6 replies to this topic

[inkar]

hi, 

 

I have an application where i see hydrate formation during blowdown. We want to inject methanol upstream of the blowdown valve during depressurization. there are about 3 blowdown points. also there is one methanol injection point upstream of a recycle control valve. Since all the injection is intermittent, whatt is the effective stratergy for injection. do we take 1 methanol pump per injection point or take multi head pump? since injection points have different initiating events, what will be effective stratergy or philosophy of injecting?

[Technical Bard]

How do ensure the methanol pump starts in time?  Or starts at all?  

[inkar]

hello technical bard, 

 

not sure. so what should be the injection stratergy in such a case? 
Can we go for continuous injection into the gas so that when blowdown comes, it has enough mixed methanol?

however that will require continuous running of pump energy wastage.

have u encountered such situation - what will be ur recommendation ??

How do ensure the methanol pump starts in time?  Or starts at all?  

[SawsanAli311]

Usually methanol is dosed for a specific duration to create a buffer volume inside the system and uppstream the blowdown device/restriction orifice. Injection is not envisaged to be required in continuous mode during the blowdown process. 

Remember that the risk of hydrate formation is mainly upstream the blowdown device and inside the system. 

The risk of hydrate formation inside large tail pipes is not as serious as inside the system and upstream the BDV. This is due to the high velocities experienced during blowdown. This is also valid when we are dealing with large size flare headers where risk of blockage by hydrates is not a major concern in addition to the fact that the low backpressure experienced downstream the blowdown orifice is too low that it might be below the hydrate formation pressure for the specific gas composition being vented. Therefore, always generate a hydrate formation P, T trend to see which region are the points falling. 

[inkar]

hi

 

in case injection is envisaged during initial phase of blowdown (not continuously)...would the preference be to activate the methanol pump from the ESD signal which initiates blowdown? 

Usually methanol is dosed for a specific duration to create a buffer volume inside the system and uppstream the blowdown device/restriction orifice. Injection is not envisaged to be required in continuous mode during the blowdown process. 

Remember that the risk of hydrate formation is mainly upstream the blowdown device and inside the system. 

The risk of hydrate formation inside large tail pipes is not as serious as inside the system and upstream the BDV. This is due to the high velocities experienced during blowdown. This is also valid when we are dealing with large size flare headers where risk of blockage by hydrates is not a major concern in addition to the fact that the low backpressure experienced downstream the blowdown orifice is too low that it might be below the hydrate formation pressure for the specific gas composition being vented. Therefore, always generate a hydrate formation P, T trend to see which region are the points falling. 

[Technical Bard]

I would look seriously at the design of the system and where you think the hydrate is going to form.  It is entirely possible for the hydrate to form downstream of the BDV - the temperature there will be the lowest.   Upstream will only happen later in the depressuring sequence.  

 

The problem is that depending on a pump to start on ESD means you need to be 100% certain nothing that could prevent the pump from happening is going on.  In a fire scenario, or loss of power, does that pump still start?  What about loss of control system?  Do you need to have the pump motor controls tied to the SIS system?  

 

Generally - hydrate formation in blowdown systems is rare (although I have seen it once in raw natural gas from a well going to flare).  

 

Just make sure you think through all the scenarios that could cause your solution to fail, and deal with them.

[SawsanAli311]

 

hi

 

in case injection is envisaged during initial phase of blowdown (not continuously)...would the preference be to activate the methanol pump from the ESD signal which initiates blowdown? 

Usually methanol is dosed for a specific duration to create a buffer volume inside the system and uppstream the blowdown device/restriction orifice. Injection is not envisaged to be required in continuous mode during the blowdown process. 

Remember that the risk of hydrate formation is mainly upstream the blowdown device and inside the system. 

The risk of hydrate formation inside large tail pipes is not as serious as inside the system and upstream the BDV. This is due to the high velocities experienced during blowdown. This is also valid when we are dealing with large size flare headers where risk of blockage by hydrates is not a major concern in addition to the fact that the low backpressure experienced downstream the blowdown orifice is too low that it might be below the hydrate formation pressure for the specific gas composition being vented. Therefore, always generate a hydrate formation P, T trend to see which region are the points falling. 

 

Depending on the criticality of hydrate formation in your system and which shall be assessed based on detailed analysis, it may be mandated to include the- automatic start-up of the methanol pump interlocked with the auto-blowdown system - as part of a detailed SIL assessment to ensure the system's reliability.