Proper selection of the materials of construction is essential for a reliable system. Improperly applied materials can lead to poor performance or even the failure of the hardware to perform its function. The points to consider are:
a) Compatibility of the materials with their environment; and,
b) Compatibility of the materials within the expected range of temperatures.
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Manufacturers may only offer certain materials in their product. It is still prudent for the user to confirm that all materials are appropriate for their expected usage. In most cases, the materials expert is the user. The user most likely has other hardware in similar service. Under normal conditions, pad valves are exposed to the blanketing gas, and the vent valve is exposed to the tank vapors. When considering this, be aware that although the blanketing valve may normally handle nitrogen, it is possible to overfill the tank and expose the valve to tank contents. Purges and check valves may be employed to limit exposure. Don’t ignore the importance of
selecting the proper elastomers and plastics. These materials are more sensitive to temperature and chemical contact than the metals. Material hardening, softening or even dissolving can cause serious problems and system failure. Further, contact a supplier’s technical department for assistance in selecting materials.
Venting
Venting refers to several different functions. It is performed by several different pieces of hardware. The tank pressure and vacuum ratings must be known in advance in order to select the vents and the operating pressures.
Used tanks, without PV ratings, must be cautiously and carefully evaluated to see if they are safe to use.
The pressure-vacuum vent (PV) is a protective device. It is also called the normal vent or tank vent. It often contains both functions in a single unit. It would have separate vacuum and pressure sections. Usually, it is has a weighed pallet construction. The weights determine the operating vacuum or pressure. The pallet functions as the valve. They are very simple in construction.
The vacuum vent provides essential protection against vacuum collapse. Most tanks have little capability to resist internal vacuum. A collapsed tank will result in a spill, a non-repairable tank, and possible damage to associated equipment. It would be a hazard to life and property – both on-site and off-site. Select this vent carefully, working with the tank and vent manufacturers. API publications offer help in this area.
The pressure portion of the PV tank vent protects against tank overpressure. It is set above the blanketing system operating range. Depending on the system, it may also function as the blanketing vent. Systems allowed to vent to atmosphere combine the blanketing upper limit and pressure vent setting into a single vent. This simplifies the installed system. This vent can be direct to the atmosphere or pipe away to a remote location. In a pipe-away vent, any backpressure in the discharge will change the vent set-point. This pressure directly adds to the pallet loading pressure.system operating range. Depending on the system, it may also function as the blanketing vent. Systems allowed to vent to atmosphere combine the blanketing upper limit and pressure vent setting into a single vent. This simplifies the installed system. This vent can be direct to the atmosphere or pipe-away to a remote location. In a pipe away vent, any backpressure in the discharge will change the vent set-point. This pressure directly adds to the pallet loading pressure.
Characteristics of these vents include small seat leakage, additional pressure buildup and reseat pressure.
Because of their construction, these products are not capable over time of remaining bubble tight. There is an additional pressure buildup in the tank to fully open the valve. During full flow, the tank pressure will have to rise to some percentage above the set pressure. Additionally, when tank pressure reduces, the vent will not reseat until the tank pressure drops somewhat below the opening set pressure. If these characteristics are not
acceptable, then consider using pilot operated vents and de-pad valves. If pipe-away venting is required, it often is for environmental reasons. The tank vapors are directed to a device to process the vapors. Several methods are in use including refrigeration and thermal oxidizers. Pipe-away venting involves backpressure buildup. The tank vent must be able to operate at its set-point, regardless of the backpressure. In this case, some
manufacturers offer de-pad venting valves that meet this requirement. Some combine the pad and de-pad valves
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into a single packaged unit.
References
1 American Petroleum Institute (API), 1220 L St., Washington, D.C. 20005-4070, 202 682-8000 2 API Standard 2000, Venting Atmospheric and Low Pressure Storage Tanks, Nonrefrigerated and
Refrigerated.
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Failure to size valves correctly can result in operational problems, damage to the stored product or even damage to life and property. It is important to determine the correct size and capacity required, for the valves and tank vents used in tank blanketing. The hardware’s purpose, in combination, is provided to protect from under or overpressure conditions during operation. Failure to do so can result in operational problems, damage to the stored product or even damage to life and property. Sizing is not difficult if one knows under what parameters the system will operate. Proper sizing results in a proper operating system.
In Part I the relationship of the various pressure/vacuum set points were discussed and how they might interact if not properly set in relation to each other. All of this hardware, combined, is a system and must be evaluated as such. I have often been asked about reducing gas usage by means of lower set-points for the pad valve.
In fact, the lower the set-point, the less gas will be used. However, this saving is very small. The volume of gas saved is in ratio to the absolute pressures, not the gage pressures.
While it is worthwhile to reduce gas usage, it should not be a primary objective. It is more important and
worthwhile to raise the pad set-point to prevent interaction with the vacuum vent. Following this train of thought, accuracy of regulation is important only when related to the deadband between valves and vents. The actual accuracy is not important unless compared to the available deadband. The more deadband available, the less accuracy required. In your design, emphasize selecting set-points in relation to the deadband between devices.