Tank Blanketing
A Versatile Tool for Prevention and Protection
Tank blanketing valves are commonly used in tank storage systems where it is desirable to reduce the hazards associated with flammable liquids or to minimize contamination or product degradation that may result from drawing air into the tank’s vapor space. In this section, some basic information on blanketing valves will be provided. It is suggested that you contact Protectoseal when reviewing a specific tank blanketing application. This article explains the function of blanketing valves, describes their method of operation and provides guidelines for sizing and specification of the valves. Definitions of terms commonly associated with tank blanketing valves are listed below.
DEFINITIONS
| Blanketing Valve – | A device that senses the pressure in the vapor space of a storage tank and controls the flow of an inert gas (usually Nitrogen) into the vapor space so that the tank pressure can be maintained within an acceptable range. |
| Deadband – | The total pressure difference between the blanketing valve opening pressure (or set point) and resealing pressure. This applies to the main valve. Some minor leakage through the pilot will occur above the main valve resealing pressure. |
| Dome Pressure – | In a pilot operated blanketing valve, the pressure in the dome volume. |
| Dome Volume – | In a pilot operated blanketing valve, the chamber between the poppet in the pilot valve and the piston in the main valve. |
| Flow Plug – | A small cylinder which may installed in the valve to partially block the flow of inert gas through the valve. |
| Main Valve – | The portion of the valve through which the supply gas flows into the storage tank. |
| Pilot Valve – | In a pilot operated valve, the portion of the valve that senses tank pressure and controls the opening and closing of the main valve. |
| Poppet – | The component in the valve which moves in response to changes in pressure in the sensing diaphragm chamber and which, when unseated, allows flow through the device. |
| Pressure Balanced Poppet – | A poppet designed so that the supply pressure will not have an effect on its opening or closing characteristics. All Protectoseal blanketing valves have pressure balanced poppets. |
| Sense Chamber – | The space below the diaphragm chamber to which the sense line pressure, from the tank, is directed. The pressure in the sense chamber controls the opening and closing of the poppet. |
| Sense Diaphragm – | A thin, non-metallic disc in the diaphragm chamber which flexes in response to changes in the sense line pressure. |
| Sense Line – | A tube running from the tank’s vapor space to the sense port of the blanketing valve. This tube transmits tank pressure to the sense chamber. |
| Set Point – | The pressure at which the main valve opens and flows. |
FUNCTION OF A BLANKETING VALVE
A blanketing valve uses a supply of high pressure gas to maintain a blanket of low pressure gas above the stored material in a storage tank. The gas is usually non-flammable and chemically non-reactive when mixed with the vapors of the stored product. The gas, usually inert Nitrogen, is injected as necessary in order to maintain a non-flammable atmosphere in the vapor space. The blanketing pressure is usually very low, less than 1 pound per square inch (PSIG).
Blanketing valves serve several purposes:
- Maintain the vapor space of the storage tank within an acceptable pressure range.
- Keep the vapors non-flammable by eliminating oxygen-rich air.
- Minimize evaporation losses (and product losses).
- Reduce product degradation and tank corrosion by keeping contaminants and moisture from entering the tank.
Blanketing Valve Operation
A blanketing valve is typically mounted on top of a storage tank along with a pressure/vacuum conservation vent and an emergency pressure relief vent. Piping from the blanketing gas supply is connected to the valve inlet, and the valve outlet is piped to the tank. A sense line runs from a remote location on the tank to the valve’s sense port, thus supplying control pressure for the valve.
The blanketing valve provides primary vacuum relief for the tank. It opens and supplies gas to the vapor space when pressure decreases to the valve’s set point. When vapor space pressure increases, the valve reseals. The P/V relief vent is sized to take care of overpressure and vacuum conditions brought about by unforeseen conditions or equipment failures. The pressure setting of the vent is set at a slightly higher setting than the blanketing pressure in the tank but below the maximum pressure the tank can withstand. Similarly, the vacuum pallet is set at a higher vacuum setting than normal operating conditions bring about and below the maximum vacuum pressure the tank could withstand. An appropriately placed flame arrester would provide additional protection in the event of inert gas failure. An emergency relief vent may also be placed on the tank, the setting being slightly above the conservation vent pressure setting.
Pilot Operated Blanketing Valve
A pilot operated blanketing valve (Series No. 20 – Pilot Operated Blanketing Valve and Series No. 10 – Pilot Operated Blanketing Valve) consists of two separate valves, working in tandem (the main valve and the pilot valve). The main valve inlet connects to the high pressure gas supply source. The valve outlet is piped to the tank vapor space. The piston in the main valve is held in its closed position by supply line pressure accumulated in the dome volume (the space between the poppet in the pilot valve and the piston in the main valve). This accumulated pressure is called the dome pressure.
Opening and closing of the main valve is controlled by the pilot valve. The tank’s vapor space pressure is transmitted, via the sense line, to the diaphragm sense chamber. Decreases in the sensed pressure result in movement of the pressure balanced poppet in the pilot valve. The poppet unseats and allows gas to flow out of the dome volume. This results in a reduced pressure in the dome volume and opening of the main valve piston to allow gas to flow into the tank. Increases in tank pressure cause the poppet to reseal, the dome pressure to increase and the main valve piston to reseal.
Pilot operated blanketing valves provide very accurate sensing of the tank pressure and also provide full open flow through the main valve at a pressure very near to the blanketing valve set point.
Spring Operated Blanketing Valve
A spring operated blanketing valve (Series No. 30 – Spring Operated Blanketing Valve) functions in a manner similar to a spring loaded valve. The valve’s inlet is connected to the supply gas and the outlet is connected to the tank. The pressure balanced poppet provides the primary seal. The tank’s vapor space pressure is transmitted, via the sense line, to the diaphragm sense chamber. Decreases in the sensed pressure result in movement of the sealing, pressure balanced poppet. This results in flow through the valve, into the tank. Increases in tank pressure cause the poppet to reseal, stopping flow into the tank.
Spring operated blanketing valves are often used on smaller tanks and vessels and in situations where the very small dead band provided by a pilot operated device is not considered necessary.
Sizing and Specification of Blanketing Valves
Data concerning the flow characteristics of blanketing valves is available from Protectoseal. This information defines the maximum flow of gas through the device for a specific supply gas pressure and a specific set point. This full flow rating through the valve can be reduced by the use of specially designed flow plugs. The proper blanketing valve to meet the flow requirements of the tank system can be determined.
Once the basic valve has been chosen, options that may enhance or simplify system operations should be reviewed. Among the most common options are:
- Optional connections for piping to supply and tank.
- Material choice for soft goods (gaskets, o-rings, etc.).
- Pressure gauges to accurately record supply and/or sense line pressures
- Integral purge system to constantly direct a small volume of supply gas through the outlet and sense line. This prevents tank vapors from propagating into the valve.
- Field test option to allow for checking and changing of set point in the field.
- Supply line filter to prevent contamination from supply line
