Installing an Underground Tank
Step 1: Sheet piling work
Conventionally, most contractors did not use this sort of method to unload the ground. Furthermore, this type of method is still not being widely used in Japan, Australia, New Zealand, and even USA. However, in Malaysia, for any Petronas Underground tank project, they insist to use sheet piles to brace the land. By doing this, it can protect any outside load from entering the tank farm and it also can hold the ground firmly, even water from outside can not pass through it.
When planting the sheet pile into the ground, there maybe some problems such as the soils are too hard for the sheet pile to go into the ground. Alternatively, we can dig the soils first using a dozer, and then the sheet piling work comes later.
A sketch of sheet pile
(Single pile)
(A row of sheet pile)
(Hammering a sheet pile)
(A joint welded to bracing) (A good bracing)
Step 2: Excavation work
The excavation should provide adequate room for the tanks and compacted backfill. Follow the manufactures’ recommendations and standard industry codes. Piping lines need to be at least 12 to 18 inches below land surface, with at least 6inches of compacted backfill (sand) placed in all trenches before lines are installed. And make sure proper precautions should be followed to avoid injuries. Barricade should be placed around the pit. The soil is being dug by a dozer until it reaches a certain level, as acquired by tank provider (tank installation worksheet). We use a “Teadher Light meter” to make sure how deep has the soil been excavated.
Bury depth for STP tank design (6000IG Glasteel II Tank)
1. Maximum soil cover depth is 3950mm (13feet) for tanks with standard wall thickness.
2. Deeper bury depth require additional structure and tank wall thickness.
3. Allow 300mm (12inches) of bedding material below each vessel)
Pictures:
(Trench for flexible pipe) (Excavating soil)
Step 3: Burying R.C pile
R.C pile is planted into the soil before concrete base work can be carried out. The uses of R.C pile is to strengthen the base and be as “a point of weight”. By doing this, the soil would not be de-leveled; as the weight from the load will be spread out to every point of R.C pile. On top of R.C pile, we put two layers of reinforced steel (BRC A9) to cover all the tank area. Thus, by doing this, the base will be stronger and the possibilities for the base to slide down would be nil. There are two types of BRC steel:
1. Y type (fabricated steel)—stronger but it is more expensive
2. N type (smooth steel)
After we finished placing BRC, we continue with fixing U-Bar(Y-20 painted with 2coats of bituminous paint) to BRC. U-Bar has to be tied up to prevent it from moving. The insertion of U-Bar must be correctly placed with tank provider’s manual. Error in placing U-Bar will favor a severe difficulty in the future.
Pictures: (on the next page)
Pictures:
(A R.C pile) (A R.C pile is being planted)
(A planted R.C pile and BRC on top) (U-Bar fixed on place)
(Counter-check of a U-bar measurement-550mm from top to concrete base)
Step 4: Concrete base work
The contactor is in charged to design and to provide base slab reinforcement. The standard grade that we usually use for any base slab work is Grade 30, but it depends on the thickness of the base slab (Refer to the tank installation worksheet). If we are making a 300mm concrete base slab, we use Grade 25 cement. It is more economical than Grade 30, and furthermore this Grade 25 can hold the slab as well as Grade 30 holds. Premixed concrete is more preferably. For four tank spaces, we need about 27meter cube of premixed cement, but this depends on the area of the tank farm. One premixed carrier can transport about 5meter cube of premixed cement at a time.
Before the tank is being filled with the concrete, the concrete provider must carry out several tests on the concrete. The first test is the slump test. It will make sure the viciousness of the premixed cement. If this test failed, the cement has to be rejected. The test is conducted by the provider after he arrives at the site. The cement is said to be failed when the premixed cement lesser than 50% of the actual height (it means the premixed is too liquefied). Then the second test is the tube test. This test is to determine pressure/ compactness of the mixture. How to do the test? The premixed cement is put into 6 tubes.3 tubes is tested on the first 7 days of the work and another test after 28days.The curing for the slab is about 28days .To make the concrete entirely mixed, we push air (high pressure) into the concrete with air pump.
Pictures:
(Concrete transfer: carrier-vessel)
(Spreading concrete and his-pressured air mixing it) (Concrete after curing time)
(Test tube) (Slump test)
Step 5: Crusher runs (Bedding material)
Crusher runs is transferred from the lorry to the tank pit by a shower vessel. We do not use a dozer or a shovel to transfer it. This is because, by “showering” it into the pit, we can obtain a good and fast densely packed of base slab. The technique to put crusher runs is by thinning it out to one tank area to another tank area. The height of crusher runs is depends on tank provider specification, usually 300mm (12inches) from the base level.
(Refer to the tank installation worksheet):
Step 6: Placing the tank into the farm pit
Before placing the tank, we need to check:
Serial number (record number for warranty form)
Orientation of the tank (towards canopy)
Shipping damage
Deliveries of STP underground tank
Deliveries are made only on or very close to public roadways. Do not expect delivery to remote or difficult to access construction sites.
Owner must inspect and sign shipping document upon delivery.
Owner is responsible for rigging and offloads in a safe and timely manner (optional).
Always lift tank from above rigging to all designated lifting lugs provided.
Never wrap chain or steel cable around a tank in order to lift it.
Never lift any tank from below with a fork truck or loader bucket.
Always use a spreader beam to properly rig and handle large tank (preferable).
Improper rigging angles MAY destroy the lifting lugs causing injury and damage.
Always use opposing tag lines to control a tank while in the air.
Have your C.O.D payment ready for tank provider prior to offload.
All primary tanks must be vented. They are intended to be operated with fluid levels no higher than the tank top. Fiberglass tanks are designed to be operated at atmospheric pressure only – except when using vapor-recovery systems – provided the pressure or vacuum does not exceed 1 psi on the primary tank. Over-pressurizing the primary tank will result in tank failure. When venting the monitoring cavity between the inner and outer tank, it must be vented independently from the primary tank (annular space venting is required for all hydrostatically monitored tanks).
Anchor Accessories is supplied by tank provider.
A. Anchor Straps – Provide glass fiber reinforced plastic anchor straps for each tank shown. Number and location of straps shall be as specified by manufacturer. Each strap shall be capable of withstanding a maximum load for each tank diameter as shown
Tank Diameter (ft)
Max. Load (lbs.)
4'0
4,200
6'0
18,000
8'0
25,000
10'0
25,000
B. Straps shall be standard as supplied by the tank manufacturer
Certification Plate – Underwriters Laboratories label shall be permanently affixed to each tank. UL labels shall be affixed to each tank
Vacuum gauge- Tank provider has to make sure that the pressure in the tank must not have much different before and after delivery. Usually, the initial tank pressure is 18Hg or 46cmHg and when we place the tank into the pit, the pressure showed by gauge meter must not have much different (tolerance below 5) and must not exceed 30Hg or 76cmHg)
Pictures:
(Unloading the tank from carrier to the pit) (Positioning the tank)
(Putting belt) (Connecting body to U-bar
using turnbuckle)
(Vacuum gauge) (Certification plate)
(17Hg shown)
Step7: Backfilling from top of the tank to the bottom of concrete base slab
Requirements for backfill material:
Backfill medium must totally bed, surround, and cover the entire vessel.
Backfill must be clean, dry, and washed aggregate.
Individual particles may range from 1/8 to ½ inch in size with minimal fines.
Aggregate must be structurally sound and weight at least 95lbs/cu.ft
Materials customarily used are “pea gravel” or crushed rock “chip”(a.k.a.chipping).
Never use soil, sand, road base, structural fill, or crusher fines as backfill.
We use “showering” method again to fill the pit with backfill. There are some precautions in doing this work:
1. Cover the manway, in order to prevent the chipping from entering it
2. Never unload the chipping directly onto the tank with unreasonable height.
3. Make sure the tank level is correct (Use a water-pointed ruler).
Pictures:
(Unloading chipping) (Leveling tank)
Step 8: Preparation for top concrete slab and setting manhole/manway
Some information about manway:
1.There are 4 holes on manway
SPARE: can use for anything include filling, out-point etc.
VENT: ventilation point where from here it will be connected to vein pipe.
STP: where oil comes up to pump and is used during the hydrostatics tank test.
FILL: where oil from tanker comes in
A. Tanks shall be tested and installed according to the current installation instructions provided with the tank provider.
B. Loading Conditions – Tank shall be designed to meet the following design criteria:
External hydrostatic pressure: Buried in ground with 7' of over burden over the top of the tank, the hole fully flooded and safety factor of 5:1 against general buckling.
Surface loads: When installed according to manufacturer’s installation instructions, tanks will withstand surface H-20 axle loads (32,000 lbs./axle).
Internal load: Primary and secondary tanks shall withstand 5 psi air pressure test with 5:1 safety factor.
Tanks shall be designed to support accessory equipment such as heating coils, ladders, drop tubes, etc. when installed according to manufacturer’s recommendations and limitations.
Tank laminate shall include silica treated with silane.
How to conduct the test?
Fully fill the tank with water through STP. Usually a tank is designed to load more than its original specification (in case of oil reservation).Beware, there may be some air that stuck on the top of the inner tank which favor to the tank for not being fully filled with water due to air pressure. In that case, we have to suck out the air using air sucker.
Then, a “bush” plug is attached to the manway. This will convert a 4” hole to a ½ inch hole.
Carefully, place a T-shaped tube into the ½” hole of Bush plug. Connect a pressure meter on top of the tube with stopper, and another junction of the tube as inning point for water to come in and also connected to stopper. Precaution in doing this test:
1. All holes must be secure tightly with plug. Make sure there is no pressure can escape from the tank.
2. Manway already connected correctly and rigidly by tank sump.
3. Use soap-water to identify any leak either during connection, or accessory faulty.
4. Pump water into the tube until the meter reaches 5psi. (Water from outside will come into the tank).
5. Quickly, stop the water from entering the tank using stopper.
6. Give 8 hours for the meter to settle down. And, if after 8hours the meter shows so much different with its initial value, the tank will be considered fail.
7. If the tank passes the test, suck out all the water in the tank and the tank is ready to be connected to flexible pipe and etc.
Pictures:
(Flexible pipe)
(Dispenser sump)
(Tank sump/manhole STP)
Step 9: Concrete top slab
1. BEFORE fully cover the tank with backfill, all the points from the tank have to be tightly connected to their own points;
SPARE: clogged by a plug provided by tank provider.
FILL: use a 4inches of GI pipe to connect the point to the Filling point. Simply place GI fitting (elbow and etc) between the point and GI pipe.
VEIN: use a 2inches of GI pipe and suitable GI fitting to connect the point to the Vein pipe.
STP: Place flexible entry boot on the manhole wall. Put in Flexible pipe into the hole of entry boot and clamp it with a hose clamp provided by tank provider. Screw tightly. Connect it to the pump dispenser using flexible pipe.
DIP and AG point: Tightly closed with Dust cap (dust cap with overclock handles EBW 777/779. or OPW 30D CALLH).Also, place dust cap on each tube holes of Filling point.
2. Fully fill the tank area with chipping/backfill until it reaches the top level.
3. Filter Fabric-Geotextile all around the tank area. The advantage of doing this is when the water from outside goes into the tank pit, it will avoid the water from goes further into chipping which will harden the chipping (turn into concrete). The hardened chipping will block the water from escaping the tank sump. If this happen, the tank sump will be flooded with water and MAY then flows inside the tank.
4. Place 2 layers of BRC on top of chipping.
5. Concrete slab can be performed. The thickness of the slab is 8inches.
6. Finishing of the top the concrete slab.
7. Cover the manway hole with composite round black manhole (covers with spheroidal graphite cast iron frames) and so for other holes (top cover).
Pictures:
(Site before the work starts) (Work done-After theproject handing over)
(Manhole) (Dip point)
(AG point) (Gauge meter)
(Vein pipe) (Well- to indicate water level in pit)
