Two-stage pressure-reducing valve

[sheiko]

Hello,

Could somebody explain (if possible) how a two-stage pressure-reducing valve (PCV) can allow to avoid choked flow through it, when upstream pressure is 10000 mbarg (10 barg) and downstream pressure is 50 mbarg?

"We don't believe things because they are true, things are true because we believe them."

 

[gerhardl]

The simplest explanation is that it won't, If I have got the theory right.

Any valve will have a max capacity given by the Cv and the driving force given by the pressure difference before and after the valve.

If the pressure difference is large enough, the valve will choke. (Choke: max velocity reached)

I believe you are talking about gas.

If you, however, are talking about liquid water, cavitation will occur, but the valve construction will allow for this, formed to accept or send cavitation to areas not (much) affected by gas-bubbels collapsing.

 

[sheiko]

Thanks,

I am indeed dealing with nitrogen.

So what is the purpose of having two-stage pressure regulator, if not to cascade the pressure down to avoid choked flow?

Also, if the flow is indeed choked through the valve and having two stages does permit to avoid that, shall the downstream piping be sized for choked flow?

"We don't believe things because they are true, things are true because we believe them."

 

[PaoloPemi]

you may connect control valves in series to keep the pressure drop across each valve below critical condition, however that requires a (very) good control system...

 

[gerhardl]

May I suggest that you give more details about your problem?

50 millibar and nitrogen suggest that you are perhaps working with nitrogen for nitrogen blanketing purposes.

If this is the case, this is an art in itself, but solved with standard commercial products, but requiring knowledge and the correct valves both for inlet and outlet for the (closed tank?) system, including correct type inlet valve, and correct type outlet (overflow or overpressure relief valve).

Anyway: you normally start the the procedure by selecting a valve after the normal flow conditions, aiming at having a normal flow in the upper middel (up to 80%) of the valves capacity upholding the given pressure (eg. no choking).

For filling up larger amounts, before starting keeping the normal pressure, it might be necessary with a split-range system or other devices for filling.

Check for your purposes with a commercial supplier of valves or N2 (or both) what is available for control devices and valves for your purpose.

 

[sheiko]

The nitrogen is used to inert and isolate the interwall of a LNG storage tank. It is a continuous process.

T = 15ºC
min flow = 5 Nm3/h
normal flow = 15 Nm3/h
max flow = 200 Nm3/h
PS: Normal conditions: 0ºC and 1 atm

So, what about the purpose of two-stage pressure regulator?

And, shall the downstream piping be sized for choked flow?

"We don't believe things because they are true, things are true because we believe them."

 

[israelkk]

From my experience the reason for two stages is that it is difficult to impossible to get an accurate regulated pressure by reducing from very high pressure to very low pressure in one stage. It is embedded in the physics of the regulator that the ratio between the regulating area and the sealing area has to very large to get an accurate regulated pressure in one stage, resulting in very large regulator.

 

[Compositepro]

Also the supply tank pressure varies over a large range as it empties. For practical reasons most high pressure regulators are two stage. But, theoretically, one stage will work.

 

[MattC1234]

The purpose of a 2 stage pressure regulator depends on the valve manufacturer and the size of the valve.

In this case the flow will be choked, however the use of multiple stages is not to prevent choked flow, but either (as stated, depending on the valve and pipe size) to reduce noise and vibration as a result of high energy levels in the valve. If the valve size is small (IE less than 0.5") then the outlet velocities of the fluid maybe high, thus a multi stage trim in the valve, would reduce noise and be able to absorb a higher amount of energy.

I would suggest that your downstream piping was designed to keep the velocity of the fluid in your pipe within acceptable limits... so avoid making the pipework smaller than the valve.

Hope this helps.

 

[stanier]

This is a standard application and the equipment can be provided by Emerson (Fisher).

 

[786392]

Dear sheiko Hello/Good Evening,
hopefully you have already got all the much needed responses to satisfy the query.

Here I add my little bit

That two valves are most usually employed to ensure sustainable controlled environment downstream of 'controlling envelope' if you define that way the segment containing them!

As otherwise It is foreseeable that

we may end-up with much poorer controlled variable(pressure) on downstream of 'control envelope' as per 'very high differential pressure' across only one P.C.V.

Hope this proves helpful!

Best Regards
Qalander(Chem)