【2026 Nitrogen Sealing System】Should the pressure gauge for the nitrogen sealing valve pipeline be installed before or after the valve?
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In the design reviews of industries such as petrochemicals, chemicals, and storage and transportation, a seemingly simple yet often confusing question keeps arising: should the pressure gauge on the nitrogen seal valve pipeline be installed before or after the valve? This issue is not only related to the compliance of instrument configuration, but also directly affects the safe operation and fault diagnosis capabilities of the tank nitrogen sealing system.
The specifications state clearly: pressure gauges must be installed in front of the valve. According to Article 4.2.2.8 of the \"Design Code for Automation Systems in Petrochemical Tank Farms\" (SH/T 3184-2017), it is stipulated that: \"A pressure gauge shall be installed on the nitrogen inlet pipeline of the nitrogen seal valve.\" ”
This sentence clearly states that the pressure gauge should be installed in front of the nitrogen seal valve, that is, on the pipe section before the nitrogen source reaches the control valve.
The fundamental purpose of this requirement is to monitor the nitrogen supply pressure, ensuring that it remains stable and meets the minimum requirements for the proper operation of the nitrogen seal valve. Typically, self-acting nitrogen seal valves require an inlet nitrogen pressure of not less than 0.2~0.8 MPa (depending on the valve model). If the supply pressure is insufficient, it will lead to the failure of the nitrogen seal; the storage tank will then be unable to maintain a slight positive pressure, which in turn can result in air backflow and the formation of explosive mixtures, posing serious safety risks.
Therefore, the pressure gauge before the valve is the first line of defense for determining whether there is gas in the nitrogen sealing system and whether the amount of gas is sufficient; it is a mandatory safety requirement.
Is a pressure gauge needed behind the valve? How do you say standard? Some argue that since the nitrogen seal valve is of the \"pressure control type after the valve\" category (SH/T 3184-2017, Clause 5.4.5.2), the pressure behind the valve is regulated with high precision by the valve itself; ordinary pressure gauges have low precision and slow response, so they cannot detect any changes, and therefore there is no need to install them behind the valve.
There is some truth to this claim, but it overlooks two key points: 1. Monitoring and debugging requirements: Although nitrogen seal valves achieve millibar-level pressure control through controllers or diaphragm mechanisms, operators still need to use on-site instruments to verify that they are operating as intended. The pressure gauge behind the valve (or the gauge on the tank top) is an important basis for commissioning, calibration, and routine inspections.
2. Value for fault diagnosis: When the pressure in the storage tank is abnormal, if only a pressure gauge before the valve is available, it is not possible to determine whether the issue lies with the valve itself (such as a damaged diaphragm or stuck valve core), or with a clogged pressure tapping pipe or faulty pressure transmission. The pressure data behind the valve can provide crucial clues, significantly improving the efficiency of fault diagnosis.
It is worth noting that Clause 5.4.5.3 of SH/T 3184-2017 further emphasizes: \"The nitrogen seal valve should be installed on the nitrogen pipeline as close as possible to the tank top inlet, and the sampling point for the external pressure measurement pipeline should be located at the tank top in order to measure the actual pressure inside the tank.\" ”
This means that what truly reflects the pressure condition of the storage tank is not the pressure on the short pipe behind the valve, but rather the pressure in the gas space at the top of the tank. Therefore, although the regulations do not require pressure gauges to be installed on the pipeline downstream of the valve, it is highly recommended to install local pressure gauges or pressure transmitters on the tank top to monitor the actual pressure inside the storage tank.
Best practice: Install in front of the valve; installation on the tank top is recommended. Taking into account the comprehensive regulatory requirements and engineering practices, it is recommended that: a pressure gauge must be installed before the valve to monitor the pressure of the nitrogen supply and ensure reliable gas supply. Pipeline downstream of the valve: It is not mandatory, but it can be installed as an auxiliary reference if space permits and maintenance is facilitated. Tank top: It is highly recommended to install an on-site pressure gauge (a vacuum pressure gauge is advised, with a range that covers negative pressures), at the same pressure sampling point as that outside the nitrogen seal valve, in order to accurately reflect the pressure inside the tank.