1.0 Tank Farm
1.1 Mixers Use while filling tank
1.2 Settling time > 24 hours desirable Ras Tanura follows an alternative approach for Arab light and tight lines the crude from the supply line to the unit.
1.3 Salts Measure and record
1.4 BS&W Measure and record
1.5 API slops None Keep slops separate from crude tanks. Feed directly to
process if needed so that problem slops can be quickly identified and isolated.
1.6 Organic chlorides Avoid
1.7 Tank dewatering chemicals Could be evaluated site by site for economic benefit.
2.0 Desalter
2.1 Temperature 120 to 150°C Target. Temperatures lower than 120°C result in less effective desalting.
Limit: Temperatures higher than 150°C damage equipment.
2.2 Residence time 15 minutes for oil Typical value. Longer residence times such as 30 minutes better.
60 minutes for water Minimum value. Longer residence times would be better.
2.3 Mixing valve ΔP 5 to 28 psi (0.35 to
2 kg/cm2) Typical values. Mixing valve adjusted on-site to optimize desalting efficiency. Re-evaluate with each crude tank switch or crude slate change. High ΔP gives more efficient mixing (good) but if too high can result in tight emulsions (bad).
2.4 Salts-out < 1PTB Target. However, consistent performance with stable
salt-out essential.
2.5 BS&W < 0.2 % Target. Saudi Aramco crude streams gain water
through the desalter.
2.6 Desalting efficiency ≥ 95% Target. Consistent operation essential. Desalting efficiency can be lower if it still results in a low salt content of < 1PTB.
2.7 Wash water feed rate 4-6 % of crude charge Higher wash water rates are usually uneconomic in Saudi Arabia. Rates lower than 3 % will result in very poor desalting efficiency.
2.8 Wash water pH-in 5.5 to 6.5 Target. The pH range 5.5 to 8.0 is normal. The more acidic pH range when achievable keeps tramp amines in the water phase.
2.9 Wash water pH-out 5.5 to 6.5 Target.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
2.10 Wash water O2 < 20 ppb Target. Highly desirable to limit corrosion but difficult to achieve due to limited water sources.
2.11 Wash water Cl- < 15 ppm Target.
2.12 Wash water NH3 < 20 ppm Target.
2.13 Source of wash water Good water sources include crude overhead receiver
boot, recycled desalter water, condensate or stripped water from sour water stripper. Water streams from other refinery sources must be used with caution as they may result in corrosion or fouling.
2.14 Demulsifiers 3 to 25 ppmv Chemical additives are not presently used in Saudi Aramco desalters. If used in the future, treatment rates would be expected to range from 1 pint to one gallon per thousand barrels of crude.
2.15 Mud wash 15 minutes per shift, 150
gpm Highly recommended for effective desalting
3.0 Caustic Quality
3.1 Source Fresh caustic only Spent caustic results in tramp compounds entering the system and causing corrosion, fouling, emulsions, and foaming.
3.2 Concentration 1 to 5 wt % (2 to 7° Baume) Typical values. Dilute caustic aids mixing. Identical concentration must be provided. Variation in caustic strength injected to process stream is a major cause of preheater fouling.
3.3 Measurement Each batch Essential. Measure the concentration of each and every batch of caustic to be used in the plant prior to use. Data must be stored in a permanent record.
Injection of off-specification caustic at one plant caused stress corrosion cracking and an economic loss of over $1 million. Injection of off-specification caustic at another plant caused excess fouling.
3.4 Dilution water O2 < 20 ppb Stripped sour water is a good source.
3.5 Dilution water Cl- Zero Target.
3.6 Storage tank Target. The larger the better to minimize batch
make-up operation and variation. Nitrogen blanket to exclude oxygen. Mechanical mixers.
3.7 Caustic delivery to unit Dedicated line Strongly preferred. A dedicated line from the bulk caustic tank to the unit day tank facilitates the correct dilution of caustic. At least two plants that uses a complex caustic header system have experienced major problems with cross contamination and delivery of out-of-specification caustic that resulted in major operational problems.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
4.0 Caustic Injection
4.1 Injection location NACE recommends and 80% of the industry injects
caustic downstream of the desalter. The suction of the crude booster pump is the normal location and assists efficient caustic mixing. In cases where preheater fouling is an issue, then caustic injection upstream of the heater is allowable. Ras Tanura Plant 15 has had good long term success with this latter methodology.
4.2 Injection temperature Maximum 350°F Best practice of the majority of the industry. Shell restricts caustic injection temperature to 350°F.
Nalco’s Best Practice restricts caustic injection to 300°F. Higher temperatures can be used but only if fouling of preheat exchangers or some other site specific need makes it essential. Fouling can be readily detected by measurement of pressure drop across the preheater. Higher temperature injection requires careful and continuing attention to every detail as failures at higher temperatures can be catastrophic.
Caustic injection rates are fine tuned based on overhead chloride levels. Initial rates for a new unit follow the rules of thumb provided. The maximum amount injected is limited to less than 2 PTB NaOH.
Amounts injected may be limited further due to effects on downstream process such Visbreakers, FCCUs, and hydrocrackers.
4.4 Injection quill material Monel 400 Saudi Aramco experience has demonstrated that this material serves well whether the injection point is located downstream of the desalter or upstream of the heater. Injection quill design shall follow the recommendations of Saudi Aramco Best Practice SABP-A-015, Chemical Injection Systems.
4.5 Injection quill length To allow injection in the center third of the pipe.
Injection quills must be the correct length to allow injection in to the center third of the pipe.
4.6 Injection slip stream Dilute caustic 1:100 with
crude slip stream Use of a slip stream aids dispersion of the caustic and helps to minimize caustic-caused corrosion problems.
Slip stream is effectively mixed prior to injection using a Monel 400 static mixer. All caustic pipe and fittings shall be Monel 400.
4.7 Injection orientation Co-current with crude flow. Nozzle discharges downstream.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
4.8 Quill installation PMI Positive Materials Identification of the injection quill and all slip stream pipe and fittings must be performed on site to verify metallurgy. Incorrect materials shall not be fitted.
Radiography Location of quill tip shall be proved after installation
by radiography of the crude pipe. A permanent record of the radiograph will be retained by the refinery.
4.9 Quill design Non-retractable with
match-mark indicator Correct positioning of the quill is critical. Therefore, a non-retractable design shall be used. A match-mark indicator is used to show the orientation of the nozzle.
4.10 Crude pipework upstream of the
injection point PWHT for minimum of 3
diameters upstream Required that the pipework, fittings, etcetera, be post weld heat treated to prevent caustic stress corrosion cracking. Failures due to CSCC have occurred in Saudi Aramco plants.
4.11 Crude pipework downstream of
the injection point PWHT all piping systems
and equipment. Required that the pipework, fittings, etcetera, be post weld heat treated to prevent caustic stress corrosion cracking. Failures due to CSCC have occurred in Saudi Aramco plants.
5.0 Overhead
5.1 Velocity 75 feet/second Target optimum value for carbon steel systems. Since corrosion is enhanced by velocity, lower velocities will provide lower corrosion rates. Velocity control in existing plants is usually a compromise between what is mechanically achievable and required system throughput.
< 100 feet/second Mandatory limit for carbon steel systems. Velocities in excess of this value will be a major contributor to premature failure.
< 150 feet/second Allowable velocity for corrosion resistant alloy systems. Higher velocities are probably achievable as industry knowledge with alloy systems develops. For mixed metal systems with some carbon steel pipework and some alloy pipework, ensure that the velocity limits for carbon steel are not exceeded.
5.2 Velocity calculation Various There are many proprietary programs available. The results are only as good as the data that is fed into the program. Remember to include all water sources including stripping steam and carryover of water in naphtha reflux, if any.
5.3 Dewpoint > 15°C lower than line
temperature Target is for the vapor dewpoint to be at least 15°C lower than the operating temperature before the water wash. Various computer programs are available from vendors or engineering services to estimate dewpoint.
Must be sure to include all stream components.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
5.4 Salt point > 15°C lower than line
temperature Target. The salt point is the temperature at which neutralizer /ammonia chloride products condense and can cause fouling and corrosion. Water wash removes these deposits. Vendors provide proprietary programs to calculate salt points for their specific products.
5.5 Pipe metallurgy Carbon steel Carbon steel is the standard material for overhead systems. Usually, hydrogen sulfide levels are below the minimum necessary to mandate hydrogen induced cracking resistant steel, but must be examined on a case by case basis versus SAES-L-133.
Hastelloy C-276, and C-22 For critical systems where other control measures have been unsuccessful; evaluate based on system
economics.
5.6 Thermal insulation Thermal insulation up to the water wash reduces
condensation but can hamper OSI programs.
6.0 Corrosion Inhibition
6.1 Pump Positive displacement
metering pump Ensure that the size is appropriate.
6.2 Filter Strainer Required on naphtha slip stream, preferred on
inhibitor line. 100 mesh typical.
6.3 Type Oil dispersible film former.
Nalco 5186 and Baker 81202 approved for use.
Contact CSD Corrosion Technology Unit and local refinery SARCOP team if there is a need or wish to chemicals.
6.4 Treatment rate 3 to 5 ppmv of total naphtha
product and naphtha reflux. Typical. Depends on product used.
6.5 Injection location After 1st elbow Typical. Most Saudi Aramco crude units inject corrosion inhibitor downstream of the 1st elbow and downstream of the neutralizer injection point. Ras Tanura primary injection point is ahead of the fin fans. Secondary injection ahead of the fin fans may be appropriate for refineries with flow distribution problems.
> 5 diameters from neutralizer
injection Rule-of-thumb but difficult to achieve with present physical layouts.
> 5 pipe diameters from
downstream elbow. Strongly preferred to minimize damage on downstream elbow but difficult to achieve with present physical layouts..
6.6 Slip stream 100 naphtha to 1 inhibitor. Dilute inhibitor in naphtha stream. Flow measurement on inhibitor and naphtha streams essential. 100 mesh screen required.
6.7 Quill design Retractable Mandatory. Allows maintenance on-stream.
12 o’clock Normal orientation.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
Inject in center third of stream Preferred to inject in the center third of the stream to ensure even distribution away from pipe walls. In large systems it may be impossible to obtain a quill that can be retractable, i.e., removed on-line and meet this criteria. In this case, the minimum insertion into the pipe flow must be no less than 6 inches.
6.8 Quill metallurgy Hastelloy C-2000, B-2 Hastelloy C-2000 quills or Hastelloy B-2 are a good choice. Monel shall not be used for inhibitor service.
Any existing stainless steel quills should be replaced at the next T&I.
7.0 Neutralizer
7.1 Pump Positive displacement
metering pump Ensure that size is appropriate.
7.2 Filter Strainer Preferred. 100 mesh typical.
7.3 Treatment rate Treatment rate is adjusted to give required overhead
receiver pH. Strong acid test can be used to provide a method to calculate the target injection rate that will assure neutralization of the first drops of condensing acid.
7.4 Injection location After 1st elbow Neutralizer must be injected into the overhead system. Injection into the reflux is bad practice.
> 5 pipe diameters from
downstream elbow. Strongly preferred to minimize damage on downstream elbow.
7.5 Steam co-injection Required. Use steam co-injection to ensure
neutralizer is vaporized and adequately dispersed.
Use lowest pressure steam that meets design need.
Inject into the overhead at no more than 5 psi over stream pressure.
7.6 Quill design Retractable Mandatory. Allows maintenance on-stream.
12 o’clock Normal orientation.
Inject in center third of stream. Preferred to inject in the center third of the stream to ensure even distribution away from pipe walls. In large systems it may be impossible to obtain a quill that can be retractable, i.e., removed on-line, and meet this criteria. In this case, the minimum insertion into the pipe flow shall be no less than 6 inches.
7.7 Quill metallurgy Hastelloy C-2000, B-2 Hastelloy C-2000 quills or Hastelloy B-2 are a good choice. Monel shall not be used for neutralizer service. Any existing stainless steel quills should be replaced at the next T&I.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
8.0 Wash Water
8.1 Source Overhead receiver Recycle water from the overhead receiver makes an excellent choice.
8.2 Quality O2 < 20 ppb Required. Oxygen in wash water results in major corrosion damage.
TDS Total dissolve solids typically are in the region of 160 ppm in Saudi Aramco operations. Low figures are preferable but not controllable.
Tramp amines Avoid the presence of tramp neutralizing amines.
Neutralizers introduced with the wash water help to control overhead receiver pH but do not help control pH in the first condensing drops of acid in the overhead.
8.3 Injection location Downstream 2nd elbow Preferred location is immediately downstream of the second elbow after the crude column.
8.4 Injection rate 5 % volume of overhead
naphtha Target is to maintain at least 25% of the injected water in the liquid phase after injection. Failure to meet this level may result in premature condensation of acid salts and severe corrosion. New wash water designs must be checked using phase behavior modeling.
8.5 Measurement and control Accurate measurement and control of wash water flow is essential especially in locations where on water feed is used to supply different parts of the system, such as the overhead and the fin fans.
8.6 Nozzle design Spraying Systems Company, Wheaton, Illinois,
(www.spray.com) “Whirljet-CX” design, standard hollow cone nozzle has been used in Ras Tanura and Riyadh Refineries.
8.7 Nozzle metallurgy Type 316L and 316F stainless steels
Inconel 600
Corrosion resistant alloys such as Hastelloy C-276 or Inconel 625 are the materials of choice for this service but only available in batches of 25 or more from the manufacturer. Therefore, Type 316L or 316F stainless steel which are available “off-the-shelf” have been accepted as a compromise. There is a small potential for crevice corrosion and stress corrosion cracking with these materials. Rabigh Refinery specified Inconel 600 wash water pipe and spray nozzle which will have improved resistance to chloride stress corrosion cracking over the Type 316 alloys.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
9.0 Air Coolers
9.1 Flow distribution Poor flow distribution due to long headers will result
in high velocity flow and possible erosion corrosion in the fin fans nearest to header inlet and low velocities and possible under deposit corrosion in low flow air coolers. Measure outlet temperatures of fin fans to indicate variations in throughput. Use values on inlet lines to fin fans to control flow distribution.
9.2 Internal coating and metallic
linings Internal coating of new air coolers will reduce
fouling and corrosion. It is an economic method to extend cooler life.
Alloy full length liners have also been used successfully for heat exchanger repair in Saudi Aramco (www.cti-ind.com).
9.3 Water wash If poor distribution or low flow is a problem,
supplemental wash water can be injected into fin fans to remove deposits. Continuous water washing is strongly preferred over intermittent water washing that may increase corrosion.
9.4 Inhibitor injection Secondary corrosion inhibitor injection may be
necessary at the fin fans in systems with mal-distribution.
10.0 Overhead Receiver
10.1 pH limit 5.5 to 6.5 Limit. Adjust pH value by adjusting neutralizer
addition.
10.2 pH measurement 1 per shift Operators should measure pH on-site a minimum of once per shift using narrow range pH paper and also a well maintained and calibrated pH meter. These measurements are in addition to regular laboratory based measurements. All pH records must be permanently recorded.
On-line pH meters have proved very hard to
maintain and calibrate in the Saudi Aramco environment.
10.3 Iron < 1 ppm Limit. Higher levels indicate excessive corrosion
which must be rectified.
10.4 Chlorides 10 to 30 ppm Limit. Too low a value results in high caustic use and possible sodium contamination of downstream processes. Too high a level results in corrosion damage. Adjust chloride level by adjusting caustic treatment.
10.5 Water boot residence time 4 to 5 minutes minimum Short residence time will result in poor separation and possible entrainment in reflux naphtha.
10.6 Water chemistry Measurement of hardness will show presence of
leaks and cross contamination from any upstream water-cooled heat exchangers in the overhead system.
LOCATION/ VARIABLE TARGET OR LIMIT COMMENT
11.0 Naphtha Reflux
11.1 Water content <0.1 % Limit. Water in the naphtha reflux causes significant corrosion in the tower and overhead. Measure water content once per shift. Ideal is zero percent water.
11.2 Neutralizer or inhibitor injection None allowed Mandatory.
12.0 Corrosion Monitoring
12.1 Corrosion management program Essential. Should include operations data, laboratory results, chemical injection rates, OSI, and monitoring results. Good information management is vital.
12.2 Injection point inspection API-570 & Inspection Alert
001/97 Essential.
12.3 Probes, mpy < 5 mpy Corrective action required if value exceeded.
Electrical resistance or MicroCor probes in
retractable fittings are useful especially in the region of the air coolers and overhead receiver.
12.4 Coupons, mpy < 5 mpy Corrective action required if value exceeded.
Coupons may be used in the overhead system or in the same locations as probes.