What is Blow Down Valve and Sizing BDV?

At upstream process (PLEM, SSIV, SSCV) to Riser, in case of emergency shutdown (ESD) or depressurizing (process shutdown – PSD), the BDV valve automatically/remotely opens to release high pressure fluid flow to flare system.

The following are guidance for developing the BDV (Blowdown Valve) to flare system:

BDV Sizing and Its Line Sizing to Flare-system

The BDV size is same with the inlet and outlet line size. How to determine the appropriate line size?

  1. You must determine the maximum flowrate through this BDV line during depressurizing. Hence, you have to conduct depressurizing study-workshop at first.
  2. Based on the maximum flow (peak blowdown rate), select the appropriate size that meet the velocity and Rho.v2 or (pV2) criteria (Momentum increased when velocity increased).
  • Most of Engineering Company consider maximum velocity of 60m/s (or 200 fps).
  • Considered maximum Rho.v2 of 200000 kg/m.s2 (But, some other Process Engineering Company use the lower of 150000 kg/m.s2 in their standard practice – for safety reason).  

The Rho.v2 to be limited to prevent turbulence – induced vibration of flare piping based on the lesson learn of experience in many company.  Considering maximum Rho.v2 of 150000 kg/m.s2 for new design

Restriction Orifice (RO) Size

Do you know, why we need RO in downstream of the BDV to flare? It is to limit blowdown flowrate to flare system (to ensure controlled flow rate). When the BDV opens to release the high pressure on its upstream, the restriction orifice plate at its downstream ensures that the flow is not excessive to overload the flare system. If there is no RO (or with RO but oversize), the actual flowrate to flare system could be higher (comparing to depressurizing simulation result, which the BDV size, and pipes are designed for). Besides, the higher blowdown flowrate, the faster depressurizing time, the colder the depressurized section system. The current material in the system may not appropriate to handle drop temperature.

Reason: There will be Joule Thompson (JT) effect at orifice due to hydrocarbon fluid expansion typically but to a high pressure let down. Generally the JT effect will produce a sub zero temperature (below 0 Deg.C) at RO and during the blowdown process low temperature will propagate to the BDV causing moisture from atmosphere will freeze on outside of the Blowdown valve.(Latent heat in the piping will transfer to cold fluid and slowly approaching this subzero temperature. The “coldness” will travel back to upstream of Restriction orifice and probably reaches Blowdown valve. It potentially cause the upstream Blowdown valve body temperature drops below subzero as well.) Operator will not be able to close the BDV after the blowdown and it could also cause backflow creating potential hazardous situation.  Those “general good engineering practice is to locate Restriction orifice, 600mm downstream of Blowdown valve.

RO to be sized correctly so that the actual maximum flowrate during blowdown is correct. Blowdown flowrate may exceeds the flare capacity due to RO is oversized. But, if RO is undersized, it will result in longer depressurizing time. Therefore it is very important that the correct data is used when sizing the RO (ensure input data is correct), so that the RO size is correctly selected.

The SelecTion and TeSTing of ValVeS for lng applicaTionS

Pipe Size Downstream RO

The pipe size of downstream RO is usually bigger than the upstream line (since gas expand when pressure reduced). Therefore the reducer (enlarger) is provided at downstream RO.

  1. The pipe size of downstream RO is selected with maximum Mach No 0.6 – 0.7
  2. The maximum back pressure at this line is about 0.5 times of upstream pressure to ensure the flowrate at throat RO is at critical condition (so that the flowrate through RO will only depend on the upstream pressure).  

BDV System Configuration

The distance from BDV to RO to be verified that the “coldness” temperature travel time is sufficient to avoid freezing of BDV is occurred.  This requirement is to prevent the moisture from atmosphere will freeze at the body BDV and potentially cause the stem stuck at position. Operator may not possible to close the BDV after blowdown activity and potentially lead to back flow.

For general practice, the distance from BDV to RO should be more than 0.6 meter (or other company use standard of minimum 3ft).

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