Requirements for off-loading of flammable fluids from tanker trucks

[Ktracid]

Hi all,

I have been working on tanker truck off-loading that handles diluent (flammable fluid). I have the following questions:

1. How do I make sure that the venting capabilities of the tanker (that we are off-loading from) are adequate? In other words, how can I determine the maximum off-loading rate from the tanker truck without causing any venting/vacuum issues?

2. I know that that the hose connection is 4"? What is the maximum recommended velocity to reach in the hose? What is this maximum velocity based on?

3. What kind of provision is required for off-loading of a flammable fluid like a diluent? I have been advised that a portable nitrogen package is potentially the solution. I have no prior experience on the topic and do not know how to assess the need for a nitrogen package. What other information do I need to make sure that the nitrogen package is definitely needed?

4. Do I need to consider any vapour recovery?

Many thanks for your responses.

 

[StoneCold]

Ktracid
I would recommend that you start by looking at NFPA 30.
I would also recommend that you make sure your tanker is grounded during the transfers.
A clean way of offloading trucks is to vapor balance with the tank you are loading into.
If you can't vapor balance for some reason, you can vent the vapor out either through a carbon scrubber or directly to air depending on where you live.
Nitrogen pad on top of the tanker is a good idea. You should be adding nitrogen to the headspace of the tank if you are not vapor balancing with the tank.

4" hose, Boy is that heavy.

Offloading rate is driven by your pumps, unless you are pressuring off the tanker.
For static reasons it is a best practice to keep the velocity under 1 meter/sec in the offloading hoses.
This is somewhat product dependent as conductive products are more tolerant of higher velocities.

If you don't vapor balance with the truck you need to add nitrogen at the same rate you take liquid off to keep the tanker from pulling a vacuum.

Regards
StoneCold

 

[Duwe6]

Most trucking companies want to "vapor balance" to prevent vacuum collapses of their tanker truck [they are delicate under external pressure]. EPA wants you to capture all stray vapors. Your operators will want a system that doesn't require them to jump up and runn to shut off a pump. Your maint. mechanics will get tired of replacing seals on pumps that have been dry-run.

Do everybody a favor, and have dry nitrogen regulated to 30 psig at the truck unloading dock, and use pressure to unload with. And pipe the vapor from your tank(s) back to the unloading station - no stray vapors; they get hauled off in the 'empty' tanker.

Check with your preferred chem suppliers and see what trucking company(s) that they use, and call the trucking folks to see what unloading pressure they prefer, and what type and size of connections their trucks will have.

and install, maintain, and test a grounding cable on a retractable reel, with a clamp that has wicked, pointy teeth Its kinda interesting to watch somebody elses' truck and unloading station burn, but you won't enjoy watching it at your business.

 

[Ktracid]

Thank you both. I have found your responses very useful.

I have been asking for padding (injecting inert gas in the vacant space of tanker) and I understand that it has other advantages. We all know that the fluid (i.e. diluent) is flammable (its flash point or vapor pressure is not know yet) but this alone is not convincing the team that nitrogen package is needed. Is there any code or standard that will have padding as a requirement for diluent off-loading? I have already reviewed NFPA 30, API RP 1007, and CSA B621.

Another design considerations that are worth contemplating:
- Let's assume that nitrogen at 20-30 psig cannot be added. It is likely to have pump NPSH issue in hot conditions. I do not believe purchasing a self-priming pump is a solution for diluent or is it? Having these options left out, do you have any other suggestions to deal with low NPSH pumping issues? Is there any Suction Specific Speed (Nss) requirement?

- How about eliminate air from the system at the pump discharge? I also think it is required especially if nitrogen is being added or the pumps can run dry (which is inevitable for fully emptying the tankers).

- We are dealing with double tanker type B trucks. How many compartments does each trailer have? I would like to use multiple connections to limit the velocity within each hose connection to around 1-1.5 m/s.

- Let's assume that vapor balancing or padding cannot be incorporated in the design. How would I know that the venting capacity of the tanker is enough to handle the design off-loading rate?

I will definitely be careful about tanker grounding and interlocking it to the pump start in the control narrative.

 

[LittleInch]

I find it a little alarming that you're asking a forum questions like this, but here's my take on your issues.

1) Venting capabilities of the tanker - See what the tanker details are as each are different, but tankers are clearly designed to load (normally with vapour recouvery lines) and un load. As a minimum you would have an open vent connected to your vapour space line and then it becomes an air flow calculation to determine what the max flow rate is that doesn't result in a pressure lower than atmosphere which the tanker can't stand (should be plated on the tanker). In most cases this isn't very much.

2) Good grounding all the way through the system is a must and connecting the tanker to the same ground as the pipe / hose before unloading is a requirement. However static in the hose is not that significant providing your connection at the end is a bigger pipe and it is all earthed. Static per se is not a big issue, it is when it is in a tank with a vapour space above it and something to flash to that you have an issue. 1m/sec in a 4" hose is OK, this is flow rate of 30m3/hr.

3) How you offload can be two ways -

gravity feed into a buried tank with a vent higher than the tanker and then pump out of the tank. This means your pump shouldn't run dry and you almost eliminate the potential for sucking product out of the tanker too fast and collapsing your tanker. you can then also shut off the pump on low level and avoid sucking air / nitrogen in. Of course you have the extra cost of a buried tank, but I reckon it's worth looking at to separate your tanker from your pump.

or

feed into a manifold and into inlet of a pump to pump it directly out of the tanker to your storage tanks. This is where it gets fun as the flow rate is quite variable as the tanker discharges and where you need good control to reduce air entrainment and avoid accidentally sucking your tanker in.

4) you need to allow for vapour handling from whatever storage tank you're pumping into yes, unless it is a floating roof tank.

A low pressure Nitrogen or inert gas blanketing system for the tanker is generally a better idea than air as it avoids any chance of explosive vapour escaping and getting into places where it could explode. However you then need to determine how to handle gas which will inevitably be blown into the piping system.

Double tankers are around 60-70,000 litres, normally in 6-8 compartments. Usually offloading is done from all compartments at the same time, usually aiming for offload in 25-30 mins.

NPSH depends on many things which are not noted here, but yes, volatile liquids can have NPSH issues in higher temperatures. Search the pump forum first where many questions have been asked on this. self priming is for wh your pump inlet is above your normal liquid level which shouldn't be the case for a direct system but might be needed for a buried tank option. Submersible pumps or can pumps are also used for low NPSH issues or add pressure to the tanker to generate extra head.

I think you need proper advise from someone who has more experience in these issues and you design the best system for your particular situation.

Be careful.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way