The oxygen venting system is one of the highest-risk sections in an air separation unit (ASU). How to effectively control construction costs while ensuring compliance with applicable standards and safety requirements has always been a key concern for designers and end users. Based on engineering practices, this article shares some practical approaches to equipment selection and system design.
Ⅰ What Standards Should Be Followed for Oxygen Vent System Design?
An oxygen vent system typically consists of control valves, upstream and downstream piping, flame arresters, filters, silencers, explosion-proof walls, and other related components. The design and selection of these systems are mainly based on the following standards:
GB 16912-2008 – Safety Technical Regulations for Oxygen and Related Gas Production by Cryogenic Air Separation
EIGA Doc 13/20 – Oxygen Pipeline and Piping Systems
JB/T 12955-2016 – Technical Requirements for Oxygen Valves
Most standards provide general principles and safety requirements, while publicly available technical literature mainly focuses on the analysis of fire incidents in oxygen pressure regulating stations. In comparison, specific discussions on valve and piping selection for oxygen vent systems remain relatively limited.
This article will present several practical engineering cases to illustrate how cost optimization can be achieved while maintaining compliance with safety requirements.
Ⅱ Engineering Practice Cases
Case 1: High-Pressure Oxygen Venting System in a Coal Chemical Project
Process Parameters：
Flow rate：45,000 Nm³/h；
Pressure：88 bar(A)；
Temperature：20°C
Customer’s Control Valve Selection：
Fisher 6" Class 900 control valve, with the valve body and trim fully manufactured in Monel alloy.
Upstream Piping of the Control Valve：
Flow velocity：8.27 m/s； PV value：72.78 MPa·m/s
Since the upstream section of the control valve is considered a non-impact service area,
the requirements specified in GB 16912-2008 are consistent with EIGA recommendations: PV ≤ 80,
allowing the use of stainless steel piping.
Downstream Piping of the Control Valve：
If the downstream piping is directly connected to the vent outlet, the flow velocity will approach sonic speed, and the resulting vibration may cause severe damage to the piping system.
Although a solution using fully Monel piping combined with a noise reduction plate can be adopted, it requires the flow velocity before the noise reduction plate to be controlled at approximately 0.3 Ma, while the velocity after the plate must still comply with the safety requirements for stainless steel piping.
However, the cost of a fully Monel piping system, including the transition from DN150 to DN450 and corresponding flanges, would be extremely high.
Our Proposed Solution：
DN150/DN250 piping and reducer：Monel
DN150 orifice plate：Silicon brass
DN250/DN450 piping and reducer：Silicon brass
DN450 orifice plate：Silicon brass
The specific pressure drop distribution is as follows:
1.First-stage orifice plate (No. 4)：
Size 10" Class 300， Rated Cv = 212， Material: Copper-nickel alloy
2.Second-stage orifice plate (No. 3):
Size 18" Class 150， Rated Cv = 550， Material: Copper-nickel alloy
3.The flow velocity conditions under each operating condition are as follows:
On-site Installation Photos:
By adopting segmented material selection, the material cost was significantly reduced while ensuring system safety.
Case 2: Medium-Pressure Oxygen Venting System for a Steel Industry Customer
Process Parameters：
Flow rate：60,000 Nm³/h；
Pressure：20 bar(A)；
Temperature：20°C
Control Valve Selection：
CenturyVal DN200 PN40 control valve，
with copper alloy valve body and Monel internal trim.
Upstream Piping of the Control Valve：
A DN300 stainless steel pipe is connected to the DN200 copper alloy valve through a copper alloy reducer.
Flow velocity：28.43 m/s
Downstream Piping of the Control Valve：
DN200/DN450 copper reducers with copper noise reduction plates.
Ensure that the downstream flow velocity and Mach number after the noise reduction plate are maintained within acceptable safety limits.
Case 3: Low-Pressure Oxygen Control in Copper Smelting Applications
In the copper smelting industry, there is another challenging issue that often causes difficulties for engineers：the throttling and supply of low-pressure oxygen from backup systems.
Process Parameters：
Flow rate：40,950 Nm³/h
Pressure reduction：From 5.2 bar(A) to 3.1 bar(A)
Temperature：20°C
Under this operating condition, GB 16912 does not distinguish between impact and non-impact service conditions. It uniformly requires the flow velocity to be ≤ 30 m/s. Therefore, copper reducers are required on both the upstream and downstream sides of the control valve.
If a flanged connection design were adopted, the overall cost would be excessively high.
Control Valve Selection：
CenturyVal DN250 PN16 control valve with copper alloy valve body.
Piping Configuration：
Upstream of the Control Valve：
DN400 stainless steel pipe connected to a DN400/DN250 red copper reducer by welding, followed by connection to DN250 red copper piping.
Downstream of the Control Valve：
DN250 red copper piping connected to a DN250/DN500 red copper reducer by welding, followed by connection to DN500 stainless steel piping.
Key Challenge：
Welding technology for dissimilar materials (stainless steel and red copper).
Flow Velocity Calculation：
Upstream DN400 stainless steel piping flow velocity complies with the requirements of GB 16912.
Downstream DN500 stainless steel piping flow velocity complies with the requirements of GB 16912.
Ⅲ Conclusion
The key considerations in the selection of oxygen venting systems include accurately identifying impact and non-impact service conditions, reasonably controlling flow velocity and Mach number, and optimizing material selection within the defined safety limits.
For high-pressure applications, segmented material selection (Monel + Silicon Brass) can be adopted as an alternative to a fully Monel piping solution, achieving significant cost optimization while maintaining safety requirements. For medium- and low-pressure applications, attention should be paid to the technical feasibility of reducer transitions and dissimilar material welding processes.
Compliance with applicable standards is the fundamental requirement, but it does not mean that costs cannot be optimized. The key lies in a comprehensive understanding of the standards, operating conditions, and engineering requirements.