SAFELY AND AUTOMATICALLY DRAINING WATER
FROM PRODUCT STORAGE TANKS
Written By Eng Reuven Perez
THE REMOVAL of water from hydrocarbon products in storage tanks have been a constant problem in the oil industry. This problem is 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.
Discussions were held with operations supervisors, oil loss and environmental engineers, and off-site designers.
An increase in the awareness of the problems associated with water in tankage was evident after the discussions.
Growing environmental pressures, particularly in the US and EU, resulted in increased interest in the issue.
It also 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.
Some of the refineries and storage terminals surveyed are testing techniques other than the
experienced eye of an operator to determine when oil and not water is draining from the tank.
Most of the oil companies require an operator to be present when the water is drained.
But these companies 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:
• Oil loss
• Soil and air pollution
• Manpower involvement
• Potential of labor intoxication
• Corrosion of tank-bottom plates
• Product degradation caused by microbial growth
• Product volumetric and mass- 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 inches 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 inches) 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 inches.
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 US at distribution terminals. However, in tanks using this form of
measurement, water causes similar problems in calculating tank volumetric or mass contents.
However, 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. The elimination of water in the tank and on the floating roof
will reduce measurement errors considerably.
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 microorganisms 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. The elimination of water in the tank will reduce prevent microbial growth considerably.
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.
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 andoperational experience in drainingthe 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, starts to drain is not easily determined.
No simple test is available to assist the operator, who depends upon their sense of smell and sight. The point at which the transition from water to kerosene 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 safe 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 they must wear a face mask while draining water from tanks containing toxic
products. The automatic and unattended elimination of water in the tank will prevent potential labor intoxication.
At the conclusion of draining water from a tank, the drain line is full of oil. The next time water needs 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 short and the quantity of oil to be purged will be relatively small (5 US gallon- 18 litres). However, a large- diameter tank with an apex-down bottom may havea 4-inche, or even
6-inch drain line extending 75 ft into the tank. The volume of oil that must be purged may exceed 100 US gallon (380 liters).
How much of the oil contained in the dead leg can be recovered?
The answer depends on whether the refinery had installed an open or closed sewer system and the type of downstream
separation- recovery system used.
In Europe, the predominant answer was that when the product is gasoline and an open sewer system is used
virtually none is recovered. The gasoline evaporates into atmosphere.
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 water drain 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.
Draining the water out through an automatic drain valve equipped with a manual pump for returning the product
from the dead leg line will dramatically reduce the amount of dead oil and will reduce operator errors.
Legislation introduced recently in the US and EU has concentrated the minds of offsite 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 sense of smell and touch, the ability of the operator 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
Spills to the open ground must be dug out and incinerated in some states in the US.
The cost for this is estimated to be $5,000 or more per tonne of contaminated earth removed.
AUTOMATIC WATER DRAINERS
Based on the survey findings it is 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 possibilityof oil being drained in a failure mode, then its benefits would be considerable.
The estimated cost savings from a reduced oil loss is approximately
$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,
• Elimination of oil spills caused by
Roof drain automatic shutoff valve
Tank bottom drain valve
The Eco-Valves system was designed for the safe, automatic drainage of water \from storage tanks.
The system is designed to operate at 52 US gallons/minute (12,000 \litre/hour) and discharges through a 2-inch line.
The low flow rate is always adequate and exceeds by far the rate at which water becomes free in the tank.
The automated drain valves are mounted on the flange of the existing mechanical manual drain valves of the storage tanks.
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.
Many failures are not publicly reported.
Those refineries whose operating standard is to leave the drain valve close drun the risk of sinking the roof after an extended rainstorm.
Most floating roof tanks, however, are designed to withstand up to 1 ft of water But thosethat 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 safety roof drain valves
will prevent oil spills and system damages.
This article was written by Reuven Perez, engineer, at Eco-Valves.
For technical data visit
1 Bottom drain
2 Roof drain
3 Roof drain automatic shutoff valve
4 Tank bottom drain valve