Sumit,

A BDV is a tight shut-off valve, generally a ball valve, operated by a single-action pneumatic actuator (though it may have some variations about it). Since the BDV’s objective is to blowdown on specific process upsets, we must ensure that this status, i.e. valve open, is reached even if the electrical signal, or the instrument air are lost. That’s why we must define the failure conditions in such a way to make the actuator’s spring to open the valve if no instrument air pressure is applied and, the three-way solenoid valve to exhaust the actuator’s air when de-energised (or the cable is cut).
To summarise: in operation the actuator must be pressurised and the solenoid valve energised, and to blowdown we must de-energise the solenoid valve to exhaust the actuator.
When a process blowdown implies a large amount of product being released to the atmosphere it is obvious that it will only be done if absolutely necessary so, to minimise spurious trips we usually add a small tank, kept at the instrument air pressure. This tank will supply air to the actuator for a while, in case the instrument air is lost, so the blowdown is avoided.
I hope this explanation helps.

Regards,

Rodolfo Foresi

Be aware that it is possible for the BDV operation to cause severe process hazards. Rapidly depressuring a system below about 25% of operating pressure may drop the temperature of a carbon steel vessel into the brittle failure region, so a fail-open fault is not necessarily a safe failure.  Read the relevant sections in the API standard. Many blowdown systems are designed to maintain about 25% design pressure to avoid this problem. Equally, one should be aware that a dry vessel exposed to fire with only a PSV is a candidate for rapid disassembly when the metal weakens with full pressure trapped inside by the PSV.

Yes, I overlooked the process aspects so you are right to talk about it. However, both aspects can be considered, say fail safe design and metallurgy limits, by adding a restriction orifice to the BDV (whenever your process admits a longer depressurisation time), and providing robustness to the safety instrumented function not to have frequent spurious trips (installation, materials, redundancy, EMI immunity, etc.)

[quote author=Rodolfo link=topic=24.msg51#msg51 date=1210093806]
Yes, I overlooked the process aspects so you are right to talk about it. However, both aspects can be considered, say fail safe design and metallurgy limits, by adding a restriction orifice to the BDV (whenever your process admits a longer depressurisation time), and providing robustness to the safety instrumented function not to have frequent spurious trips (installation, materials, redundancy, EMI immunity, etc.)


A-correcto...
Also material selection is very much taken into consideration in a system that is being designed for a blowdown case. It is usually specified in the datasheets for e.g in a range of -45/80[sup]o[/sup]C

The blowdown valves are a very popular category of pipe valves, used in a wide variety of applications. They are used for operation in open position. The main function of blowdown valve is mainly to control a continuous flow of steam /fluid under high differential pressure. The outstanding feature of this type of valve is that it can maintain fluid tightness and it is easily operated without the help of any wedging action.



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