19 replies to this topic

[bhdrsc]

Dear all,
i have a heat exchanger working with the pressure7 barg in order to supply ammonia gas. After transporting this ammonia for a certain distance, i want to decrease the pressure 0,05 barg. is it possible to do this? Yet, the flow rate of ammonia should be same after the pressure relief.

[thorium90]

So let me get this straight. You have an unknown amount of ammonia gas at 7barg that you want to reduce by 0.05barg and you want it to go through a pressure relief valve and want it to have it at the same flow rate? I'm sorry, but could you rephrase your question?

[bhdrsc]

i am transporting 1000 Nm3/h ammonia gas from a heat exchanger to an ammonia tank. The outlet of ammonia gas from the exchanger is 7 barg. I want to decrease this pressure from 7 barg to 0,05 barg at the inlet of the ammonia tank under the condition that flow rate of ammonia is same. Diameter of pipe is 4". İs it possible to do this?

[bhdrsc]

Do you have any suggestion to decrease the pressure from 7 barg to 0,05 barg?

[thorium90]

Is this tank of yours a pressure vessel? Or is it really a tank, like a fixed roof tank or something for liquids?? And that you want to put 1000Nm3/hr of ammonia gas into the tank?

If its just warm gas ammonia, shouldnt a simple regulator work? So this tank of yours can only withstand 0.05barg? If it is not a pressure vessel, where will all that ammonia gas go to? Unless your tank is enormous or I'm missing some more details.

Edited by thorium90, 17 January 2013 - 09:05 AM.

[latexman]

The gas will expand by about an 8:1 ratio. Does the ammonia go right into the tank for expansion or does it stay in the 4" pipe a while. Check out the upstream and downstream velocities. The velocities need to be < Mach 1, preferably << Mach 1.

[thorium90]

Dont blow up the tank. I doubt atmospheric tanks can hold 7barg...

[fallah]

I doubt atmospheric tanks can hold 7barg...

But i am sure atmospheric tank cannot hold 7barg...

[gegio1960]

bhdrsc,
just to understand a litle bit more, which is the temperature at the outlet of the heat exchanger?

[bhdrsc]

Outlet of the heat exchanger: +35 celcius 7 barg. In order to fill an ammonia tank, i just have this heat exchanger. can i decrease the pressure to 0,05 barg prior to ammonia gas is entered the tank.?

[kkala]

NH3 vapor will have a temperature of ~ 15 oC, when throttled down to 0.05 barg (enthalpy=1506.4 kJ/kg, http://www.engineeri...onia-d_971.html '> http://www.engineeri...onia-d_971.html .
Upstream/ downstream (absolute) pressure ratio would be 8.01/1.06, so there will be choked flow; that is mass flow rate F will depend only on upstream pressure; downstream pressure will be the result of pressure drop for flow F, up to a point of known pressure. See http://www.cheresour...-orifice-sizing '> http://www.cheresour...-orifice-sizing and attached pdf file.
According to it, flow of 1000 Nm3/h NH3 (g) (7 Barg upstream) can pass through a properly designed flow nozzle. Downstream pressure P2=0.05 Barg would not be controlled, but could be more or less so (order of magnitude) for 1000 Nm3/h by specifying proper diameter of downstream pipe. Is it important for P2 to be exactly 0.05 Barg? Probably not.
However it is critical to clarify role and data of destination tank, already asked in post no 5. Is it a fully refrigerated ammonia storage (http://www.cheresour...a-storage-tank/ '> http://www.cheresour...a-storage-tank/ )? There is no hint about it, such rather huge tank is not common.
Expecting info on this existing tank, it is noted that a control valve could be installed instead of nozzle, if more sophisticated solutions are needed. What is the purpose of transferring this NH3 (g) into the tank? Mentioned exchanger heats or cools NH3 (g)?

[bhdrsc]

The purpose is that the commissioning of a new tank will be done.

[S.AHMAD]

Bhdrsc

1. Does the ammonia storage tank has PCV to maintain the pressure at 0,05 barg? and PSV installed for overpressure protection?

2. If it does! go ahead and do it - No problem. If the pressure rises, the PCV will control it to maintain the pressure. The PCV control is to flare line then you will be loss a lot of ammonia.

3. The problem here is that you did not present you case clearly and insufficient information is given. For example, the storage tank is it full of gases or liquid ammonia? Is there BOG recovery system? How the tank pressure is controlled etc.

4. Therefore you do not get the right answer to solve your problem.

5. The answer to your basic question to reduce the pressure from 7 barg to 0.005barg IS YES but we must use the correct method so that it is safe to do so. You must use right instrumentation etc.

Edited by S.AHMAD, 01 March 2013 - 02:55 AM.

[kkala]

Indeed, more information is needed, already requested in previous posts. Ammonia is stored as liquid in vessels, what is the purpose of passing gaseous NH3 from said "ammonia tank" (post no 10) during commissioning? There may be a special reason. Most probably NH3 is not stored permanently there, and this is a common atmospheric tank without refrigeration. Please help others understand the case.

[bhdrsc]

i am not responsible for commissioning the tank. i heard this siuation from my friend. i just wonder how to reduce the pressure.

[kkala]

Limiting the topic on "how to reduce the pressure":

1. An orifice on the 4" pipe could result in 1000 Nm3/h downstream flow for 7 barg upstream pressure. But you have to arrange diameter (and configuration in general) of downstream piping, so that ΔP=0.05 bar up to atmosphere (supposing tank is atmospheric). The latter cannot be realized exactly, but you can have a rough approximation through the design (post no 11); and better by closing/opening a valve at downstream piping, which would not affect flow rate (chocked flow).

2. Alternatively we could do all above without orifice, but place a control valve linked with downstream pressure instead. Downstream piping to be designed as before. In this case downstream pressure will be 0.05 barg for upstream pressure 7 barg, or even some variation of the latter. But flow will have some variation and it will be roughly 1000 Nm3/h (not precisely) at 7 barg  upstream pressure.

Valve size to be low enough, corresponding to the orifice at normal conditions. Rules of control valve sizing (e.g. ΔP = higher between 20% of total pipe frictional ΔP and 10% of operating pressure) are not applicable in this case of chocked flow.

3. In fact example (1) tries to control flow, example (2) controls downstream pressure. A valve is a variable size orifice and could replace the latter in example (1). A flow control valve (sort of example 1) followed by a pressure control valve (sort of example 2) in series could balance drawbacks of each example. It is not known whether this more sophisticated scheme is necessary and worth while. More general info would help that.

4. Comments and additions on the above would be welcomed.

Edited by kkala, 01 March 2013 - 09:31 AM.

[S.AHMAD]

1. Take note kkala comments post#16.

2. Several methods are available. If more information available, we may propose the right method that minimize hazards.

[Dacs]

You can also have an orifice upstream and have some sort of control valve in tandem downstream of it.

[DB Shah]

Ammonia storage tank is a highly specialized job to carry out any modification in and around this tank. I never will worry how you reduce the pressure from 7 to 0.05 barg, but more worrying is introudcing something warm in Ammonia storage tank. (-15°C in -33°C) You should be aware about "Tank roll over" phenomenon. " Accident in a Lithuanian plant In 1990 Symposium, Bengt Orval Andersson reported on Lithuanian fertiliser plant accident happened in March 1989, destroyed a 10,000- tonne-ammonia storage tank. Whole ammonia tank slid from its foundation,smashed with great force through a surrounding wall of reinforced concrete, finally landed 40m from the foundation. Enormous devastation around the tank, 70cm deep liquid ammonia. Large quantities of ammonia evaporated, vapour caught fire and whole plant engulfed in flames. About 32,000 people evacuated from a nearby town, rescue operation continued for three days. Seven people died, 57 were injured. Ammonia tank over-pressured when its contents rolled over. Warm ammonia supplied to tank bottom, caused the rollover while tank refrigeration compressors out of service. "

[kkala]

Thanks, DB Shah, for the information on Lithuanian ammonia storage accident (1989), which is a nice complement to <http://www.cheresour...a-storage-tank/>.

For the present case, it is not clear whether NH3 will be stored in the tank or just pass from it for some reason (e.g. passivation?, cleaning?).  The latter seems more probable, according to previous post discussion.