The selection of nitrogen seal valves: a new issue for veteran engineers
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For the nitrogen sealing valves in storage tanks, under normal circumstances, self-acting control valves are selected in accordance with HG20570.16; however, a directive issued by an authoritative company specifies the use of ordinary pressure cut-off valves (control? For tanks with a design pressure of -0.5 kPa to 2.0 kPa, the following control scheme is adopted: Each tank should be equipped with a nitrogen sealing valve assembly and a bypass for the flow control orifice plate. Under normal conditions, the nitrogen sealing valve assembly is used to maintain the pressure in the gas space inside the tank at around 0.3 kPa; when the pressure in the gas space exceeds 0.5 kPa, the nitrogen sealing valve closes, thereby stopping the supply of nitrogen ； When the pressure in the gas phase space is below 0.2 kPa, the nitrogen seal valve opens to start supplying nitrogen. When the nitrogen seal valve needs maintenance or fails, a flow-limiting orifice plate bypass is used to supply nitrogen to the tank. Is this plan feasible? What are the set values for H/L/LL in PIA, and for the pressure behind the nitrogen seal valve?
H1.0 L 0 LL -0.3
At 0.2 kPa, the nitrogen seal valve opens to start supplying nitrogen. LL should be 0.2, right? -0.3 kPa is the value at which the breathing valve opens, isn’t that right?
Well, it depends on the level of work pressure you need
It is best if L is 0.2 and LL is 0
This document is Sinopec Document No. 2016-127, and I also think the solution mentioned in it – which is to use a shut-off valve – is definitely incorrect. Small storage tanks simply cannot have a switch function! It’s my personal understanding; I’m not sure if it’s correct, haha!
This is the challenge posed to PCV performance by the new requirement (using nitrogen sealing in atmospheric-pressure tanks for VOC treatment). Let’s first focus on several characteristic pressure values of PCV versus flow rate: 1. To activate the pressure PCV, it is necessary to initiate an injection flow rate, but one that is lower than the desired rate, in order to ensure that the suction valve does not activate. It is generally greater than 0. 2. Flow point pressure: At this pressure, the flow rate passing through meets the process make-up requirement. Your request is: 0.2 3. Set pressure: This pressure is a nominal value; no flow rate is specified. After all, 2 has already met the requirement for replenishment. Your requirement is: 0.3. 4. The shut-off point pressure is at the PCV shut-off level, and it meets the requirements for the leakage grade. Your requirement is: 0.5. In practice, the issue is that the shut-off pressure for many PCVs is too high, causing the exhaust valve to remain slightly open and resulting in continuous leakage. If the liquid level remains constant, then the PCV is continuously filling air in. If the set pressure is reduced, the pressure at the flow point also drops, but there is a risk of negative pressure during discharge (the flow requirement is met, but the pressure requirement is not). There is no fixed relationship between the values of 3 and 4; it depends on the design, CV value, sealing mechanism, and pressure-sensitive diaphragm. I have encountered problems with imported brands as well; only by specifying the pressure at the shut-off point did the manufacturer manage to find a suitable solution. The requirements for micro-pressure nitrogen sealing indeed pose challenges to domestic PCV systems, with unstable quality being the key issue. Therefore, SH issued a notice: at least the control valve can still be operated remotely for emergency use. If the tank is large and the residence time is long, control is relatively easy to manage; but for small tanks under low pressure, it’s better to keep blowing and releasing air continuously, which results in only minimal nitrogen consumption.
This setup has a high cost, but with the same goal, the approach is feasible
Sinopec’s configuration includes pressure sensors, while many tanks do not have this feature. Without pressure sensors, if the nitrogen seal fails (i.e., there is no gas supply), the protection of the tank will be insufficient.
……The nitrogen sealing system we use involves installing separate pneumatic control valves for nitrogen intake and exhaust, with interlocked automatic control based on the tank pressure; it works quite well……:)