Chapter 23. Summary Checklist for Reuse of Process Equipment.pdf

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Chapter

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Summary Checklist for Reuse of Process Equipment This chapter summarizes methods I have used in refinery and petrochemical plant process retrofit and expansion projects. The objective is always to reuse the existing equipment rather than to purchase new equipment. As long as a piece of process equipment can be used in place, internal modifications will normally have a minimal environmental impact compared to installation of a new pump, vessel, or heat exchanger. As may be seen from my tabulation below, expanding existing process equipment will also improve energy utilization of fired heaters, compressors, motors, pumps, and turbines. We should also try to avoid installing new plant utility systems, as described below. EQUIPMENT CHECKLISTS Fired Heaters 1. Minimize the draft to 0.05 to 0.10 in. H2 O as measured below the bottom row of convective tubes. Pinch on the stack damper and open the air registers. 2. Reduce air leaks in the convective section. 3. Blow or wash soot off the finned and studded tubes in the convective tube banks and shock tubes. 4. If burning fuel oil, knock or blow vanadium ash deposits off the radiant tubes. 5. Increase excess air. Unfortunately, this wastes energy. Process Engineering for a Small Planet: How to Reuse, Re-Purpose, and Retrofit Existing Process Equipment, By Norman P. Lieberman C 2010 John Wiley & Sons, Inc. Copyright


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Note: Air preheaters will save energy but will reduce the overall heater capacity if the heater is limited by the radiant heat density or the radiant tube coking rate. Heat Exchangers 1. Re-tube the bundle from carbon steel to stainless steel so as to retard fouling deposit accumulation. 2. Increase the tube-side passes if the pressure drop permits. 3. Replace the tube bundle with twisted tubes or helical baffle tube bundles. 4. Thermally shock (i.e., spall) tubes so as to remove the fouling deposits. 5. Back-flush or acid clean the tube side of water coolers. 6. Replace the bundles with low-fin tubes if both the shell and tube sides are clean. 7. For reboilers with the process fluid on the shell side, lightly sand-blast the tubes’ exterior, to provide nucleation sites for bubble formation. 8. Add shell-side seal strips for improved shell-side sensible heat transfer. Air Coolers 1. Maximize the fan blade pitch. 2. Increase the size of the motor pulley wheel. Check the torque rating of the fan blades first, and the motor FLA point. 3. Wash the fins from underneath the fin tube bundle. 4. Use vane tip seals to reduce air recirculation through the forced-draft fan. 5. Seal leaks between the tube bundle itself and the tube bundle support structure. 6. Reverse the rotation of the fan for a few minutes, to blow loose deposits off the underside of the tube fins. Distillation Tower Trays 1. Replace valve or sieve tray decks with Sulzer push-type MVG tray panels. Reuse existing downcomers and bolting bars. 2. Convert existing side downcomers on two-pass trays to segmental downcomers (but not segmental weirs). 3. Reuse the downpour areas, as bubble areas, using Nye-type inserts, or panels, below the downcomers. 4. Increase the downcomer clearances, but do not unseal the downcomers. 5. Increase the hole area by drilling holes in the tray deck periphery, but do not exceed 14% of the tray bubble areas. 6. Increase the downcomer areas by sloping downcomers using Z-bar clamps on existing downcomer bolting bars.

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Vapor–Liquid Separators 1. Retrofit with demisters, but only for clean, noncorrosive services. 2. Inject silicon defoaming agents, but only if the downstream silicon contamination can be tolerated on the catalyst. 3. Improve liquid-level control using radiation-type foam density-level detection, such as neutron backscatter. 4. Control the vessel bottom’s pump suction pressure, and drop the liquid level out of the vessel altogether. 5. Raise the vessel operating pressure so as to reduce the vapor velocity. 6. Add a vapor horn or impingement plate at the feed nozzle to dissipate the excessive inlet momentum. 7. Add a chimney tray to promote uniform vertical vapor velocities above the vapor inlet nozzle. Centrifugal Pumps 1. Replace the impeller wear ring. 2. Reduce impeller-to-case internal clearances back to the original design specifications. 3. Increase the impeller size. Note that power requirements increase with the cube of the impeller diameter. 4. Increase the trim size in the downstream control valve. 5. Increase the density of fluid pumped by reducing liquid temperature. 6. Subcool liquid at the pump suction, to increase the available net positive suction head. Reactors (Fixed-Bed Refinery Hydrotreaters) 1. Reduce upstream use of silicon defoaming agents. 2. Reduce upstream corrosion to reduce catalyst bed plugging. 3. Install baskets on top of the catalyst bed. Immerse the baskets in the top few feet of the catalyst. 4. Avoid exposing olefinic-type feeds to air, which will cause them to form gums and plug the catalyst beds. 5. Minimize entrained asphaultines in the feed, as they cause nickel and vanadium ions to fill the catalyst pores. (Note: High-conradson carbon feed that is not due to entrained asphaultines is never bad for the catalyst.) 6. Increase hydrogen partial pressures. 7. Improve mixing between the hydrogen and interbed flows. (See U.S. patent 3,855,068 “Apparatus for Inter-bed Mixing of a Fluid Quench Medium for a Vapor–Liquid Mixture,” by N. P. Lieberman, Dec. 17, 1974.)

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Electric Motors 1. Increase the full-limit amp set trip point. (Caution: This may reduce the time between motor rewindings.) Only small increases (i.e., less than 10%) are typically made. 2. Clean the screen in back of the motor. 3. Lubricate the motor bearings. 4. Trim the size of the impeller of the pump being driven by the motor. 5. Have the motor coils rewound. Gas-Fired Turbines 1. Clean the suction filter combustion air intake screens. 2. Detergent-wash the front-end axial air compressor blades. Steam Turbines 1. 2. 3. 4.

Make sure that all hand valves are open. Increase the size of the steam inlet nozzles in the nozzle block. Condensate-wash or degrease the turbine wheels. Make sure that the governor speed control valve is 100% open, by minimizing the
P value across the governor. 5. For condensing steam turbines, improve the vacuum in the surface condenser. Centrifugal Compressors 1. Increase the molecular weight of the gas if limited by the maximum rated compressor speed. 2. Minimize the molecular weight of the gas if limited by the compressor’s driver horsepower. 3. Spray a heavier liquid into the compressor suction to keep the wheels from drying out and fouling. 4. Suction-throttle the gas if limited by the compressor’s driver horsepower. 5. Clean the rotor wheels, especially if limited by the compressor’s maximum rated speed. Reciprocating Compressors 1. Remove the pulsation damper plates (or reduce their
P value by bigger orifices) from the suction and discharge lines. 2. Obtain an indicator card survey to identify bad valves and leaking piston rings. 3. Install adjustable head-end uploaders and remove valve disabler unloaders. 4. Optimize the spring tension in valves to minimize leakage, but without excessive pulsation losses.

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Air Blowers 1. Bypass the air-intake silencer if the measured pressure drop is more than a few centimeters of water. 2. Clean the air-intake suction filters and modify the intake screen to reduce
P. 3. Water-wash the rotor wheels. 4. Control the flow on the air intake suction, if limited by motor amps, rather than on the discharge airflow control valve. Water–Oil Separators 1. Increase the height of the riser inside the separator vessel. 2. Introduce the two-phase mixture via a distributor, which reduces turbulence in the separator vessel. 3. Back-flush the water draw-off connection with clean water. 4. Back-flush the interface-level tap connections with clean water to maintain an accurate level indication between the aqueous and hydrocarbon phases. 5. For electrical precipitators, clean the insulators so as to maximize the voltage across the grid, without excessive amperage. Piping Pressure Losses 1. Remember that piping pressure losses increase inversely with the diameter raised to the fifth power. Example: One mile of 3-in. pipe has the same
P value as 32 miles of 6-in. pipe. 2. Repair concrete liners in seawater piping systems used for cooling. 3. Chemically clean freshwater circulating cooling piping systems. 4. Replace the none full-ported isolation valves. An 80% ported valve has 2 12 times the
P, as does a full ported-valve.

EQUIPMENT LIMITATIONS Mechanical Limitations 1. Pump temperature limits are typically due to mechanical seal design temperature limitations. Change the seal, not the pump. 2. Pressure limitations can often be overcome by rerating MAWP using corrosion allowance to recalculate the mechanical strength of the vessel. 3. Specified heater temperature limitations are often process specifications rather than actual mechanical temperature limitation. Utility System Limitations 1. Instrument air. Find and fix air leaks. Disconnect air-operated tools and other noninstrument users.

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2. Cooling water. Repair holes in cooling tower distribution grids. Clean and repair cooling cell distribution decks. Repair slipping belts on fans. Isolate water coolers not in service. 3. Steam systems. Repair weatherproofing and insulation. Check the increase in the plant steam demand during a rainstorm. 4. Electric power. Eliminate control valves and replace with variable-speed, frequency-adjusted electric motors. Trim impeller sizes on pumps. Avoid compressor spill-back control, and use suction throttling. 5. Process water. Stripped sour water can be used as boiler feedwater for generating low-pressure steam. Steam condensate recovery should be 70 to 80%, not 20 to 30%! Check cooling-water cycles of concentration to minimize the cooling tower blowdown rates.

SAFETY NOTE Sometimes, my objective in the reuse of process equipment requires inspecting tower internals to determine plugged and coked internals. The following recent story is a reminder to be careful when performing this work. Kumar, the unit engineer, and I were inspecting a wash oil spray header for plugged nozzles and proper spray pattern in the gas oil section of a large coker fractionator (Figure 23-1). Based on coking of the wash oil grid, we expected to find several of the nozzles plugged. Our plan was to crawl under the spray header and observe the water flow of the spray nozzles. To access the wash oil header, we crawled down through a chimney. We were outfitted with oversized slicker suits, rubber boots, safety glasses, and hard hats.

Figure 23-1

Coker fractionator wash oil section.

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The fractionator had been open for three days. Water and cleaning solutions had been circulated through the spray header, and entry permits were in order. Kumar, being 40 years my junior, slipped through the chimney tray easily. The combination of old age and the slicker suit made it a challenge for me to follow him through the chimney. Regardless, after 10 minutes of effort, I was sitting on the packed bed with my head poking through the spray header. I had committed my first unsafe act. I had entered a confined space that I could not easily exit. I suspected that I could not climb back up the chimney without Kumar’s help. Now for the almost fatal error. As we suspected, some of the nozzles were plugged. I had asked a pipefitter to loosen the spray nozzles so that I could remove the plugged nozzles without tools. But those sections of the spray header piping upstream of the plugged nozzles had never been cleared of hydrocarbons with flush water. The next error was one of communication. I wanted the water pressure at the spray header inlet to be 10 psig, to match the normal spray nozzle
P of 10 psi. But this information was not relayed to the responsible operator. Thus, we had the full water system pressure of 100 psig at the spray header inlet. As a result of the 100-psi pressure drop through the nozzles: r The nozzles made a roaring sound that prevented Kumar, who was on the other side of the tower, from hearing me. r The nozzle produced a mist that fogged my safety glasses. Even after removing them, I could see only 2 or 3 ft. r The high water pressure blew hydrocarbons, trapped in the spray header lateral arms, out of threads around the loosened spray nozzles. Kumar’s H2 S alarm went off at once. I, as a contractor, did not have an H2 S alarm. Kumar decided to wait a few minutes to see if the H2 S alarm cleared. Not a good decision on his part! Within 2 minutes, the hydrocarbon fumes had made breathing difficult. My eyes were burning, I was coughing, and I became confused. In retrospect, both Kumar and I had suffered a loss of judgment. I recall thinking, “I can’t get out myself, and Kumar can’t hear or see me.” I could still communicate with the “hole watch” man. His H2 S alarm had also gone off outside the manway. I asked him to shut off the water and help pull me through the chimney. Stopping the water flow did not reduce the overwhelming hydrocarbon odor but did allow me to tell Kumar that we had to get out of the tower immediately. Unfortunately, Kumar could not do so, because I was stuck in the chimney. I was close to the manway and could get some fresh air. Trapped below me, Kumar was breathing hydrocarbon vapors. If we had more foresight, we could have provided an alternate exit by removing an additional hat above one of the other chimneys. With the hole watch guy pulling and Kumar pushing, I was extracted from the chimney. Both Kumar and I had headaches and felt tired the next day. The safety lessons I learned from this “near miss” are: 1. Do not enter a confined space if it is very difficult to obtain access. If the unexpected happens, getting out will be just as difficult and take just as long.

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SUMMARY CHECKLIST FOR REUSE OF PROCESS EQUIPMENT

2. Do not wear protective clothing that inhibits movement or that catches on metal parts. For example, my rubber boots filled with water, and I wasn’t able to pull my way through the chimney, due to their additional weight. 3. If a screwed or flanged connection is broken inside a vessel, it should be water-washed again and new entry permits issued. Breaking any process piping connections inside a vessel should void existing hot work or entry permits. 4. Contract personnel should have the same H2 S monitors as those issued to refinery personnel, even if the contractor will be working closely with the refinery representative. 5. When an H2 S alarm goes off, the person should not try to exercise judgment as to whether evacuation is really necessary. Just clear out! 6. Alternate escape routes should be established. 7. If a parameter is to be adjusted (i.e., the spray water pressure), make sure that the operator actually controlling the parameter is advised as to his or her responsibilities.

CHECKLIST SUMMARY As the reader may well imagine, after 46 years I have developed many hundreds of such methods as described above, to reuse existing process equipment in expanded service. These methods are, to a large extent, detailed in my books, which are listed in the preface. The general idea that I apply has always been the same. Use the existing equipment in place. Reusing process equipment in a new location is almost as wasteful as building new process equipment, as new piping, valving, instrumentation, and foundations are still going to be required. Whenever I start a new project, I always say to myself, “What I have now is what I’ll use.” Only as a last resort will I take my red pencil and sketch in on the process flowsheet a new heat exchanger, vessel, turbine, or centrifugal pump.