In a dual-pressure nitric acid plant, how should extremely high pressure in the ammonia evaporation system be handled?

A safety valve is installed to activate in case of overpressure

The pressure parameter for the ammonia evaporation system is 0.5–0.55 MPa; the setting pressure of the safety valve is 0.95 MPa, while the actual operating pressure can reach 0.75 MPa. What should be done in this situation?

May I ask the original poster: What is the heat source for your ammonia evaporation system? Is it steam, or some other heat source?

0.8-1.2 MPa liquid ammonia is fed into the ammonia evaporator to be vaporized into gaseous ammonia. About 80% of the liquid ammonia is evaporated by the closed-loop circulating water that flows between Evaporator A and the upper part of the absorber cooler ； About 20% of the liquid ammonia is evaporated in Evaporator B using cooling water from the high-pressure reaction water cooler, with the evaporation pressure controlled at 0.52 MPa; the two evaporators are connected in parallel. Currently, the evaporator at Unit A cools water from 21°C to 17°C, and then sends it to the absorber cooler to remove the heat generated by the absorption reaction. In the case of the B evaporator, due to the high pressure of gaseous ammonia, the inlet water valve has been fully closed. In other words, the entire system has excessive heat and high ammonia pressure. How should it be handled?

Based on the analysis provided by expert Lcy0214, it is possible that the high thermal load on the acid absorption tower is the cause; this leads to a high thermal load in the cooling water entering ammonia evaporator A, resulting in an increased amount of ammonia vaporized in that evaporator, and consequently an increase in the pressure of the ammonia vapor.

Floor 6 shares the same view as me; I wonder if other manufacturers experience this phenomenon as well； Especially in summer, when the temperature is high, the pressure in the ammonia system tends to exceed the specified levels a bit.

“In summer, when the temperature is high, the pressure in the ammonia system exceeds the specified value by a bit. ”So, may I ask: what is the normal operating pressure range for an ammonia system? How high will the pressure in the ammonia system rise during the hot summer months?

When the ammonia system is operating normally, the ammonia pressure is 0.5 MPa; the operational range is 0.5–0.55 MPa. When the temperature is high, the water supply valve of evaporator B is closed, and the ammonia pressure can rise to 0.75 MPa. Meanwhile, the oxidation furnace cannot handle an increased amount of ammonia, and it becomes impossible to adjust the water temperature in evaporator A.

I wonder what the liquid level is in your ammonia evaporator; couldn’t you lower it a bit?

Generally, the ammonia evaporation pressure is high; the method to address this is to reduce the water flow into the evaporator, thereby lowering the liquid level in the ammonia evaporator； In any case, that method has been tried; now the only option is to install a branch line at the outlet of the ammonia evaporator leading to other ammonia pipes outside the unit, in order to reduce the pressure within the system ；