14 replies to this topic

[Sharma Varun]

Hi,

 

Generally hydrocarbon pressure vessel LG/LT drains are connected to closed drain system. In a running plant, normal operating procedure for taking these instruments in maintenance, shall be as follows:

• Isolate the instrument.
• Depressurise to flare system.
• drain to closed drain vessel.

In case, there is a difference in rating & design pressure of system, I have seen a practice of providing flow restriction orifice in these drain lines. Reason given is to safeguard the drain system against a possible gas blow by, from the pressere vessel through instrument line.

 

My underatnding w.r.t. these ROs is limited, & I have few queries:

1. I have worked for @ 4 years in operations in refinery & petrochemical complex in India, but I don't remember seeing these ROs. Also in my similar process design experience I have never come across these. I may have missed the same due to limited exposure. Now I want your opinion on the same that - Is it a good rather acceptable & common practice to do so?
2. I understand that the requirement of these ROs is to restrict the flow rate so that you don't need to size the drain system relief for the purpose in case this becomes the governing case. If yes what shall be the criteria for sizingsuch ROs?
3. Also I guess choked flow scenerio shall be envisaged as generally we encounter pretty small valve sizes in instrument drain system & we do have critical pressure scenerio. Now in case the choke flow equatin gives a smaller amount shall we specify the same flow for orifice also if this value comes pretty high & clealy surpasses the amount for which drain relief is designed, what shall be specified for RO.

I request veteran members to provide their views on the same. seniors can also guide me, if this is not the right place to post such query.

Edited by Sharma Varun, 16 August 2013 - 02:17 AM.

[ankur2061]

Varun,

 

The question that arrives from your post is what are you draining from the instrument (LG/LT) drains  of a vessel?

 

To my understanding this would be very small quantities of liquid after isolating the LG/LT for the purpose of maintenance. For such small quantities why would you require a RO to restrict flow. It just doesn't make any sense.

 

However, despite what I have mentioned above, ROs are sometimes used in closed drain systems from process drains of large equipment such as pig traps and separators. I had written a standard on "Drain Systems" for a well known oil & gas operator in the middle-east and I will reproduce what I had written:

 

Controlled Draining in Closed Drain Systems

 

For controlled draining from pressurized hydrocarbon equipment (e.g. pig traps, separators) to closed drains, one of the valves in the equipment drainpipe shall be a pressure balanced plug or globe valve. Alternatively a restriction orifice (RO) may be provided downstream of the double block valve which shall ensure controlled draining. If the RO needs to be installed in the horizontal section of the drainpipe, then it shall be an eccentric type to ensure complete draining of the RO upstream piping. The minimum orifice size for controlled draining shall be 6 mm.

 

Note:  For hydrocarbon liquids containing moderate to heavy solids and/or tending to form gels and hydrates, restriction orifice or any other form of pipe restriction shall not be used in the drain piping.

 

From the above it is clear that ROs are not to be installed indiscriminately in any drain system but require careful evaluation before installing them in a drain system. It is also obvious that installation of RO for controlled draining is meant for process drains and not for instrument drains due to the fact that the liquid quantities encountered for an instrument drain are very small.

 

Hope this helps.

 

Regards,

Ankur.

[Sharma Varun]

Varun,

 

The question that arrives from your post is what are you draining from the instrument (LG/LT) drains  of a vessel?

 

To my understanding this would be very small quantities of liquid after isolating the LG/LT for the purpose of maintenance. For such small quantities why would you require a RO to restrict flow. It just doesn't make any sense.

 

However, despite what I have mentioned above, ROs are sometimes used in closed drain systems from process drains of large equipment such as pig traps and separators. I had written a standard on "Drain Systems" for a well known oil & gas operator in the middle-east and I will reproduce what I had written:

 

Controlled Draining in Closed Drain Systems

 

For controlled draining from pressurized hydrocarbon equipment (e.g. pig traps, separators) to closed drains, one of the valves in the equipment drainpipe shall be a pressure balanced plug or globe valve. Alternatively a restriction orifice (RO) may be provided downstream of the double block valve which shall ensure controlled draining. If the RO needs to be installed in the horizontal section of the drainpipe, then it shall be an eccentric type to ensure complete draining of the RO upstream piping. The minimum orifice size for controlled draining shall be 6 mm.

 

Note:  For hydrocarbon liquids containing moderate to heavy solids and/or tending to form gels and hydrates, restriction orifice or any other form of pipe restriction shall not be used in the drain piping.

 

From the above it is clear that ROs are not to be installed indiscriminately in any drain system but require careful evaluation before installing them in a drain system. It is also obvious that installation of RO for controlled draining is meant for process drains and not for instrument drains due to the fact that the liquid quantities encountered for an instrument drain are very small.

 

Hope this helps.

 

Regards,

Ankur.

well Sir, thanks for the prompt response, I do understnad that the amount of liquid will be very less, but as I have writtn the vey purpose of these ROs is to restrict flow in the event of gas blow by from the vessel. Say if some one opens the LG/LT drain line in a running system & high pressure gas from the contineous system flows to low pressure drain.

I do understand that in case of drain system RO one shall be very careful as these are dirty services & RO may get blocked or choked due to solids accumulation. However, I have gained from your reply as I was wondering about the minimum orifice size which I was required to give in some other case. Can you also tell me the basis for this 6 mm limit.

Edited by Sharma Varun, 16 August 2013 - 02:50 AM.

[ankur2061]

Varun,

 

If gas blow-by is a concern for a LG/LT drain then the best way would be to provide a spectacle blind in the drain line.

 

The attached sketch shows an instrument drain from a vessel to a closed drain system with the spectacle blind in the open position. This can be kept in the closed position if a gas blow-by scenario is found to be credible.

 

Regards,

Ankur

Attached Files

•  Instrument_Drain_to_Closed_Drain_System.jpg   53.57KB   38 downloads

[Sharma Varun]

Varun,

 

If gas blow-by is a concern for a LG/LT drain then the best way would be to provide a spectacle blind in the drain line.

 

The attached sketch shows an instrument drain from a vessel to a closed drain system with the spectacle blind in the open position. This can be kept in the closed position if a gas blow-by scenario is found to be credible.

 

Regards,

Ankur

You are right Sir, this is what we have been practicing in India, I mean I found the same concept while working in operations in refinery & during other basic as well as detail engineering projects. That spectacle blind is usually kept in closed position avoiding operational negligence & a postive isolation is ensured.

However I found usage of these ROs in middle east. I have encountered these in two cases:

1. For one of the clients ( A major in onshore oil & gas production), I was involved in detail engineering. FEED was done by an american firm, which suggested consideration of gas blow by from compressor suction/discharge KOD level instruments for governing relief scenerio identification for closed drain vessel. Also during HAZOP study such ROs came up in drain line from LG/LT of glycol dehydration column.
2. Second case is for an offshore oil producing giant, where again FEED prepared by a MNC specifies ROs in such LG/LT drain from high pressure vessels. This is the same company for which you have written the standard, as I could see that, content reproduced by you & above sketch exactly match the standard, which I am referring for the project.

Thanks regards,

varun Sharma

Edited by Sharma Varun, 16 August 2013 - 05:03 AM.

[fallah]

varun,

 

Drain from LT/LG is a pressurized drainage of a minor inventory toward closed drain system. It is among operational draining system not to be a maintenance drain. Drain line can be composed of a block valve followed by a globe valve (or a RO) then a check valve to avoid any back flow due to a high pressure in downstream...

[Sharma Varun]

varun,

 

Drain from LT/LG is a pressurized drainage of a minor inventory toward closed drain system. It is among operational draining system not to be a maintenance drain. Drain line can be composed of a block valve followed by a globe valve (or a RO) then a check valve to avoid any back flow due to a high pressure in downstream...

Well you have pointed out the right thing weather to consider it an operational drain or maintenance drain, because if we consider it operational drain we can not provide a blind in the circuit.

But we are discussing about gas blow by scenerio where higher gas flows are envisaged at high pressure from vessel. Please note I am talking about a scenerio where back flow is not an issue as upstream pressure is much much higher  than downstream drain.

[fallah]

varun,

 

I am agree with you that in such operational drain line spectacle blind shall not be provided...

 

Then in my opinion in an operational drain line from LT/LG that have been done manually to release minor inventory, the gas blow by case might rarely be happened and cannot be a credible scenario.

Such drain line normally to be sized to be able to release minor inventory means has small size and even if operator left the drain valve opened, the drain flowrate is low enough that operator has been announced by low level alarm in vessel and has adequate time to handle the matter...

Anyway as i mentioned  a globe valve installed next to block valve will play the role of a RO to prevent (or mitigate) any issue in this regard even though in my opinion it can rarely be happened...

[Sharma Varun]

varun,

 

I am agree with you that in such operational drain line spectacle blind shall not be provided...

 

Then in my opinion in an operational drain line from LT/LG that have been done manually to release minor inventory, the gas blow by case might rarely be happened and cannot be a credible scenario.

Such drain line normally to be sized to be able to release minor inventory means has small size and even if operator left the drain valve opened, the drain flowrate is low enough that operator has been announced by low level alarm in vessel and has adequate time to handle the matter...

Anyway as i mentioned  a globe valve installed next to block valve will play the role of a RO to prevent (or mitigate) any issue in this regard even though in my opinion it can rarely be happened...

hmm, but just because its rare can we say its not a credible scenerio? If not a RO is required, and question is how to size that RO?

[fallah]

varun,

 

As i mentioned a globe valve can handle the issue instead RO...

 

Anyway, if you insist to use RO you can size it based on delta P (vessel operating pressure+liquid static head-back pressure) and required flowrate. Some RO sizing softwares can specify needed size...

[Sharma Varun]

has no one else faced this situation ????

[Sharma Varun]

come on guys please respond................................

[curious_cat]

Why aren't all the options discussed feasible alternatives? They sound all OK to me.

 

There isn't really a constraint that a best practice needs to be a unique solution. 

 

One downside I see of having a blind versus an orifice is that it becomes yet another item that might get overlooked. An orifice is better than a spectacle blind in the wrong orientation. OTOH if you are afraid of plugging, yes an orifice is unsuitable. 

 

Overall, I'd say an assessment really depends on the relative probabilities you assign to each scenario. 

Edited by curious_cat, 04 September 2013 - 03:40 AM.

[curious_cat]

Varun,

 

If gas blow-by is a concern for a LG/LT drain then the best way would be to provide a spectacle blind in the drain line.

 

The attached sketch shows an instrument drain from a vessel to a closed drain system with the spectacle blind in the open position. This can be kept in the closed position if a gas blow-by scenario is found to be credible.

 

Regards,

Ankur

 

One question I have about Ankurs setup is, in case a mistake does get made, how do they now depressurize before opening the blind? 

[shekhar dhuri]

_Hi, 

 

_{I was going through the Post. I quickly checked arrangement of Instrument Drains in 4-5 Project P&IDs. All of them support Mr. Ankur's & Mr. Fallah's views.} 

 

_{Such drains are treated as maintenance drains & hence Spectacle blind is sufficient & RO is not envisaged.}    

 

_{Mr. Curious Cat, as mentioned by Varun, normal operating procedure for taking these instruments in maintenance, shall be as follows:}

 

_{·         Isolate the instrument.}

 

_{·         Depressurise to flare system (By Opening Vent Valve)}

 

_{·         Drain to closed drain vessel.}

 

 

 

 

 

 

 

 

 

 

Edited by shekhar dhuri, 15 September 2013 - 07:37 AM.