unleveled or uneven floating roof in maintenance position 1

[processrj]

A 72' diameter tank x 48 feet height, single deck floating roof tank has been overhauled (overall roof, support legs,and roof drain line replaced,along with some parcial repair made to the bottom plate by patching some small portions). After conducting hydrotests, we observed that the roof landed unevenly causing the damage of several tension springs and the 2" thermowell (attached to the 12" stilling well) touching the stilling well sleeve. Thinking that the drain line might be influencing this behavior we shortened it and conducted another hydrotest, which was succesfull; however, after draining the water to the operating position and filling it back to the maintenance position and draining again for inspection, we still find the thermowell touching the stilling well sleeve. What are the possible / most likely causes for this situation? Thank you in advance.

 

[jimstructures]

A picture is worth a thousand words. Plans and elevations might help illuminate the problem.

Jim

 

[HTURKAK]

I understand that the deck was replaced ..Does the overall roof include replacement of the whole pontoons ?


Did you replace thermowell and the 12" stilling well also ?. What about verticality of thermowell and the 12" stilling well ?

Some descriptive sketches and photos will be helpful to see the case .

 

[r13]

Seems the roof has uneven load. Can it be caused by the patch works, or defective springs?

 

[LittleInch]

Someone didn't put the support legs down in the "long" position??

How are we supposed to guess without drawings sketches or pictures?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Dhurjati Sen]

Your repair of the drain line by shortening it was not proper. Does it have swivel joints?


DHURJATI SEN
Kolkata, India

 

[IFRs]

The only reason for the deck landing in a not level condition is some issue with the support legs. If however the deck is level and you are seeing the deck move such that the vertical pole is no longer in the center of the well and is instead up against one edge, the deck is either rotated or translated. Rotation or translation are usually symptoms of tank deformations, seal weakness, strong winds, drain hose pulling, rolling ladder binding, uneven weight or buoyancy distribution, lack of adequate anti-rotation, jet mixers, etc. We can't help much without a lot more details of the floating roof and tank.

 

[LittleInch]

Processrj.

You haven't bothered logging in to see the replies but when you do listen to IFRs. He or she really knows what they are talking about.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[r13]

She? Do you know her?

 

[processrj]

Apologies for late response. I had internet connections issues. I appreciate all of your valuable insights and comments.

Answering some of the questions:
a) Yes, the pontoons, stilling well (SW) and thermowell pipe (TWP) are new.
b) Verticality of SW and TWP was checked and are ok.
c) Drain line is a 4 inch. pipe of the master joint type. 4 pipe sections, the closest one to the outlet being rather fixed, while the other move up or down with the FR movement.
d) The support legs lengths have been adjusted to compensate for any irregularities / bulges imposed by the patching done on some portions of the bottom plate
e) As mentioned, when the FR is lowered to the operation position (4 feet from the bottom) it is noticed to be centered, but when it is raised back up to 10 feet and lowered back down to its maintenance position (6 feet from bottom) the TWP touches the SW sleeve.

I include some sketches and photo.

Thanks again for your feedback and kind regards

Processrj

 

[processrj]

Sorry, I meant to send this other file as well....

 

[HTURKAK]

This situation implies , the supporting legs do not supporting the roof evenly.. If you provide sketch showing no .of the supporting legs and lengths and positions, the connection detail of roof plate to inner rim of the pontoons and info. regarding the plate thicknesses (rim, bulkhead, top and bottom deck for the pontoons we may see the effect of uneven supporting .

 

[LittleInch]

So is what type saying that the roof isn't centred?

Can you post a detail of that drain pipe in section.

It looks like it is forcing the roof over as it descends but need to see the details.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[r13]

Due to relatively light weight of the roof structure (quite flexible), the other possibility is the imperfect lifting and lowering operation. Can you add more lifting points, or add removable weights evenly around the perimeter?

 

[IFRs]

This is a steel pontoon floating roof (from the image maybe an external floating roof), I would not generally classify it as "lightweight and flexible" at least in comparison to pan or aluminum internal floating roofs. Presumably the raising and lowering is done with liquid, most likely water. Assuming the deck is flat when floating and flat while landed, what's happening is that the deck is shifting when floating and the rim space at the pole is changing. As LittleInch suggests, the first thing to look at is the water drain system, especially since it is in the same orientation as the floating roof movement. Easy to tell if you can do a quick test: disconnect the water draw off piping from the floating roof, center the deck, fill up to 10 feet and lower down again. Floating roofs tend to float in the tank center unless acted on by an external force. Other places to look are the seal (although the seal will have a hard time moving the deck in a short timeframe), rolling ladder, any centering devices, anti-rotation devices, check for leaks in pontoons (look in each pontoon manway for liquid), if this is an external floating roof wind can have an effect but is usually only evident when the tank is over half full.

Let us know what you find, and decide to do...

 

[r13]

IFRs,

It is about scale factor, length contributes to flexibility, and flat floating object usually is lightly built. The OP has already re-adjust/balance the weight twice, I think maybe it's time for him to look outside of the box. Since this roof equipped with a drain line, I guess it is an external floating roof rather than internal.

 

[processrj]

Thanks for all feedback so far, they are really helpful.

Providing some more clarifications:
A) Yes, indeed is an external FR
B) A summary of tests and their results follows:

a. With drain line (DL) 68 ‘ long, tank hydrotest (THT) passed, but after draining completely, 10 tension springs (TS) got damaged,
secondary seal (SS) got misaligned, and thermowell pipe (TWP) attached to stilling well (SW) found touching SW sleeve SWS).

b. With SW re-aligned and re-welded, TS’s replaced, and SS replaced, with same 68’ DL, hydrotest passed, but after draining, 3 TS
got damaged near the SW-TWP set, and again TWP found touching SW sleeve (SWS).

c. With no DL (it was removed to explore issues with it) and filling the tank 50%, FR was found centered (no SWP touching SWS)

d. With DL increased (I was wrong when I said before it had been shortened) in size to 76’, tank hydrotest was ok. Drained and
landed on “operation” position (low legs, 4 ‘ from the bottom), no SWP was observed touching the SWS; however, after raising
with water to 10’ from the bottom and lower it down to “maintenance” position (6’ high from the bottom) SWP was found touching
the SWS.

Question: Is there specified filling / emptying rates that should used to avoid potential unleveling of FR?

Thanks for your further insights.

Processrj

 

[LittleInch]

I think it's all about the design of your drain line.

Can you supply proper details.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

The filling rate should not really matter unless the inlet stream is directed against the deck and could move it sideways. Good industry practice (API and NFPA in the USA) is to limit the velocity in the pipe network to 1 meter per second until the floating roof is floating or there is 2 inlet pipe diameters of liquid above the inlet pipe, whichever is greater.

LittleInch is (as usual) correct - it's your drain line. Since it floated properly without it, your drain line certainly seems to be the issue. Change the design or go with a different type altogether. If it causes this trouble when it is new, I would not be comfortable using it for years. You may also benefit from centering devices or at least bumpers to keep the seals from being crushed. It is disappointing that both seals failed, but hopefully the drain line manufacturer is standing behind you and will compensate you for what may prove to be their fault.

 

[processrj]

Yes, it looks that there is some issues with the drain line. Would it has to do with the orientation and/or relative position with the stilling well-thermowell pipe (SW-TWP)set? Should it be a defined angle relative to the SW-TWP set?

Please find attached some drain line details.

Thanks,

Processrj