Crude 2 Questions Flashcards
- What is the purpose for the C-6101 Recycle Wash Water?
Why do we inject ammonia into the Wash Water?
The purpose of the Recycle Wash Water is to protect the overhead-to-crude exchangers from ammonium chloride corrosion and fouling.
To do this enough recycle wash water must be injected so that some of the water remains as liquid water instead of vaporizing.
It is necessary to have liquid water throughout the E-6101s to dissolve chloride.
If liquid water is not present, the ammonium chloride plates out on the tubes. This increases the corrosion
rate considerably.
The electric motor driven water injection pumps, P-6150 and P-6150A pump water from D-6101 into C-6101 overhead vapor line.
An injection nozzle is used in the vapor line for better dispersion of the wash water into the vapor stream.
This injection rate is controlled by FC-1038.
This water keeps the ammonium chloride salts dissolved and thus prevents chloride pitting of the overhead exchangers.
Excess water from D-6101 is routed through
LC-1049 to D-6106 on level control.
- What should we do if the level in C-6101 reaches the fifth rams horn?
Explain why this is important.
Anytime the liquid level is above the 5th Ram’s Horn, stripping steam
should be pulled to prevent tray damage in the stripping section.
If the level rises above the steam inlet, then the steam throws the liquid against the stripping trays.
This usually damages and displaces these trays.
- Why should we limit our exposure to Benzene?
What is the permissible exposure limit – time weighted average (PEL-TWA) for an eight-hour workday?
Present in LSR and HSR streams.
Hazards:
Known Carcinogen
Exposure must be limited
Slightly toxic to internal organs
Can cause severe damage to lungs
Exposure should be limited.
Permissible exposure limit – time weighted average (PEL – TWA) is 1 PPM for a normal eight hour workday.
- What are the Triconex chopper valve trips for F-6101? Include LOPA conditions.
- Low fuel gas header pressure (PT-428) *Start up permissive only
- Low fuel gas burner tip pressure (PT-4159 / 5159)
- Low pass flow (FT-181B/C through FT-188B/C)
- Loss of draft fan (FT-374/375)
- C6101 over pressure (PT-307A/B/C)
- Big Red Pushbutton (HS-432A/B)
- C-6101 High Level (LN-0106A/B)
- C-6102 High Level (LN-0138A/B)
F-6101 is equipped with a Triconex safety shutdown system.
The system monitors critical process information and will automatically shut down the furnace if the process violates established Triconex trip parameters.
In most cases dual redundancy is provided for greater
reliability.
Triconex will require both transmitters indicate a shutdown condition before the
system will activate a shut down sequence.
The LOPA Safety System, installed in 2010, is part of the Triconex safety shutdown system.
Triconex is equipped with a pretrip warning system.
If pretrip conditions are reached, an alarm activates alerting the operator of the process approaching shutdown conditions.
In order to activate the pretrip alarm both transmitters must be in pretrip condition.
Triconex pretrip is only a warning and will not initiate a system shutdown.
A furnace shutdown is accomplished by closing the fuel gas supply chopper valves, XV409, XV4006, and the LER II waste gas chopper valve.
Closing these valves stops all fuel supply to the furnace burners.
When a Triconex shutdown is activated, a solenoid trips the latch at each fuel gas chopper valve causing the valve to close.
To reset the system, process conditions must be corrected before the chopper valves can be latched.
Under certain conditions Triconex will take the furnace to minimum fire for a designated time period before initiating a full shut down by closing flow controller.
If process conditions can be corrected before the system times out, a shutdown can be avoided.
When a Triconex trip activates calling for minimum fires, a solenoid trips the latch on the fuel gas control valve causing the valve to close.
To reset the system, process conditions must be corrected before the fuel gas control valve can be latched.
- What are the Triconex chopper valve trips for F-6102? Include LOPA conditions.
- Low fuel gas header pressure (PT-433) *Start-up permissive only
- Low fuel gas burner tip pressure (PT-4158 / 5158)
- One low pass flow (FT-284B/C through FT-291B/C)
- Loss of Draft Fan (FT-374/375)
- C-6102 Overpressure Shutdown (PT-317A/B/C)
- C6101 Overpressure Shutdown (PT-301A/B/C)
- Emergency Shutdown Switch
- C-6102 High Level (LN-0138A/B)
F-6102 is equipped with a Triconex safety shutdown system.
The system monitors critical process information and will automatically shut down the furnace if the process violates established Triconex trip parameters.
In most cases dual redundancy is provided for greater reliability.
Triconex will require both transmitters indicate a shutdown condition before the system will activate a shutdown sequence.
Triconex is equipped with a pretrip warning system.
If pretrip conditions are reached, an alarm activates alerting the operator of the process approaching shutdown conditions.
In order to activate the pretrip alarm both transmitters must be in pretrip condition.
Triconex pretrip is only a warning and will not initiate a system shutdown.
A furnace shutdown is accomplished by closing the fuel gas supply chopper valves.
Closing the valves stops the fuel supply to the furnace burners.
When a Triconex shutdown is activated, a solenoid trips the latch at the fuel gas chopper valve causing the valve to close.
To reset the system, process conditions must be corrected before the chopper valves can be latched.
Under certain conditions Triconex will take the furnace to minimum fire for a designated time period before initiating a full shut down by closing the fuel gas control valve.
If process conditions can be corrected before the system times out, a shutdown can be avoided.
When a Triconex trip activates calling for minimum fires, a solenoid trips the latch on the fuel gas control valve causing the valve to close.
To reset the system, process conditions must be corrected before FC-167 can be latched.
- What is the purpose of the Preheat Train?
The Primary Preheat Train raises the temperature of incoming crude to 270-280°F prior to entering D-6125A Desalter
It also reduces duty on F-6101 Atmospheric furnace thus reducing fuel gas demands and cost
- What function does PC-102 serve?
Where is it located and where does it sense its signal from?
Crude Feed Pressure Controller
It controls the pressure of the primary preheat train.
It senses it’s signal from the outlet of E-6175 or Upstream of LC-1026
- Why is it important to routinely sample unit products?
- What is the purpose of the Desalters (D-6125A&B)?
Crude oil contains impurities that would cause problems in CU2 equipment if not removed.
Salts, solids, and water are removed from the crude in a process called desalting. Desalting is carried out in crude units to control three problems:
* Corrosion
* Fouling
* Water in crude oil
The most typical salts are chlorides, sulfates, and carbonates of sodium, magnesium, and calcium.
Most of the salts in crude are dissolved in water, but, in some cases, crystalline
and/or organic salts may be dispersed in the oil.
Although salt composition depends on the crude and the formation it was taken from, in most cases the primary salts are chlorides.
Chlorides are acid producing salts that create hydrogen chloride.
Hydrogen chloride causes corrosion in the overhead systems of crude columns.
Sulfates and carbonates are hardness salts that form scale when water vaporizes.
Scale and solids cause fouling.
Corrosion: Magnesium and calcium chloride are the primary sources of corrosion due to the hydrochloric acid that is produced.
Sodium chloride also contributes to corrosion but to a much lesser extent.
The high temperatures in CU2 promote a chemical reaction known as hydrolysis in which chloride salts break down to form chloride gas (Cl).
This mixes with water vapor to form hydrochloric acid (HCl), which is extremely corrosive.
The other products of hydrolysis, calcium and magnesium hydroxide, leave in the Bottoms stream of the C-6101 along with un-reacted calcium chloride.
These may cause problems in downstream processing of heavy ends if they are left in high concentrations, for example, sodium poisoning of catalyst in the RDS.
Caustic injection is used to help stabilize the magnesium and calcium chloride salts that break down due to hydrolysis.
Fouling: Hardness salts produce different kinds of problems than chlorides do.
The sulfates and carbonates of magnesium and calcium form scale when water vaporizes in the heat exchanger trains, causing fouling.
Fouling in heat exchangers reduces heat transfer and therefore requires extra fuel in the Furnaces to achieve the same crude temperatures.
Water vaporization explains why the crude must be desalted before the crude oil temperature is preheated beyond 300°F.
Water should not vaporize at this temperature given the pressure present in this exchanger train, which should be at least 250 psig.
The other contributor to fouling is solids. Solids are divided by size into two categories:
* Sediment - larger than 20 microns
* Suspended solids - smaller than 20 microns
Sediment has the following characteristics:
* Generally 0.05 to 1.0 percent by volume of crude oil
* Particle size ranges from 20 to 2,000 microns
* Derived from formations or production techniques
* Water insoluble inorganic solids such as sand, silt, clay, and silicates
* Surfaces that are water wettable
* Relatively easy to remove from the crude oil
Along with brine, sediment is one of the main impurities in crude oil.
More than 90 percent of sediment is normally removed in the desalting process, which is why mud washing the desalters on a routine basis is necessary.
Solids removal efficiency depends on residence time, temperature, oil viscosity, wettability, and particle size.
Larger particles settle out faster than small ones.
Suspended solids of less than 20 microns are small, inorganic solid particles that remain suspended in the oil.
They originate primarily as corrosion products and as contaminants introduced during transportation.
Suspended solids have the following characteristics:
* Amounts range from 1-200 ptb (pounds per thousand barrels)
* Size smaller than 20 microns
* Inorganic solids, insoluble in oil or water
* Gather in the surfaces of brine droplets and prevents their coalescence
* Composition (silicon, sand, silt, alkali metal salts not dissolved in brine)
* Surfaces are saturated with oil
One of the most notable characteristics is that these solids have surfaces saturated with oil.
Instead of collecting in water droplets, suspended solids gather around the water’s surface and prevent coalescence.
Stabilized by these particles, small brine droplets tend to gather at the desalter interface layer where they cause the emulsion band to grow, making the separation of water from the oil more difficult.
Because these very fine particles are saturated with oil and attached to the outside of water drops, removal requires water wetting the particles so that they will migrate into the water droplets and be removed.
The wetting agent is called a surfactant. Another aid in wetting these particles is to ensure adequate mixing of the crude oil and the injection water.
Water in Crude Oil: The last function of desalting is removing the water that came in with the crude oil.
Mixing and the use of the surfactant chemical are important.
Water that carries over the top of the desalter with the oil is called carryover.
The amount of carryover must be minimized because of the wasted energy required to heat it in the preheat train and Furnaces.
Large amounts of carryover also results in vapor loading of the Furnace
and Column.
Oil that does not separate from the brine at the bottom of the desalters is
carried out to a tank where it is skimmed off the top and called skim oil or recovered oil when it is fed again.
Feeding this oil again is also wasteful and can upset Column operation if it
brings brine back with it.
The brine stream leaving the plant is sampled.
The oil residue is called carry under.
The separation of the oil and water is aided with a chemical called demulsifier, which helps eliminate the oil/water emulsion or cuff at the interface of the two.
This chemical is injected into the crude in front of the Primary Feed Pumps to give good mixing.
Desalter summary.
The Desalter system is the most important in terms of long run-time for
2CU.
It is possible to run the plant with them by-passed, but fouling and corrosion increase.
The currently-projected five-year runs could never be realized without crude oil desalting.
Some important concepts of Desalter operation are:
- The acceptable salt content in the desalted crude is set by the processing
requirements of the receiving Unit. Salt contents of 0.3 ptb or below are always
acceptable. If lab analysis indicates that salt content is too high, Desalter operation
must be corrected. - The main control on salt content is water make-up rate. Use of seal drum
condensate, 27 Plt. & 84 Plt. waters should be maximized. Then bring in process
water after making sure that the other Desalter operation variables are within their
normal ranges. Desalter temperature should be at maximum limited by the Teflon
bushings. - The main controls on oil carry-under and cuff thickness are desalting chemical
addition rate, mixing valve pressure drop and pH. The pH should be checked first.
If pH has increased above 7, check pH of Desalter make-up water sources. If pH
is normal, increase desalting chemical addition rate. If this does not clear up water
after two hours, decrease mixing valve pressure drop.
If Desalter temperature is too low, carry-under will increase.
Sufficient Desalter crude preheat is more of a problem at low feed rates.
There are several strategies used to try to trade heat around under this condition. It may be impossible to achieve a Desalter temperature close enough to 300°F to assure good separation at low feedrates.
- A high percentage of heavy crude oil in the plant feed also makes good separation
more difficult. Remember that all oil carried-under to 184tk has to be eventually
re-fed.
Check the Desalter effluent for carried-under oil frequently each shift.
The Desalters remove water, salts, and suspended solids from the crude prior to additional heat exchange.
Salts found in crude are primarily sodium chloride with small amounts of
calcium, magnesium, iron, aluminum chlorides, sulfates, and bicarbonates.
Chloride salts decompose to form hydrogen chloride (HCl) in the atmospheric and Vacuum Furnaces.
HCl forms hydrochloric acid in the presence of water.
This acid can severely corrode Atmospheric and Vacuum Column overhead systems.
Salts and solids foul heat exchanger and furnace tube surfaces resulting in reduced heat transfer and higher pressure drop in these units.
The desalters operate at 280-320° F because a higher temperature in the primary train would promote salt lay down in the exchangers.
Also, any temperature above 320°F would affect the Teflon insulators on the electrical grids.
The crude oil should contain, on the average, about 30-70 pounds of salt per thousand barrels (PTB).
It will also contain about 0.5 LV% of water and solids (BS&W).
The desalters are located in the primary preheat train between exchangers E-6104 and E-6105.
There are two desalting stages.
The oil/water mixture flows through the two desalters in the series.
The first stage desalter, D-6125A, removes about 96% of the salt.
The second stage, D-6125B, removes the remaining 3.3%.
The two stage desalter will decrease the salt level in the crude to about 0.5 PTB from an incoming level of 30-70 PTB.
The BS&W leaving the desalter should be 0.1 LV%.
Water mixes with the crude at three locations.
The total amount of water added to the crude is about 5 liquid volume %.
Three fifths (3 LV %) is added at the discharge of the feed pump, P-6101.
This water helps protect the heat exchangers ahead of the desalter from salt
deposits.
The remaining two fifths of the water are added to the crude between the first and
second stage desalters.
Water recovered in the second stage is recycled back to the inlet of the first stage where it mixes with the incoming crude.
The water recovered in the first stage desalter is the net water removed from the crude.
The water flows through exchanger E-6119, which preheats the fresh water going to the inlet of the second stage desalter.
The water is cooled to about 200°F in E-6119 and flows to the API separators or tankage where the water is de-oiled.
- What is the purpose of the mix valves?
Ensures water contacts salt compounds in crude feed for removal
Mix valve operation:
Too much mixing allows salt and water carry over to Flash Drum
Too little mixing allows salt and water carry over to Flash Drum
The degree of mixing imparted to the oil and water is a function of the pressure differential, Delta P, across the mix valve.
Mix valves are designed to shear the large water droplets in to many smaller droplets.
Mix valves.
The key to successful salt and solids removal in the desalting operation is water contact of the contaminants in the oil phase.
This is primarily accomplished through a good mix/water rate ratio.
The degree of mixing imparted to the oil, water, and chemical as they flow through the mix valve is a function’ of the pressure differential Delta P across the valve.
Mix valves are normally double ported globe valves that are designed to shear the large water droplets into many smaller droplets.
This action creates greater water surface contact area and produces patterned spray dispersion for maximum water coverage inside the pipe diameter.
The goal is to disperse the fresh process water as thoroughly as possible without over mixing and forming an emulsion so highly stabilized that it is difficult or impossible to break.
The mix Delta P is the most important and frequently used variable in the desalting process utilized by the Crude unit operators to control the performance of this system.
Optimum mix Delta P’s vary with different crude and different crude combinations so a “feel” for each feed composition must be acquired through operating experience and adjustments made to maximize the effectiveness in that particular situation.
The recommended procedure to establish the optimum mix requirement for new crude is as follows:
Set wash water rate at a given point normally 4-5% to start and leave this rate and
other variables as constants while mix requirement is being determined.
Both “A” and “B” stage mix valves should be started wide open and then increased in 3-5# increments.
Testing for salt and BS&W should be conducted on the desalted crude out of each stage after a sufficient time at each Delta P setting normally 45 minutes to 1 hour.
Continue to increase the mix Delta P until an increase in water and/or salt is noted in the desalted crude sample.
When this occurs, mix valve should be opened slightly to eliminate the increase and
maximum mix is considered established.
It should be pointed out that with some oil/water mix may be attained by velocity of crude though the primary train and a naturally greater tendency of the higher salt and BS&W raw crude entering the first stage to form emulsions; typically the mix on D-6125A will be less than with the cleaner crude entering D-6125B. For this reason, mix requirements for each stage should be established separately.
If excessive mix is required to attain desired salt removal by the above procedure, mix valves should be opened, wash water rate increased 1-2%, and procedure repeated at the higher water rates.
- What purpose does mud washing serve?
Uses clean water from MP-6124/A to help remove sediment and solids from the Desalters
Mud washing:
To assist the Utilities Area with the effluent treating system, the mud washing
will be performed between 12:00 midnight and 02:00 daily.
Many of the solids that are removed from the oil phase have a tendency to settle to the bottom of the desalting vessel.
This reduces volume capacity of the desalter and affects valuable residence time.
Clean water at the Tricocks and dirty effluent is normally an indication that solids have accumulated on the bottom of desalter and the effluent has to channel through this buildup to get out.
This condition would indicate a need for mud washing.
- How can the interface level in the Desalters be tested to assure the LC is reading correctly? Explain, be specific.
Each desalter has five Tricock sample valves which extend into D-6125A from the bottom of the vessel from 24” to 72” which can be used to determine exact level of oil/water interface, cuff thickness and to double check level transmitters.
Two capacitance level alarms, LH-5017 and LL-5018, have been installed in D-6125A,
Desalter, which alarm when the interface level rises and drops.
The probes are permanently installed and are not to be retracted.
- Where can the Desalter effluent water be routed? What is the normal routing?
Ballast Tank
OWS
slip stream to suction of P-6101/A
- What is the purpose of the Flash Drum (D-6130)? Where are the streams from the flash drum routed?
The purpose of D-6130 Flash Drum is to vaporize ”flash” all of the water in the desalted crude oil along with the light ends.
This reduces fouling in the downstream exchangers.
Water that is not flashed causes fouling since the dissolved salt is converted to salt crystals that plate out on the exchanger tubes.
Flashing the crude also removes light ends from the crude.
This reduces the pressure drop through the secondary preheat exchangers and the furnace thus reducing pumping cost.
The vapors from the flash drum flows directly to the atmospheric column flash zone.
The Flash Drum liquid flows to the P-6102 Crude Booster Pumps.
This flashing process also reduces the load on F-6101.
This subsection discusses Flash Drum control mechanisms for:
* Pressure control
* Level control
* Foaming
* Temperature control
* Flow control
Pressure Control.
The Flash Drum pressure controller, PC-1027, is located on the vapor line leaving the Flash Drum. It is routed directly to the Atmospheric Column flash zone.
The Flash Drum pressure is determined by C-6101 pressure.
The controller is locked open allowing the Flash Drum to ride on the atmospheric column pressure.
This will maximize the vapors sent directly to the atmospheric column.
PC-1027 was removed during the 2007 rebuild.
Level Control.
The level controller for the Flash Drum is located downstream of the E-6105 exchanger.
The Flash Drum is elevated to hold good suction head on the secondary crude feed pumps without increasing Flash Drum pressure.
The Flash Drum level controller, LC-1026 regulates the desalted crude oil flow to the Flash Drum.
The level should be kept at about 65 percent of scale to provide surge volume for the
crude booster pumps.
This level provides about five minutes of surge.
Foaming.
The most serious problem that can occur in the flash drum is foaming.
If it foams and carries over into the atmospheric column, the crude will contaminate the fourth sidecut draw.
This crude contaminated HDN feed will seriously damage the Isomax catalyst.
When indications shows the flash drum is starting to foam, then the fourth sidecut stream should be watched closely for crude contamination.
If this occurs switch the fourth sidecut to off test until it clears up.
Foaming causes the LC transmitter to give a false indication.
This could cause lose of level and cavitation would occur in the secondary feed pumps, P-6102’s. Also the false indication could cause the crude level to go up and result in crude going overhead in the vapor line ending up with cool crude in the flash zone of C-6101.
Having too low a temperature in the flash drum usually causes foaming.
Normal operating temperature range for the A.
Heavy crude mixture is 315°-320°F.
Temperature Control.
A hot crude recycle stream from E-6109 is used to maintain the bottoms temperature at around 310°F.
TC-2134 controls the flow (usually around 16MBPD) of hot crude recycle that enters the bottom of the Flash Drum which provides the correct temperature to prevent foaming and the right amount of flashing.
Flashing cools down the crude oil because vaporization is a heat-consuming process.
When the Flash Drum is operating properly, the Flash Drum bottoms temperature should be about 5°-15°F below the Flash Drum feed temperature.
Flow Control.
Desalted crude oil from the Flash Drum flows to P-6102/A, Secondary Feed Pumps.
Immediate action is required if one of the P-6102 pumps shut down and the APS
pump does not come on line.
P-6102s are also responsible for maintaining sufficient
pressure upstream of the furnace pass flow controllers to prevent vaporization.
If flashing were to occur, the pass flow control valves would lose control.
This would greatly increase the chances of coking up one or more furnace passes.
The minimum crude oil pressure is about 340 psig.
This is sufficient for crude oil leaving the secondary preheat train at 570°F or below.
If the pressure falls below the minimum, check spill-back of P-6102.
- Where do the PRD’s from the Desalters relieve to?
The Flash Drum
- What product stream does the secondary preheat train primarily cool?
Where does this stream come from, and what equipment is it routed through?
The flashed crude from the bottom of the Flash Drum (D-6130) is pumped through the secondary preheat train with the secondary crude feed pumps P-6102 and P-6102A.
This increases the preheat temperature from 315°F to approximately 480°F.
