10 replies to this topic

[Afshin4451]

Dear Experts,

 

A LPG drain drum has been designed to install near LPG loading arm in jetty area. After every ship loading, loading arm and associated piping should be drained to drain drum.There are 3 options can be considered for venting of lpg gas in drain drum.

 

1-  Venting lpg gas to safe location

2-  Venting lpg gas to existing refinery flare header via 2 km flare header 

3-  Installing a separate flare stack in jetty area

 

Option 1 is not recommended by our technical safety due to lpg flamabilty and hazard.

Option 2 can be considerd but problem is due elevation difference between jetty area and refinery (+53 m), new flare header should be routed to existing refinery flare header with 5% slope toward lpg drain drum.

Option 3 seems is very costly because the source to flare is only from lpg drain drum and lpg loading arm PSV.

 

From engineering point of view option 2 seems more practical but my problem is to keep positive pressure in new flare header purge gas require to inject in inlet of new header close to loading arm and drain drum.

 

As purge gas also can sweep liquid in header, purging line can avoid of free draining of liquid in new flare header toward drain drum.

 

It seems this design is not correct but i don't have any idea to design it in proper way.

 

Appreciated if somone could shade a light in my problem.

 

 

 

 

 

 

[fallah]

Dear Afshin,

Would you please upload a simple sketch of the system you described?

[Afshin445]

Dear Fallah,

 

Enclosed please find schematic for jetty flare header which has been designed for LPG loading arm and Drain Drum. Kindly inform me if you need more information.

 

 

 

Attached Files

•  Jetty Flare Sketch.xlsx   14.28KB   34 downloads

Edited by Afshin, 29 November 2016 - 01:34 AM.

[fallah]

Dear Afshin,

 

At a glance, considering the required positive pressure in new header which depends on the pressure in existing flare header, if the LPG drain drum is equipped with an electrical heater with proper duty and the new header is electrically traced (both can be activated in each ship loading) the free draining liquid toward the drain drum will be minimized and the matter might be resolved...

 

Purge gas can be connected to the header through a PCV to compensate the relieving gas pressure shortage...

Edited by fallah, 29 November 2016 - 03:43 AM.

[Afshin445]

Dear Fallah,

 

First of all, thank you for your prompt reply.

 

Good point.By following your suggestion and maintaining drain drum and jetty flare header temperature above C4 dew point (worst case scenario)  we can assure lpg drain drum and flare piping is free of liquid.

 

Some more question:

 

1- C4 normal boiling point is -1 C and min. ambient temperature is 4.5 C, do you think still tracing and heater is required? 

2- What about if there is any any water content in lpg composition?

3- What do you think about N2 purge gas injection point? Is not better to move upstream of drain drum to protect all of lpg system?

4- There is available steam in jetty area, is it any problem if we use steam trace and heater instead of electrical?

Edited by Afshin, 29 November 2016 - 04:41 AM.

[fallah]

 

1- C4 normal boiling point is -1 C and min. ambient temperature is 4.5 C, do you think still tracing and heater is required? 

2- What about if there is any any water content in lpg composition?

3- What do you think about N2 purge gas injection point? Is not better to move upstream of drain drum to protect all of lpg system?

4- There is available steam in jetty area, is it any problem if we use steam trace and heater instead of electrical?

 

Dear Afshin,

 

1- Header tracing appears to be not required but electrical heater for drum is required if the required positive pressure in new header is higher than corresponding vapor pressure of 4.5 C as B.P. of C4

 

2- The water should be drained by a boot at the bottom of the drum...

 

3- The N2 purge should be evaluated based on more info in hand...

 

4- If it's needed to trace the header, using steam for this purpose is possible...

[Afshin445]

Dear Fallah,

 

Thank you for your reply again.

 

Please refer to my reply as per below:

 

1- Maximum flare back pressure is 1.5 barg which is corresponded to C4 dew point of 26 C, it seems heater is required.

2- In tis case we should install heater in boot or above it?

3-  What more information is required ?

4- OK, is it aplicable for heater too?

Edited by Afshin, 30 November 2016 - 12:58 AM.

[fallah]

Dear Afshin,

1-Yes,appears EH is required
2-The EH should be installed inside the shell above the boot
3-N2 may be required to equalize the pressure in new header during the times of no ship loading or compensate the pressure shortage of header in ship loading, but it should be evaluated if existing flare system can tolerate the required rate of this inert gas
4-No, it's better using EH rather than steam tracing due to the risk of condensate freezing

[Afshin445]

Dear Fallah,

 

1- OK.

2- Agree.

3- During ship loading there is no connection between drain drum and loading arm, could you explain how ship loading can make pressure shortage in new jetty flare header? In this regards, if there is no any purge gas injection in jetty side, how existing flare purge gas can pressurize new header and drain drum through 1 km pipe.

4- Good point,agree.

 

In addition, there is a LPG vapor return line from loading arm to storage tank, which I forgot to show it in first sketch, I modified it and attache it again. As an alternative case, is not better to route drain drum vent to this line instead of existing flare? In this case loading arm PSV should be routed to atmosphere in safe location.

Attached Files

•  Modified Jetty Flare Sketch.xlsx   14.49KB   37 downloads

Edited by Afshin, 30 November 2016 - 12:15 PM.

[Saml]

It is the storage tank a cryogenic one or a pressure vessel?

 

If it is a cryogenic one,  can the tank refrigeration system handle the extra loading generated by the drain drum? (my guess is that it should).

 

However, I'd ask a more basic question: What is not acceptable about the current situation?, Why "it is required" that you install a drain tank?  Why, specifically, this solution?  May be, there are easier options.

Edited by Saml, 30 November 2016 - 11:42 AM.

[Afshin445]

It is the storage tank a cryogenic one or a pressure vessel?

 

If it is a cryogenic one,  can the tank refrigeration system handle the extra loading generated by the drain drum? (my guess is that it should).

 

However, I'd ask a more basic question: What is not acceptable about the current situation?, Why "it is required" that you install a drain tank?  Why, specifically, this solution?  May be, there are easier options.

Saml,

 

The storage tank is spherical with operating pressure of 6 barg,if we following your sggestion and routing vent gas to vaour return line then new lpg drain drum should be pressure vessel protected by PSV and under pressure control also liquid LPG need to pump seaparatly to LPG storage tank which need separate piping(about 1km).

 

About your basic question, I am not sure if I understand your question but please note that both loading arm and drain drum are new and drain drum is required to drain loading arm and associated piping inventory after each ship loading.   

Edited by Afshin, 03 December 2016 - 09:11 AM.