2 replies to this topic

[SabinaA]

Hello, 

 

I'm hoping somebody on here may be able to offer some guidance. I have the task of replacing a dip pipe in a Carbon Steel storage tank. I can't find any information on google etc regarding the design of dip pipes, and i have searched this forum but i am really looking for the basics and those tend to be glossed over. 

 

The tank has no agitation, is at ambient temperature. It's flat bottomed with a dished top. There is no fixing bracket at the base of the tank and there is no access to install one.

 

The details are as follows; 

 

Process fluid - Xylene 

Vessel flange- Table E 3"

Process connection - ANSI 150 1 1/2"

Dip pipe length - 3850mm

Dip pipe MoC - StSt

Vessel volume - 10,000L

Vessel height - 4000mm

Vessel Moc - Carbon Steel

 

Is a 1 1/2" pipe suitable, is Schedule 10 suitable? If not why? Are there design calcs, astandard or a book i can refer to?

 

Thanks in advance for your help!

[Steve Hall]

I think you have to ask or answer some questions before going too far:

 

- Why is the dip pipe being replaced?

- Will your new dip pipe mitigate whatever problem resulted in the need for replacement?

- What is the purpose of the pipe?

- How is it connected to the tank, and how is the feed pipe connected to the dip pipe?

- What is the flow rate (or range of flows) through the pipe?

- Is the incoming xylene at the same temperature as the tank?

 

It doesn't sound like you have any unusual bending stress on the dip pipe inside the tank since there's no agitation. But there could be a bending moment on the connection to the pipe at the top of the tank. If so, you might need to calculate the stress to be sure there is no structural problem.

 

The purpose for the dip pipe might be to reduce static charge by introducing the feed below the liquid surface. You'll just want to be sure it is grounded and connected (by conductance) to the tank itself (not insulated from the tank due to gaskets).

 

You also need to evaluate if there is a potential for syphoning xylene out of the tank through the pipe, especially in the event of a process upset or operator error (for example, if a drain valve external to the tank is inadvertently left open the entire tank contents could syphon out). If there is a potential, there are ways to prevent the syphoning -- drill a hole through the pipe in a location within the head space of the tank, provide an air break outside of the tank, install a foot valve on the dip pipe, or some other means.

 

Only you can decide if 1-1/2" pipe is suitable since you haven't provided any process information. Pipe wall thickness is dependent on bending stress, but with no agitation or thermal contraction/expansion I'd hazard to say that Schedule 10 is fine (or even Schedule 5) assuming you can weld it to the connection flange.

[Art Montemayor]

Sabina:

 

I've designed, specified, and had fabricated a lot of dip pipes for storage tanks when I did DuPont projects in the last century.  In fact, I've sketched out some of those installations in past threads of many years ago.

 

You ask for "information", but don't specify exactly what it is that you seek.  Please be specific.  Do you want a detailed sketch? - or a calculation for the pipe's diameter?  I can furnish a sketch, but you should calculate your diameter in accordance with your design fill rate.

 

Why use a Stainless dip pipe in a carbon steel tank?