Venting Cryogenic Vessels- Is Product Lost?

“OK- So you recommend venting cryogenic liquid cylinders and portable vessels down “fully” after filling. But- Doesn’t this waste product?”

Answer: Define “product.”

If the “product” you are referencing is liquid oxygen, nitrogen or argon then No. Venting off gas created during the fill process will actually help preserve liquid in your 4L DOT Liquid Cylinder, Medical Liquid Oxygen delivery vessel or other portable container. The fact is that the gas in the vessel that you would be venting off has already been lost and is no longer liquid product.

Remember- Pressure building tends to be an accelerating curve the higher the pressure in you vessel the shorter the time before you start venting. If you vent down all the way back to right at zero then you stabilize the liquid in your tank moving you back away from the venting time point. You also remove gas which is by definition hotter than liquid. Getting this gas out also helps remove energy that raises the temperature of your liquid product. As a last point- A filled vessel with pressure on it also has a potential to push liquid into the venting system which will boil off and create pressure.

If the “product” you are referencing is gaseous oxygen, nitrogen or argon then you might be wasting a little product. BUT- this is only true if you plan to immediately use the vessel and you will be able to harvest the oxygen, nitrogen or argon gas right away for cutting, purging or some other use. If you are not using it right away then leaving gas pressure in the vessel will just lead to faster venting and the product will be lost anyhow. You will also lose additional gas created by liquid boiling off sooner than it needed to.

Overfilling Portable Cryogenic Liquid Vessels

There are several negative consequences that can result from overfilling portable cryogenic liquid vessels used for transport or delivery of liquid oxygen, nitrogen or argon. While there are also concerns with 4L-DOT liquid cylinders and stationary bulk tanks this article is more focused on horizontal vessels from approximately 119 gallons up that are used for medical oxygen delivery, construction and demolition or other portable applications.

If you fill portable horizontal cryogenic tanks do you always stay in close attendance while filling?Do you use the Full Trycock as your indicator for when to stop? Or do you have a “full” mark on your liquid level indicator to tell you when to stop filling that is well below the point that liquid would come out the vent?

If you answered “yes” to these questions then you are less likely to have some of the problems that are common with overfilling.

First, let’s note that most portable horizontal cryogenic liquid vessels are fitting with a Full Trycock. This line and the vent are to be fully open when transfilling liquid into the portable vessel. The trycock and vent lines run into the top of the vessel. The Trycock line is lower inside the vessel than the vent so that liquid will come out the trycock before reaching the vent. Shutting off the fill as soon as liquid starts to come out the trycock reduces the incidence of liquid getting all the way up to the fill line.

It is not uncommon to hear of situations where the trycock was not used or the liquid source was not shut off quickly enough. Liquid was then allowed to flow out the vent. This can result in:

a) Liquid running up through a roof or side vent and ruining paint or cracking the metal or windshield of a vehicle in hot weather.
b) Uncontrolled liquid oxygen flow running to areas which will be saturated increasing fire risks;
c) Liquid being trapped in the vent line when the fill line and vent line are closed with inadequate head space left in the vessel.

Incident “C” is a common reason for vessels building excessive pressure. Liquid trapped in the line will rapidly vaporize creating pressure in the vessel and causing it to vent excessively. It can also result in blown head safety rupture disks. Users often mistake this overfilling for a vacuum related problem as the net result (over pressuring) is very similar.

A lot of discussion occurs in this post about the importance of fully venting cryogenic liquid vessels when they are filled. This means that after shutting off the liquid fill source you need to vent until there is no evident positive air flow from the vent line prior to closing it. There are few more important things that can be done to help prevent premature and excessive venting of cryogenic liquid vessels. Venting will also help correct some of the ills of over filling and hot filling.

But- Is product wasted when we vent fully?

Quality- What is it?

“Trifles make perfection and perfection is no trifle.”
- Michelangelo

Last week provided an opportunity for one of the more pleasant dinner evenings ever. Besides the general joy of being with a large group of gas sales and marketing executives there was also a terrific discussion on the mid-90’s Austin/ Dallas/ San Antonio semiconductor surge. For those less familiar, this was the when the electronics Third Coast was flourishing with the construction of many new fabs for companies like AMD, MOTOROLA, TI, SEMATECH, HITACHI, ST MICROELECTRONICS, and NATIONAL SEMICONDUCTOR. Of equal import for many of us was the great influx of suppliers to the industry such as Applied Materials and Tokyo Electron not to mention the dozens of specialty mechanical and cleanroom contractors, chemical suppliers, and analytical services.

An almost cliché, but largely unmet maxim is: “People don’t buy because they understand. They buy because they feel understood.” This is largely unmet mostly because developing a true understanding of client needs and then actually meeting or exceeding them can be hard work.

Inherent to success in the electronics market rush was the ability to meet the stringent quality demands of the industry. Those of us who prospered found opportunities to go beyond keeping up and actually become drivers of the standards expected for service providers. First, though, we had to come to grips with what was actually needed and what could be done. For many of us, the leap from our normal life to Semiconductor-Grade was a jolt.

Dan Driessen formed Ultrapure & Industrial Services as a division of Driessen Culligan to develop a team focused on the needs and opportunities inherent in the growing markets for semiconductor, electronics and medical device manufacturing. As a novel first step in truly understanding client needs the Ultrapure group set up a series of roundtable discussions with some of the area companies most recognized for formal quality. These included 3-M, Motorola, Sematech, AMD, Huntsman Chemical and The UT Microelectronics Research Center. Over the course of three separate monthly round table events these clients shared their definitions of quality, their perceptions of the industry’s specific needs and the gaps Ultrapure would need to close to become a recognized player.

What these clients had to say definitely upset our homeostasis and created huge doubts regarding both the ability to perform as needed and the ultimate profit to be gained in consideration of costs. Yet, the decision was made in short order to move forward. The next several years proved to be both challenging and rewarding beyond expectations. And a lot of lessons were learned that will benefit us and those we have the opportunity to serve for the rest of our lives.

Last week’s dinner brought up a lot of great memories. It especially helped with connecting the dots to the needs of clients using cryogenic vessels and their service support needs. CryoNews is intended to help meet those needs for both suppliers and users. The underlying themes of all articles in this post are Quality and Best Practices as they relate to meeting Producer, Distributer and User needs and eliminating waste. Your suggestions related to topics that will advance this mission are always appreciated.

Spec Gas- The Eyes Have It

The gases business has an incredibly broad reach into industry and medicine. Visits to gas producer web sites can be real eye openers. A little blip viewed yesterday referenced perfluoropropane. Do you already know all about it and what it is used for?

Would you believe it is used as a tool in pneumatic retinopexy?

This is a surgery used for repair of some retinal detachments. According to http://www.webmd.com/, “It uses a bubble of gas to push the retina against the wall of the eye, allowing fluid to be pumped out from beneath the retina. It is usually an outpatient procedure done with local anesthesia.” Full healing and recovery takes weeks, but the gas is eventually absorbed into the body.

And why do we care?

Mostly, we care from a big picture stand point. Specifically, most of us are widely unaware of the amount of specialty gas products produced by U.S. manufacturers and shipped all over the world. Take a peek sometime and you’ll find an amazing volume of specifications for these products related to purity and product consistency. This applies to both Healthcare and Industrial applications where tiny variables can make a huge difference. Personally, I’m thankful for the chemists, scientists, engineers and techs who work quietly everyday in relative obscurity doing things that make our lives better.

Cryogenic Liquid Cylinder Operation- Part 3- Pressure Building System

The April 16th article http://cryonews.blogspot.com/2009/04/cryogenic-liquid-cylinder-operation_16.html discussed why it is necessary to have a gas use coil that pulls liquid from the bottom of a 4L DOT Cryogenic Liquid Cylinder and vaporize it for gas use supply.

And- It was noted that a pressure building system is needed to help maintain pressure within a specified operating range. So, how does the pressure building system work?

Liquid cylinders equipped with gas use coils also have another set of coils brazed to the inside skin of the outer jacket. Just like the gas use coils- the pressure building coils pull liquid from the bottom of the liquid cylinder and vaporize it. Vaporization happens by exposing the liquid to temperatures above its boiling point.

Unlike the Gas Use system, the Pressure Building system returns vaporized gas back to the head space of the liquid cylinders. Crowding gas molecules into this head space raises the pressure of the liquid cylinder. The pressure building regulator controls the flow of liquid into the coils and stops the flow of gas back into the head space.

It is important to note, though, that this does not stop pressure from building. Once pressure is raised on a liquid cylinder and warmer air is introduced into the top of it normal evaporation of liquid nitrogen, argon or oxygen will increase. Be sure that the valves feeding pressure building regulators are closed if the vessel is not going to be used over night or over a several day period. Venting of pressure and release of hotter air can also help minimize product lost due to normal evaporation.

Cryogenic Liquid Cylinder Operation- Part 2D- Gas Withdrawal

Is there a limit to how much liquid can be vaporized by the internal gas use coil in a standard 160liter to 200liter 4L DOT cryogenic liquid cylinder?

Yes. And that limit ranges between 300-400 standard cubic feet per hour (SCFH) depending on the make/model of the liquid cylinder, the gas in question, the external humidity and outside temperature. Direct exposure to the sun will also increase gas flow. This rating is for an 8 hour period and it diminishes rapidly with excessive flow or time.

Why does this matter?

The failure to use this information is one of the biggest contributors to dissatisfaction with liquid cylinder operations. Users who start with high pressure cylinders and “move up” to liquid cylinders are not accustomed to limits on how much gas they can draw. This is particularly true for operators of large tip cutting torches and oxygen lances. Drawn by the relative convenience and weight of liquid cylinders they do not realize that their operation requiring 700 to 1500 scfh gas flow simply cannot be supported by one or two liquid cylinders linked together with only internal vaporizing coils.

What happens if flow is too high?

Very simply- if the liquid is not vaporized then it comes out as liquid. This can be disastrous for many operations unable to withstand extremely cold cryogenic liquid temperatures. For instance, a carbon steel tank that is being purged with nitrogen gas can easily split when sprayed directly with liquid nitrogen.

Is there a way to get more gas flow from a cryogenic liquid cylinder?

Yes, absolutely. All you have to do is add downstream ambient vaporization. Remember, though, not all vaporization is the same. Some vaporization packages for liquid cylinders (ex- Dragons) take into account connecting vent line connections to balance pressure and withdrawal rates. A vaporizer added straight on to the gas use line will be more limited than one tied to the liquid line. The liquid line connection is larger internally and has less flow restriction allowing for more gross flow of molecules. Withdrawing product from the liquid connection also prevents chilling of the outer vessel skin enhancing the ability of the pressure building coil to produce gas that replenishes the head space.

Pressure building coil?

More next time…….

Homecare Medical Oxygen- Fuzzy Math

The Medicare Benefits related to oxygen were changed three years ago limiting collection of rental for respiratory oxygen equipment to 36 months for individual patients. There are a number of articles and talks discussing the direct impact of this new guideline on oxygen providers. And there is some fuzzy math being used. As mentioned by an old boss, figures lie……

It is purported by the Office of the Inspector General that 22% of Medicare patients receive care for over 36 months. Does this mean then that 22% of oxygen rentals just went away at the36 month point?

This is not likely.

Medicare does not account for 100% of rental billing. Even if it did, the only way January could account for 22% instant reduction would be for providers to have set up no new accounts since January 2006. Not.........

Yes. The Medicare guidelines are most certainly having a big impact on oxygen providers. But- there are homecare oxygen companies who continue to grow and very many who do not have business plans built around reliance on Medicare.

Cryogenic Liquid Cylinder Operation- Part 2C- Gas Withdrawal

A typical customer service call:

“Hello. This is ___________ and there is frost on the outside of our cryogenic liquid oxygen cylinder. Is this a problem?”

How do we determine without a game of 21 questions?

Q: Is the ice or frost all over the vessel or is it more localized?

A: Oh no. It is just at the bottom and some up on the pipes?

Q: Does the unit frost up all the time or only when in use?

A: Only when in use. And, the more gas used the worse it gets.

Conclusion- In all likelihood this is normal…. Especially in humid environments like the Gulf Coast and Northwest U.S. .

Remember. If the pressure of the liquid cylinder contents is below the economizer set point then Gas used is originating from the bottom of the liquid cylinder as liquid. This liquid is being exposed to outside heat via a set of coils brazed to the outside wall of the vessel. Exposing it to heat raises the temperature of the liquid above the boiling point to make it gas.

This liquid is coming out of the bottom of the liquid cylinder at something around -300 degrees Fahrenheit. It is only normal for outside cold are to frost up on the outside of a container that is exchanging energy at this level. Also, It is completely reasonable that the more gas you use the more frost will grow.

Is there a limit to how much liquid can be vaporized by the gas use coil?

Manana….

Cryogenic Liquid Cylinder Operation- Part 2B- Gas Withdrawal

The last two posts covered 4L DOT Liquid Cylinder Liquid Withdrawal and Vent operation. Summarizing- When the liquid valve is opened, liquid is drawn up from the bottom of the vessel through a dip tube. When the vent valve is opened gas is the top head space of the vessel is released. And- it was determined that the gas use valve cannot withdraw gas in the same manner as the vent or pressure would rapidly drop below needed levels.

“If gas doesn’t just pull right off the top, then where does it come from?”

Would you believe “It depends?”

And, in this case, what does it depend on?

It depends on the pressure of the contents compared to the setting for the pressure builder and economizer regulators which will be discussed next time. For now- the economizer setting will typically be about 15psi higher than the pressure builder setting. So, if your pressure builder regulator is set at 125 psi the economizer regulator actuate at about 140 psi.

When the contents pressure is below the economizer set point then the gas coming out of the cryogenic liquid cylinder actually originates from the bottom of the liquid cylinder. It comes out of the inner vessel through a small copper tube that is brazed to and circles around the inside of the outer vessel. Confusing? Not too. The key here is that, up to the point of needing gas, the purpose of the cryogenic liquid cylinder has been to protect the liquid oxygen, nitrogen or argon from outside energy (heat) in order to keep it in liquid form.

When gas is needed, all we have to do is expose the liquid to energy (via direct exposure to the outside wall of the vessel) to bring it above its boiling point. Now we have gas. By withdrawing liquid from the bottom and converting it to gas the head pressure of the liquid cylinder contents can be maintained far more easily. This provides more consistent output and allows the supported process to run normally.

The other aids to maintaining pressure within a usable range are the pressure building (PB) regulator, PB coil and economizer regulator. Stay tuned for more…….

Cryogenic Liquid Cylinder Operation- Part 2A- Gas Withdrawal

Open the Gas Use valve on a filled cryogenic liquid cylinder and gas will come out. Interestingly enough, the same thing happens if you open the Vent valve. So……. Does the gas coming out of each valve come from the same place inside the vessel?

If you answered “yes” or “no” you are sometimes right. As they say, though, even a broken clock has the right time twice each day. It is more accurate to say “It depends.” To understand the “why” of this, it is helpful to first look at the Vent connection.

We previously affirmed that liquid withdrawn from a 4L-DOT liquid cylinder comes from the bottom of the vessel. This is easy since the liquid is heavier. Gas that is formed by the normal evaporation of liquid nitrogen, argon or oxygen collects in the top of the liquid cylinder inner vessel “head space”. As more and more molecules crowd into the head space the inner vessel contents pressure increases. This pressure is useful to push liquid back out through the dip tube when the liquid valve is opened.

When the Vent valve is opened, free gas molecules are allowed to escape and the pressure goes down. If the Vent valve and Gas Use valves were both connected to the head space only then there would be no reason for both. There would also be a big operational problem.

Let’s consider a case where Mike the Scrapper is using oxygen from a cryogenic liquid cylinder to feed a cutting torch or oxygen lance. If he were using a simple LS (Liquid Service Only) vessel then his work cycle would look very much like the profile of a wedge or the topography of the White Cliffs of Dover. He’d have a long wait while pressure built in the liquid cylinder. Then, once the pressure was high enough he’d fire up his torch and start pulling off gas. The pressure would drop precipitously until it got too low and the torch would go out. Mike would then have to wait a long time again for pressure to build and then fire up for a little while, lose pressure and go out. This would be unlikely to occur more than a couple of times before Mike would give up on the liquid cylinder and go back to high pressure cylinders that would give him a constant supply of gas until empty.

If gas doesn’t just pull right off the top, then where does it come from?

Cryogenic Liquid Cylinder Operation- Part 1- Liquid Withdrawal

Is pressure good or bad in terms of cryogenic liquid cylinders?

A) Good
B) Bad
C) It Depends? (The Millennium Answer)

Right- It depends. Pressure is necessary for liquid and gas withdrawal from a liquid cylinder. Too much pressure, though, can cause excessive venting or even severe damage if safety devices are absent or malfunction.

As a first simple step it is important to understand that when liquid is filled into a cryogenic liquid cylinder it goes in through a dip tube that goes down to near the bottom of the liquid cylinder. When liquid is withdrawn it comes back up from the bottom of the tank through the same dip tube. Since the valves and connections for a liquid cylinder are at the top it is necessary to have some head pressure on the liquid in order to push liquid oxygen, nitrogen or argon back up and out the liquid valve.

The simplest liquid cylinders then contain only two valves (Liquid and Vent) and are often referred to as “LS” vessels. They are used for liquid service only and typically fitted with 22psi reliefs to avoid spraying liquid out with too much force when withdrawn. They really on normal product evaporation to create head pressure for liquid withdrawal.

This is all pretty simple, right?. Next we’ll discuss “VGLs” and Gas Withdrawal.

Nitrogen Pipeline Purge and Testing

A few client calls have been received recently related to the possible combined use of high pressure nitrogen tube bundles and liquefied cryogenic nitrogen as sources for higher volume testing requirements. The most specific application related to the use of high pressure nitrogen gas for high pressure purging and testing of pipelines. Nitrogen offers significant advantages for this application over water or air by providing an inert atmosphere with a very low dew point. Ie- It is dry, clean and reduces oxidation. As with any compressible medium, nitrogen has significant risks associated with it in the event of a pipe or component failure. It is therefore very important that high pressure gases only be used for testing by professionals with carefully reviewed and administered operational safety procedures.

The discussions related to testing or purging with pipeline pressure requirements at 1200-1800 psi. Total volume requirements ranged from 50,000 to 150,000 cubic feet of nitrogen. A conventional simple approach for these applications is to roll up with a high pressure (~3000psi) tube trailer and directly transfer nitrogen from the trailer to the pipeline. There is inefficiency here as only a certain percentage of the gas in the tubes will be available for the pipeline at the pressure needed. Cascading of the tubes increases the consumable gas from the tubes, but still leaves a significant residual.

Getting pressure up is directly related to the volume of molecules you can get into a space. One thought on pressurizing pipes is to use portable liquid nitrogen vessel (ex- 850 gallon GOrilla) to provide the initial low pressure mass of molecules into the pipe. Once the pipe is up from atmospheric pressure to the level that can be reached from the liquid source then tubes can be used to get to the required test pressure. The advantage of this approach is that each vessel source is used for what it actually does best. That is- Volume from the Liquid Vessel and Pressure from the Tubes.

Alternatively, for a project requiring multiple tube/liquid sources for a series of tests some added efficiency could possibly be achieved by adding a high pressure pumping system to the liquid source. The tubes could be used to provide fast flow and pressure to the pipeline. The portable liquid nitrogen vessel could be used to replenish the tubes between tests. It could also be used to discharge an initial volume of molecules into the pipeline. This approach could be advantageous in cases where it is more convenient and cost effective to deliver liquid to a site for refilling the portable cryogenic vessel than it is to swap out or refill tubes. There is also a good possibility that an 850 gallon GOrilla-type vessel with trailer, vaporizer pumping system and power source is less expensive that a tube trailer with comparable volume. Use of less expensive portable cryogenic vessels in place of high pressure tubes (where feasible) could allow more valuable assets to be used where they are required.

There is also the side benefit that the portable cryogenic vessel could be used for many other applications when not tied to high pressure testing and purging. The ability to keep capital invested in cryogenic vessels working for pipeline freezing, liquid delivery, welding purge, harvest, or high volume cutting applications warrants consideration. This is particularly true in the event that the highly versatile liquid and gas use vessel requires less capital investment, less ongoing testing/ recertification cost and provides more positive financial return than a set of tubes holding a comparable volume of nitrogen.

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…….

Cryogenics Industry- Great People; Opportunity

It is just really hard to imagine an industry with nicer people overall than those in the cryogenic and atmospheric gases business. From major producers to distributors and dealers there is an almost universal kindness, generosity, helpfulness and humility. And this is said for a lot of reasons, but mostly on the basis of how pleasant it is to visit, lunch, work with and serve people you enjoy. Maybe this pervasive goodness is a result of so many multi-generational families in the industry. Maybe the pervasive goodness is the cause of so many multi-generational families and so many people that have been in the business their full careers.

Either way, it is an industry that should be strongly considered by people looking for a career with good people, plenty of challenges and an unlimited supply of opportunities for innovation and learning. From building ships to harvesting grapes; from hospitals and universities to aerospace research the cryogenic gases industry is constantly developing new applications to help industry, healthcare, research and more. Consequently, projections for the industry tend to forecast continued (albeit controlled) growth for the foreseeable future.

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.