Pressure before and after the liquid ammonia control valve
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Liquid ammonia control valves are widely used in the nitrogen fertilizer industry. There is one issue that has been bothering me, and I would like to bring it up for discussion among everyone. Generally, there is a liquid level control valve before the liquid ammonia enters the evaporator in liquid ammonia pipelines. The pressure in front of this valve is easy to understand: it is naturally the pressure in the liquid ammonia storage tank. What about the pressure behind the valve? One theory is that it should be at the same evaporation pressure as the evaporator, but if liquid ammonia loses pressure in the pipes, it has neither a space for pressure to escape nor the heat required for vaporization – how is this possible? . Another approach is to consider only the pressure drop across the valve body, as regardless of whether the pressure of liquid ammonia decreases or not, it will naturally lose pressure once it enters the evaporator. Both explanations make some sense; which one is correct?
After passing through the control valve, liquid ammonia undergoes throttling expansion, resulting in a decrease in pressure; as a result, it vaporizes and turns into gaseous ammonia. Vaporization requires heat absorption, so there is no apparent change in the pipeline ahead of the control valve, but the temperature drops behind the valve, causing the air in the surrounding environment to condense and form ice.
After passing through the control valve, part of it vaporizes; it should be a gas-liquid mixture, with a pressure close to that of your evaporator.
The pressure behind the control valve is equal to the pressure of the liquid ammonia evaporator plus the resistance in the piping. The liquid behind the valve undergoes adiabatic flashing, with part of it vaporizing into gas; as a result, the velocity of the two-phase flow behind the valve increases, so the pipe diameter does not change much.
From a design perspective, the pipes downstream of the control valve should become progressively larger, and this is also the case in practical applications; however, in some smaller projects, the diameter of the pipes after the control valve remains unchanged.
This control valve should function as a throttle valve; the pressure behind the valve is the sum of the evaporator pressure and the pipeline resistance
1# Brass Knight Approximate evaporation pressure behind the valve
After being depressurized by the regulating valve, part of the liquid ammonia vaporizes (it is impossible for it to vaporize completely in the absence of a heat source), forming a low-temperature vapor-liquid mixture that enters the ammonia evaporator. The pressure behind the regulating valve should be: the pressure inside the evaporator + the liquid column height in the liquid ammonia pipeline + the pipeline resistance.