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Knowledge>Medical Gases and Vacuum>Cryogenic Liquid Manifolds

Cryogenic Liquid Manifolds

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Figure CLM-1 Schematic of a Cryogenic Liquid Tank.

Figure CLM-2   Components of a Cryogenic Liquid Tank:

(1)   Gas Use Valve ˇV used for gas withdrawal

(2)   Fill / Liquid Valve ˇV used for liquid filling or withdrawal operations

(3)   Pressure Control Valve ˇV used to isolate (on/off) the pressure control
       regulator

(4)   Vent Valve ˇV used to vent pressure

(5)   Pressure Control Manifold ˇV used to automatically maintain pressure

(6)   Pressure Gauge ˇV for indicating cylinder pressure.

(7)   Combination RegulatorˇV used to automatically maintain pressure.

(8)   Pressure Relief Valve ˇV used to limit pressure in the liquid cylinders

(9)   Liquid Level Gauge ˇV used to check the liquid content


MEDICAL USE OF CRYOGENIC LIQUIDS 

When oxygen (normal boiling point, nbp = -182O C), nitrogen (nbp = -196O C), and nitrous oxide (nbp = -88.5OC) are cooled to below their boiling points, they will condense to become liquid. If the original gas temperature is 69O F (15O C), the reduction ratios in volume will be 1 to 854, 690, and 662, respectivelyˇXopposed to a ratio of 1 to 151 if they are stored in pressurized cylinder at 2200 psig (151 atm).    

In fact, cylinder gas is typically the liquefied gas allowed to warm back into vapor state and packaged in cylinders.

 Furthermore, since cryogenic liquids can be stored at much lower pressures, the containers do not need to be as strong and can thus be made in much larger capacities. A typical cylinder used on a medical system is capable of storing about 6,700 liters. By comparison, cryogenic containers that can supply from 114,000 liters to millions of liters are available. 

Some advantages of cryogenic containers are: 

1.    Very larger supply capacity,

2.    Liquids are stored at lower pressure, and

3.    Less frequent container changing results in reducing labor cost and improving operational safety. 

The drawbacks are:

1.    The container design and its operations are much more sophisticated and complex than gas cylinders, and

2.    Special care must be given to handling the extremely low temperature tanks. 

CRYOGENIC LIQUID STORAGE TANK 

The heart of a cryogenic liquid system is its storage tank(s).  They come in different sizesˇXfrom small portable ones to permanent large bulk installations. A typical portable tank, for example, measures about 60 inches in height and 20 inches in diameter.  

While the basic operating principles are similar, their designs can be very different.  Key components of typical cryogenic liquid storage tanks are discussed below. Note that, because of the internal design of the containers, portable tanks should be kept in up-right position when they are being transported or changed.   

Double-walled Vessel 

Cryogenic liquid storage tanks are specifically designed for two functions: 

ˇP      To safely insulate the cryogenic liquid and ensure the user is well protected from the extreme cold while handling the tanks, and

ˇP      To allow the user to withdraw the liquid either as liquid or as gas for use. 

If a cryogenic liquid is exposed to temperatures higher than its boiling point, it will rapidly and even explosively convert into gas. Hence, preventing (minimizing) ˇ§heat leakageˇ¨ is critical to the effective use of any cryogenic container.

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KSY2007 Copyright 2006-8 by Gentec Systems Co., All Rights Reserved.

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Figure CLM-3 Cut-away view of a cryogenic liquid storage tank

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Figure CLM-4 Pressure Relief Valve

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To achieve this, cryogenic liquid tanks are typically made of two layers of insulated wall with the space in between evacuated to a very deep vacuum. 

Relief Valve and Rupture Disk 

However, as no insulation can be perfect, liquid will still evaporate continuously to gas due to the energy leak, no matter how small it is. The vaporization rate is called ˇ§Natural Evaporation Rateˇ¨ (NER). A relief valve that will open and close at its preset pressure is needed to vent off NER excess gas. Hence, hearing a slight hiss from a liquid cylinder is usually the normal operation of its pressure relief device. Should the relief valve fail or not be able to handle the volume, a burst disk will blow out and vent the pressure to prevent explosion. 

PRESSURE RELIEF VALVE DEVICES 

Pressure relief devices for vacuum-insulated cylinders are prescribed based on the formula:

  Maximum Pressure Relief Device Rating = (Cylinder service pressure X 1.25) ˇV 15psi.   For example, Example (200x1.25) ˇV 15psi = 235psi

Hence,

4L 292 = 350 safety 4L 200 = 235 safety

 

Liquid cylinders should always be stored and used in areas with appropriate natural or mechanical ventilation. Furthermore, field operators should never adjust, block, plug or attempt to repair anything on a liquid cylinder. 

Pressure Builder 

On the other hand, at high supply demands, NER alone often is not capable of providing enough vapor to sustain the tank pressure as the liquid level drops. When this happens, a controlled small amount of liquid is introduced into an internal or external pressure builder vaporizer. The liquid is turned into gas and delivered back into the top vapor space of the tank. The result is a rise in pressure inside the tank.

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Figure CLM-5 Operation of an Internal Pressure Builder: A controlled amount of liquid is withdrawn and vaporized to increase the gas in the headspace

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Figure CLM-6 Operation of an External Vaporizer: Liquid is withdrawn from (1) inside the tank, evaporates and becomes warmer in (2) and (3), and gas is released for use at (4).

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Figure CLM-7 Operation of an Internal Vaporizer: Liquid is withdrawn from the bottom of the tank, vaporized in the vaporizer, and released for use.


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KSY2007 Copyright 2006-8 by Gentec Systems Co., All Rights Reserved.

Figure CLM-8 Schematic of Vaporizer with a Ceramic Heating Chamber

Figure CLM-9   Operation of an Economizer: Gas in the headspace is withdrawn for use first when the tank has sufficient pressure.
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Figure CLM-10 A More Sophisticated Ultrasonic Liquid Level Sensor


Vaporizers 

The extremely cold liquid in the tank must be vaporized and warmed up before it can be used. Internal or external vaporizers with much larger capacities than those of pressure builders are used to withdraw liquid and covert it to gas. External vaporizers are typically used with large containers and installations demanding large outputs. Bulk gas installations are of this type. (Note that, as described in the previous section, some liquid has to be vaporized to provide the necessary volume and pressure of the headspace above the liquid level inside the tank to push out more liquid.)  

Vaporizers come in many different designs, including atmospheric, powered (forced-air, steam, and electric), waste-heat, and hybrid. The selection of vaporizers should be based on demand, intermittent or continuous usage, energy costs, and temperature zones. 

Poorly ventilated sites or undersized heat exchangers can cause ice to form on vaporizers during the conversion process. Excessive ice formations can clog and damage the vaporizer. Also, ice could allow extremely cold gas or the cryogenic liquid to enter the piped system; damage the valves, alarms, and medical components; and even injure patients. 

Automatic Controls 

Automatic controls furnished with the tanks regulate the flow of liquid through the vaporizers. When there is a demand for the gas, the supply system draws liquid from the bottom of the cryogenic storage tank and passes it through the vaporizers. At last the gas moves through a final line regulator. Thus, a constant supply of gas at a regulated pressure is provided. 

Economizer 

An economizer can be used to reduce some of the NER gas loss. When the operating pressure is above the economizer setting, a regulator will be opened so that the gas that is being supplied to the application will be pulled out of the vapor space in the top of the tank first. When the operating pressure is reduced to the economizer setting, the regulator will close so that the gas will now be supplied from the liquids through the vaporizer. 

Content Gauge 

The container contents gauge is mostly a float-type liquid level sensor that indicates the level of the liquid.  

The float in liquid cylinders is typically an aluminum rod of a specific length, weight and density. As the liquid level changes inside the tank, the float rod allows the spring loaded gauge to rise or fall proportionately.  

The content gauge is an indication of approximate container content, and should not be used for judging the weight of the container. (Containers are always filled by weight.)ˇ@

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KSY2007 Copyright 2006-8 by Gentec Systems Co., All Rights Reserved.

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Reserve 

Typical NER is in the range of 1.5% per day for a new portable tank in good condition to 3.0 % or more per day for older ones. The average is about 2.0-2.5% per day under ideal conditions. NER for a large well-maintained bulk is about 0.6% per day.   

This should be taken into account seriously in the inventory control system. For example, NER will cause a tank to lose its 40 to 60% capacity in thirty days even if it is not used at all. 

One safeguard NFPA 99 mandates is the provision of gas cylinders enough for 24 hour supply and called the Emergency Reserve.

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SYSTEM CONFIGURATION 

Both Liquid (Primary) - Liquid (Secondary) - Gas (Reserve) and Liquid (Primary) - Gas (secondary) - Gas (Reserve) configurations as shown in Figure CLM-9 are permitted by NFPA. However, careful evaluation of the actual demand profile and detailed operating cost analysis are needed to select an optimal system configuration. 

In fact, cryogenic liquid systems are not cost-effective or suitable for many hospitals and clinic centers.

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(A)  Liquid (Primary)-Gas (Secondary)-Gas (Reserve) Configuration:

(1) Primary header with cryogenic liquid tank(s)

(2) Secondary header with high pressure cylinders

(3) Automatic changeover control box

(4) High pressure cylinders as the reserve

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(B)   Liquid (Primary)-Liquid (Secondary)-Gas (Reserve) Configuration:

(1) Primary header with cryogenic liquid tank(s)

(2) Secondary header with cryogenic liquid tank(s)

(3) Automatic changeover control box

(4) High pressure cylinders as the reserve 

Figure CLM-11  Schematics of two Possible NFPA 99 (2005) Compliant Cryogenic Manifold Systems

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KSY2007 Copyright 2006-8 by Gentec Systems Co., All Rights Reserved.

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ˇ§TO BE OR NOT TO BEˇ¨ ISSUE 

To decide if a cryogenic system is a right choice for a particular hospital, one has to at least understand the following factors:   

1.    Due to unavoidable energy leak, the liquid inside a cryogenic liquid storage tank will evaporate continuously.  The resulting pressure built-up has to be dealt with by releasing some gas from the tank.  Unless it can be used before being discharged by the pressure relief valve, it will be wasted.  Hence, a facility which does not use each day at least the NER for the number of containers installed should be considered unsuitable for liquid.

2.    NER is a function of ambient temperature. Heat and solar radiation will drive up NER.

3.    Sizing of the liquid system will have profound effect on the systemˇ¦s cost effectiveness.

4.    Demand surge beyond design specification as well as poor ventilation may cause ice to form in a cryogenic liquid system and create a serious safety problem. Even ice formed on the tank surface could become insulator and hinder the liquid evaporation operation.

5.    On the other hand, high pressure gas cylinders need to be changed when its pressure drops to below a threshold.  Otherwise, there will not be sufficient pressure head to distribute the gas to the sites of use. Hence, cylinders are usually sent back still partially full.

6.    Gas cylinder systems require more frequent cylinder changes due to their much smaller capacities.

7.    Environmental Impact: Continuous release of the gas to its surroundings can be a serious safety concern. NFPA guideline on the issue, however, could be a limiting factor in optimal system selection. 

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ALARM REQUIREMENTS 

    Per NFPA, the following alarms are required for all master alarm panels:

ˇP      Changeover, which indicates the primary header is empty and the secondary header is in service.

ˇP      Reserve in Use, which indicates the Primary and Secondary headers are empty and that the Reserve header is now supplying the system.

ˇP      Reserve Low, which indicates the Reserve Header is below one average days supply.

ˇP      System Pressure High, indicating system pressure is 20% or more above normal.

ˇP      System Pressure Low, indicating system pressure is 20% or more below normal.

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Last UpdatedˇG 2008-12-18