The ventilation device installed on the top of the fixed storage tank can reduce the evaporation loss of oil and other liquids, control the pressure of the storage tank, and prevent the tank from being damaged by overpressure or vacuum. The types are pipe away breath valve, single suction valve and breathing valve. All kinds of structures can be combined with flame arrester, and the required connecting pipe can also be prepared. How many breathing valves should be installed in a storage tank?
Selection Strategy of Breather Valve
- For the requirements of installation position and temperature range, for example, all weather breathing valve shall be selected in cold area, while pipeline type breathing valve shall be used for those installed in the pipeline.
- The control pressure of mechanical breathing valve shall be suitable for the relevant pressure bearing capacity.
- The specification of mechanical breathing valve (flange diameter) shall meet the maximum flow requirements of oil breathing gas in and out of the oil tank.
Type of tank breathing valve
1. gravity mechanical breathing valve
Gravity mechanical breathing valve is composed of valve body, pressure valve, vacuum valve, guide rod, etc. it works by balancing the weight of valve disc and the thrust generated by pressure difference inside and outside the tank. When the pressure in the tank is larger than the valve disc weight, the valve disc rises along the guide rod, and the oil tank exhausts (or inhales) gas; After pressure balance, the valve disc falls on the seat under the action of self weight. When the pressure change in the tank is relatively slow, the valve disc is continuously beating on the valve seat; Only when the air volume in the tank is large, the valve disc can be lifted by air flow and suspended above the seat.
In order to prevent the valve disc from beating and collision, most valve discs are made of aluminum alloy. The guide rod of valve disc is generally made of stainless steel, and the installation must be vertical to prevent the guide rod from rusting or tilting from hindering the valve disc from moving up and down.
In order to prevent the valve disc from freezing on the valve seat in winter, the width of the top of the valve seat (sealing surface) is not more than 2mm. However, it is proved that when the breathing valve is used in cold areas, there is still a phenomenon of freezing the disc and the valve seat together, and the tightness is poor, and there are always micro leakage in the closed state.
2. spring type mechanical breathing valve
Spring type mechanical breathing valve is composed of valve body, upper and lower valve disc, spring, guide rod, etc. It mainly relies on the balance of spring tension and thrust generated by pressure difference inside and outside the tank. Spring type mechanical breathing valve has no strict requirements on the weight of the disc, so it can be made of non-metallic materials (such as polytetrafluoroethylene) to reduce the risk of freezing the valve disc. The control pressure of the breathing valve can be adjusted by changing the compression length of the spring. When the pressure inside the tank reaches the control positive pressure of the valve, the lower valve disc drives the upper valve disc to rise together, and the valve seat is separated and the oil tank exhaled; When the negative pressure in the tank reaches the control negative pressure of the valve, the lower valve plate drops, and it is separated from the upper valve disc, and the oil tank inhales. The breathing valve is compact in structure, small in volume and light in weight, but after long-term use, the spring is easy to rust, and the valve disc is easy to deform, which makes its air tightness reduced. When used in cold areas, frost on spring may affect spring activity, and valve disc can not be opened according to preset control pressure.
3. gravity spring compound mechanical breathing valve
The composition of gravity spring compound mechanical breathing valve is similar to that of spring type breathing valve. It is usually used to control the positive pressure of the oil tank by the weight of valve disc (including weight block), and the suction negative pressure of the oil tank is controlled by the tension of spring. The pipeline breathing valve belongs to this kind of breathing valve. When the pressure in the tank is greater than the positive pressure controlled by the breathing valve, the pressure inside the tank overcomes the gravity of the valve disc and supports the valve disc to inhale outwards; When the pressure in the tank is less than the atmospheric pressure, the atmospheric pressure compresses the spring, overcomes the spring tension and causes the valve disc to drop and the gas enters the tank.
Installation method of breathing valve oil tank
- All breathing valves shall be vertically installed at the highest point of the air source of the storage tank, so as to reduce the evaporation loss of materials and smoothly provide the most direct and largest channel to the breathing valve. Generally, for vertical tanks, the valve should be installed within the central roof of the tank top as far as possible. For tanks with heat insulation layer on the tank top, it can be installed near the ladder platform;
- When two breathing valves need to be installed, they should be symmetrically arranged with the center of tank top;
- If the breathing valve is used on the nitrogen sealed tank, the connecting pipe of the nitrogen supply pipe should be far away from the valve interface, and inserted into the tank about 200 mm from the top of the tank, so that the nitrogen will not be discharged directly after entering the tank, so as to achieve the purpose of nitrogen sealing.