Control Valve Sizing

[Chemitofreak]

Hello Friends,

I have a system in which a column(A) top vaopurs are fed to another column(B).

During abnormal operation, if column(A) gets pressurised, column(A) top gases are also routed to the Fuel Gas System (Refer attachment for the scheme).

Can anyone guide me in sizing the control valve on the fule gas line (refer sketch for details). I mean what shall be the basis for sizing. I am intereseted in the approach/concept for sizing the pressure control valve.

Thanks in advance.

 

[Chemitofreak]

Refer the diagram in the attachment

 

[georgeverghese]

The sizing basis is some what straightforward, but there seems to be something missing in the process controls - there should be a downstream high set PIC ( downstream of the PCV) that limits the spillover (through an autoselect ) into the fuel gas system in the event there is no room to accept this spillover from this column - if that happens, there should be an alternate spillover to flare or somewhere else?

 

[EmmanuelTop]

There is no flow (NNF) through the control valve under normal operating conditions - the valve is supposed to dump the excess gas in case of column over-pressure.
In order to size this control valve properly, one would have to know all the scenarios that could (and will) lead to over-pressure of the column, as well as the resulting flow rates. So dig in the HAZOP of this unit and see what those scenarios are, and get the corresponding flow rates.

Also, ability of the fuel gas system to receive full quantity of gas in all scenarios should be thoroughly evaluated. If it happens that, for whatever reason, fuel gas system cannot absorb the entire amount of gas coming from this unit, the resulting consequence could be over-pressure of the column with possible catastrophic failure.


Dejan IVANOVIC
Process Engineer, MSChE

 

[sshep]

Dear Chemitofreak,

I bet there are relief valves protecting both the column and the fuel gas system, so this is not the last safety system which protects the tower. I think this valve is to allow recovery as fuel in the event of upset- i.e. it will backout fuel gas coming from elsewhere.

If the piping losses are negligible, the control valve at 80% open can be sized for the entire process flow (or as much as the fuel gas system could reasonably take) at the envisioned pressure controller setpoint (dP= setpoint - fuel gas pressure). Given the Flow, dP, fluid properties, and a linear (probably) characteristic; this is just a straight forward sizing problem to calculate the required flow coefficient (Cv). Really even the calculation is just for your own benefit of a reasonability check- i.e. the control valve vendor will actually size the valve from their line of offerings.

That is how I see it from the data given.

best wishes,
sshep