The operation of safety valves in any type of tank, whether it is a liquid gas tank, boiler or water heater, protects the tank from the risk of high pressures, and in the case of liquid gas tanks, installing a safety valve is associated with the release of gas, but if this valve is not there and the tank cracks, there is a risk. It multiplies.
For basic safety measures, all tanks that are approved for liquefied gas must be equipped with a safety valve of adequate capacity. In order to be sure that the safety valve provides full protection at all times, several measures have been taken. They are as follows: They are installed on the tank, which are directly connected to the steam space of the tank and can discharge it indefinitely to the outside air. In this way, the steam is discharged, which is significantly less dangerous than the discharge of liquid. In addition, the release of steam reduces the temperature of the liquid and thus the pressure.
The safety valve must be protected and kept away from tampering, which is especially true for pressure gauges, and if the gauges are located outside, they must be sealed with an approved method.
It is obvious that between the tank and its safety valve, an additional shut-off valve should not be installed. When 2 or more safety valves are installed on the tank and the condition of the valves is such that it is possible to close one of them at any time, this operation can be done exceptionally. The valve or other valves must have the necessary capacity to protect the tank. Using a multi-way safety valve makes it easy to pull out, test, and perhaps replace the safety valve because it eliminates the need to empty the tank. Testing the safety valve is usually not necessary, but it has been observed that in the case of tanks with a capacity of more than 2000 gallons, it is better to perform such tests at intervals of 5 years.
Figure 5. A section of a spring safety valve whose different parts are:
Page, the amount of sealing, the spring part and the valve, what is not shown is a “drain” hole at the bottom of the upper part from which the liquid vapor escapes.
Capacity of the safety valve
Safety valve capacities may be indicated in two ways on the rating plate attached to the valve. Older taps were marked with a number, such as 3046, without using any other symbol. This number represented the capacity of the safety valve in cubic feet per minute of liquefied gas. Since 1952, the specification pages have the word “air” which means that it is clearly shown that the capacity is in cubic feet per minute of air.
This issue simplifies the determination of the size of the safety valve because the required capacities are also expressed in this way.
Causes of safety valve opening
1. An obvious reason for the opening of the safety valve is the presence of a fire near the fuel tank. In such cases, it is better to burn the volatile gas before it accumulates in sufficient quantity and causes an explosion.
2. In some areas, the high temperature in summer causes the safety valve to open. At temperatures greater than 100 degrees Fahrenheit, the relief valve assembly pressure may increase from 220 pounds to 250 pounds. In order to prevent such discharge, a material with a lower vapor pressure may be used. With colors that reflect the heat or creating shade for the tanks or cooling them with the help of water splashing.
3. Since overfilling the tank is accompanied by a sufficient increase in temperature, it will certainly lead to the opening of the safety valve, and as soon as the liquid expands and completely fills the tank, the hydrostatic pressure of the tank increases and a further increase in temperature causes a further increase in pressure and As a result, the opening of the valve is ensured.
4. If “hot” material or inappropriate material is poured into the tank, it may cause the pressure to rise and the safety valve to open. As mentioned earlier, the fuel supplier must be sure that the tank is suitable for his product.
5. Sometimes quickly filling the tank causes the safety valve to open. When the liquid enters the tank, the vapor inside the tank is compressed and takes up less space. If the vapor pressure is higher than the vapor pressure of the substance, the vapor turns into liquid and reduces the pressure. However, if the pressure build-up is faster than the steam condensate, it may reach the limit of the safety valve opening pressure. It is in order to minimize the possibility of creating this pressure that for tanks with a capacity of between 30 and 2000 gallons of water, filling the vapor space is a definite rule. Using a pressure equalizing line is an effective way to prevent excess pressure when filling the tank.
6. Inaccuracy in cleaning the tank for the first time may cause an abnormal rise in pressure. The pressure of the compressed air in the tank combined with the normal vapor pressure of the material, even at moderate fuel temperatures, will certainly cause the safety valve to open.
In all cases, the discharge of the relief valve must be such as to protect it from physical damage.
Safety valve discharge criteria
DOT (ICC) ASME (120 gallons or less): Relief valves must be installed to minimize the possibility of draining onto the tank.
ASME (125 gallons or more) and portable tanks (120 gallons or more) gas trucks: Discharge should be upward away from the tank and released to the open air. Safety valve vents should be protected from natural influences and if protective caps are used, they should not be removed except when draining.
Above ground (over 2,000 gallons): Relief valve discharge must be vertical and at least 7 feet above the open air tank. If necessary, drain pipes with certain criteria should be installed for waste materials. The inlets and outlets of waste materials should be designed in such a way that the possibility of the flame approaching the tank is eliminated. L