Q & A > Question Details
What are the likely effects of water carry over from desalter on Crude heater and distillation column? What steps should be taken if this happens?
03/01/2012 A: Saugata Palit, Intercat India Ltd, spalit@intercatinc.com
For water carryover, the immediate effect is that there will be drop in feed temperature to column, drop in column botom temperature, loss in column bottom level, colouration of lighter products, shootup in column top pressure, etc... If the water carryover is in excessive amount, the column bottom pumps will vapour lock and the plant will have to be shutdown for better measures.
In the long term effect, column top trays and overhead lines will face corrosson. The furnace tubes will also face shock due to expansion of carry over water to steam in the furnace. This will have negative impact on furnace tubes and over the times may lead to tube failure.
For remedial action, if there is a water shot accompanying crude, the desalter temperature will drop sharply. Taking this as an indication, go immediately for water draining from deslater. Maximize demulsifier dosing (shock dosing) and check the width of rag layer from sample points regularly. If water level touches the transformer grids the transformer will trip. This should be avoided, by draining desalter to maintain the level. The charge rate should be lowered to avoid plant shutdown. Stop water injection to desalter.
If the conditions do not improve even after all these steps and water gets carried over to column, it's better to shutdown the plant as the bottom pump will cavitate and starve the downstream heaters.
16/05/2011 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Water carry over into desalted Crude is likely to carry small amounts of salts and fitlerable solids that float on the Crude-water intephase in rag layers. As salts leads to corrosion issues which are normally addressed by Crude tower overhead corrosion prevention practices, the presence of filterable solids leads to preheat train fouling for which specific solutions are yet to be in place. Water carries the filterable solids into the the downstream equipment. As the water evaporates in the preheat train, the filterable solids tend to deposit on heat exchange surfaces and lower the preheat recovery. This will lead to increased fired Fuel consumption which is a permanant loss. Filterable solids are contributed by high BS & W and slops processing in CDU.
02/03/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Mr. Bowers concerns about safety issues from water carryover are greatly overstated. The water leaving the desalter is under pressure and the heat up process through the preheat train is gradual so you don't tend to get the explosive type vaporization he describes. The vaporization in this situation is much more like what happens as the crude oil begins to flash. It is possible to concoct some scenarios where explosive vaporization could occur in a crude preheat train or furnace, but they are very unlikely.
The phenomena of explosive vaporization is very real, but not in this type of situation. Explosive vaporization occurs when the water is suddenly introduced into a very hot system at low enough pressure that allows the water to rapidly vaporize and expand. A classic example is when a slug of wet stripping steam enters a hot fractionator, where the resulting pressure surge tends to blow out the trays above and below the injection point. In a very severe case I know of a slurry oil separator in an FCC that exploded shortly after start up when a line that had not been properly purged of water was opened up into the vessel. The water hit the hot slurry at over 600 F and instantly flashed, ripping open the vessel and killing several workers. These types of scenarios are very unlikely when the source of water is as far upstream in the process as a desalter.
27/05/2010 A: SUNDEEP GUPTA, BECHTEL, sundeepgupta1@rediffmail.com
Excess water carryover is undesirable. It creates problems of corrosion in the crude column overhead circuit. It is important therefore to minimize this carryover; however heavier crudes being processed nowadays can lead to an increase in water carryover. Differential condensation in column overhead employed in newer designs can reduce this problem to an extent.
22/05/2010 A: Vineet Singh, IOCL Panipat Refinery, SINGHVK007@GMAIL.COM
Water carry over in main fractionator may lead to corrosion in top trays over period of time. in such case immediately reduce the column stripping steam, reduce desalter interface level, reduce delta P across mixing valve, reduce water injection to crude, start manual water draining from desalter, check colour of top products.....
21/05/2010 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
There is always water coming out of the desalter with the crude, it varies with crude gravity, lighter the crude less water will be carry (between 0.1 to 0.3%), the heavier the crude higher the amount of water (0.8 -1.2%), these values should not create problems either the heater not the column; the problem with higher water carryover is the associate salts with the water that will produce more HCl, also the sodium will increase the heater tubes fouling (standard criteria is to keep sodium content in the residue below 20 - 30 ppm.
An upset at the desalter is a different issue, in this case a slug of water will be entrained with the crude causing an increase in pressure drop at the heater, also the outlet temperature of the heater (COT) will drop (because water is being vaporized, so the amount of fuel being burned will increase), this will create a pressure surge at the flash zone of the crude tower which could dislodge the upper trays.
21/05/2010 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There can be many effects and severity of problems will depend on how bad the water carry over is. Water will waste energy - each barrel of water takes roughly 6-7 times the energy to heat as does a barrel of crude oil. Having a preflash tower does not impact the energy penalty as much as might seem on the surface. In addition salts and solids carried over with the water can lead to fouling of downstream equipment when the water eventually vaporizes. Salts carried with the entrained water will also hydrolyze and increase the mass of chlorides going overhead. Because the water make off the overhead accumulator is also going up the increase in OH chloride concentration may not be dramatic even though the total mass of chlorides is up dramatically.
If the water carryover is bad enough it can over pressure the atmospheric column. I know of one case several years ago where water carry over from the desalter overloaded the overhead fin fan hydraulically. Enough of the steam condensed that the accumulator never reached relieving pressure, but the DP across the cooler was so high that the atmospheric column was over pressured (PSV on the accumulator was supposed to be protecting the tower too.) The tower operated for several hours this way because no one realized that the tower was being over pressured.
If the problem is not severe, you may just need to reduce the mix valve DP, up the emulsion breaker dosage, and/or lower the water level in the desalter to reduce the water carry over from the desalter. You would probably also want to check the OH water pH and may need to increase neutralizer until the desalter gets back to normal. If the problem is severe as described above you would try to rapidly lower level in the desalter, probably turn off the wash water until things came back under control and maybe cut crude charge rate.
21/05/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
If the quantity of water is fed as a continuous phase (a water bubble) and is larger than a pint or so, the near explosive pressure surge on its evaporation in the heater will likely upset/bend/destroy the trays above the feed nozzle.
If a gallon or more of water suddenly enters the fired heater tubes, they will likely rupture and may destroy the heater.
In any event if liquid water carry-over (not finely dispersed haze or droplets, but a slug of water) is detected before it reaches the heater, immediately stop feed and stop heater fuel, and then pray to HE WHO KNOWS ALL or whomever your Supreme Deity is---because real nasty things are about to happen and depending where you are right then, you may die a fiery death.
21/05/2010 A: Luis Marques, Galpenergia, luis.marques@galpenergia.com
If this happens for sure you will damage your bottom column distillation packing or trays the heater tubes can also be damaged. If this happen there is not much you can do. To avoid this situation you have to act upstream. Normally these kinds of blow ups happen during feed changes. A good practice is to have a good communication between process people and storage operations and be sure that the tanks that are feeding the unit were well purged to avoid excess of water carry over into the unit.
21/05/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Because water is the solvent for the extraction of salt from the crude, any carryover of water from the desalter allows salt through the equipment in the rest of the unit. Corrosion and fouling will be accelerated. When emulsion problems occur in the desalter, either water carries into the unit as you describe or oil carries under to the biological treatment unit. Both are undesirable long term. Emphasis should be on improving the operation of the desalter.
21/05/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
If water gets feed to the Crude Fired Heater (and distillation column) the adverse effects depend almost entirely on HOW MUCH. Small amounts well dispersed will be noticed only in the water boot on the top reflux drum. How small is small? 1% water by distillation after desalting is a lot, but if well dispersed and uniformly fed, the primary impact will be an increase in fired heater duty by about 3-4% and a very large increase in duty on the overhead condenser (the ratio of total overhead/crude X 1% x 4 is approximate percent increase 4 being the approximate ratio of water heat of condensation / that of light straight run.
However, if significant water,as a separate phase (slugs of) is fed to the fired heater, severe damage or even total destruction of the heater is possible or probable, depending on the volume of the water slug.
Water has an expansion ratio of approximate 1200:1 when evaporated to steam---which is many times that of hydrocarbons. The crude furnace tubes operate near or above the critical temperature of water, and tremendous pressure will develop almost instantly when the slug of water reaches the hot furnace tubes.
So, your question has a common answer of IT DEPENDS. Small amounts of well dispersed water will mostly effect overhead condenser duty and top trays water handling capacity.
A slug (bubble) of water larger than about 100ml will cause a near explosive rate pressure surge, that may or may not damage the furnace and upset the lower trays in the tower. A water bubble of one (1) gallon or more will LIKELY cause damage to the tower trays. Still larger quantities will almost certainly cause major damage to and even total destruction of the fired heater and distillation column trays, and the ensuing fire may well destroy the entire unit and......
Uncontrolled water carry-over into a fired heater is DEADLY. even a small quantity fed as a water phase (bubble in the oil) can cause massive damage.