3 replies to this topic

hello,

I have a lot of confusion regarding the topic. So let me start with the situation,me n my teammates are assign to design a transport system using pipline for transporting ammonia from a storage tank to a plant which is 500metres away with the rate of 500,000 tones anually and in need to consider the shutdown days of the plant.

So,from our discussion,we've decided to transport the fluid in liquid form which need a rather low temperature. But somehow,theres a confusion in choosing the material of the pipes since the boiling point and melting point of the fluid is rather low(-33 C<Temperature range<-77 C).

And regarding on the case,is it enough to just use a single pipeline?and what is the average range velocity of liquid ammonia transport?

I would be very glad and appreciate if someone could assits me and my teammates,thanks~

[astro]

Well you're off to some kind of a start with the P-T curve for saturated ammonia.

EFMA have published on booklet on world best practice for NH3 production:
EFMA NH3 Production Booklet
Give it a read.

In there you'll find that storage of NH3 is typically done at atmospheric pressure. The reduced pressure provides a reduced hazard because there's less stored energy in the fluid. Very important in the event of loss of containment.

Shutdown & Tank Size
You mention shutdown considerations but don't go into detail. I'm guessing you'll need to set a basis for the size of the tank and plant availability.

Based on the annual production rate of your plant, I reckon your name plate capacity will be around 1400-1500tpd. However, that's based on a production profile of 340 stream days per year and does not account for tank sizing considerations. You need to work out what the required plant availability needs to be, what allowance in time is fair for major maintenance and whether the market provides you a downturn which would help reduce your inventory need.

Ammonia plants are typically shut down for major maintenance based on either machine overhaul or catalyst replacement (or a combination of both). You also need to factor in statutory pressure vessel inspection. Industry in my part of the world is largely self regulated and tends to apply risk based inspection and appropriate use of condition monitoring, rather than the old school approach of regimented time frames. This is relevant for you to set out a basis of planned downtime and unplanned downtime. If you took the 93%-ish availability used above, you could assume 20 days planned and 5 days unplanned. However, you should search the www for current industry experience if it's available.

If you don't have the detail above to hand or the lecturer won't provide it to you, then all you can do is make an assumption or set of assumptions. Assuming that market demand is constant, then from your planned downtime duration and the average daily market demand you'll end up with a tank size (best to add some margin to allow for the unexpected). You can reduce the tank size and overall plant cost by reducing the production demand during a shutdown period. This is reasonable given that ammonia production can be seasonal so if a shutdown is properly timed, the demand on daily export to market can reduce.

NH3 Rundown Line
I would size the NH3 rundown line for full plant production with some margin to allow for process dynamics. You'll export a liquid so seek out in your references a recommended maximum velocity. Provided the liquid is sub-cooled typical rules of thumb should suffice (size the ammonia pump for the liquid rise and line loss with a margin). Also, I see no reason why a single pipeline would not suffice.

Ammonia plants typically have the refrigeration section as part of their synthesis loop sized to liquefy 100% of the make for rundown to the tank. So, you'd refrigerate the product to the tank storage temperature as a basis. This will mean that you'll need to insulate the line to avoid heat in-leak to the product and icing up the line itself.


Design Temperature and Materials of Construction
Assuming you select an atmospheric storage tank, then the minimum operating temperature is simply determined by the P-T curve for pure ammonia. It is theoretically possible to drop below this temperature under vacuum conditions or if the vapour space is not 100% ammonia.

Consider a nitrogen purge event over an ammonia liquid pool (inerting for maintenance). The liquid will come to equilibrium with partial pressure of ammonia in the vapour. The temperature of the liquid will reduce in accordance with the P-T curve (assuming neglible heating from the incoming N2).

Assuming that preparation for maintenance is done diligently so that pressure is vented and then liquids are drained prior to purging with N2, then take your minimum operating temperature (MOT) and add a bit of margin. I've seen some designs use 0 and some use a couple of degrees below MOT.

Consult piping material selection data to pick the materials required. In this service, I've seen low temp carbon steel ASTM A-333 or stainless steel A-312 (TP304 is good enough) used. A-333 being the most common due to cost.

Well you're off to some kind of a start with the P-T curve for saturated ammonia.

EFMA have published on booklet on world best practice for NH3 production:
EFMA NH3 Production Booklet
Give it a read.

In there you'll find that storage of NH3 is typically done at atmospheric pressure. The reduced pressure provides a reduced hazard because there's less stored energy in the fluid. Very important in the event of loss of containment.

Shutdown & Tank Size
You mention shutdown considerations but don't go into detail. I'm guessing you'll need to set a basis for the size of the tank and plant availability.

Based on the annual production rate of your plant, I reckon your name plate capacity will be around 1400-1500tpd. However, that's based on a production profile of 340 stream days per year and does not account for tank sizing considerations. You need to work out what the required plant availability needs to be, what allowance in time is fair for major maintenance and whether the market provides you a downturn which would help reduce your inventory need.

Ammonia plants are typically shut down for major maintenance based on either machine overhaul or catalyst replacement (or a combination of both). You also need to factor in statutory pressure vessel inspection. Industry in my part of the world is largely self regulated and tends to apply risk based inspection and appropriate use of condition monitoring, rather than the old school approach of regimented time frames. This is relevant for you to set out a basis of planned downtime and unplanned downtime. If you took the 93%-ish availability used above, you could assume 20 days planned and 5 days unplanned. However, you should search the www for current industry experience if it's available.

If you don't have the detail above to hand or the lecturer won't provide it to you, then all you can do is make an assumption or set of assumptions. Assuming that market demand is constant, then from your planned downtime duration and the average daily market demand you'll end up with a tank size (best to add some margin to allow for the unexpected). You can reduce the tank size and overall plant cost by reducing the production demand during a shutdown period. This is reasonable given that ammonia production can be seasonal so if a shutdown is properly timed, the demand on daily export to market can reduce.

NH3 Rundown Line
I would size the NH3 rundown line for full plant production with some margin to allow for process dynamics. You'll export a liquid so seek out in your references a recommended maximum velocity. Provided the liquid is sub-cooled typical rules of thumb should suffice (size the ammonia pump for the liquid rise and line loss with a margin). Also, I see no reason why a single pipeline would not suffice.

Ammonia plants typically have the refrigeration section as part of their synthesis loop sized to liquefy 100% of the make for rundown to the tank. So, you'd refrigerate the product to the tank storage temperature as a basis. This will mean that you'll need to insulate the line to avoid heat in-leak to the product and icing up the line itself.


Design Temperature and Materials of Construction
Assuming you select an atmospheric storage tank, then the minimum operating temperature is simply determined by the P-T curve for pure ammonia. It is theoretically possible to drop below this temperature under vacuum conditions or if the vapour space is not 100% ammonia.

Consider a nitrogen purge event over an ammonia liquid pool (inerting for maintenance). The liquid will come to equilibrium with partial pressure of ammonia in the vapour. The temperature of the liquid will reduce in accordance with the P-T curve (assuming neglible heating from the incoming N2).

Assuming that preparation for maintenance is done diligently so that pressure is vented and then liquids are drained prior to purging with N2, then take your minimum operating temperature (MOT) and add a bit of margin. I've seen some designs use 0 and some use a couple of degrees below MOT.

Consult piping material selection data to pick the materials required. In this service, I've seen low temp carbon steel ASTM A-333 or stainless steel A-312 (TP304 is good enough) used. A-333 being the most common due to cost.

thanks astro for the informations..
but i think there might be a slight of misunderstanding whereby the 500000tpy rate is actually the transfer rate from the storage facility to the plant and not the production rate of the plant.(sorry if the description wasnt that clear)

so,my project was not actually assigned for an ammonia plant facility,but it was just a design of the pipeline(including the pumps,valves n etc) for the fluid transport.

just for the information,yah,im quite new with all this P-T curves.

[astro]

No worries.

You will still need to set an availability / downtime for the pipeline to arrive at its instantaneous rate for you to plug into your calculations.