For over 25 years, we have been asked many questions about vacuum jacketed pipe (VJP). The following article answers the most frequently asked questions about VJP and cryogenic equipment.
What is the different between foam insulated pipe and vacuum jacketed pipe (VJP)?
Foam insulated pipe is usually constructed using a copper pipe and foam insulation covered by a protective polyvinyl chloride layer. The foam insulation is inexpensive and works well when new. However, it degrades substantially over time making the foam ineffective and the pipe inefficient. The reason is that during manufacture, the foam typically bonds to the inner pipe. As the foam becomes cold during cool down, it shrinks, forming small cracks and pulling away from the pipe. This situation allows water vapor to fill the void and increases the insulation’s thermal conductivity causing attrition in efficiency. Over time the expansion and contraction cycle causes the foam to crumble. After about five years, the foam has deteriorated to the point where it loses all effectiveness.
Vacuum jacketed pipe is a conduit in conduit fabrication with a vacuum annual space. The inner pipe transfers the liquid cryogen at temperatures of -3201 and below, while the outer pipe remains at ambient temperature, safe to touch. VJP is designed to reduce all three forms of heat transfer, convection, conduction and radiation, into the liquid cryogen as it travels from the tank to the end use point. VJP uses multi-layer insulation to deter radiation and convention transfer. Due to the low heat leak, VJP is highly efficient about 40 times better than foam insulated pipe. In many applications, VJP has a product life of greater than 20 years.
Rigid versus Bendable vacuum jacketed pipe, which is better?
Both types of pipe have advantages; and many projects require a combination to enable installation and delivery of the cryogen to the end use point. They may be interconnected using close fit bayonets which reduce the heat leak into the system and allow for future reconfiguration if required. Vacuum jacketed pipe may be used with all cryogens in industries such as aerospace, semiconductor, pharmaceutical, food and beverage packaging, biological research and storage, rubber deflashing as well as original equipment manufacturers applications. ASME B31.3 Certification is available from many manufacturers.
Rigid pipe is available in tube and pipe options. Vacuum jacketed tube is applied when low flow and quick cool down are required. Vacuum jacketed pipe (VJP) is used for larger flow applications beginning at ½” IPS up to 10” IPS diameter in some system designs. The advantages of rigid VJP include the lowest heat leak, highest efficiency, longest life cycle and easiest maintenance.
Bendable pipe is manufactured in sizes ¼” through 2” ID. These systems simplify the VJP layout and reduce the need for precise system measurements. The pipe is easier to layout and install than rigid, less costly to buy and lighter, which reduces the packaging and shipping expenses.
Are there different types of bayonets?
In general, the industry supports three types of bayonets. The Linde style or tip seal bayonet has become an industry standard for use in liquid helium transfer especially when filling MRI machines. The other two styles are prevalent in the industry, but are usually proprietary to the manufacturer. These are the close fit and nose seal bayonets. The most efficient bayonet design is the close fit type because it relies on small, precise tolerances between the male and female to prevent a leak path. This design offers the lowest heat leak meaning the least amount of vapor generation. Because each manufacturer has a proprietary designed bayonet pair, competitors’ bayonets do not normally connect to each other.
Why does vacuum jacketed pipe take so long to make?
Vacuum jacketed pipe (VJP) is complex to manufacture and special fabrication procedures are required to insure a reliable and high quality product. The VJP design is pipe-in-pipe with a vacuum in between. To begin with, the incoming materials are inspected for adherence to quality specifications. Then, two sets of pipe are fabricated for each line, an inner and outer. The inner and outer pipes are cleaned to remove contaminants, which helps with creating vacuum integrity and preventing long term off-gassing in the annual space where the vacuum is maintained. Both must be welded and each are leaked checked. Before assembling the two pipes, the inner is wrapped with multilayer insulation. The next step is to assemble pipes to create the spool. The spools are leaked check and the vacuum process begins. The vacuum process includes heating the spool and pumping the annular space. This process alone may take up to several days. To insure the spools maintain vacuum integrity, they are allowed to sit prior to final inspection. The spools are then flowed with liquid nitrogen and inspected to rigid quality standards.
Because not all manufacturers have the same quality standards, we recommend asking questions about your supplier’s fabrication processes, quality procedures and inspections.
What is the difference between a gas vent, cryovent, vapor vent, keep full, and keep cold?
The answer is simple. They are different names for the same device. All provide the same function. The purpose of a gas vent is to release gas boil off generated in your cryogenic delivery system as the system sits idle while maintaining liquid in the system to satisfy immediate equipment demands.
Where are SRV’s needed?
A safety relief valve (SRV) is required anywhere in a liquid cryogen delivery system where liquid has the potential to be trapped. The reason for relief valves is clear, liquid cryogens expand at a very high rate. For example, liquid nitrogen expands at a rate of 700 to 11. If a safety relief valve is not used where liquid cryogen is trapped, it will expand and rupture its container.
To further illustrate this point, standard practice includes a safety relief valve between any two shut off valves located in a VJP system. If both valves are closed, the normal heat leak into the pipe will cause the liquid to vaporize and raise the line pressure until the pipe ruptures. Including the required safety relief valve in the line prevents damage to the spool, vents the excess gas to the atmosphere, and maintains a safe system pressure.
Note: The cracking pressure of the relief valves must not exceed the rated Maximum Allowable Working Pressure (MAWP) of the pipe. Operating the system at a pressure greater than the MAWP is dangerous and with most manufacturers will void the equipment warranty. Standard practice dictates that at least one relief valve is required in a transfer system and that it must be set at or below the MAWP.
Where is the serial number?
Serial number placement varies with the manufacture. Technifab Cryogenic’s serial numbers are located on the nameplate under the evacuation port. The number usually begins with a “T” such as T011887.
What is the metal cap on each section of pipe?
The metal cap is a cover for an evacuation port. The port is sealed with a vacuum plug. The evacuation port should never be tampered with and only accessed by a trained technician.
What is the price per foot?
Due to the complexity of fabricating a vacuum jacketed pipe system, cryogenic delivery systems are usually priced per the project, not per foot. The price of a foot of VJP may vary significantly depending on the pipe type and number of components. To illustrate, a single 100 foot run of rigid pipe delivering liquid cryogen from a bulk tank to a machine will be a certain price per foot. However, a 100 foot rigid run that has several drops delivering cryogen to multiple machines will cost more per foot. This is because building vacuum jacketed tees into a system is more time consuming to fabricate. The extra time and materials increase the manufacturing costs and cause the price per foot to increase. Consequently, the industry usually prices VJP project on a system basis.
What is a phase separator?
A phase separator’s purpose is to supply high quality liquid to a use point while, at the same time, delivering a customer specified pressure and flow required by the downstream equipment. Another less frequent application is using the phase separator as an accumulator in systems where the flow requirements are high for short durations, but still need quality liquids.
Some phase separators offer adjustable liquid level limits and pressure settings. Liquid level limits must be considered with regard to the application. Settings where the high and low liquid levels are close together result in rapid pressure fluctuations of low amplitude. Conversely, settings with wider limits result in pressure fluctuations less frequently but of higher amplitude. Narrow level settings are best for controlling pressure closely, while settings that are father apart offer longer phase separator component life and quieter operation.
To ensure the most efficient use of the phase separator, place it upstream and as close to the end use point as is practical. This minimizes pressure drop, heat leak, and cool down losses, all of which degrade the liquid quality.