Anhydrous Ammonia vents

[lingwood]

Just become involved in a new project utilising Anhydrous Ammonia, and my question relates to the potential to lock-in liquid ammonia between two valves and the potential for expansion.

To relive this it has been advised to use hydrostatic vents, but are there any other options?

 

[pennpiper]

1. The expansion rate for Ammonia is higher than the rate for water to steam so the pressure will develop faster and higher.

2. You may be able to just vent to atmosphere or you may need a closed collection system.

3. You need to determine the HSE (Health, Safety & Environmental) requirements for Ammonia venting for the surrounding area of this plant.

 

[lingwood]

Given sch 40 stainless pipe of 1.5" diam has a burst pressure of approx 11,000 psi, but hydro vents set at ~400 psi, is the desing pressure rating of the pipe not sufficent alone to hold the expansion?

PS this is for an non refrigerated system

 

[pennpiper]

Is your system made up of ONLY 1-1/2" schedule 40 Stainless Steel pipe? There are no fittings, no control valves, no unions, no PI (Pressure Indicators)connections, no pumps, no joints or no other items except 1-1/2" SS pipe. Is this really true?

Please think about the whole system. Even the two block valves that lock-in a section of the piping system that is the concern of your original post. They have weak points such as seats and packing that will not survive the alleged 11,000 psi surge that you say the pipe can take.

Now, you asked for advice. You got some advice. You do not have to follow that advice but it would not be unreasonable for you to just say Thank You. Then go do what ever you want to do.

Good luck to you and your project. I hope you choose to do what is right.

 

[lingwood]

THANK YOU!

 

[lingwood]

Back to my original question! Are there any other options other than hydro vents?

A vent point will become a measurable emission point, maintenance item and other source of leaks (like also valves, pumps and gauges etc).

Other option considered is consider the system as a whole and therefore minimise number of isolations to one, and then combine works to one time.

Yes more will be vented down to work on one item, but will give 100% sure that the whole system is same.

It would also minimise human error or draining/isolating the wrong section.

Accumilators have also been discussed.

 

[ash9144]

A single isolation for an entire system is unreasonable and a bad design from the start. I seriously doubt you will shutdown and decontaminate your entire ammonia system if a local pressure gauge starts leaking.

 

[StoneCold]

You would get better answers if you told us what type of project it is. Unless it is some super secret govermnent design.
Anyway You should look at ASHRE for some advice on handling ammonia. You can also look at the international institute for ammonia.
Your question leads me to believe you really need some outside help on piping design, pressure rating, etc. Though maybe not.

Regards
StoneCold

 

[BenThayer]

you better start reading the OSHA requirements:

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9757

also look at various suppliers literature:

http://www.tannerind.com/sto-anh-ammonia.html

basically, the hydrostatic reliefs are not intended to be used. they are there if an operational error occurs and the line is blocked in liquid full.

with regards to your question about 11,000 psi vs. the 400 psi relief. does your system have any flange gaskets, valve packing, etc.?

i agree with StoneCold. you need more assistance than this forum can provide.

 

[bchoate]

BChoate

OSHA requirements in 29 CFR 1910.111 cover ammonia systems. A requirement you mention, thermal relief for liquid ammonia trapped between two valves, is part of 1910.111 (which is ANSI K61.1 in CFR form).

You didn't mention if the liquid ammonia is under pressure or is atmospheric at refrigeration temperatures (-28F). One can use hydrostatic reliefs or standard relief valves. The hydrostatic valves are "burp" valves which only relieve a small amount of liquid. Standard relief valves will release more ammonia. Hydrostats are small (3/4" X 1/2") and can be connected to discharge headers with SS tubing.

You mention stainless steel pipe. Carbon steel is acceptable. A106B is good down to -20 F. A333 has to be used for lower temperatures. Threaded fittings are only allowed if they are SCH 80 minimum.

Hydrostats are available in 250 psi, 350 psi, or 450 psi standard sizes. The discharges can be connected to a relief header which is connected to an atmospheric vent stack. CL 150 is rated to 285 psig; CL 300 is rated to 720 psig. Liquid ammonia at normal temperatures requires under 150 psig pressure.

Bill Choate

 

[BenThayer]

please note that the ratings Bill is giving is for carbon steel and not stainless flanges.

i peronally am not a fan of CS in high purity applications because of ammonia SCC. you can overcome that by stress relieving but that is an additional cost.

the A333 fittings are typically hard to find on short notice and makes repairs and modifications difficult and poor change control can (has) resulted in the wrong metallurgy being used for repairs.

 

[AMMGUY]

lingwood - I have been involved in a lot of ammonia storage and forwarding plant designs for Power plant.

As per ANSI K61.1 and NFPA you have to have thermal relief valves at pipe portions that can be isolated.

We have used a lot of Rego 8022 hydrostats for this which have a very small release. In some cases we have connected all the hydrostats in to a common vent and then take that vent into a water tank etc.