Cryogenic Bulk Tanks- Vacuum & Leaks (d)

The principle differences between Bulk Tank and 4L-Liquid Cylinder vacuum investigations?

A) Size and B) More parts and Penetrations

Bulk Tank size matters a great deal when it comes to the ability to conduct and effective vacuum investigation. A liquid cylinder can typically be thoroughly investigated in less than an hour. A perlite bulk tank can take weeks just to get a vacuum good enough on it to perform an effective investigation. Investigating in place will also frequently require the use of a manlift or other device to get all the way up and around the vessel seams, lift plate, etc. Add just a little breeze and it can be well nigh impossible to get a accurate investigation completed due to helium dispersal. The only answer is tenting or bagging section of the vessel which further increases the time required.

Add to Size issues related to additional parts- especially the probe tube. Probe tubes frequently leak through causing vacuums to go bad just by trying to read them. This has been true in many cases even with new probe tubes. There have also been a number of leaks in the probe tube isolation valve handles that are sometimes only present with the valve open. Then there is the vacuum lift plate used on many bulk tanks. This plate is o-ring and lube sealed by the vacuum pulled on the tank. And unlike 4L- Liquid Cylinders which have all piping run through a single head boss, bulk tanks are loaded with line penetrations. None of these issues are overly challenging for a capable investigator using helium mass spec equipment. But they do help explain why those not performing full investigations can get a good vacuum pulled on a tank and leave believing the tank is fixed.

We go back to size and semi-permanent installation. Unlike a liquid cylinder it is a huge undertaking to remove a bulk tank, send it in for vacuum investigation and restoral and reinstall. And, there is typically not a ready supply of spares to pop in place while a bulk tank is being repaired. Hence- It is almost always easiest to send 4L- Liquid Cylinders in for repair while it is frequently justifiable to perform bulk investigations in the field. Justifiable…… but often costly nonetheless making it all the more critical that the right tools and right people are used.

Cryogenic Bulk Tanks- Vacuum & Leaks (c)

How do bulk cryogenic vacuum investigations differ from liquid cylinder vacuum investigations?

The principle differences relate to the size of the vessels and the more permanent nature of bulk cryogenic tank installations. There is also an issue with added penetrations and connections on a bulk tank which mean more potential leak points. There are also a couple of other nuisances related to probe tubes, lift plates, vacuum valve and perlite insulation. First- The principles of helium mass-spec leak detection are the same for almost all double-walled, vacuum-insulated cryogenic liquid storage vessels and include:

A) A vacuum evacuation port of some type must be installed on the outer vessel allowing a vacuum pump to be connected to pull molecules from the annular space. These are specialized connections capable of pulling down to very low atmospheric (high vacuum) levels without leaking past. On smaller 4L-DOT type vessels this is typically a port and plug assembly that are pulled on using a fixture that allows the plug to be pulled and replaced under vacuum. A larger vacuum diaphragm valve is used with bulk tanks

B) A helium mass-spec leak detector is set up on a slip stream of the vacuum line to allow sampling for helium molecules. The leak detector can only be used effective after vacuum has been pulled down far enough to get a helium reading. If the vacuum will not come then the leak will have to be found by other means and some repair performed prior to performing the full helium mass spec leak investigation. Vacuum valves, vacuum probe tubes and probe tube isolation valves are often replaced prior to performing an investigation due to their propensity for being large leak points. It is also common to break the vacuum seal on a vessel and reseal lift plates where present (including o-ring replacement and vacuum lube addition). Perlite is also added at this point for perlite insulated vessels. It is worth noting that breaking vacuum improperly can be a major source of moisture contamination and can also cause permanent damage to the vessel.

C) The initial leak investigation consists of spraying helium molecules around the outside of the vessel and all penetration points. This process can be painstakingly slow and tedious as a leak can both provide a positive indication and flood the annular space with helium that can take hours (or days) to evacuate. Also- getting a positive reading is only a start. It is often necessary to work through a series of tests where small portions of the vessel seams or other penetrations are masked off to ultimate isolate the leak point. And- repairs requiring welding will likewise require careful breaking of the vacuum, completion of the repair, and then pulling the vacuum back down on the vessel to start testing all over again.

D) Once the outside of a vessel has been determined to be leak-free then helium is added to the inner vessel and some pressure added behind it to look for possible inner leaks. Fortunately, inner leaks are not that common on cryogenic bulk tanks- especially stationary tanks- since they are not subjected to the kind of rough handling that 4L Liquid Cylinder and smaller open top nitrogen dewars are. I say “fortunately” as isolating and repairing inner leaks is most complicated and worthy of another full set of articles.

But what about the differences between Liquid Cylinder and Bulk Tank vacuum investigations?

We’ll hit more on this next.

Cryogenic Bulk Tanks- Vacuum & Leaks (b)

Shortly after posting yesterday’s article I saw an advertisement for cryogenic equipment noting, “We Do Revacs.” And….. so do lots of people. But- Does this include full vacuum investigation and repair?

As mentioned previously, there are very frequent occurrences of people putting a vacuum pump on a tank and pulling the vacuum down within specification limits. Because the vacuum pulls down to where it needs to be the assumption is made that the vessel must be good now. The fly in the ointment here is that a tank with a small leak can easily be pumped on to the point that the vacuum reading is good. This is fairly simple as all you have to do is have a pump that pulls molecules out of the vessels annular space faster than they come in through the leak.

This is very much like a leaky boat. My good friend, John, told the story of an off-shore fishing trip where they struck a piece of driftwood or some other debris and started to take on water. This was particularly problematic since they were catching lots of fish and did not want to quit. So, they turned on the bilge pumps and fortunately were able to pump out water faster than it came in. Returning home the leak grew and they had to pull the plugs in the back of the boat to allow water to flow out. All was OK until they stopped at the boat ramp and jumped out to go get the trailer. Once stopped, the boat sank almost immediately and had to be floated up from the bottom with lift bags.

Interestingly enough, we never talked about it, but the final excess flow might have come from a failure to get the plugs in fast enough once stopped. This has a parallel to cryogenic vessels that will be touched on in a moment.

But, first- If we assume that a cryogenic vessel was originally constructed in a leak tight fashion and this was proven out over years of operation then where did a leak come from? There are several primary sources of vessels leaks:

a) The Vacuum Rupture Disk and/or Lift Plate
b) The Vacuum Probe Tube and/or Probe Tube Isolation Valve
c) The Vacuum Evacuation Valve
d) Piping Penetrations, Doublers and Other External Welds
e) Occasional Inner Leaks

Time, corrosion and/or damage (intentional or otherwise) can cause leaks in each of these areas. Unlike a boat leak, though, it is rare that they are visible with the naked eye. In most cases, these are leaks that have developed very slowly over time and are allowing just a few molecules to seep in little by little. And- as vacuum loss is typically a boiled frog progression (slow and with almost imperceptible progress) it is also difficult to link the leak to any cause or event.

So how do you find the cause and location of a leak?

Helium Mass Spec Leak Detection.

This testing utilizes mass spectrometry equipment specially calibrated for detection of helium molecules along with a vacuum pump to sniff out leaks. Helium is used since the molecules are so small they will travel through the tiniest leaks.

Next we’ll do some review of Helium Mass Spec Testing and the principle causes of leaks in tanks.

Cryogenic Bulk Tanks- Vacuum and Leaks

"Nature abhors a vacuum."

When conducting cryogenic vessel classes we often discuss the fact that, other than occassional flashes between my ears, natural vacuums do not exist within the earth's atmosphere. Given time and opportunity, all vacuums will draw in molecules to the point that atmospheric balance is restored.

It is no great wonder then that vacuum insulated cryogenic bulk tank and other liquid cryogenic vessels lose their vacuums and related insulation capacity over time. And, when the vacuum is lost, the vessel loses all or the greatest part of its insulating capacity and can no longer protect the liquid from heat resulting in excessive loss of oxygen, argon, or nitrogen liquid product. The mystery (and fun CSI part) of the job comes from determining the how, where, and why of a lost vacuum.

All too often, vessels with bad vacuums are hooked up to a vacuum pump until a good reading is once again achieved. The pump is then disconnected and the vessel returned to use only to find that it is again losing product within days, weeks or months. The process is repeated with the unsurprising recurrence of the problem.

There is another way and we'll discuss it next.....

Implosions Are Also A Mess......

There have been several posts in Cryogenic News regarding cryogenic tanks rupturing as a result of over-pressuring. This is important due to the real danger posed by tanks and cylinders that are improperly modified or otherwise malfunctioning.

Implosion is also a very real concern and Wil Ferch (Owner/Principal at Nordstern Associates LLC) has graciously allowed me to reprint his recent article "Don't Let This Happen to You!!" below without endorsement.

Interestingly enough, as I was reading Wil's article I remembered that Dan Ariely (of Predictably Irrational fame) is also a director with The Center for Advanced Hindsight which you can visit at http://web.mit.edu/ariely/www/MIT/thecenter.shtml.

Ideally, employing the lessons shared by Wil and others can help you avoid status as a case study for Dan's group so here it is:

"Don't Let This Happen to You !!!

The industrial gas industry, (producers of liquid oxygen, nitrogen and argon...amongst other products of air), require large site-erected tanks to be built in order to store these liquids for later use. These tanks can range in size up to 2 million gallons or more, and cost millions of dollars. The stored product, like liquid oxygen, exists at about minus 300 degrees Fahrenheit.

In the process of bringing such a newly constructed tank into service, it must first be carefully "cooled-down" such that it can accept incoming cold liquid later. Here is a photo of a typical tank on a representative plant site---> http://gazettextra.com/photos/2008/oct/02/10382/

Nordstern Associates LLC - CASE STUDY # 7

A major global producer of industrial gases, recently "cooled-down" such a tank as part of their commissioning process, utilizing their own qualified personnel and historical procedures. This time, however, things went horribly wrong, and their 450,000 gallon tank went from being a multi-million dollar asset... to nearly scrap metal value, in a fraction of a second ! In the process of cooling down the tank, it vacuum collapsed ! To illustrate the violence, sudden-ness and uncontrolled nature of the collapse, here is a YouTube film clip showing a similar ( this time, on purpose ! ) tank collapse of a railway tanker.---> http://www.liveleak.com/view?i=72e_1208694365&p=1

Also, when such a flat-bottom tank vacuum collapses, it may implode the top and side walls, and also start to "lift" the center of the flat floor. See here of what such damage looks like in yet another example ( once the web page is opened...move down to title, " Plastic bag vs. Storage tank" for photos and text.)----> http://www.driedger.ca/dp-1/Dirty%20Pictures%201.html

After the damaged tank was repaired, Wil Ferch of Nordstern Associates LLC was enlisted to provide both a revised cool-down procedure, and on-site supervision of the new attempt to cool-down the tank. Result?--> a perfect cool-down with no damage ! Wil Ferch, principal at Nordstern Associates had over 20 years of experience cooling down these types of tanks with a 100% recorded success rate throughout his career !

So....don't let this happen to you !

The industrial gas business has lost much in the way of retained skill sets and nuance-driven knowledge with long-term staff changes. Enlist people who have the training and knowledge to do this correctly. Call upon Nordstern Associates, LLC.

Contact--> Wil Ferch, principal at Nordstern Associates, LLC and let us discuss this.... or other support needs. ( ferch.wil@gmail.com .... business tel : +1 716 983 4089, alt tel: +1 716 662 7173). "

Bulk Cryogenic Portability and Utility

Can any horizontal cryogenic storage tank be fitted with a road valve and used for liquid transport?

No. Many horizontal cryogenic liquid storage vessels are manufactured for stationary use only. The support structures for their inner vessel, outside shell strength and other factors are built more simply than those designed to handle the stresses and strains of portable service. Vessels manufactured for portable service have advertising, specifications or other material specifically designating them as portable. Since portables will inevitably be a bit higher priced than stationary vessels the manufacturer will certainly want to make a point of their portability.

Also, some portable vessels are designed for transporting liquid only and are not intended for gas use. Others might have gas use outlets, but they need to be properly piped in order to use them for gas withdrawal service. Just connecting to an outlet marked “Gas” might not assure you a proper operation for gas service. Many vessels have the outlet present to make gas an available option, but lack the proper plumbing to be used effectively for gas service such as pressure building regulators, economizer regulators and economizer circuits.

More and more users are becoming aware of the convenience of operating with mid-sized portable cryogenic bulk tanks at 500 to 850 gallons or more capacity. If you are purchasing a new or reconditioned cryogenic tank it will be helpful to define your needs and be sure the vessel meets them. Odds are you might want to use the tank for something else one day or sell it so you might also want to be sure is fitted with everything you need which could include:

- Full-sized standard fill connections,
- Liquid withdrawal connections,
- Gas use connections,
- Pressure building vaporization and regulator,
- Economizer regulator and plumbing,
- Sufficient external gas use vaporization,
- Dual reliefs,
- Road valve,

While all this might sound a bit trite or redundant, it might surprise you to know how often costly assumptions are made.

High Pressure Options for Oxygen, Nitrogen & Argon

How can efficiency be optimized in cases where high pressure is needed for purging, testing or other high pressure applications? First, it is important to understand the application and the factors that play into system design. Once the application is better understood then consideration needs to be made for the specific installation and location. Sometimes, the “best” method is not the most practical or cost effective. And supply of materials or availability of services can be a restraint.

One of the major justifications for liquid cylinders or cryogenic bulk tanks is the ability to get molecules to a user in a lighter and more compact package. Depending on the gas used, one liquid cylinder might be a direct replacement for a 12- or 16- pack of high pressure gas cylinders. This one 4L DOT cryogenic liquid cylinder full takes up only a 21” square footprint and weighs well under 1000lbs versus the high pressure pack with a 4x4 or so footprint weighing 3000 pounds plus. And- a 500 gallon GOrilla portable cryogenic vessel could take the place of ten high pressure packs at only 8000 pounds or so full.

An application can be a real bear when the minimum pressure is high enough to require high pressure gas cylinders or tubes and the volume of gas used is fairly high. In many cases, this can result in the need for a dozen or more high pressure 12- or 16-packs on site with a like amount committed back at the plant for exchange. This becomes an even bigger issue when the volume of oxygen, nitrogen or argon molecules needed necessitates a high pressure tube pack or trailer. If tubes are exhausted it becomes necessary to:

a) have a complete replacement pack;
b) have a spare for exchange stock or
c) have the ability to “bump” the tubes in the field.

More exploration of these options and consequences soon…….

Cryogenic Liquid Bulk Drops- Part III- Good for the Customer?

Having determined that bulk cryogenic convenience drops are usually a good thing for the vendor the question is “Are they good for the customer?” And, of course, comes the Millennium Answer: “It Depends.” So- what are the customer costs and benefits of convenience drops?

First- the potential negatives:

N-a) These drops are often commonly referred to as “dumps” where clients have a negative view. The biggest concern is that the liquid left in the transport is “hot” and will raise the temperature of the bulk tank receiving the product. In some cases a company is concerned that the transport has traveled from say Phoenix to Tucson to Albuquerque and back and now has 500 gallons they need to get rid of. When filling the delivery guy might blow down some of the pressure on the receiver of there might be a good deal of venting which could translate as wasted molecules. Or- the liquid did, in fact, heat the tank up a little it might cause product to boil off faster and there would be a net loss of product.

N-b) Most bulk agreements also include a “per delivery” fee and it is undesirable to be billed a delivery fee for partial drops.

N-c) There can occasionally also be logistics issues arising from getting a partial delivery on Tuesday when the regular delivery is Thursday resulting in less than a full drop on Thursday.

All these things can result higher total cost of goods.

And the potential positives:

P-a) A well placed client working closely enough with their supplier could see some significant cost savings if they are lucky enough to be strategic. This is especially true if the supplier is trying to expand into new territory and the client has a large enough tank to receive most of the excess product the transport might ever have. This could result in both good upfront pricing from the supplier and some waivers for delivery or other fees that significant reduce the total cost of liquid oxygen, nitrogen or argon.

P-b) A strong enough relationship (and big enough cryogenic bulk tank) could also almost eliminate the need for scheduling deliveries triggered by getting down to re-order levels.

P-c) And then there is the whole aspect of operating closely enough that “the right hand washes the left” where doing each other favors benefits both the supplier and client.
Jim Sherrow was one of my most influential members and he’d regularly point out that it is critical not to let the Aggravation Factor exceed the Incremental Dollar. Mostly he meant this as career counseling, but it also applies to relationships of convenience.

Are Bulk Cryogenic Spot Drops a Good Thing?- Part II – Supplier Perspective

In the last episode it was determined that it is costly for cryogenic bulk transports to return home with any of the liquid they departed with. One way to make it home empty is to drop off any liquid remaining after scheduled deliveries at some other customer tank that has some room available. Ideally for the supplier, this would convert the left over liquid from a cost to income. It was asked, “Is this a good thing?” First, we’ll look at the supplier side with the question, “Did the revenue generated offset the cost of an 800 gallon delivery considering that Company X typically hates to deliver under 1500 gallons at a time?”

Here, again, comes The Millennium Answer. The Millennium Answer came into vogue shortly after some high placed U.S. politico said something to the effect of “It depends on your definition of the word ‘is.’” Ever since then it seems everything is contingent. i.e. “It depends.” In this case- It depends on the cost of getting to a drop location and making the fill compared to the revenue that might be generated. Pretty simple, eh? But, not necessarily as simple as it appears on the surface and especially when asking about dropping off 800 gallons where 1500 is preferred.

The dependant aspect of whether 800 gallons is a good drop or not relates to whether it is the (first/only) drop or whether it is the last drop. 800 gallons as a first or only drop is not necessarily a profitable proposition for a bulk supplier. Costs associated with running a 4000 or 6000 gallon transport out on a unscheduled single drop delivery might just exceed any revenue that could be made. This is especially true with lower priced products like liquid nitrogen.
On the other hand, delivering 800 gallons (or even significantly less) as a convenience drop at the end of a route is markedly different. In this case, the costs associated with getting the transport loaded and out into the market place have already been incurred and the liquid is out there. Now, there is just the incremental cost of getting to another location and offloading versus the revenue that could be generated from the drop. Playing to the profit side of this equation are the opportunity costs of not doing the delivery:

- Liquid that would just vent off if left in the transport or be partially lost transferring back in and out of plant storage;
- Slightly higher fuel costs with the load,
- The need to return to the convenience drop site sooner by not having made the drop and not having enough liquid to completely fill the tank when the delivery is made.

All things considered-convenience drops are usually good for the bulk cryogenic liquid supplier as long is the drop is within relatively close proximity of the last scheduled stop or the return route home.
But then……. Are convenience drops good for the customer?

Golf as a Metaphor for Cryogenic Liquid Delivery

Are Bulk Cryogenic Spot Drops a Good Idea?

For the sake of collective sanity- Let’s agree that business profits are good and necessary. Bulk Cryogenic Liquid Suppliers have a number of challenges to overcome in order to turn a profit at the end of the day. Others will be explored over time, but one that is universal is transportation costs. One element of this is how many miles must be traveled to deliver a given quantity of liquid oxygen, nitrogen or argon. For more effort than most of us can imagine goes into trying to optimize this ratio.

Yesterday a good friend used a golf metaphor for business that applies. To wit- He wants his business to operate like a golf ball. When you hit a golf ball out there it stays out there and doesn’t come back. All returns are costly and one of the most important things that can be done with cryogenic liquid delivery is to be sure that all of the liquid that leaves the plant stays out.

One way that delivery companies help assure they are able to do this is to identify bulk tanks that can be used as field receivers for liquid that is left in a truck at the end of its route. So, for instance, a liquid nitrogen truck drives out to a suburb area and delivers 5200 gallons of its 6000 gallon load to clients. The 800 gallons that might be returned to the plant is now a liability. To avoid returning it, the driver might go to a client that did not call in for delivery, but has a 9000 gallon tank that always has some room in it and drop off his remaining liquid He now comes back empty and converted the 800 gallons he had from a cost to revenue.

Is this a good thing?

Did the revenue generated offset the cost of an 800 gallon delivery considering that Company X typically hates to deliver under 1500 gallons?

Is this a good thing for the client that received this drop?

Manana (later).

Cryogenic Vessels- Vacuum and Moisture- Part 2

So- How do you get frozen moisture molecules out of the annular space of a cryogenic vessel?

And- If they are frozen, they are immobile right? So are they hurting anything?

The first answer is heat. The only simple way to get molecules freed up and moving so they can be sucked out of a cryogenic vessels annulus is by applying energy. The problem is that this must be done indirectly. More specifically, we cannot blow hot air into the annulus to free up the molecules as this just puts molecules in and can tear up the super-insulation in a SI vessel. This means that you have to apply heat to the inside of the inner vessel and possibly also to the outside of the outer vessel.

With super-insulated cryogenic liquid cylinders this is fairly easily accomplished by inserting a thermostatically controlled electric heat rod into the inner vessel. With larger vessels and cryogenic bulk tanks it is necessary to blow in filtered, hot air or pure nitrogen. This is pretty inefficient and can be expensive, but might be the only option. Heat lights or strips can also be used for heating the outer vessel.

In any case, it is super-critical to avoid overheating in order to stay within specifications and avoid damage or stress to the inner vessel which will weaken it. By the way- this is one more reason why there is a DOT program for inspecting and authorizing DOT-4L vessel repair facilities. And- if you want to know if your vendor is actually a DOT 4L authorized repair center just ask them for their DOT “K” number. Real prudence will prompt you to also look and make sure they are affixing their “K” tag to all serviced vessels as required.

As far as hurting anything...... next post.....

Cryogenic Vessels are Moisture Magnets

It happens almost every day that someone accidentally cracks open a vacuum evacuation valve on a cryogenic bulk tank or springs a leak in a vacuum probe tube and WHOOSH! The vessel just lost all or a lot of its vacuum.

Referring back to one of the #1 Laws of Science—“Nature Abhors a Vacuum.” Given any opportunity at all, a vacuum space will find molecules and pull them in to recover atmospheric molecule levels. And, we’ve discussed this before.

Untouched, though, is the element of just what kind of molecules get sucked in when a vacuum goes south. The millennium answer: It Depends. What it depends on is the kind of molecules that are available to suck in. In humid environments, a lot of those molecules are going to be moisture (H2O). The problem with moisture is that it is a bear to get out of a vacuum insulated vessel’s annular space once it gets in. This is particularly true if the vessel has perlite instead of super-insulation in the annulus.

Q: Once molecules get into an annular space, how do you restore the vacuum?

A: The only way to restore really strong vacuum in the annulus of a cryogenic vessel is with a really good vacuum pump.

Q: And what is the central dilemma?

A: The stronger the vacuum the less energy there is in the vacuum space. Since energy equals heat that means that the moisture in the annular space will freeze out making it almost impossible to withdraw all the molecules.

We’ll discuss this, the effects and cures some more next post………