Help - Blocked liquid outlet scenario - Surge time

[sheiko]

Hi all,

I have a couple of basic questions:

1/ How do you calculate the surge time in a partially liquid filled drum: is it V/Q, with V being the empty volume (liquid free) and Q the volumetric flow rate of the liquid leaving the drum?

2/ Then after, if the surge time is less than 10-30 min, how do you calculate the relief load?

Thanking you in advance

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[CMA010]

1) If you mean with surge time the time it takes to overfill the vessel than V/Q seems the way to go.

2) The relief rate depends on the source, e.g. pump or high(er) pressure vessel. It's the maximum flow that can occur based on the relief pressure.

 

[JoeWong88]

sheiko,

You have defined
i) ... partially liquid filled drum
ii) ... V being the empty volume (liquid free)
iii) ... Q the volumetric flow rate of the liquid leaving the drum
iv)... surge time ... is it V/Q ?

A few question to be clarified :
a) the definition of "V being the empty volume (liquid free)" is not very clear to me. Do you mean V is drum volume OR V is Vapor space when liquid is High liquid level ?

b) Liquid leaving drum at a rate of Q during normal operation should be same as liquid feeding drum to maintain a balance...Nevertheless, pressure increases in the drum may affect the vapor-liquid equilibrium and hence the Q... Is the Q is at normal operation or at increased pressure (Qr) ?

I understood your scenario as follow :
A stream FEED feeding a drum (Gas-liquid separator ??). Liquid being withdrawn from the drum continuously (so do the vapor if Gas-liquid separator) on level control (vapor is pressure control...). In the event of liquid withdrawer line blocked due to whatsoever reason i.e. liquid level control valve failed to close position, feed is continue leads to liquid start to accumulate in the drum. In addition, you also consider the vapor line is blocked as well and lead to pressure built-up in the drum and potentially lead to liquid (or vapor-liquid) relieve via PSV protecting the drum...

Am i correctly interpreted your scenario ?

You may have to state your basis (please verify) :
i) Liquid is at possible high liquid level
ii) PT to favor possible maximum liquid generation
iii) Pressure built-up in drum till relieving pressure...

The surge time (as i understood) is the time from maximum liquid level to drum completely fill.

Surge time = Remaining Vapor space / Maximum liquid feed to drum

Relieving flow being maximum feed flow...You have to careful here...The feed may possibly two phase...initially could be liquid relieve (possibly boiling liquid) due to liquid displacement, also two phase relief due to what being fed into drum shall be relieved via the PSV...

Above has not considered the vapor-liquid disengagement, fluid characteristic, nozzle location of PSV, etc...



JoeWong
Chemical & Process Technology

 

[sheiko]

Thank you for your detailed answer

- Yes, the empty volume is the vapor space

- Q is during normal operation

- Yes you are correct interpreting my scenario but saying "In addition, you also consider the vapor line is blocked as well and lead to pressure built-up in the drum" and "PT to favor possible maximum liquid generation", does this mean that we have double jeopardy? or that we take credit to instrumentation with the PT?

- Indeed, total input flowrate = total output flowrate during normal operation. But in case of multiple outlet, which stream shall i consider to calculate the surge time?



"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[JoeWong88]

Yes you are correct interpreting my scenario but saying "In addition, you also consider the vapor line is blocked as well and lead to pressure built-up in the drum" and "PT to favor possible maximum liquid generation", does this mean that we have double jeopardy? or that we take credit to instrumentation with the PT?

Double jeopardy... Subject to how you define the credible scenario and how your system works...

Let take an example...
Say your system consist of Gas-Liquid Separator (A). The liquid level of Separator A is maintained by a LIC and LCV before it is fed to Separator B. Similarly, The liquid level of Separator B is maintained by a LIC and LCV before it is fed to Separator C.

In the event of LIC and/or LCV failure in Separator A lead to LCV full open, large amount of liquid will feed to Separator B. This liquid could be flashed and substantial amount of vapor generated in Separator B. Now larga amount of Vapor and liquid in Separator B would increase the pressure and level in the Separtor B, and may subsequently reach High-High Level (LAHH / LZAHH) in separator B.

On LAHH, to avoid liquid carry over into next equipment on vapor line, vapor outlet SDV may have to be closed base on ESD logic (and inline with API 14C). The vapor line from Separtor B is blocked.

For Separator B liquid outlet line, the LCV will wide open due to increased level. Large amount of liquid will feed from Separator B to the next separator C. As this is abnormal large amount of liquid from A into B and into C, the separator level will increase and finally reach LAHH and trigger SDV (inlet to Separator B) shutdown . Thus liquid line in Separator B is blocked.

Continue liquid feeding into Separator B and vapor generation would lead to High-High pressure trip (PAHH/PZAHH).

Both LAHH & PAHH shall initiat shutdown of separator B inlet SDV. Infact, closing inlet SDV would solve all problems related to pressure and liquid level built in the Separator B & C. However, there is still reliability issue related to the PAHH and/or LAHH shutdown loop, logic solver, ESD panel, SDV, etc. failure of any of these elements would left the SDV remain open. Thus, the SDV at inlet to Separator B will have to be considered unreliabile (SIL study can prove this) and remain open for conserativeness.

From above scenario defintion, you may notice that all steps occur sequentially and not mutually exclusive. Thus, they are not double jeopardy.

JoeWong
Chemical & Process Technology

 

[sheiko]

Thank you it is clearer
And in case of multiple outlet, which stream shall i consider to calculate the surge time? is it: (total input) minus (not blocked outlets)?


"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[JoeWong88]

Take credit on multiple outlets... this very much subject to protection system... Hard to advise.

By the way, you are considering the filling time. Don't you ?

JoeWong
Chemical & Process Technology

 

[sheiko]

Yes

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."