rebar in place, ready for pouring the side and end walls.

Phase 1b – Tank Internals

Most roof openings are for either steam jacketed tank connections, steam or condensate.  Even the pumps have steam jacketed discharge risers.  Heated surfaces all penetrate the roof through liners, so that no concrete was cast around or is in direct contact with those heated surfaces.

Most steam jacketed connections penetrating the roof have jacketing that continues through the thickness of the roof.  This prevents restrictions or blockages due to crystalizing or freezing sulfur.  Connections with such extended jacketing include the tank corner air inlets, the air outlet going to the eductor, the natural draft air outlet, the sensing tubes connected to the DP level transmitter, and a 30" tank manway.

Heating of the SRU sulfur tank and various lines is accomplished by two levels of steam.

50 psig (295°F sat temp) is used to heat the following.

  1. The sulfur seal legs and line into the SRU#1 tank.
  2. The sulfur pumps in the tank.
  3. Lines from the sulfur pumps to the loading rack.
  4. 150 psig (365°F sat temp) is used to heat the following.
  5. The heating pipes in the bottom of the sulfur tank are heated
  6. The four jacketed corner vent stacks are heated
  7. The jacketed air inlet line that starts 44 ft above grade is heated
  8. All jacketed connections penetrating the tank roof are heated.

The use of 150 psig (365°F) steam is not usual for heating liquid sulfur lines.  However, the Cheyenne refinery has for many years used 150 psig steam in the above services in both the old SRU#1 tank, and in equivalent services in the SRU#2 pit.  This has caused no apparent problems.

Pure sulfur undergoes a huge viscosity increase as it heats above 318°F increasing from 6.4 cP to 948 cP at 319°F to 2940 cP at 320°F, to 93200 cp at 365°F. This viscosity increase is due to breaking the S8 ring molecules as the liquid sulfur heats up. The broken rings re-assemble into extremely long chains of sulfur atoms, which cause the viscosity increase.  However, Claus plant sulfur always contains a small amount of dissolved H2S.  The majority of that H2S inserts into these chains, breaking them up into shorter H2Sx chains, which dramatically lowers the increase in viscosity.

A side view and an end view of the new sulfur tank are on the next two pages.  Locations are shown of the various connections, the pumps, and internal steam heating pipes.

SRU

sulfur recovery unit engineering

Finished view of SRU#1 tank from north, looking south.

SRU sulfur engineering work

Finished view of SRU#1 tank from north, looking south.

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Finished view of SRU#1 tank from southeast corner, looking north.

Design Improvement – Revised Ventilation of Sulfur Recovery Unit (SRU) #1 Sulfur Tank

Sulfur produced in a Claus SRU always contains some amount of dissolved H2S. This H2S slowly evolves from the liquid sulfur into the tank vapor space. It is important that either air or an inert gas sweeps through the sulfur tank, in order to keep H2S concentration well below its lower explosive limit.

A 3”x4” jacketed line brought air from 44 ft above grade into the south end of the old SRU#1 tank. 25 ft north of that, a 3”x4" steam jacketed line routed air from the tank to an eductor which exhausts into the incinerator. Sulfur from the SRU#1 seal legs entered the tank 8 ft further north of the outlet to the eductor. There was no other air inlet. Thus, 8 ft of tank length from its inlet was not swept with air. However, there was a small nitrogen purge piped into the sulfur entering the tank. There was no known history of vapor space deflagrations or explosions in the SRU#1 tank.

The new sulfur tank has air inlet vents near each of its roof corners. It is common to see sulfur tank inlet vents at the surface of the roof with no jacketing or other heating. However, some sulfur tank/pit installations in cold climates have jacketed inlet vents. This includes the SRU#2 pit at the HollyFrontier Cheyenne refinery. It is suspected that heating the air coming through such jacketed pipe helps prevent corrosion near the vents and, possibly, on other steel surfaces within the tank.

sulfur tank

An above-ground sulfur tank in Wyoming, but not at the Cheyenne refinery, with non-heated air inlet vents. Note the partial collapse of those vents, caused by corrosion.

The new SRU#1 tank has 6 ft tall 3"x4" jacketed air inlet stacks near each of the four roof corners.

The 44 ft high air inlet and the vent line to the eductor are still in place. Normal operation is with the eductor operating to pull air into the tank from all 5 of these inlets. If the eductor is not operating, natural drafting will result in the 44 ft high stack pulling air through the tank from the four lower roof corner stacks. In both cases, air is swept over the whole area above the sulfur in the tank. Also, for both cases, flow rates of sweep air were calculated for a wide range of ambient temperatures. In all cases the sweep air kept H2S concentration well below 20% of the lower combustible limit.

The two following diagrams show example tank sweep air flows and estimated emissions for operation with and without the eductor in service. In both cases, air within the tank is estimated to be well above ambient.

SRU tank

SRU tank replacement

Detail for the four corner air inlet stacks. The restriction (1" i.d. "washer") at the very top of this inlet is designed to allow sufficient vent air, but not excessive air. Excessive air could somewhat cool the tank interior.

SRU tank engineering

Closing

The new SRU#1 sulfur tank has operated without problems since October of 2016. The SRU#1 to SRU#2 pit sulfur line is still in place and can be used if needed.