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protectoseal

STORAGE TANK VENTING FOR CONSERVATION, 

SAFETY & ENVIRONMENTAL PROTECTION

Protectoseal vents are intended for use on flammable liquid storage tanks that operate at pressures of 15 PSIG or less. This section explains why tank venting equipment is needed and how it may be properly specified. The hazards associated with pressure and vacuum accumulation in a tank storing flammable and combustible liquids are stated. The operation of vents, their role in safe plant operations, the method of sizing and specifying vents and their importance in minimizing evaporation losses and fugitive emissions are discussed. Definitions of terms commonly encountered in the tank venting industry are provided, along with links to other useful and informative sites.

DEFINITIONS

Atmospheric Tank - A storage tank that has been designed to operate at pressures from atmospheric through 0.5 PSIG.
Combustible Liquid - A liquid having a flashpoint at or above 100° F.
Diaphragm - The sealing (gasket) material that is part of the pallet assembly and which seals against the seat surface when the vent is closed.
Design Pressure - The maximum pressure or vacuum that a storage tank can withstand without damage to its structure.
Flammable Liquid - A liquid having a flashpoint below 100° F.
Flashpoint - The minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid.
Leak Rate - The leakage of vapor from the vent prior to reaching the set point.
Low Pressure Tank - A storage tank which has been designed to operate at pressures above 0.5 PSIG but not more than 15 PSIG.
Pallet Assembly - The weight or spring loaded disc housed within the vent that moves in response to the tank pressure, allowing flow into or out of the tank. The pallet assembly covers the vent seat when in the closed position.
Pressure Vessel - A storage tank or vessel which has been designed to operate at pressures above 15 PSIG.
Set Point - The tank pressure/vacuum at which the vent begins to open.
Seat - The machined orifice within the vent housing on which the pallet assemblies sit when closed.
Tank Vent - A device intended to provide pressure and/or vacuum relief for atmospheric or low pressure storage tanks. The set points of the vents may be provided by weight loading, spring loading or buckling pin.

 

PRESSURE/VACUUM ACCUMULATION 
The use of large capacity tanks and vessels for the temporary storage of flammable or combustible liquids is a common practice in a wide range of commercial and industrial enterprises. These tanks provide fixed volume containers to hold liquids transferred (filling and emptying) through connected piping systems. In any such fixed roof tank, the volume above the liquid level is known as the vapor space.

Assume that a tank is completely vapor tight and that liquid is being pumped into and out of the tank. Filling the tank raises the liquid level and causes the vapor space to decrease (vapors are compressed), with a resulting increase in the pressure in the vapor space. Alternatively, if liquid is withdrawn from the tank, the vapor space increases (vapors are allowed to expand) and the pressure in the vapor space decreases.

Now assume that the tank is again completely vapor tight, no liquid is being transferred (the liquid level does not change), but the liquid in the tank is being heated or cooled. The addition of heat causes vapors to be generated and evolve into the closed vapor space. The result is an increase in pressure in the vapor space. Cooling of the liquid leads to contraction of the vapors and a corresponding pressure decrease in the vapor space.

The scenarios outlined above reflect common hazards associated with the storage of flammable liquids in fixed roof tanks. Unless the tanks are equipped with properly designed and specified venting devices, excessive pressure and/or vacuum accumulations in the vapor space can result in severe tank damage. Protectoseal pressure and vacuum relief vents are specifically designed to address and eliminate this potentially hazardous situation.

Normal Venting- In day-to-day tank operations, changes in the liquid level are caused by routine filling and emptying of the tank. Changes in the temperature of the vapors and liquids in the tank are the result of variations in the ambient atmospheric temperatures (e.g. higher temperatures during the day; cooler temperatures at night). Discharging the volume of vapors generated (pressure relief), or inbreathing the volume of make-up air required (vacuum relief), during such activities is defined as normal venting (Vents That Provide Normal Pressure/Vacuum Relief).

Emergency Venting- The temperature of the stored liquid and vapors may also increase as a result of the tank being exposed to an external fire. A significant amount of heat may be transferred through the tank shell and the volume of vapors generated as a result of this heat input can be substantial. Providing a means of discharging this large volume of vapors and prohibiting an increase of pressure within the tank is defined as emergency venting (Vents That Provide Emergency Pressure Relief).

EVAPORATION LOSSES 
In addition to protecting a tank from excessive pressure and vacuum, Protectoseal vents also play a key role in the reduction of product evaporation losses and fugitive emissions. The vents are designed to remain closed until they must open to protect the tanks. Vapors are contained and are not released into the atmosphere. The reduction in product loss as compared to an open vent pipeline is significant. The emission of vapors into the atmosphere is minimized. Tank vents are an important tool in any company’s attempts to comply with the Clean Air Act mandates concerning air pollution.

VENT OPERATION 
The method of operation of Protectoseal pressure/vacuum vents is straightforward. The vents are mounted on a nozzle connection that leads to the tank’s vapor space. Each vent includes a machined seat that is closed by a moveable sealing disk (pallet assembly). The pallet assembly is held in its closed position by weights, springs or buckling pin (depending on the vent style). The amount of closing force applied determines the set point of the vent. The pressure in the tank’s vapor space pushes against the pallet assembly, in opposition to the closing force. When the tank pressure reaches the vent set point, the pallet assembly lifts and vapors are allowed to escape from the tank through the vent. The pressure and/or vacuum in the tank’s vapor space is maintained within a safe range.

presusre pallet assemble closed

pressure pallet assembly open

vacuum pallet assembly closed

vacuum pallet assembly open

 

 

 

 

 

 

 

 

 

SIZING AND SPECIFICATION 

Pressure/Vacuum relief vents are available in a range of sizes. Larger size vents provide greater flow capability than smaller size vents. When choosing a proper size venting device the following information is significant:

1. THE AMOUNT OF VAPOR/AIR THAT MUST PASS THROUGH THE VENT.
The amount of vapors that must be relieved is usually stated in Standard Cubic Feet of Air per hour (SCFH). Methods of calculating these volumes for specific normal venting and emergency venting situations can be found in 29CFR – OSHA 1910.106.

2. THE DESIGN PRESSURE/VACUUM OF THE STORAGE TANK.
Storage tanks are mechanical structures. There are limits as to how much pressure and vacuum they can withstand before they are damaged. These limits are known as the tank’s design pressure and vacuum.

3. ANY OPERATING CHARACTERISTICS OF THE TANK SYSTEM THAT REQUIRE A SPECIFIED PRESSURE OR VACUUM TO BE MAINTAINED IN THE TANK (MINIMUM VENT SET POINT).
The relief vent will remain closed until its set pressure is reached. If there is a need to maintain some pressure in the tank during normal operations, the vent must be set so that it will not open and begin relieving below that pressure.

4. THE FLOW CAPABILITY OF THE VENT BEING CONSIDERED FOR USE.
Each size and style of vent will flow specific volumes of vapors at a given pressure. These vent flow capabilities are available from the manufacturer.

The key to sizing a vent for pressure or vacuum relief is to make sure that the vent (with set point) chosen will flow the required amount of vapors at a pressure less than the design pressure of the tank. This insures that the tank’s design pressure or vacuum are never exceeded.

Although the vent sizing procedure can be done manually, The Protectoseal Company has automated the calculation and specification process through the ProFlow® Sizing/Selection Software.

MATERIALS OF CONSTRUCTION
Protectoseal venting devices are available in a wide range of materials (aluminum, stainless steel, ductile iron, hastelloy, PVC, FRP, etc.). The material must be compatible with the service conditions. Improper material choice can lead to contamination of the product being stored or reduction in the vent’s ability to operate safely. Information on the corrosion resistance of materials under various service conditions is available in corrosion handbooks and chemical dictionaries.

 

UNDERSTANDING / SPECIFYING
FLAME & DETONATION ARRESTERS
 

When properly applied, Protectoseal flame arresters and detonation arresters are effective in preventing the propagation and transmission of a flame or flame front in locations where flammable vapor/air or gas mixtures are present. In this section, an explanation of how flame fronts develop will be provided. The significance of different classifications of flammable chemicals will be described. The function of an arrester and the key parameters in sizing and specifying a device are explained. The importance of independent third party testing and approval of flame arrester and detonation arrester designs is documented. Definitions of terms commonly encountered when discussing arresters are provided, along with links to other useful and informative sites.

DEFINITIONS

Arrester Element - The portion of a flame arrester or detonation arrester comprised of parallel spaced plates or crimped metal windings. The element provides the mechanical barrier to flame passage. The arrester element is mounted in the arrester housing.
Arrester Housing - The portion of a flame arrester or detonation arrester that houses the arrester element and that provides the flanged or threaded connection to the pipe/tank being protected.
Flammable Liquid - A liquid having a flashpoint below 100°F.
Combustible Liquid - A liquid having a flashpoint at or above 100°F.
Confined Deflagration - A deflagration (see below) propagating in a location where expanding combustion products are confined. A flame traveling within a pipe may be a confined deflagration.
Deflagration - A flame front propagating through a flammable gas or vapor at a velocity less than the speed of sound in that gas or vapor.
Detonation - (Also “Stable Detonation”) A flame front propagating through a flammable gas or vapor at a velocity equal to the speed of sound in that gas or vapor.
Detonation Arrester - An arrester designed to prevent the propagation of unconfined deflagrations, confined deflagrations, stable detonations and overdriven detonations.
End-of-Line Arrester - A flame arrester that is mounted at the end of a pipe (flanged or threaded inlet connection) and which vents directly to the atmosphere. The arrester is designed to stop unconfined deflagrations.
Explosive Range - The range of values between and including the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL) for any vapor/air mixture.
Flashpoint - The minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid.
Lower Explosive Limit - (LEL) The lowest volumetric concentration (expressed as a percentage) of flammable vapor in air that is capable of sustaining and transmitting a flame throughout the vapor mixture, at a specified temperature and pressure. Mixtures below the LEL are considered to be too “lean” to burn.
Overdriven Detonation - An unstable flame front that propagates through a flammable gas or vapor at a speed in excess of the stable detonation velocity.
Stoichiometric Mixture - The flammable liquid/air mixture where the fuel and oxygen are totally consumed if the mixture is ignited.
Unconfined Deflagration – A deflagration propagating in a location where the expanding combustion products are not confined. A vapor cloud ignited in the open atmosphere is usually an example of an unconfined deflagration.
Upper Explosive Limit – (UEL) - The highest volumetric concentration (expressed as a percentage) of flammable vapor in air that is capable of sustaining and transmitting a flame throughout the vapor mixture, at a specified temperature and pressure. Mixtures above the UEL are considered to be too “rich” to burn.
Vent-Line/In-Line Arrester - A flame arrester that may be mounted upstream of a pressure/vacuum relief vent, or that may be located upstream of a specified maximum length of vent piping to atmosphere. This arrester is suitable for stopping a confined deflagration that has propagated through a pipe for some specified maximum distance.

 

FLAME FRONT GENERATION
If any flammable mixture of vapor or gas comes in contact with an ignition source, a flame front will develop. This flame will burn through the vapor or gas until:

  1. The supply of fuel (vapor or gas) is consumed.
  2. The heat necessary to sustain combustion is removed.
  3. The oxygen concentration becomes either too high or too low to allow continued burning.

Flame Front Generation Graph pic

 

 

 

 

 

 

 

 

 

 

 

If a flame front is propagating at a speed less than the speed of sound in the vapor, it is known as a deflagration. A flame front that propagates at a shock wave at the speed of sound in the vapor is known as a (stable) detonation. An overdriven detonation is a flame front propagating at a speed in excess of the speed of sound in the vapor. Such an overdriven detonation is a short lived phenomenon and usually occurs as the flame front is transitioning from a high speed (near the speed of sound) deflagration to a detonation.

A deflagration may develop in the atmosphere as an unconfined deflagration, or in an enclosed area, typically a piping system, as a confined deflagration. Detonations and overdriven detonations are most commonly encountered in closed piping systems.

pic that goes above an unconfined deflagration reuslts in relatively low flame speeds

An unconfined deflagration results in relatively low flame speeds and virtually no pressure increase. A confined deflagration (e.g. – an ignition in a run of pipe) starts at low speed and pressure. As the flame front propagates in the pipe, its speed and associated pressure increase. In long or complicated (multiple bends) pipe runs the flame accelerates until it transitions through an overdriven detonation state into a stable detonation. In a 4.3% propane/air mixture the stable detonation velocity is 5800 ft/sec and the associated pressure is approximately 300-400 PSIG.

Flame arresters and detonation arresters that are designed and tested to withstand and stop these various categories of flame fronts are available.

FLAMMABLE VAPOR/GAS CLASSIFICATION 
Common flammable chemicals have been examined and arranged into groupings on the basis of their burning and explosion characteristics. In the National Electric Code (NEC) chemicals are categorized in Group A, B, C or D. Group D contains the least volatile flammable chemicals. Groups C, B and A contain, respectively, chemicals of increased volatility. Similar chemical groupings have been developed by the International Electrotechnical Commission. Their categories are designated as IIA, IIB, IIC, with IIA containing the least volatile and IIC containing the most volatile chemicals. In general terms, Group D is equivalent to Group IIA. Propane/air is a representative Group D (IIA) vapor. Group C is equivalent to Group IIB. Ethylene/air is a representative Group C (IIB) vapor. Group B is equivalent to Group IIC. Hydrogen is a representative Group B chemical. Group A contains only acetylene. The classification of the chemical in the flammable vapor is a significant parameter in the choice of a flame arresting device.

HOW AN ARRESTER FUNCTIONS 

pic that goes under how and arrester funtions

 

 

 

 

 

 

 

 

 

Flame arresters and detonation arresters are passive mechanical devices that are mounted to threaded or flanged connections on a tank or in a process piping system. In normal operation, vapors in the pipe are directed through the arrester. An arrester consists of a housing and an arrester element.

Arrester elements are available in a number of different configurations (parallel rectangular metal plate, wound crimped metal, parallel round metal plate). One common feature of all flame arresters is that the flammable vapor mixture is forced to pass through a series of small openings as it flows through the arrester. The size of the openings and their length of passage can vary, depending on the arrester style.

If the flammable vapor should ignite, the flame burns towards the arrester/element. As the flame attempts to pass through the element, it is slowed and cooled by contact with the metal walls of the small passages. Heat is transferred to the element until combustion cannot be maintained. The flame front is extinguished.

pic that goes above sizing and specifications

SIZING AND SPECIFICATION 
The primary function of a flame arrester or detonation arrester is to provide protection against an approaching flame front. In their typical applications, however, they must also allow vapors and/or air to pass through the openings in their elements so that pressure and vacuum relief may be provided and so that normal processing of the vapors can be conducted. The resistance to flow through the arresters is based on their size and configuration. The arrester must be sized to allow the required flow rate at some acceptable resistance (pressure drop). Although the sizing procedure can be done manually, The Protectoseal Company has automated the calculation and specification process through the ProFlow® Sizing/Selection Software.

The optimum location for the arrester must be determined. End-of-Line flame arresters are mounted on outlet flanges and they vent directly to atmosphere. Vent-Line/In-Line flame arresters may be installed at some maximum distance (specified by the manufacturer) from the end of a section of open vent piping. Detonation arresters are designed so that they may be installed anywhere in a flammable vapors piping system. Specific information on the restrictions to location of any arrester is available from the manufacturer.

Flame and detonation arresters are rated for use with chemical vapors of appropriate groups defined by the National Electric Code (NEC) and the International Electrotechnical Commission (IEC). The suitability of the arrester for service with a particular vapor group must be verified. The initial pressure and temperature of the vapors in the system being protected are also significant factors that must be reviewed. The materials of construction of a flame arrester or detonation arrester must be selected to insure compatibility with the process vapors being handled. The possibility of corrosion of the arrester components or contamination of the process materials must be minimized.

APPROVALS AND LISTINGS 
The Protectoseal Company has submitted their flame arresters and detonation arresters for inspection and testing by nationally recognized independent, third party approval agencies. We have been granted acceptance of our arresters by Underwriters Laboratories, Inc. (UL), Factory Mutual Research (FM), The United States Coast Guard (USCG), in the United States and by the Federal Institute for Physics and Technology (PTB), in Germany.

MATERIALS OF CONSTRUCTION
Protectoseal flame arresting devices are available in a wide range of materials (aluminum, stainless steel, ductile iron, hastelloy, etc.) The material must be compatible with the service conditions. Improper material choice can lead to contamination of the product being stored or reduction in the flame or detonation arrester’s ability to operate safely. Information on the corrosion resistance of materials under various service conditions is available in corrosion handbooks and chemical dictionaries.