Present paper discusses case study for strengthening of bulk urea silo for a leading fertilizer manufacturing company of India. Paper discusses need for retrofitting of silo. Analysis and design process followed for arriving at retrofit solutions. Comparative study of various retrofitting systems and specialty materials considered and selected for final execution. Site challenges and solutions for the same to execute the job in given time frame by maintain zero safety incident are also presented. Entire work was executed by Sanrachana Structural Strengthening Pvt. Ltd. on design-built basis.
Reasons for structural retrofitting of urea storage silo
The bulk urea storage silo is an existing structure 240 m long X 55 m wide of 40,000 ton capacity. Presently, urea from G-1 and G-2 plant is fed to the silo through existing conveyor BC1. Urea from new G-3 plant will also be stored in the existing storage silo after the modification works. After the modification works, urea from G-1 and G-3 plant will be fed through existing conveyor BC-1 and urea from G-2 plant will be fed through new tripper conveyor BC-2. The modification works envisaged in the bulk urea storage silo are as follows:
A new tripper conveyor BC-2 for storing G-2 Urea will enter through the east side of existing storage silo, the length of the new tripper conveyor BC-2 will be such that 25 % of storage area is covered by the new conveyor.
The head pulley and drive of existing tripper conveyor BC-1 will be shifted about 85 m to accommodate the new tripper conveyor BC-2.
The capacity of existing conveyor BC-1 has to be enhanced to 320tph.
The above increase and rearrangements in loading on existing hanger structure demanded rechecking of adequacies existing capacities structural elements and designing of retrofit system if demand is more than capacity of various structural elements.
Existing silo structure
Existing silo structure is precast-RCC parabolic arch structure. Parabolic arch consists of three precast segments connected with steel connection. Span of arch is 55m and height of arch is 25m. Arches are placed 8.013m c/c. Pre-cast slabs are placed between arches to cover the structure. The conveyor is provided at top along the center line of silo. Conveyor is supported on conveyor beams and conveyor beam is supported on hangers. Walkway is provided on either side of conveyor for maintenance. Walkway is also supported on hanger. Refer Figure 1 for Silo section and Figure 2 for Silo plan. Following structural elements – Walkway beams, Hanger horizontal beam, Hanger vertical and inclined members, Hanger pin with arch beam and Arch beam were analyzed and checked for design adequacy based on data available with client. Concrete strength was decided based on NDT tests and reinforcement details which were missing for few elements were obtained by GPR scanning.
Analysis and design adequacy checks
To find out reaction due to tripper and conveyor, moving load analysis is carried out in STAAD PRO. The output results from STAADPRO moving load analysis and various load combinations are compared and designed for worst combinations.
Following Load combinations for limit state design were considered.
Option 1 –With no earthquake load
1.5DL + 1.5EQPL + 1.5LL
Option 2 – With earthquake load
1.5DL + 1.5EQPL + 1.5LL
1.5DL+ 1.5EQPL + 1.5EQ
1.5DL+ 1.5EQPL – 1.5EQ
1.2DL+ 1.2EQPL + 1.2LL + 1.2EQ
1.2DL+ 1.2EQPL + 1.2LL – 1.2EQ
Where, DL – Dead load, EQPL – Equipment load, LL – Live load, EQ – Earthquake load
As discussed above, Following structural elements – Walkway beams, Hanger horizontal beam, Hanger vertical and inclined members, Hanger pin with arch beam and Arch beam were analyzed and checked for design adequacy.
Based on the detailed analysis and design following conclusions as presented in table No. 1 were made.
Discussions on various retrofitting strategies for various structural elements:
As we are aware various retrofitting strategies like Jacketing with concrete, Steel plate jacketing and carbon fiber wraps and laminates were available. Selection of particular system was governed by following parameters.
Long term durability of retrofit system- As silo is going to store Urea which is hygroscopic material and emit fumes that can damage concrete and steel alike. It was important that retrofitting system is resistant to this. This will ensure
Speed of construction: It was not possible to take shut down while executing the work. It was necessary to select method which is less intrusive. Any retrofit option selected should ensure that workers will have minimal exposure to urea fumes while working.
Cost: Whichever system we chose should not be prohibitively expensive.
Table No.2 gives comparison for various retrofitting system considered for various structural elements. This helped in selecting particular retrofitting option by weighing its pros and cons.
Challenges during Site execution
As cited earlier, it was not possible to take shutdown. This pose many challenges while planning execution strategies. Broadly following challenges were there,
Height: Entire work to be executed at the height of 55 m. However, as plant was running it was not possible to erect scaffolding from ground. So special suspended type scaffolding was designed and hanged from top. Also, for wrapping verticals members of hangers, sky lift was used as shown in figure no. 4.
Urea fumes: As production was in progress, silo was filled with urea. It is difficult and pose health hazard to work in this system. Special mask were introduced for workers while working (Figure no.5) and workers were sent out side silo in fresh air after every one hour.
Speed of construction and long term durability of retrofit system.
While selecting retrofit methodology special care was take for ease of construction. Instead of conventional form work which would have taken long time for fixing and removing, specially designed GFRP stay in form work was used for jacketing of hanger beam bottom figure no. 6, 7. This not only ensure speedy construction but also durability-GFRP lining will protect concrete from urea ingress.
Work was executed in sink with requirements of client and as per the designed methodology of the selected retrofit option (Figure 8).
Detailed engineering is the best approach to arrive at optimal retrofit solution. Depending on site condition we can select various retrofit strategies. It is essential to consider work environment work safety and health concerns before arriving at proper retrofitting solution. This will ensure safe and time bound project completion. In present case various methods like RCC jacketing and CFRP wraps were successfully used to achieve the different level of strengthening. In present case construction of new storage silo for new production line would have incurred huge cost. However, by proper engineered retrofitting approach same silo was put to use at fraction of cost of constructing new silo.
- Toyo Engineering India Pvt. Ltd.
- Vijna Consulting engineers Pvt. Ltd- Mr. SachinAgnihotri, Nikhil Jadhav.
- Specialty Material Supplier- Specialty Reinforced Matrix Pvt. Ltd.
- Sanrachana Structural Strengthening Pvt Ltd. -Site & Safety Team- Mr. Manish Yadav, Mr. Prashant Kamble, Hemraj Patil.