6 replies to this topic

[Karina1992]

Hi,

 

I am a new chemical engineer in my plant and I have a question.

 

I have a container full of water ( 1.5 m water and 1.17m3 volume of the container) that have also a vent.

a also have a pressure relief valve that discharge liquid ammonia to the water tank. 

 

how can I calculate if the pressure inside the tank will raise when relief valve dischare?

I want to understand the principle.

 

Tnx

[Qasem215]

Can you elaborate more?

Is the height of the water in the container 1.5m?

Also what is the temperature of the water?
Do you want to know the pressure at the bottom of the tank or the pressure at the top that will develop by ammonia evaporating and also the vapor pressure of the water?

Is the discharged ammonia a pure liquid?

The volume is constant, you can calculate the final Temperature, and you're half way to getting the pressure.

Qasem

[breizh]

Hi,

Can you provide a sketch ( simple PFD) for better understanding ?

Did you check the specification sheet of the PSV ? With the name plate you should be able to find the set point of the PSV .Check with your maintenance team ..

Good luck,

Breizh

[astro]

Hi,

 

I am a new chemical engineer in my plant and I have a question.

 

I have a container full of water ( 1.5 m water and 1.17m3 volume of the container) that have also a vent.

a also have a pressure relief valve that discharge liquid ammonia to the water tank. 

 

how can I calculate if the pressure inside the tank will raise when relief valve dischare?

I want to understand the principle.

 

Tnx

Karina, I'd suggest that you get your hands on ammonia-water heat of solution data and understand the heat balance associated with your process. From a quick web search, here are some hints to help you down that path:

https://www.physicsf...thermic.899774/

https://inyoprocess....qua_ammonia.pdf

 

Where this all ends up will depend on the mass balance but in simple terms, if you dump anhydrous ammonia into a container of water, the heat of solution will result in significant evolution of ammonia vapour. So, I've got no doubt you'll see a pressure rise in your system. The mass and energy balance will dictate by how much.

 

So, once you've established your vapour evolution rate, it's then a case of understanding the system design pressure that your PSV is working to protect, plus ensuring that the PSV has sufficient capacity to handle the relief rate.

 

From there you need to determine your system hydraulics up to the PSV and downstream from the PSV and confirm that PSV's sizing criteria is not compromised.

 

This all needs to be referenced back to recognised industry standards such as API STD 520 Part 1 & 2 and API STD 521, along with reference to the governing pressure equipment design standard for your jurisdiction.

 

I've guided many fresh face chemical engineers through this journey in my career and it's a path full of various steps that cannot be dealt with adequately from a short internet forum post and some of the steps can end up being nasty pitfalls for the unitiated. To underscore this point, look at the number of pages in the API standards referenced above. You should look to the senior engineer(s) within your organisation for guidance. If they are not available in your situation or they don't have the relevant experience, you should recommend to your organisation to contract the requisite capability from an external source. Where I'm based, PSV relief calculations are legally required to be signed off by a registered professional engineer. Your location may have similar requirements. It would be wise for you to understand your situation in this regard to minimise rework that can result if you need to back track.

 

 

As an aside from a design review perspective the liquid ammonia relief systems that I've dealt with before discharged to a pressure vessel acting as a knock out drum. The vapour outlet discharged to vent/flare (safe location with consideration of vapour dispersion) and the liquid side was recovered/pumped away. What couldn't be recovered, weathered off to the vapour outlet as it slowly soaked up ambient heat. If you have a process need to produce aqua ammonia, then I'd suggest taking the liquids from the KO drum and combine it with water using an appropriately designed mixing system.

 

All the best. I wish you success.

Edited by astro, 29 June 2020 - 07:08 AM.

[kphenry]

Karina1992

 

I am a mechanical engineer who has been involved in producing aqua ammonia for the petroleum and metal refinery industry.  I am not going to try to go over the process aspects of what you are doing, I am only going to caution you, addition of liquid ammonia to a vessel of water will result in severe vibration to the equipment and piping in the circuit.  In both projects I was involved with there was resultant cracking of piping and repeated failure of heat exchangers.

[breizh]

Hi,

A few documents about Ammonia water solution.

Breizh

[Karina1992]

Thank you all.

I will read all your answers.