Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

0

Is It Safe To Open The Bypass Line


5 replies to this topic
Share this topic:
| More

#1 J_Leo

J_Leo

    Gold Member

  • Members
  • 262 posts

Posted 29 September 2018 - 11:32 AM

Dear Friends,

 

There is a 2" bypass line around a manual valve on a 30" LNG transfer line. It is planned to provide temporary cooling to the downstream LNG line when the manual valve is closed during a planned event. 

 

Information of the 2" bypass at the time of the event:

Upstream P of the bypass: 130psig

Downstream P of the bypass: 30psig

LNG T: -250F

Isolation ball valves on the bypass

Bypass length: a couple of feet

MOC: stainless steel

 

The max design velocity is generally around 13 to 15 ft/s. Based on a simple Hysys model, the velocity can reach 60 ft/s without adding restriction orifice or globe valve. The bypass itself is working like a flow restriction component.

 

Is it safe to open the bypass? Vibration and noise might be the issues. Will the pipe break even if it is well supported? Can a restriction orifice help if the flow required is not a limiting factor?

 

Thank you,

-LEO


Edited by J_Leo, 01 October 2018 - 08:35 PM.


#2 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 3,529 posts

Posted 29 September 2018 - 10:09 PM

There must be someone at your site who knows the answer. Probably one of your operators. If this is a new installation, your design engineer should answer your query. If you don't get a positive response here, what will you do?

 

Bobby



#3 J_Leo

J_Leo

    Gold Member

  • Members
  • 262 posts

Posted 30 September 2018 - 01:11 PM

Hi Bobby,

 

The site operator said no problem. But he still asked the process to check. I worked mostly on the process design side before, so what I know are all the normal practices from the EPC companies.

 

For the time being, I am considering two options:

(1) Recommend adding a restrictive orifice on the bypass

(2) Enforce the piping support on the bypass to minimize the potential vibration. Measure the vibration level when the operator opens the bypass. 

 

I just hope any engineers who had the experience before can give some advice.

 

Thanks,

_Leo



#4 Art Montemayor

Art Montemayor

    Gold Member

  • Admin
  • 5,780 posts

Posted 30 September 2018 - 04:34 PM

Without a detailed piping diagram, the results of the Hazop that must have preceded this “by-pass” operation, and a complete, accurate explanation of what the operation entails, it is not possible to risk giving advice on this query.  This is a serious query about a serious operation.

 

Just review what you have written - without a detailed sketch or piping diagram of any kind:

 

“It is planned to provide temporary cooling”
You don’t provide “temporary” cooling to any pipeline being prepared to receive LNG at a temperature of -260 oF.  You must first pre-cool the pipeline to at least -260 oF, before attempting to use it as a transport line.  If you don’t totally pre-cool the line, 2-phase flow will ensue and you will have to handle the resulting mixture either downstream or in the pipeline itself.  You make no mention of this procedural requirement and what it entails.  It isn’t just about using the by-pass valve; it’s about what will result downstream when you do open the by-pass and allow saturated LNG to come into contact with a relatively “hot” pipeline.

 

“Upstream P of the bypass: 130psig
Upstream P of the bypass: 30psigLNG T: 250F”

Come on J_Leo, explain how both upstream pressures can be different.  I probably can guess the response: it’s a typo.  My point here is that there shouldn’t be any “typos” with such a critical and serious query.  Is the temperature of “250F” also a typo?  Please be accurate in your basic data.  That is a minimum requirement of any process design engineer.  Are we supposed to respond to typos?  You really meant -250 oF, right?

 

An LNG flow velocity of 60 ft/sec is ultra high in stainless steel in anyone’s design.  I normally would not design for any velocity higher than 10 ft/sec.

 

If the downstream initial pressure is really 30 psig I presume it is all gaseous natural gas and what happens to this gas volume when a subsequent 2-phase flow follows it is not explained in detail - nor how the downstream pressure is kept constant.  All these items and how they are controlled and secured as safe should be documented in the customary and required HazOp.  And I wouldn't refer the local query to an operator (who possibly was not present at the required HazOp).

 

One thing I can positively tell you, based on my LNG experience:  you are going to generate 2-phase flow and you will have to handle the resulting vapor.  How you do it is what determines the flow rate that you can tolerate into the un-cooled pipeline.  But your haven't told us that.  First find out how much vapor you can safely generate and that will set the max initial LNG injected into the downstream pipeline.



#5 J_Leo

J_Leo

    Gold Member

  • Members
  • 262 posts

Posted 01 October 2018 - 09:14 PM

Hi Art,

 

Thank you so much for your great inputs. I have corrected the mistakes in the original post.

 

I am in the process of understanding the cooling down process, which is part of the recommission of one LNG storage tank after repair. The recommissioning will not occur until early next year. The LNG transfer lines to the jetties should be filled with liquid LNG and cooled at all time, so the ship loading will not be affected by the LNG tank recommissioning. 

 

I am temporarily helping with the estimation of the flow through the bypass line. I am not sure if the HAZOP has been done or not. Let me find it out.

 

The total flow through the bypass should be sufficient based on the experience of a senior worker who is in charge of the procedure. Of course, I don't mind doing the estimation if I am asked to by my lead.

 

The purpose of closing the manual valve on the transfer line is to provide sufficient pressure at the cooling ring inside the tank, to make uniform spray and cooling. The bypass will provide the cooling flow through the transfer line when the manual valve is closed.

 

Regards,

_leo



#6 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 3,529 posts

Posted 01 October 2018 - 10:50 PM

A sketch would help immensely. Show what's in service & conditions. We can only guess with what you have told us. It sounds like this facility has been around for quite some time. Is the facility in the US?

 

Bobby






Similar Topics