6 replies to this topic

[Dummy]

Three PSVs are connected to a common header on a gas compression package . Header needs to be sized for 0.7 Mach and I'm using the relief load of the largest psv to size it but not sure which equation to use ?

Can somebody help on this ?  

Edited by Dummy, 04 April 2015 - 12:42 AM.

[ChemEng01]

mach number = flowing velocity v (m/s) / speed of sound  c of your fluid) (m/s)

 

Speed of sound c for ideal gas = (k * R * T)^0.5

 

where k = ratio of specific heats

R = gas constant J/K.kg for your gas (for methane this = 518.3 J/K kg

T = temperature in Kelvin

 

Your mach number will vary along your vent line due to changing velocity (i.e pressure / density / vol flow changes)

 

Directly downstream of your PSV you'll have built-up backpressure. 

At the vent outlet the pressure will be ATM (as you are sizing for non choked flow)

 

Worst case for you would be calculating the velocity based on the density of your gas at atmospheric pressure. The temperature will be dependent on your pressure drop across your PSV. If your set pressure is less than around 140-180 barg (i.e not supercritical) you could assume 0.5degC temperature drop per barg. Otherwise model your properties in HYSYS. 

Edited by ChemEng01, 03 October 2015 - 10:40 PM.

[S Manikandan]

Dear all 

 

In a flare system analysis, I have query on determining back pressure for a PSV in a particular scenario (eg: Fire scenario, having simultaneous relief of other PSVs in the same fire zone). In my case, the max operating pressure of flare header is 0.35 kg/cm2g. Since, I do not have any information about flare tip at this moment, i have fixed the system back pressure as 0.35 kg/cm2g in flarenet and proceeded for tail pipe sizing. Now my question is, from flarenet how to determine variable superimposed back pressure and built-up back pressure for a particular PSV in fire scenario.

 

 P.S: I understand that both system back pressure and constant superimposed back pressure are same (0.35 kg/cm2g).

 

Thanks in advance.

[S Manikandan]

Attached is the sceen shot of flarenet model

Attached Files

•  Fire.png   187.76KB   6 downloads

[Bobby Strain]

It's often overlooked, but you can use backpressure for a fire case of 21% of set pressure.

 

Bobby

[Atul jain]

I have a query regarding aspen flare system analyzer that can we put properties in the model without selecting the component. Because we want to use the properties that licensor has provider not the simulator

[cylai]

Hi,

 

It is rather important to specify the relieving scenarios to us. Assuming all 3 PSVs open at the same time, not likely. Assuming the largest flow PSV, I would think it's the compressor outlet blocked discharge case. Some client would prefer Gas Blowby at the suction scrubber instead of Fire case, which I don't agree. As long as the recycle / spill back valve is operational, I don't see a possibility of the suction / discharge pressure going up. There are possibilities that the suction can arrive at high pressure but let's take it else where.

 

Reason is... The higher the back pressure, the smaller the pipe (gas is compressible afterall). But it is NOT always the case that I choose to use the maximum backpressure because I want a smaller pipe size or because the client never specified in the contract and I do what I want.

 

Put it this way, if it is Fire Case... which will cause an EDP (emergency depressurization) and all the blowdown valves will open. The flare header pressure will increase and hence obtaining maximum back pressure. But Fire case rarely governs the header size.

But if it is a blocked discharge or gas blowby scenario, the flare system operates at near atmospheric. I.E. in my opinion, your flare header will need to be sized at near atmospheric condition.

 

Hope we are on the same page, we're talking about sub flare headers right; since the Mach no. is 0.7 and you mentioned common header on compression package? Do also check if the specification also requires you to check against momentum criteria.

 

Thanks.

Lai