Roof air rising 1

[Marinel1]

I woukld like to know how to calculate the right pressure for roof air rising of a tank.
I have been involved in some but only as an observer. I would like to know if someone can help me on this issue.

 

[bimr]

Depends on the weight of the roof. Take the area of the roof times the pressure.

 

[Marinel1]

I don't know if it is so easy as you say. The roof is not flat,is more like a dome, and if we consider the total area of the spherical calotte, then the force up wards is not the area X pressure. Could you please let me know about your previous experience?

 

[fegenbush]

bimr is correct. Weight / cross sectional area = pressure. This would apply regardless of the radius of the roof since the radial forces will cancel out due to the axisymmetric geometry. Of course, one must also take into account any frictional forces from the perimeter seals.

 

[Marinel1]

Thanks Fegenbush!
In

https://ferrysibarani.wordpress.com/2009/10/29/air-raising-process-dome-roof-tank/

you can see also the balancing wires in order to keep the roof in equilibrium.
Is there is any rule to calculate how many points around the tank and type of cable to be used?

 

[bimr]

On digester covers, we typically see a guide every 15 feet. The distance will depend on how thick the steel is and if there is a stiffening ring around the edge of the dome.

Interesting project. Thanks for posting the pictures.

 

[Syahar1975]

Good morning Gents,

1. Get from the engineering drawing the roof weight + roof nozzles + other accessories. Total weight = W Ton. Let say = 200 Ton.
2. Calculate the roof surface area Just calculate the flat surface. It is not necessary to calculate the dome. The area is A M2. Let say = 1500 M2.
3. Pressure P = W / A Ton/M2. = 200 / 1500 = 0.1333 Ton/M2 To get mm H20 this number must be multiply by 1000. Hence = 0.1333 x 1000 = 133.3 mm H20
4. There is also friction between the seal and the shell. It is estimated about 13 mm H20
5. Total pressure required to raise the roof = Item 3 + Item 4 = 133.3 mm H2O + 13 mm H20 = 146.3 mm H20
6. Get the blower or fan performance chart.
7. From the blower perfomance chart we will know how much air can be produced by the blower at total pressure 146.3 mm H20
8. Estimated leakage could be estimated 70 M3/hr per M tank circumferential.
9. Net air produced by the blower : item 7 - item 8.
10. Roof raise speed could be calculated : Item 9 / Item 2.
11. How long it will take to raise the roof could be calculated = Tank height / item 10.

The blower,seal, roof balance during raising, tank roundness and plumbness are the main factors during air raising.

 

[Marinel1]

Thanks Syahar1975!
Your comment confirm my calculations.
But what about the balancing wires? How many we must use and type of cables?From I saw in the past and from the pictures it looks that they are around 9 to 11mm diam. which looks to small in order to do the equilibrium of the roof during the way up.
Could you comment on this?

 

[Syahar1975]

Good evening Marine,

The size of the wire rope is 1/2".

The empiric method to calculate how many wire rope we need is : tank diameter in Feet / 10. Must be even number. Let say tank diameter is 30 M or 98.4 Ft. So the number of wire rope or A-frame would be 98.4 / 10 = 9.84 round up to 10 .

We did many air raising by using the above calculation.

 

[Marinel1]

Thanks Syahar!
It means that balancing wires is a empiric data accordingly your experience.No maths involved.

 

[bimr]

Something is incorrect in your calculation as the tank in the picture shows 16 wire ropes. The number of ropes should be proportional to the circumference, not the diameter. It looks like the ropes are approximately 12 meters apart, about the same length as the steel panels.

 

[Marinel1]

To be proportional to the circunference or diameter is the same because we have just a factor of scale ( PI=3,14.
Concerning the number of balancing wires and accordingly thumb rule of Syahar we need to have 20 wires instead of the 16 used ( diam:206´/10=~ 20).
I do not know if I am right but 16 or 20 are in the same range if we consider it as a "thumb rule" which of course depend of the experience of each company.
Am I right Syahar?

 

[bimr]

Perhaps. The friction of the double pulley would also be added.

 

[Syahar1975]

Good morning Marinel1 / BIMR,

Number of A frame (wire rope) = tank diameter in FT / 10 and round up to the next even number is minimum requirement. Like your case 206' diameter you can use 22 A frame.

It is right each company has their own experience and it was proved. Off course there is a factor the easiest and economist way.

Let me give you an example. One company ask us to cut the shell on the first shell ring to make an opening for their temporary door sheet. On this temporary door sheet they will install blowers and access for the crew going to inside the tank during initial lifting. After roof air raising we have to fit up and welding and NDT that cut out shell plate. They don't like to utilize shell man way for access.

Other company they are utilizing shell man way for blowers and other shell manway for crew access. If they need 2 blowers just make tandem. Or they fabricate adaptor like Y from 24" pipe. The vertical leg of this Y connected to the shell man way. The diagonal of this Y connected to 2 blowers.

The 13 mm H20 is additional pressure need to be added for friction including pulley to wire rope.

 

[IFRs]

Very interesting! What does the seal between the tank and dome look like (if not secret) ?

 

[Syahar1975]

Good evening IFRS,

Off Course no secret in today technology. Before only CBI the one who did roof air raising. Now many companies can perform roof air raising.

I have been involved for roof air raising preparation, pressure calculation and execution when I was young .

I have seen and installed 2 type of seal.

1. Wire mesh on the shell side and plastic on the wire mesh side. So the wire mesh is between the shell and plastic.

2. Metallic sheet on the shell side and plastic on the metallic side. So the metallic sheet is between the shell and plastic.

 

[Marinel1]

Good afternoon,

As far I remenber, we welded some wire pins on the border of the roof in order to fix the wire mesh. This mesh was like a roll covered by plastic.
Is what you have done in your first option Syahar?
From your experience what is the avarege distance between the roof and the course? Is it the same for metalic shell and concrete shell?

 

[Syahar1975]

Good afternoon Marinel1,

Yes that is the 1st option.

I have seen and installed 2 type of rod to roof underside in order to fix wire mesh or metallic sheet.

1. Use weld rod 3.2 mm bend it to U shape and tack weld to the underside roof.

2. Tack weld 6 mm bolt to underside roof. Fix the wire mesh or metallic sheet and install the nut.

I have seen from almost 0 to 25 mm the gap between the roof and shell.

Unfortunately, I have never seen air raising for tank with concrete shell. But if the shell is steel with concrete on the external surface of the steel, it would be the same principle.

 

[PersonalProfile]

Although I do not have first hand experience, from a distance I have seen this method used, and have pondered how the potential hazards [power, and or, fan failure etc] are managed.
Perhaps: the corresponding system discharge results in the roof lowering without risk, or NRVs / valves etc are installed.

Obviously these hazards may already be under control in your application, though maybe they need to be considered in your application.

Regards,
Lyle

 

[Marinel1]

In order to avoid or minimize the safety problem, normaly we do the roof air rising with redundant fans and supplied by exteranl generators with also an extra generator in stand-by.
Nevertheless, if everything goes wrong, the roof can be goes to the original position slowly as the air escapes between shell and the roof via the wire mesh instaled. The balancing wires will keep the roof in a stable position during the descend.