Spiral staircase on storage tank 1

[monopoly]

Hello,
I’m new to the above ground storage tank.
I’m looking for a design guide for spiral staircase on storage tank.
1. When to use staircase vs ladder? Please provide reference.
2. Min & max dimensions of run and riser. Max landing distance. Min stair case width. Do we have to follow OHSA requirements.
3. Mostly i saw the old tanks have outside guardrail. Do we need guardrail and handrail on staircase for storage tank? Do we need inside handrail?
4. Do we have typical standard details for spiral staircase on tank?

If you can point out any references, engineering practices, client’s specifications so i can read and learn on this topic.

Thank you.

 

[HTURKAK]

Some points;

- When you write ( Standard Details of Circumferential Stairway for Cone Roof Tanks ) and search the web , one of the outcomes
-

https://1filedownload.com/library-drawings-dc-950102-001-api-asme-resources/

- Pls look API 650 (Table 5.17—Requirements for Platforms and Walkways ) and (Table 5.18—Requirements for Stairways )

- If this is a small size tank, you may look OHSA requirements for ladders and details.

- Some clients have company own standards .






Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[IFRs]

1. When to use staircase vs ladder? Please provide reference. Owner preference, most seem to use spiral. Ladder may make rescue difficult and you can't carry anything.
2. Min & max dimensions of run and riser. Max landing distance. Min stair case width. Do we have to follow OHSA requirements. In the USA follow OSHA, some global companies require US OSHA worldwide, that will answer all, there is no max landing distance.
3. Mostly i saw the old tanks have outside guardrail. Do we need guardrail and handrail on staircase for storage tank? Do we need inside handrail? Per current OSHA you need a 3-rail system on the outside (top, hand and mid rails), the gap between the tank and the stairway determines if you need a railing on the inside near the tank.
4. Do we have typical standard details for spiral staircase on tank? See OSHA.

 

[shvet]

Note that there is "Storage tank engineering" thread at this forum. Specialists might not look into a petro forum.

7.9 Tank access
Fo safety reasons a tank have to means of egress from the roof. For a single tank, which is not interconnected with another, then the second means of access is usuallu by a vertical-caged ladder.
The BS 2654, API 650 and prEN 14015-1 Codes all specify similar design requirements for access ways but in using these the designder must also be aware of any local and/or client requirements ans safety issues.
Whilst there are some differences between the tank Codes, the principal requirements are as folows:
*Min clear width of a staircase, platform or walkway shall be 600 mm.
*Min height to the top handrail of a horizontal platform or walkway shall be 1070 mm.
*Min depth of a stair tread shall be 200 mm.
*Max slope for a staircase 45° (50° in API)
*Handraling is required on the inside stringer of a spiral staircase where the gap between the stringer and the tank shell exceeds 200 mm.
*The normal "going" and "rise" for treads of a spiral staircase is 200 mm.
*The max vertical rise between intermediate platforms of a staircase is 6 m.
...
*Vertical ladders over 4m high shall be fitted with safety cages. BS 4211 allows a max height between intermediate platforms of 9m but it is normal to limit this to 6m on tanks.
...
7.9.1 Spiral staircase
Probably the most common means of access is the spiral staircase. This type os staircase is simple to fabricate and erect. ...
There are long-term disadvantages with this type of staircase, and these are:
*Being welded directly to the shell makes corroded treads difficult to replace (galvanized treads cannot be used because of the health risk in welding on to a galvanized surface).
*Where tank shells are thermally insulated, there are numerous penetrations in the cladding where the dogleg supports and trads pass through and offer a path for the rain to get in and cause corrosion on the shell.
*The tread replacement issue can be solved by using bolt on treads where a short lenght of drilled angle is welded toe on the shell to which the tread is bolted. Similarly at the outer tread support a short length of drilled flat is welded to the support to carry the tread.
*Because of the shortcommings of the weld on staircase, most spiral staircase today are constructed with an inner and outer stringer and bolted galvanized treads. The stringers are supported off brackets welded to the shell but the limitations in the Codes regarding the welding of permanent attachments to shell musr be observed.
*The double stringer spiral staricase is to be preferred for thermally insulated tanks because of the smaller number of penetrations in the cladding.
...
7.9.2 Radial staircase
This type of staircase is often used to access large diameter tanks, which have large bunded areas. ...
7.9.4 Vertical ladders
Tank operators do not favor vertical ladders as a main means of access to a tank roof because they are tiring to climb and require the full use of all limbs during the ascent, hence the carrying of any sundry equipment is difficult. However as a secondary means of escape from a tank roof under emergency conditions when the primary route is blocked or otherwise unavailable, then they are most welcome. ...

 

[IFRs]

SHVET has a good summary.

"*Vertical ladders over 4m high shall be fitted with safety cages. BS 4211 allows a max height between intermediate platforms of 9m but is is normal to limit this ti 6m on tanks."

OSHA has gone away from from cages and requires a lifeline / fall protection device.
Some states in the US have more strict requirements on rise / run and railing extensions.

 

[LittleInch]

For me the decision is based around size and proportion of the tank.

Tall vertical tanks with a H/D ratio of 2 or more and not more than 5-8m diam are probably better served by vertical ladders.

Larger diameter tanks >8m with H/D <2 are going to be circular.

Shvets list is pretty good and normally this is to be discussed with the tank vendor, taking into account local regulations and many companies have their own specification.

Ladders in particular tend to have intermediate platforms every 7-10m.

Some stairs have intermediate platforms every 6-10m, but I've been up many tall tanks who don't have any.

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

 

[IFRs]

OSHA has rules on intermediate platforms, on vertical ladders they are required at certain spacing, on winding stairs they are suggested but not required. OSHA uses "spiral" for very small diameter stairways and "winding" for the rest.

 

[MJCronin]

Some brief commentary on tank spiral staircases ...

1) They have been discussed before on eng-tips .... Do a search

https://www.eng-tips.com/viewthread.cfm?qid=330297

2) There exists a practical "minimum diameter" for tanks to have properly designed spiral staircase its about 30 feet

https://www.eng-tips.com/viewthread.cfm?qid=450837

MJCronin
Sr. Process Engineer

 

[monopoly]

Thank you everyone for the responses.

 

[monopoly]

Thank you shvet for the reference document. The Bob Long & Garner book " Guide to Storage Tanks & equipment" is great. I was able to find the reference and read it.

In my case, the tank will be 58ft dia. x 50 height with a spiral staircase. API 650 does not provide specific requirements for maximum vertical landing platforms but references to OSHA, which requires 12ft max between intermediate landing platforms. Does it seem like a lot of landing platforms? I saw some spiral staircases from the bottom to the tank roof without intermediate landing platforms.