Rise in gas pressure at abrupt valve closure 4

[Iomcube]

In this p&id there is a constant feeding of H2 into the circuit. Now only one consumer of H2 gas exists i.e acid furnace, which requires a bit pressurised H2 gas. If there is any tripping at the furnace at that instant vessel pressure will increase abruptly beyond mentioned setpoint. How can we estimate that pressure rise?

I know volume of vessel & holdup volumes of piping as well. I can convert mentioned feed volumetric flow to furnace to mass flow & thereby can calculate rise in density of gas in the vessel for instance. Perhaps packages like CoolProp (or any equation of state package) can back calculate rise in pressure at that density (assuming T remains same)

https://files.engineering.com/getfi...9e9-43d5-8de4-db378e3bf233&file=H2Circuit.PDF

 

[LittleInch]

You really need a transient analysis program of some sort.

And feed in the capabilities of the compressor as the feed element.

But you should be able to estimate what is going on if you do it in small time steps.

Just convert things to the same value so I'm not sure what 101 kg/hr is at 160 mbarg in terms of volume to be able to judge how rapidly the pressure will rise. Is is ~935 nm3/hr? So your 5m3 vessel will start to rise in pressure quite quickly to 1 bar and 2 bar.

Presumably you need the PRV by Note 3 to act quite quickly to prevent over pressure of your vessel. There is though no design pressures mentioned here or the capability of the compressor.

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[Iomcube]

@LittleInch

Just convert things to the same value so I'm not sure what 101 kg/hr is at 160 mbarg in terms of volume to be able to judge how rapidly the pressure will rise. Is is ~935 nm3/hr?

For calculation purposes ...it is pure H2 gas circuit.

There is though no design pressures mentioned here or the capability of the compressor.

Will provide in next reply.

P.S I want to estimate this without surge analysis. If there is a way

 

[Latexman]

Use the Joukowsky Equation.

Good Luck,
Latexman

 

[LittleInch]

You don't get surge in gas systems if that's what you're worried about.

I'm a bit confused about what your issue is. Can you explain a bit more.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Iomcube]

@LittleInch

You don't get surge in gas systems if that's what you're worried about.

Weird, this company has a special software for gas transients:

https://www.aft.com/products/xstream

I'm a bit confused about what your issue is. Can you explain a bit more.

I want to calculate max. pressure rise in order to see if it is higher than the 5m water seal in the P&ID

 

[Iomcube]

@Latexman
@LittleInch

In the layout the vessel is at 250mbarg, 40 - 30C at these conditions we can calculate ρ1 of H2 gas using ideal gas:

https://www.thoughtco.com/how-to-calculate-density-of-a-gas-607847

Now assume the FCV shuts down abruptly & pressure relieving instrumentation did nothing for 2sec
Now in 2sec; vessel will accumulate additional 101/3600 * 2 = 0.05611 kg of H2
The new density will be ρ2 = ρ1 + 0.05611 kg/m3 [bcz physical constraints / volumes remain same]
Now we can back calculate P from ρ2 again using ideal gas eq.
Is my understanding correct?

 

[LittleInch]

OK, lets rephrase this - you don't get classic liquid surge in gas systems but as a compressible fluid you will get transient pressures in different parts of the system when something happens to affect flow or pressure, especially when it happens very fast.

Your key inputs here are the speed of closure of your isolation valve or FCV into the furnace ( and don't say "instantaneously - it will take a finite amount of time) vs the speed of response of the PRV on the inlet into the vessel.

To go from 250 mbar to 500 mbarg if you have no outlet flow and incoming flow is the same looks to be about 10 seconds. Your PRV should be able to react that fast to reduce and stop flow going in.

But that AFT tool will allow you to simulate that - it's a transient analysis program like I said in my first reply.

Not quite sure why you have the 500mbarg (5m) pressure limit but is the vessel not designed for higher than that?

But yes that's basically it - just remember to use absolute temps and pressures.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Iomcube]

@LittleInch

But yes that's basically it - just remember to use absolute temps and pressures.
Extremely thanks. So this relationship is indeed correct?: The new density will be ρ2 = ρ1 + 0.05611 kg/m3 [bcz physical constraints / volumes remain same]

Not quite sure why you have the 500mbarg (5m) pressure limit but is the vessel not designed for higher than that?
HCl furnace has quartz glass burner which burns Cl2 / H2 ...pressure must be lower than 600mbarg per instruction manual

 

[LittleInch]

Your FCV can also control on downstream pressure. Just add a pressure input into the valve control positioner and run it through a low selector block.

Or stick that weird 5m high tube thing downstream your FCV.

Or both. Then you might not need to worry so much?

Yes mass / density change is correct as volume doesn't change. Just when converting this to pressure using gas laws use absolute pressure and temp.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Iomcube]

@LittleInch
Thanks.
What do you mean by: and run it through a low selector block.

 

[Latexman]

Pressure fluctuations are generated from any sudden flow disturbance in a piping system, whether the piping is handling a gas or liquid. The dP[sub]SURGE[/sub] of liquids >> dP[sub]SURGE[/sub] of vapors and gases due to differences in density and wave speed in the media. As such, surge in vapors and gases is not a problem a lot of the time, but there are cases that are a problem.
Good Luck,
Latexman

 

[LittleInch]

A low selector block automatically choose the lowest position of the valve from the different control loops being fed by the different inputs.

So whilst it might mainly control on flow, if the pressure went above your setpoint then that control loop would seamlessly take over control of the valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

What about the vent control valve immediately downstream of the PD compressor ? What operates it and what setpoint? Presumably it a PIC - PCV set at < 5m of water column ? If it is not fast enough to deal with sudden loss in demand at the reactor - furnace, add a volume booster at the PCV.

 

[shvet]

Dynamic simulation is recommended. A challenging option I should say.

https://www.aspentech.com/en/products/engineering/aspen-plus-dynamics

https://www.aveva.com/en/products/dynamic-simulation/

 

[Iomcube]

@ georgeverghese
What operates it and what setpoint?
It is a smaller valve designed to handle 1/2 of total H2 mass (101 kg/hr). Its setpoint will be 20mbar higher than setpoint of downstream PRV (vessel pressure maintained at 250mbarg). At 270mbarg this vent valve MV% automatically will be 30% (single shot signal from an safety interlock) afterwards operator control it

 

[LittleInch]

Tom,

you're drip feeding us information here and we are not seeing what you are seeing.

So why is this vent valve set so low and so close to the vessel set point. As soon as the PCV closes even a bit then the pressure upstream will be > 250 mbarg and hence your vent goes off.

Your drawing is simplified, but then doesn't provide enough information to follow the process. So e.g. Note 1 says 101 kg/hr at 40C and 160 mbar. That's all we know. So on that basis the pressure can't exceed 160 mbar as we don't know what the control system is upstream of your diagram...

You need to start from the pressure source and its controls and set points and work downstream from there. Looking at a little part of the system won't get you anywhere.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Iomcube]

@ LittleInch

It is not my intention to hide information so in the first post I included information which according to my comprehension was enough although I can be in error.

However, this avoid overflow of information & perhaps doesn't create an interactive dialogue where latter you can inquire aomething & I reply to it.

As soon as the PCV closes even a bit then the pressure upstream will be > 250 mbarg and hence your vent goes off.

Bcz furnace continously consumes H2 so upstream pressures do not increase however furnace when suddenly stops then upstream pressure will rise so we eject H2 using either water seal or vent valve just after compressor

pressure can't exceed 160 mbar as we don't know what the control system is upstream of your diagram...

That information will be too much for me to share. However upstream pressures you can assume are controlled & cannot increase 180mbarg. All water seals before compressor & safety systems are designed at this pressure limit

 

[Iomcube]

@ shvet

I can make a dynamic simulation using Hysys (an old version I do have). If I upload here will you be able to run it & thereby review it?

 

[LittleInch]

" However upstream pressures you can assume are controlled & cannot increase 180mbarg."

So in this case you could simply forget about your water seal and vent and the PCV at 250 mbar because it will never get there.

So all you need is your FCV before the burner, or maybe your cooler, but nothing else in terms of vents or pressure control.

you're not an inexperienced user of this site so I would have expected a bit more as all this to ad fro tends to drive many posters to give up answering your questions. But anyway, this point above is pretty critical to understanding what your issue is.

As latexman says, there is an element of surge but this is so low for gases, especially Hydrogen which is so low density, that it is usually ignored unless you're getting close to sonic velocity or have a decent density.

Also be aware that "safe vent" for hydrogen is a bit of a misnomer - this stuff catches fire and explodes as soon as you look at it and your vent needs to be designed as a flare as the chance of it igniting is very high according to everything I've read recently. A lot of people are suddenly getting into Hydrogen and don't always appreciate its properties and issues, though maybe you do.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.