Richard Ludlam
Rotork plc
Bath, U. K.
A patented packaged system is now available for safely and automatically draining water from product storage tanks. It can operate on a single storage tank or, with a manifolding system, on as many as four tanks.
BACKGROUND
The removal of water from hydrocarbon products in storage tanks has been a constant problem in the oil industry-a problem still largely solved by nothing more advanced than an operator opening the drain valve and closing it when he sees the appearance of oil in the water.
Not only is this operation incongruous in today's industrial setting, but it is also wasteful, potentially hazardous, and can be environmentally damaging. Clearly a better solution is long overdue.
In a survey designed to identify the need for and suitability of a completely automatic tank-water draining system, problems other than oil loss and environmental issues were identified. It became evident that the automatic removal of water from product tankage, as the water becomes free, is essential for a number of reasons.
Equipment to effect this reliably and without product loss is now available.
SURVEY
Depending on the type of product tank, water enters through leaking roof seals or by condensation. It may also enter as a result of processing, or be imported with intermediate products.
A survey was carried out in the U.S. and Europe to understand, in detail, the consequences of this ingress of water and to determine the need for an automatic drainer. Visits were made to oil refineries, distribution terminals, and oil and petrochemical company engineering headquarters.
Twenty-eight companies were surveyed having the following distribution:
- By operation. 13 oil refineries, 9 oil or petrochemical engineering headquarters, and 6 distribution terminals.
- By country. 14 in U.S., 6 in U.K., 5 in Holland, 2 in France, 1 in Italy.
Discussions were held with operations supervisors, oil loss and environmental engineers, and off site designers. The initial survey was conducted in 1988 and many of the sites were revisited in 1990.
An increase in the awareness of the problems associated with water in tankage was evident after this interval. Environmental pressures caused the increased interest, particularly in the U.S.
Although irrelevant to the studies being conducted, it became apparent that there are wide differences in the way tanks are managed-not only between different oil companies, but also between refineries owned by the same company.
Some companies are aware of the need for a consistent approach to tank management and are setting internal standards on how tanks should be gauged for oil and water and how they should be manually drained. Only one refinery surveyed was testing a technique other than the experienced eye of an operator to determine when oil and not water is draining from the tank.
Some companies require an operator to be present when the water is drained. But these refineries are also concerned about the toxic fumes that can be released when draining, say, a benzene-storage tank.
Other companies do not insist on the presence of an operator during the entire time a tank is being drained.
The survey revealed that the presence of water in tankage caused the following problems:
- Product volumetric and mass-measurement errors
- Product degradation caused by microbial growth
- Corrosion of tank-bottom plates
- Manpower involvement
- Oil loss
- Pollution.
MEASUREMENT ERRORS
An accurate mass-balance measurement system is essential in determining oil losses and pinpointing where they occur. With the exception of some plants where hydrostatic tank gauging is installed, the calculation of the mass of oil stored is based on oil level measurements.
Tank oil level measurement to an accuracy of 1/16 in. is claimed by some level gauge manufacturers in Europe. However, if water is present in the tank, the accuracy to which the oil-water interface can be measured (surely no better than 1/2 in. is more relevant than the accuracy of the oil level measurement.
Thus, where water is present, the accuracy to which the mass of oil stored can be calculated is related to the combined inaccuracies of the oil level and oil-water interface measurements.
Oil level measurement is affected by water lying on floating roofs. Many oil companies leave roof drain valves closed and open them only after rain has ceased falling. Two inches of rain lying on the floating roof of a tank containing gasoline will cause an apparent increase in product level of about 2 1/2 in.
It was surprising how many operators and instrument engineers were unaware of this phenomenon and the effect it can have on mass-balance calculations.
Hydrostatic tank gauging is becoming widely used in the U.S. at distribution terminals. However, in tanks using this form of measurement, water causes similar problems in calculating tank volumetric or mass contents,
But if a pressure transducer is installed to counteract the mass of the roof, then at least the effect of water on the roof can be nullified.
MICROBIAL GROWTH
Several sites mentioned the contamination of products caused by microbial growth. The presence of free water in storage tanks creates a fertile place for these micro-organisms to grow, causing product degradation.
Diesel and light heating oils are more likely to attract microbial growth than aviation fuel.
Treatment with biocides is usually successful, but the use of biocides on aviation turbine fuels is not permitted if the ASTM specification D1655 is to be met.
Consequently, the elimination of water from tankage is highly desirable to prevent product degradation and is essential in the case of aviation turbine fuels.
CORROSION
Corrosion of the bottom-plates can occur as a result of the action of air entrained in the free water. This corrosion is exacerbated if microorganisms are present.
Micro-organisms feed off sulfates in the oil and are reduced to sulfites, which cause sludges and slimes to form.
These in turn cause accelerated corrosion of the tank bottom plates.
The cost of taking a tank out of service to replace corroded sections, including the repair work itself, can be excessive. The elimination of water in the tank will reduce corrosion considerably.
MANPOWER INVOLVEMENT
The quantity of water to drain is usually determined by hand dipping to find the interface level (some automatic tank level gauges can identify and measure the interface level). The length of time it will take to drain is based on the head of liquid and operational experience in draining the particular tank.
In some cases, tank draining is a fairly lengthy process and may necessitate the operator's presence during the entire operation. The point at which oil, and not water, commences to drain is not easily determined.
No simple test is available to assist the operator, who depends upon his senses of smell and sight. The point at which the transition from water to kerosine or jet fuel occurs is particularly difficult to assess.
In the case of jet fuel, it is obviously essential that all water be drained. Therefore, a liberal quantity of jet fuel may also be drained to be on the conservative side.
Tanks that suffer from differential settlement and have an apex-up bottom configuration may need to be drained at more than one point.
Draining is a tedious task for the operator, particularly when he must wear a face mask while draining water from tanks containing toxic products.
OIL LOSS
At the conclusion of draining water from a tank, the drain line is full of oil. The next time water has to be drained, this dead leg of oil must be purged.
If the tank has an apex-up bottom configuration, the drain line length will be fairly short and the quantity of oil to be purged will be small. However, a large-diameter tank with an apex-down bottom may have a 4, or even 6-in. line extending 75 ft into the tank.
The volume of oil that must be purged may exceed 100 gal (U.S.).
One refinery visited had modified its tanks to have two drain lines-the second having a smaller diameter. Draining the majority of water out through the larger line and completing the operation with the smaller line considerably reduced the amount of dead oil.
How much of the oil contained in the dead leg can be recovered? The answer depended on whether the refinery had installed an open or closed sewer system and the type of downstream separation-recovery system used.
In Europe the answer was frequently that, when the product is gasoline and an open sewer system is used, virtually none is recovered.
If the survey was representative of the industry it would appear that, after the oil required in furnaces for reprocessing is taken into account, the net recovery of gasoline, for example, is only 50%.
But the major losses occur as a result of operator errors or equipment failure. All refinery off site personnel were able to give details of at least one occasion when a waterdrain valve was closed too late. Some had experienced the failure of the articulated roof drain. One of these failures had caused a catastrophic problem because the roof drain valve had been left open.
ENVIRONMENTAL
Legislation introduced recently in the U.S. has concentrated the minds of off site designers to devise schemes to prevent oil spills onto open ground. Solutions involving hard piping of tank-bottom drain lines to separation equipment can be very costly. (One Gulf Coast refiner has budgeted approximately $10 million for the work.)
But such a solution will cause more oil to be reprocessed because the operator must be able to see the oil start to flow through an armored glass section of pipe before closing the drain valve.
Without the senses of smell and touch, his ability to assess the transition point will be reduced. Further, the glass section of pipe will soon become opaque, requiring removal, cleansing, and the opportunity for more spillages.
Spills to the open ground must be dug out and incinerated in some states. The cost for this is estimated by one refining company to be $1,000/ton of contaminated earth removed.
AUTOMATIC WATER DRAINER
Based on the survey findings, it appeared essential that product storage tankage be operated dry. Water should be evacuated from the tank as soon as it becomes free.
If a packaged system could be designed to operate automatically, with low maintenance and absolutely no possibility of oil being drained in a failure mode, then its benefits would be considerable.
One European refinery estimated cost savings from reduced oil loss to be $30,000/year for a tank in catalytically cracked naphtha service.
Other benefits include improved mass-balance calculations, reduced tank corrosion, elimination of product degradation due to microbial contamination, reduction in manpower requirements, and the elimination of oil spills caused by operator error.
A system was designed for the safe, automatic drainage of water from up to four product storage tanks.
The system is designed to operate at 15 gpm (U.S.) and discharges through a 1-in. line. The apparently low flow rate is always adequate and in most cases exceeds by far the rate at which water becomes free in the tank.
The schematic diagram of the Autodrainer is shown in Fig. 1. The system comprises a filter, a gear pump, two capacitance probes, a turbine meter, a 3/4-in. solenoid valve through which the water discharges to drain, and a recirculation line back to the tank via a pressure-regulating valve.
The logic to control the system is contained in an explosion-proof enclosure with a window. Through this window can be seen status and alarm lamps together with indicators displaying the percentage of water detected and the cumulative volume of water discharged to drain.
The system is mounted on a skid measuring only 5 ft high, 3 1/2 ft wide, and 2 1/2 ft deep. It is certified suitable for use in Zone 1 hazardous locations (international Electro-technical Commission standards), or Division 1 locations (National Electric Code standards). The system includes all necessary isolating valves and facilities for checking the probe calibration with oil for the zero check and water for the span.
SYSTEM OPERATION
At any time the system can be in one of three modes: standby, recirculating, or draining.
In the draining mode, the water is pumped through a special pipe assembly containing two Teflon-coated capacitance probes, and then through the 3/4-in. solenoid-operated valve to drain. The capacitance probe output signals represent a range of 0-100% water, with zero corresponding to 100% oil.
The output signals are inputs to two trip amplifiers whose identical trip points are adjusted to the percentage of oil in water at which draining must terminate. If either probe detects oil at a concentration above the trip point, the solenoid-operated valve closes and the oil-water mixture recirculates via the back-pressure regulating valve to the tank, for a time sufficient for the logic to check that the two probe output signals are essentially the same.
If during the recirculation period the output signals do not agree, the system shuts down and will not restart. An alarm is also activated, indicating that a probe fault exists.
If the probe output signals are essentially the same, the pump stops and the system goes into the standby (resting) mode for a preset time period. This time duration is adjustable and is based upon the rate of ingress of water into the tank. Typically, the standby time will be set between 4 and 8 hr.
At the end of the standby mode, the pump starts and the system enters the recirculation mode to monitor whether additional water has settled in the tank. Recirculation continues until at least twice the dead volume of the tank drain line has been purged.
If water is not detected by both probes during the recirculation mode, the system shuts down and returns to the standby mode. However, if both probes detect water for more than 3 sec, the system goes into the draining mode.
The turbine meter monitors that the pump is running and, in the draining mode, its output signal is used to log the quantity of water drained.
SYSTEM INTEGRITY
System integrity is achieved by:
- Using dual detectors in a "2 out of 2" role.
- Presetting into the logic the maximum permitted volume of water to drain in any one cycle. (If this preset volume should be reached, the pump stops and the solenoid-operated valve closes.)
- Operating an alarm if the solenoid-operated valve limit switch indicates that it has failed to close or open.
Should the 3/4-in. solenoid-operated valve fail to close, the leakage to drain will be minimal because the stationary gear pump will hinder liquid flow.
SYSTEM TESTING
The capacitance probes were carefully tested by Rotork Analysis Ltd., Faringdon, Oxon, U.K., to assess their performance on a variety of light hydrocarbon oils and different types of water. Salt water, tap water, and rain water, as well as water taken from a refinery gasoline tank, had an insignificant effect on the probe output signals.
The variations in output signal due to the type of oil and to its temperature were marginal.
System testing was also performed for several weeks. This was sufficient time for microbial contamination to occur in the 300-gal tanks that were used. It was interesting to note that the contaminated water was drained, leaving behind the test oil, which was substantially clear of contamination.
TRIP POINT
Prior to the survey, Rotork believed that the trip point setting should be such that as soon as a very small quantity, say 0.25%, of oil is detected, the solenoid-operated valve closes. This is desirable to ensure that virtually no oil be drained.
However, if the trip point were set at 0.25% oil (i.e., 99.75% water) then the aggregated effects of temperature variations, change in product composition, and drift in the system calibration could cause the trip point to effectively become 0% oil (i.e., 100% water). The autodrainer would then never drain water.
The survey revealed that it is very unusual for there to be an emulsified interface layer between oil and water in intermediate or product tanks. Even in gas oil tanks where a mixer has been operating vigorously, the interface level becomes measurable within minutes after the mixer has been stopped. This meant that there would be no slow increase in oil concentration in the water as it is drained. The change was anticipated to be very rapid, with the probes detecting 100% water at one moment and 100% oil a few seconds later.
If this were true, a trip point of 5% oil in water would be acceptable and any error in the effective trip point caused by the aggregated effects discussed previously would be insignificant.
We were told by operations staff at several refineries that the refinery operator is unlikely to be able to detect drainage of a light oil product until its concentration reaches 50%. Clearly 5% is an improvement over 50%.
FIELD TESTS
Field testing of the system was essential to evaluate:
- Frequency of cleaning the filter to protect the gear pump. (A gear pump was chosen so that flow rate would be essentially the same, irrespective of tank oil level.)
- Drift in capacitance-probe calibration. During the survey it was clear that even gasoline tanks can contain slimes and gum at the bottom. Therefore, deposits were expected to adhere to the probes. (The design takes this possibility into account so that the probes do not become fouled.)
- The theory was that with a trip point setting of 5 or 50% oil in water, there would be no noticeable time difference before the solenoid-operated valve closed and therefore no significant difference in the quantity of oil drained.
- Reliability and refinery operator acceptance.
By courtesy of Shell U.K. Ltd., the Autodrainer was installed at its refinery in Stanlow, England, on a fixed-roof jet fuel run-down tank, which had been routinely drained twice per week. Fig. 2 shows the installation at the Stanlow Refinery.
TEST RESULTS
The Autodrainer was put in service at 5:00 p.m. early in May 1990, with a 2-hr resting time and the trip point set at 5% oil in water. On commissioning it immediately drained 3 gal (imperial) of water, which was substantially less than expected. The following morning, it was found that it had drained only another 3 gal (Imperial). After 4 months of continuous operation without a single failure, it had drained only 97 gal (imperial), or 440 l. of water.
As anticipated, there was a rapid change from 100 to 0% water. This change occurred in less than 3 sec, proving the theory that it is virtually irrelevant whether the trip point is set at 5 or 50% oil in water.
During the 4-month period, the quick release filter was inspected several times and no significant buildup of scale was found. Calibration was checked at the end of this period and was found to have shifted by only 0.1%, as measured by a digital voltmeter.
At the time of writing (10 months after commissioning), the system has continued in operation without failure and has been accepted by the off sites operations staff. Shell U.K. Ltd. is now considering manifolding the Autodrainer to drain automatically a second jet fuel tank.
WINTERIZATION
Winterization can be effected by lagging or lagging and heating, with self-regulating electrical heat-tracing tape. Unless winter weather is severe, heat tracing should be unnecessary because at the end of each draining mode, the system recirculates for several minutes, leaving the system full of oil.
MULTITANK DRAINING
Several tanks containing the same, or very similar, products can be drained by a single Autodrainer. This is achieved by running a line from each tank to an inlet manifold incorporating electrically operated and interlocked firesafe ball valves.
Similarly, an outlet manifold incorporating interlocked fail-closed solenoid valves is provided to select the tank to which product is to be recirculated.
The Autodrainer, complete with its manifolding, is supplied skid-mounted, thereby reducing the effective cost per tank, compared with using one Autodrainer per tank. The Multidrainer, for a maximum of four tanks, incorporates cumulative volumetric registers; one for each tank.
ROOF DRAINS
During the survey, the position of roof drain valves was noted. On some tanks the valve was closed, on others open, and some even had the valve partially opened. All operations staff were concerned that a failure of the articulated or flexible roof drain line could occur when the drain valve is open.
Such a failure was responsible for the catastrophic spill of oil into the San Francisco bay several years ago.
Those refineries whose operating standard is to leave the drain valve closed run the risk of sinking the roof after an extended rain storm. Most floating roof tanks, however, are designed to withstand up to 1 ft of water. But those that have the valves closed in harsh environments run the additional risk of the water freezing in the drain line and cracking the knuckle joints.
Automatic control and protection of roof drains is possible with a similar technique incorporating capacitance probes and electrically actuated firesafe ball valves. But a simpler method, which does not require electrical power or air, is to use Exxon-patented drain valves manufactured by Rotork Analysis under a licensing agreement.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
