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Wednesday, August 27, 2008

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"I was doing Depressuing using HYSYS and flare network back pressure calculation using FLARENET for conventional Blowdown valves with restriction (BDV/RO). Peak flow of Depressuring unit in HYSYS which obtained from initial depressuring and based on initial depressuring pressure to Atmospheric pressure, is used as input to FLARENET.

I have two questions :
What I really concerned is back pressure calculated from FLARENET is higher than ATM, is the back pressure will affect the peak flow ?

What I had done for those cases is to assume a back pressure in HYSYS to calculate the peak flow and input it into FLARENET. Newly calculated back pressure from FLARENET will be re-enter into HYSYS depressuring unit to recalculate new peak flow in HYSYS. Above will be iterated until both of back pressure and peak flow are match in HYSYS and FLARENET. Am i in right track ?"

Above was a question raised by an engineer who was conducting plant blowdown and flare network studies.

First far most important thing is that there will be total plant or segregated zone blowdown. This type of blowdown will results simultaneous opening of all BDVs within a zone and simultaneous blowdown of multiple sections. One shall not consider SINGLE BDV opening ONLY. Simultaneous blowdown will results high total blowdown rate and induced high back pressure to each BDV / RO within the blowdown zone.

In many events, the flow passing the RO is CRITICAL flow where the back pressure from flare network is lower than the fluid critical pressure (Pc). Read more in "A refresh to Process Engineer on few phenomenons in restriction orifice". Under critical flow condition, the back pressure has no impact to the flow rate passing through the RO (assume backpressure does not affect the vena contracta cross sectional area). Thus, peak flow calculated in HYSYS depressuring unit considering backpressure of ATM is still remain same if the backpressure (Pb) is lower than critical pressure (Pc).

However, for many rare cases when the system pressure is low, hence the critical is much lower. This potentially results back pressure high than critical pressure and cause SUBCRITICAL flow condition. Under this condition, backpressure has impact to the peak flow calculated by HYSYS.

Knowing back pressure may affect peak flow under subscritical flow condition, some level of checking and iteration may required.

Iteration as proposed above could be very time consuming and not all the BDV/RO required iterate update. Infact only those BDV/RO experiencing Subcritical flow would need to involve in the iteration process.

The following steps may be considered :

i) For initial step, use ATM in HYSYS for depressuring to estimate the peak flow
ii) Peak flow estimated in HYSYS will be entered into FLARENET to calculate back pressure at each BDV/ROs.
iii) Compare each BDV/RO Critical pressure (Pc) and Back pressure (Pb). If Pc is higher than Pb, that particular BDV/RO need not involve in the iteration.
iv) Identify those BDV/RO with Pc less than Pb. Re-adjust back pressure in HYSYS base on calculated Pb from FLARENET to obtain a new peak flow.
v) Reenter and rerun in FLARENET.
vi) Repeat step (iii) to (v) till backpressure in HYSYS and FLARENET are matched.

Above will significantly reduce the iteration time.

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posted by Webworm, 3:16 PM


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