Confused! Help! Issues regarding the nitrogen seal pressure and saturated vapor pressure of ammonia storage tanks
Thread Content
Gurus, I have a few questions regarding the ammonia storage system that I can’t figure out; I’m turning to the experienced experts for help! The ammonia concentration is 30%; in summer, when temperatures are high, it should be considered as 40 degrees. The saturated vapor pressure is 150 kPa, which is higher than one atmosphere. 1 What are the principles for setting the nitrogen sealing pressure of ammonia storage tanks? To suppress the evaporation of ammonia, must the nitrogen purge pressure in the ammonia storage tank be higher than 150 kPa? Otherwise, can the evaporation of ammonia not be prevented? 2 Does a nitrogen sealing pressure higher than the saturated vapor pressure of ammonia mean that ammonia does not evaporate at all? Is the partial pressure of ammonia in the tank’s atmosphere 0, with the total pressure being the pressure of nitrogen? What is the impact of low nitrogen seal pressure on suppressing volatilization? I also looked at the settings for the gas seal in HG T20570.16-95; it seems that its main purpose is to prevent vacuum-induced air intake as well as to stop air from entering. In my case, however, it’s used to suppress the evaporation of ammonia – so can’t I use those settings? 3 Because the saturated vapor pressure of ammonia is too high under these conditions, it is not possible to place a 25 m3 ammonia storage tank at a great height above the ground. In order to prevent cavitation in the centrifugal pump at the tank outlet, it is currently necessary to raise the nitrogen purge pressure inside the tank to 180 kPa. Previously, it was assumed that only a slight positive pressure would exist inside the tank, with a breather valve installed on the tank top; but now that the pressure is too high, the setup has been changed to use a control valve linked to a pressure sensor inside the tank, so that the pressure can be released when it reaches 190 kPa. Is this approach reasonable? 4 There is another 15% industrial ammonia storage tank, whose top outlet is directly connected to the exhaust gas absorption tower (without a breather valve, control valve, or nitrogen seal). In the absence of a nitrogen seal, would the ammonia remain in a state of volatilization, resulting in a decreasing ammonia concentration that eventually makes it below the required standard? Is it necessary to add a shut-off valve in the absence of nitrogen sealing? It’s written a bit messily; I earnestly ask the experts to provide detailed answers. . . Thank you! !
I’m facing the same issue; looking forward to a reply from an expert.
Raising the pressure does not necessarily suppress evaporation; ammonia solution has a saturation pressure that is dependent on concentration and temperature. When the partial pressure of ammonia reaches this saturation pressure, it stops evaporating; otherwise, it continues to evaporate, and this is unrelated to the total pressure in the tank. The purpose of nitrogen sealing is to isolate air and prevent combustion explosions. The ammonia solution with a concentration of 15% does not have a vent valve; it is likely that, based on calculations, the degree of vaporization of this concentration of ammonia is not sufficient to reach the explosive limit, so a vent valve is not necessary (I haven’t done the calculations yet). However, the evaporated ammonia gas needs to be absorbed and cannot be released directly into the atmosphere. Whether breathing is necessary depends on the capacity of the storage tank. If a tank with a pressure tolerance of -0.1 to 1.0 Mpa is used to store ammonia solutions at less than 40% concentration, it is not necessary to implement a breathing function. It would be reasonable to modify the setup by using a switch valve in conjunction with a pressure sensor inside the tank, so that the pressure can be released when it reaches 190 kPa. Is such an arrangement appropriate? In this way, there should be no problem with the high pressure. But how to refill the low pressure? What if all the liquid in the tank is drained and the tank becomes deflated?
Thank you, expert. It needs to be nitrogen-sealed, so nitrogen is continuously supplied to prevent a vacuum from forming. The problem now is that the 15% industrial ammonia tanks are not filled with nitrogen; as a result, pressure cannot be applied when using a pump to transfer the liquid. Under the intense heat of summer, at temperatures as high as 50 degrees, the saturated vapor pressure of 15% ammonia reaches 70 kPa, which makes cavitation in the pump very likely to occur. How is this usually addressed?
This post was last edited by Inamemu on 2017-11-24 at 11:46. Ammonia is prone to volatilization; there’s nothing to do about it except store it at low temperatures. Either apply pressure. A conventional nitrogen seal doesn’t completely prevent the evaporation of the medium either; its main function is to ensure safety. As for the amount of evaporation, it depends on the specific substance in question. https://attach01.hcbbs.com/forum/month_1102/11021423346109eb72e26f32f4.jpg
15% is acceptable; the ammonia pumps and liquid ammonia pumps that operate continuously in the freezing station are all functioning normally. I have been involved in the operation of ammonia transfer systems that use low-temperature methanol wash for ammonia separation; this pump is not used frequently, and it is difficult to start it. It requires a cold pump as well as exhaust ventilation, and it also fails to deliver liquid when it generates heat during operation. It’s just about exhaust, cold pumps, and adjusting flow rate and pressure – technical skills are required
How was it resolved? Was the nitrogen seal setting set too high?