HDPE Acid Tank Spec - ASTM D1998-06

[Blazer406]

I've been going round and round with trying to figure this out.

Product: 37% HCL
Tank: 6500 gal HDPE tank

There is an venting "study" where an engineering company studied the potential internal pressures and ACFM while loading HCL from a tanker truck into the storage tank. You can find this study on PolyProcessing's website. They suggest that ASTM D1998 tanks are designed to 10" W.C. internal pressure. Some of the people I have asked quotes for that sell D1998 tanks are balking at us asking for a tank capable of 10" W.C. internal pressure. In a nutshell, the study calculates the recommended "vent" size which in most cases is 6". The more simple fume scrubber is a small pot off to the side with water inside and the 6" vent coming off the top of the HCL tank is submerged in 6" of water. This automatically makes the internal tank pressure have to rise above 6" W.C. to get any flow of fumes into the scrubber. This is where the rub is.....

Section 1.2 of D1998-06 specifically states that the standard does not apply if the tank is for use at any pressures other than atmospheric pressure.

Section 1.3 says "Special design considerations not covered in this specification shall be given to vessels subject to super imposed mechanical forces.......; and vessels subject to superimposed pressures exceeding 10 in. of water."

If I take it literally, any internal pressure that is not exactly equal to atmospheric..... requires some "special design consideration" and hence.... isn't covered by D1998.

I don't actually think that is exactly what they intended.

Anu thoughts?

 

[MJCronin]

Install a spring-loaded emergency vent set to operate at 10" WC

Or ensure that a top manway is equipped with a "loose bolt" lid

Pay no attenetion if the vent actuates during the last 30 seconds of pressure loading.

Most people would not believe the number of FRP/poly HCL tanks that have failed during tanker truck pressure offloading.

Almost all FRP/Poly tank manufacturers warn about this type of failure

Here is some fun reading......

http://www.epa.gov/osweroe1/docs/chem/surpass.pdf

Respect us....come back to us later with your final design

 

[Blazer406]

I have since found out a little more detail about one installation. We installed a conservation vent set to begin opening at 9" WC and be fully open by 10" WC. On the very first loading cycle, with the scrubber set to hold 6" WC internal pressure in the tank, the conservation vent opened and was rolling fumes out while we had just liquid going into the tank.

Is there some sort of reaction taking place generating vapor when the HcL enters the top 2" fill line and falls through the vapor space in the tank to wherever the current liquid level is? On our tank, we have a vacuum breaker in the top of the tank, so as we use acid, it is opeining the vacuum breaker and letting outside ambient temp air inside the tank to mix with the HcL vapor in the tank.

Any thoughts?

 

[JStephen]

It sounds to me like you need better recommendations from the supplier of the acid in question (or maybe a trade association for it).

I understand that HCL is basically hydrogen chloride gas dissolved in water, which is why, unlike sulfuric acid, you never see "pure" HCL sold. Maybe it behaves like soda pop where splashing it around tends to release the gas? Perhaps the original tank contents are warmer than the contents being added and release gas during the mixing. Perhaps the material coming from the bottom of the tank truck actually has a tad higher vapor content due to hydrostatic pressure than what can be held at atmospheric pressure. In any case, you need more input from someone familiar with the details of handling it.

It is not uncommon to use a riser pipe (or "downcomer") pipe when material is pumped into a tank so that it is pumped into the liquid part rather than the vapor part, and that may help with the problem.

Normally, vent sizing involves calculating the flow rate required to relieve the tank, and then selecting a vent that will accommodate that flow rate at the pressure differential required. From the description, it is not clear that either part of that process has been satisfied.

Normally, "atmospheric" means just that, at the same pressure as the outside air. But occasionally, I'll see specifications where they make a distinction between "atmospheric" and "pressure vessel" and in that case, "atmospheric" may include up to, say, 2.5 psi. In a case like that, it should be up to the source to define what they mean by atmospheric. Any kind of vent has to have some pressure differential to move a vapor through it, even it's just an open hole on the top of the tank.