NH3 storage tank decommissioning and commissioning 2

[QUEENCHEM]

Dear all,

in one of our project we shall inspect internally a NH3 storage tank.
In order to allow such inspection we'll have to carry out a properly decommissioning and after the inspection the commissioning as well.
Do you have some hints for this specific case?

Thank you in advance for your valuable suggestions.

 

[EngAbdelkader]

Hi QUEENCHEM, I think that in your case it would be better to use "shut down" instead of "decommissioning" as the later definition is used for the equipments that are going to be dismantled!

before the inspection, the first thing to do is to shut down the unit to inspect, that's what I think you meant by decommissioning.
we mean by shut down is carried out through:
1. stop feeding the tank (close the control valve of the feed
line)
2. lowering the level inside the tank by opening the blow-
down valve or through the normal discharge line (pump if any)
down to the lowest level that is possible to reach
(regarding the LL alarms and the suction of the pump if
any)

Using a pump to lower the level can be problematic (vacuum issue) so you have to take in account this problem also by opening the blanketing (N2) line or to provide the tank with one, it will insure a sufficient pressure inside the tank during the pumping out period to avoid collapsing the tank and to avoid the opening of the vacuum breaker action (if the vessel is provided with a vacuum breaker) that would open as soon as the vacuum is reached which can be dangerous regarding the explosive mixture with air that can form inside your tank.

3. before anything we have to secure the environment where
we are going to enter so to avoid an explosive mixture
knowin the characteristics of the NH3 (LEL 16% UEL 25%)
the tank has to be purged!!!
4. the purging is done using an inert gas as Nitrogen, so
we connect the tank to N2 line, open the vent and the
drains of the tank and let the nitrogen flow at a
a sufficient pressure (greater enough than the
atmospheric one obviously)
the flow rate is to be calculated knowing the volume to
purge.

5. now that the tank is "secured" it's completely insulated
from the units it's working with, so blind or better
spectacle flanges are turned in closed position in all
the lines (in and out) connected to the tank.
6. at this moment the manhole can be opened safely, the
access has to be forbidden to the tank until the
environment inside is checked and safe for the workers
or inspectors (HSE issue - SAFETY FIRST!) in the sens
that it contains the right mixture O2 and N2 to be
breathable.
7. at that point the tank can be washed, inspected,
maintained,etc

here I've tried just to give you a general view of how and what should be done before opening any process vessel!!

After inspection and depending on why it has been carried out, the commissioning is run and this is also a complicated issue (piping, mechanical, electrical, instrumentation) depending on the type of storage tank you have to take in account the ASME section VIII recommendations for the pressure testing of the vessel!

finally comes the start-up time obviously before that point, all the auxiliaries related to that process have to be started first, also take in account the instrumentation (insuring the levels inside the tank and the operating pressure)

sorry but your question needs more time, more preparation, and I need to review also some notes to answer you clearly but I hope it gives you an idea of what you are going to face!

Don't forget! first and always! take a look at your P and ID's and study the process well before to run this type of job. Ask questions regarding the process to the operators, don't hesitate! they know better than anyone else the plant they are operating (Normally)

 

[BenThayer]

what grade of ammonia, what is the tank MAWP and how big a tank?

 

[BenThayer]

i thought there was a detailed answer somewhere here on the site but i can't find it. i agree with the outline that kader007 has above.

i have been involved with inspecting 30,000 ton atmospheric tanks that we had to empty of 0.5% water agricultural grade ammonia as well as 2000 ton Horton spheres filled with 99.999% pure ammonia.

the large ag grade tanks have peculiar issues since there is some water left in the tank as well as the issues outlined with regards to pulling in air.

and you will need time and more time as you go through the purge cycles. we had a standard of getting the tank to something like ~1% O2 to prevent SCC when returning to service.

google ammonia scc and look for articles like these:

http://www.nationalboard.org/NationalBoard/Articles/Classics/classic8.aspx

and the vapor space of ag grade tanks are still 99.999% pure and so SCC can occur near the roof areas.

also, for the return to service on large efrigerated tanks, you will have to review the use of a cool-down spray, etc. to prevent thermal shocking the floor, etc.

it would take us ~2 months to get into a 30,000 Ton tank from start to return to service with about 2-4 weeks inside the tank. our tanks were ~170 feet in diameter and 65 feet straight side. a fairly common size.