Blowdown Valve (BDV) to Flare System

Hi my friend.
Today, In this post, we will learn about Blowdown Valve (BDV) to flare system configuration.
During emergency or depressuring, the BDV valve opens to release high pressure fluid flow to flare system.


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

BDV Size and Its Line Size

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 depressuring. So, you have to conduct depressuring study at first.
2. Based on the maximum flow (peak blowdown rate), select the appropriate size that meet the velocity and Rho v2 criteria.

• Most of Engineering Company consider maximum velocity of 60m/s (or 200 fps).
• Consider maximum Rho v2 of 200000 kg/m s2. (But, some other Engineering Company use the lower of 150000 kg/m s2 in their standard practice).  

The Rho v2 to be limited to prevent turbulence – induced vibration of flare piping based on the lesson learn of experience in many company.  

I suggest you to consider 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 depressuring simulation result, which the BDV size, and pipes are designed for). Besides, the higher blowdown flowrate, the faster depressuring time, the colder the depressurized section system. The current material in the system may not appropriate to handle drop temperature.
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 depressuring 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.

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 0.7,
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).

See the following figure.
  

That's all,,.

Warm regard from me ^_^ to all of you, please feedback if you have any concern.

Hope it useful for you.