vapour return line - Is this possible ? 1

[saumian]

We have two 5000 US gal tanks; one is used for FRESH aqua ammonia and the other is used for SPENT. We empty the fresh tank slowly over 3 months for usage. when we fill fresh Tank from trailer ( filled in 30 minutes), no one wants wants to stay inside the facility due to ammonia fumes. The bulk trailer is pressurized so that the aqua ammonia flows from trailer to fresh tank; similarly, the trailer gets flow from spent tank by creating vaccum inside trailer. I am wondering whether I can hook a vapour-pipe line from fresh tank to the compressor-inlet so that the ammonia vapour form fresh tank goes back to the trailer during this fill up. We have a vertical 2" PVC duct, with a ball check valve, running from tank roof to facility roof and above to sky. This venting system does not help to get rid of the ammonia vapours.

Saumian

 

[pmureiko]

Why can't you vapor balance between the trailer and the fresh ammonia tank when filling it? This may just involve some short jumper piping and valves to use the existing vapor balance line with the spent tank.

I don't see why you would vapor balance with the off-line spent tank when filling the fresh tank. Is there a reason for that?

Assuming the next step is to load the trailer from the spent tank, you would use the existing vapor balance line to keep from gassing everyone.

Lastly you could install an ammonia scrubber on the tank vents. This would be a way to recover the ammonia lost to the atmoshpere during the 3 month storage period. There may be some dilution, but you could benefit by recovering the ammonia that you paid for.

 

[Montemayor]

I think there is some confusion over the description given by saumian. What I understand he is doing is nothing more than using a gas pressure (air or N2) imposed on the Aqua Ammonia (AA) full tank truck and using that pressure to force the fresh AA down and out of the tanker truck and into his storage tank. This is the old "acid egg" principle used over the years for transferring hazardous or dangerous liquid fluids. If I am correct, then saumian can't install the vapor balance line suggested by pmureiko. The recommended balance line only works when using a transfer pump.

I've used both methods - and I certainly agree with pmureiko on the installation an ammonia scrubber on the tank vents if the transfer operation is going to continue using air/N2 instead of a pump. I would not recommend using the traditional closed-loop compressor system to transfer the AA fluid. I've also used this method - but only on saturated liquids where I have an abundance of vapor available for compression and recirculation. Aqua Ammonia is limited on the amount of vapor that can be generated for that purpose - as well as limited on the rate at which the vapor is generated. saumian says he wants to use "the compressor"; yet he doesn't identify what kind of compressor he is talking about. Is this the same compressor that is pressurizing the tanker truck? Is it an NH3 compressor? He hasn't identified the pressure gas either. This is confusing, so I have to assume he is using an air compressor. If this is true, then he shouldn't use it to compress NH3 vapor - the materials of construction aren't compatible with NH3.

I can't critique the operation because I don't have enough basic data. But if saumian is pulling a vacuum on the tanker truck, he should certainly check on the vacuum pressure rating there. Tanker trucks are normally not built or equipped to handle a vacuum condition inside the tank. That is why in all the installations that I have worked with and also designed, we have either:

1) contracted with a tanker truck carrier that has a built-in transfer pump to off-load the fluid into our tanks; or
2) I design my storage tank transfer pump to also off-load the truck contents into my storage tank by installing appropriate piping and valves.

saumian, as I have mentioned, I have designed and installed successful, N2-driven Phenol transfer systems. So I know the hazard under-pinnings of such a type of operation. The hazard created is at the moment that the pressurized gas "breaks through" the tanker truck at the end of the transfer and enters the now-filled storage tank. This sudden on-rush of pressured gas into an atmospheric vessel can rupture the same vessel if not relieved instantly. Instrumentation should be installed to quickly shut down the liquid transfer before the break through occurs. Additionally, the over-pressurization of the tanker truck is also a hazard should the regulator or control valve on the pressure gas fail open.

Sorry I can't remark on the recycle compressor usage, but I hope the rest helps.

Art Montemayor
Spring, TX

 

[saumian]

Montemayor,

The tanker truck is equipped with air compressor which transfer FRESH AA from tanker truck to fresh tank, and by pulling vaccum, transfer spent ammonia from spent tank to tanker truck.

The Carrier for this AA takes care of everything, and they have been doing this kind of operation for a long time. We, as receiver, do not do any compression at all.

I begin to understand that we have to install exhaust and scrubber, even if we recycle compressor usage, because our spent tank will end up collecting vapour at the eng of this delivery and pickup. Therefore we have to go for an exhaust with scrubber for the delivery portion of this situation.

But I have a question: For AA, is there tanker truck Carriers with built-in transfer pump?

Saumian

 

[Montemayor]

saumian:

I can assure you that if tanker truck Carriers with built-in transfer pump are not available in your area of the world, then they can be made available very easily. All we are talking about is an economic maneuver similar to the tactic of V.A.T. (value-added-tax). The cost of the pump is not absorbed by the carrier; it is passed on to the customer(s) as part of the service he furnishes. It's always the same story: the customer pays for everything. It's the eternal and universal law of business and perfectly acceptable.

If you can obtain a tanker truck with an air compressor, you certainly can obtain the same vehicle equipped with a simpler pump. In fact, it is far less expensive to install and operate a pump on a truck than it is for a compressor! You can be certain that you are paying for the service of the air compressor on the carrier's tanker truck.

The following is why I recommend the transfer pump system. I have found it:

1) to be the safest method - as analyzed and scrutinized in actual HaZops;
2) it transfers the product faster and more economically; the speed of transfer is very important to me because it signifies the lowest time duration that an operator is potentially exposed to the tranfer fluid (& it associated vapors);
3) it requires the least maintenance because it has the fewest moving parts;
4) it is more rugged and reliable than the compressor system and can withstand more outside exposure and neglect;
5) it is less expensive in capital and maintenance costs.

I also repeat my concern: I do not believe you can succeed in designing a compressor to suck the ammonia vapors emmitted from the AA at the same rate the AA is releasing them. This is not a pure, saturated compound. It is a mixture made of a limited amount of DISSOLVED ammonia that will be expelled at a rate different from the compressor's capacity and dependent on various factors. During those times of low ammonia emmission rates, the compressor will be starved for vapor and will start to pull a vacuum. This is the hazard zone. How deep will the vacuum result? Can your tank(s) resist this unknown vacuum? You are unable to predict or calculate this vacuum with certainty, so you are left in a quandary and vulnerable to tank vacuum failure. I can't keep knowledge of this potential scenario without alerting you to it.

Art Montemayor
Spring, TX

 

[saumian]

Montemayor,

I appriciate your opinions. I myself not in favor of using vapour return line for the air compresor of the tanker truck, unless we have a liquid transfer pump.

However, I would like to disscuss with you the feasiblity of having a system like that at least in theory. Assume we have a ammonia & air compatible compressor in the tanker truck, then I think if we simply add a vaccum relief valve at the top of the FRESH tank, then we should not have any vaccum problem at all when we pull AA vapour and air together to the air compressor through the FRESH tank during this tranfer.

I would like to add my opinion to general liquid tranfer operations: It may be okay to use gas ( air or anyother gas) compressor to tranfer non-volatile compounds, but it is not wise to use air compressor to tranfer volatile liquids like our AA. Air compressor will increase the evaporation rate of the compound being tranferred, and we do not want this.


Saumian

 

[Montemayor]

Saumian:

As I stated before: a vapor return line (actually, it a vapor balance line) will not work with an “acid egg” or air pressure type of liquid transfer. We may be having understanding problems because of a language difference. I wish we could communicate with drawings and sketches because that is the most effective and positive way for engineers to communicate on this type of problem, but this is not possible on this forum.

Aqua Ammonia is a mixture of water and NH3 gas to the point of saturation. The gas is dissolved in the water and will escape when the pressure on it is reduced. This is not a pure, saturated liquid – such as pure liquid NH3 or LPG – and it will lose its NH3 content as you pull a vacuum on it. It no longer is Aqua Ammonia if you continue. That’s why I don’t recommend the air compressor type of transfer system for your operation. I think I was pretty explicit about it. For example, I give 5 strong reasons for recommending what I do and I also state why pumps would be just as readily available. Putting a vacuum breaker in your Fresh tank and sucking with the air compressor doesn’t improve the operation other than protect the Fresh tank. You still have the build-up of NH3 vapors and its problem to the personnel. I would never use nor recommend the gas/vapor type of liquid transfer unless TOTAL, SAFE containment is ensured. That, the 5 reasons, and the fact that a tanker truck pump is feasible are the points that I emphasize as leading to the logical use of a pump instead of an air compressor. Of course, the pump system should have a balance vapor line.

The best and simplest manner of resolving the problem (exposing operating personnel to NH3 vapors and pulling a vacuum on atmospheric vessels) is to use a cast iron, conventional, centrifugal pump with a vapor balance line to transfer Aqua Ammonia. I have been exposed to NH3 vapors and I have seen the effects of severe exposure to the same. It is a painful and needless experience and I can’t blame your operators for leaving the building! The terrible damage and pain that NH3 can inflict to the mucous membranes (like your eyes, mouth, nose, etc.) is defined as a definite human hazard and should be avoided at all costs.

In 1976 I witnessed a tragedy here in Houston, Texas that involved the overturning and rupturing of an NH3 tanker truck on a Freeway overpass. Many motorists and other people were overcome by the resultant, toxic, and terrible NH3 vapors that blinded them and burned their bodies. It was horrible to see humans killed in this manner, with so much pain and no recourse for help. I have personally resolved to never allow this type of disaster again. It need not happen if one takes the time and controls to engineer around it.


Art Montemayor
Spring, TX