5 replies to this topic

[lokesh_09]

Dear all,

PSV on the water filter (cooling water coarse filter) has been sized for the fire case. PSV outlet has been routed to overboard dump header.
what is velocity criteria to be followed for this PSV outlet line.
Normally we consider mach no <0.7 or 0.5 for vapour, whereas in this case during fire relieving fluid will be steam.
Shall i consider mach no for this relieving case or need to limit the velocity for normal steam lines.

Regards,
Lokesh

[ankur2061]

Lokesh,

Refer the link below about a great discussion on steam relief valve and the velocities at the outlet:

http://www.cheresour...m-relief-valve/

Regards,
Ankur.

[lokesh_09]

Dear ankur,

Thank you for your response.

But the link doesn't have answer for my query.

Regards,
Lokesh.

[ankur2061]

Lokesh,

You are not limiting the velocity. The velocity is sonic (Mach 1) if the ratio of the actual downstream pressure to the actual upstream pressure is less then the critical pressure ratio where critical pressure ratio is defined as

(P₂ / P₁)_critical = (2 / κ + 1)^{κ / κ - 1}

where:

P₂ = downstream pressure

P₁ = upstream pressure

κ = isentropic expansion coefficient

If you are using a conventional safety valve (allowable total back pressure - 10%) then in all likelihood the velocity is already sonic at the relief valve outlet nozzle.

If you are really interested in limiting the velocity to say Mach 0.7 then you might have to go for a balanced-bellows or pilot-operated relief valve to do so. This might allow you to have your (P₂ / P₁)_actual to be greater than (P₂ / P₁)_critical. It might then be possible to have sub-sonic velocities in the tail pipe at least at the outlet connection of the PSV.

So the question arises that what are you trying to achieve by limiting the velocity in the tail pipe of the PSV? If noise is the concern then that can be addressed by providing an in-line or end-of-the line silencer on the PSV tail pipe. The other concern could be to limit vibrations in the tail pipe. This again can be addressed by providing proper pipe supporting based on the maximum momentum (ρv²) the pipe can see during a discharge from the PSV.

Hope this helps.

Regards,
Ankur.

[lokesh_09]

Dear ankur,

As per API STD 520, i dont see any relation to relate the Back pressure and PSV tail pipe sizing criteria. In our case the percentage back pressure .625 %. We have conventional type PSV, since the percentage back pressure is less than 10%. For PSV outlet gas lines which is routed to flare, we normally limit the velocity for sonic velocity(mach no).
But in this case PSVoutlet is routed to overboard dump header. Calculated Velocity for PSV outlet tail pipe is app 250 m/s and mach no is only 0.2 mach.
My worry is about the velocity of 250 m/s, i think it is too high.
Please advice me if my understanding is wrong.

Thank you,

Regards,
Lokesh

[Dacs]

Mach # of 0.2 is normal, if not a bit low. Of course knowing the definition of Mach # (speed of fluid / speed of sound in fluid), the velocity will be high, given the normal range of speed of sound.