Issues with pressure transmitters in the tank area

Nitrogen pressure regulator? It should be a self-acting control valve; do not install the pressure transmitter at the back – it is more accurate to take the pressure reading from another location on the top of the tank.

This question is a bit confusing. Is the transmitter a pressure transmitter? The pressure to be measured is the required pressure after depressurization, right? It is recommended to draw a simple diagram, marking the relevant pressure sampling points as well as the principle of operation for fluid inlet and outlet to the storage tank, which will help clarify things. For reference.

I was wrong; it’s a self-acting control valve

The pressure at this point cannot be equated with the pressure on the top of the tank; it is recommended to install it on the other side of the tank’s top

It is virtually impossible for the pressure transmitter to detect negative pressure at its location, unless the self-acting valve is closed and cannot be opened, the nitrogen supply is unavailable, and liquid material is happening to be discharged at the same time; in such cases, the air intake from the breather valve is not sufficient to maintain normal pressure inside the tank, resulting in negative pressure inside the tank and thus a negative pressure reading by the pressure transmitter.

The installation location of the pressure transmitter cannot accurately measure the pressure inside the tank; generally, the pressure in front of the self-acting valve is several thousand pascals, while the pressure behind the valve is below one thousand pascals. The pressure inside the tank is several hundred pascals, ensuring that a slight positive pressure exists inside the tank, allowing nitrogen to be discharged through the breather valve. When the liquid material is discharged, a large amount of nitrogen is supplied, resulting in high pressure behind the valve; when no material is discharged, less nitrogen is used, and the pressure behind the valve is low. What was said upstairs is correct; it’s most appropriate to install the pressure transmitter on the top of the tank.

In normal circumstances, the differential pressure transmitter located on the top of the tank is used to measure the pressure there; the pressure measurement point should be on the tank top, away from openings such as the nitrogen inlet and the vent valve. This is merely an ideal situation. If proper attention is not paid during operational procedures, the nitrogen supply line operates at pressures below 40, and even when fully opened, it cannot accommodate the sudden changes in volume that occur. Therefore, proper operational practices are crucial. It is prohibited to add cold oil to hot tanks. Generally, the rate at which material is removed does not cause sudden changes in the pressure in the upper space. However, when adding material, care must be taken: if the temperature inside the tank is high, it is absolutely necessary to avoid adding cold substances, as the hot air in the upper space will contract rapidly upon being cooled, and the speed of this contraction is beyond imagination. I’ve encountered this before: a cylindrical storage tank with a capacity of 5,000 cubic meters and a diameter of 23 meters. Theoretically, such a large volume should not cause any significant changes in the pressure in the upper part of the tank. Therefore, normally the nitrogen seal is not used, and instead an oil level gauge with a diameter of 250 mm is used at the top. The tank is insulated to maintain a temperature of 80°C. As part of the operational process, waste oil at 30°C is added to the tank, which results in the tank expanding inward; as a result, the workers stop the operation.