Authors: H.Akbari1, G.Khaneh zarin2, S.Gheasi3, M.Siahkamari4
1.inspection department of KPIC, firstname.lastname@example.org, 2. Inspection department of KPIC, email@example.com,3. Construction engineer of NDEC, S.Ghiasi@ndec.ir, 4. Construction engineer of KPIC, firstname.lastname@example.org
Kermanshah Petrochemical Industries Co. (KPIC) is constructing a new ammonia storage tank at Kermanshah, Iran. The new tank with capacity of 16,796 m3 has been designed according to API 620 + ANNEX R. This tank has two shells (inside diameter r= 38m, outside diameter = 40 m, height = 16,610 mm). Present report explains procedure of roof air rising of the mentioned tank that was conducted by KPIC, NDEC (Namavaran Delvar Engineering Co.) and HEDCO (Hampa Energy Engineering and Design Co) (height of air rising= 15,350 mm). After assembling the roof on the outer tank bottom, it was led to final position using air rising method. Air rising procedure allows most of the roof erection work to be constructed at a low elevation, resulting in a high quality roof as well as limiting personnel exposure to dangerous heights.
Assembling procedure of the roof and suspended deck
After installation of the outer bottom plate simultaneously with the installation of the outer shell, deck plates installed on bottom plates and after checking their dimension, welding procedure initiated. After completion of the deck assembly and welding, central ring was assembled on a temporary column and by means of temporary support around the internal circumference of the shell, ring girder and all of rafters and gusset plates were erected and welded. Afterwards, roof plate’s installation commenced according to sketch plate erection and then roof plates welded to each other. At the next step, hanger clips on the rafter, girder and stiffener of deck, were installed and finally suspended deck hanger was erected (see fig.1-4). Also six nozzles were cut and then welded on roof plates on proper positions.
Necessary accessories for air rising
In order to perform air rising correctly, some preliminaries shall be implemented comprising installation and welding of compression bar at top of the last shell course, installation of levelling cables, sealing system, preparing and installation of manometers, installation of blowers and etc. Fig 5 shows a schematic view of all necessary items that shall be supplied prior to starting the operation.
After installation, fit-up and welding of the compression bars, sixteen temporary supporting arms were erected on compression bar for installation of levelling cables. Cable levelling system was installed to assist roof stabilization as it travelled from bottom of the tank to its final top position. The end of levelling cables were connected to supporting arms and annular plates of outer bottom plates by drilling the roof plate (see fig 6-7). Each wire pulled toward the bottom plate (500 mm away from the shell of the tank) and stretched by each turnbuckle until the tension reached to 1,000 kgf, the tension load was checked by using resonant frequency method, according to the following formula:
Arrangement of air blowers
The necessary air for operation was supplied by two blowers that were erected on temporary plate of the shell. One of these blowers was main and the second was standby. Standby blower operated by one generator separately (see fig 8).
Sealing system arrangement
To prevent air leakage during air rising process, all the gaps needed be sealed. Thus, a sealing system was designed to prevent escaping of the air between the roof and the shell. In addition, all of positions that were prone the air leakage were identified and sealed using adhesive tape or plaster.
To seal the space between the roof and shell, temporary bent plates were designed and fastened to the end of roof plates. Then, temporary seals consisting of polyethylene sheets and pre-assembled wire mesh were erected on temporary bent plates using nuts and bolts (see fig 9).
Arrangement of measuring equipment
A measuring system for recording the levelness of the roof structure was installed at four positions using four meter tapes along the shell (,, , to control the levelness. All data were measured at the same time. For this purpose, four persons were present on deck plates during lifting the roof. These persons had separate walkie-talkies. Commander should receive data from each person every 5 minutes to record the roof slope. During lifting the roof, maximum tolerance for levelness of the roof was five mm. finally the roof raised successfully without any problem.
Arrangement of manometers
The inside pressure of the tank should be checked during air rising. So three manometers were installed around the tank. The first, near the blowers, the second, on the roof and the third, on the last course of the shell. Three persons checked the inside pressure of the tank using mentioned manometers. According to calculations, inside pressure of the tank was kept at 20 mm H2O.
Preparation for roof fixing work
Necessary tools were arranged for the angular alignment of the roof, such as levers, jacks, jigs, tools and welding equipment around the top of roof for fit-up and welding of roof to compression bar. In addition, ten welders were arranged around the compression bar for fixing and welding the roof to compression bar.
Prior to actual rising operation, the pre-air rising of the roof was performed up to 40 cm, Fig 12, to check the following conditions:
Pre–air rising performed successfully, air leakage was identified in some areas and were sealed accordingly with adhesive tape and plaster (see fig 13).
Roof Air rising
Air rising of the roof started and continued as per below procedure:
After tack-welding the roof to compression bar and without decreasing the internal pressure, the circumferential welding process started. 10 cm on the area of T joints were left un-welded due to huge air leakage.
Fig 15-18 shows some different views of the roof during the air rising.
Safety considerations prior to air rising
Before initiation of air rising, all necessary items were checked as below: