In modern air separation units (ASUs), the reliable transport of cryogenic liquids from the lower column to the upper column is critical for operational efficiency and product purity. This article provides a comprehensive technical analysis of gas lift valve technology - its purpose, working principles, and a field-proven retrofit approach that enables minimally-invasive upgrades to existing installations.
I. The Role of Gas Lift Valves
The primary function of a gas lift valve is to assist in transporting cryogenic liquids (such as liquid air and liquid nitrogen) from the lower column to the upper column in an air separation unit. It addresses liquid transport issues caused by insufficient column height differential or inadequate lower column pressure.
- Historical Context
Historical Context: Interestingly, many early air separation units were designed without lift valves. At that time, liquid transport relied entirely on sufficient pressure differential to occur naturally. It was only with the industry's growing focus on energy efficiency and consumption reduction - leading to lower air compressor discharge pressures - that gas lift valves became necessary. In a sense, gas lift valves exist precisely because of the drive for energy conservation.
- Critical Role in Argon Systems
In argon production systems, the role of gas lift valves becomes even more critical. The condensers at the top of both the crude argon column and the pure argon column require a stable supply of liquid air to function properly. Once the liquid air supply becomes insufficient or unstable, argon product yield and purity are directly affected, and in severe cases, the entire argon system may shut down.
- The Operational Dilemma
For large-scale steel mills or coal chemical enterprises, oxygen and nitrogen are the key products supporting the main production process, while argon is often treated as a by-product. When liquid transport issues are discovered after an ASU startup:
- Main column issues -Typically addressed immediately due to their impact on core production.
- Argon tower system issues -Often deferred because traditional retrofit solutions require production shutdown, insulation removal, and extended downtime with high costs. Companies often choose to sacrifice argon production to maintain overall operation.
II. Working Principle of Gas Lift Valves
The working principle of a gas lift valve can be summarized in one sentence: Dry gas is introduced downstream of the control valve in the cryogenic liquid transport pipeline, increasing the vapor fraction of the liquid flow, thereby reducing the density of the liquid column and enabling the liquid to overcome height differential and pipeline resistance.
- Core Mechanism
In the distillation column system of an ASU, there is a significant height differential between the lower and upper columns. As cryogenic liquid (e.g., liquid air) is transported upward from the lower column, it must overcome both the hydrostatic head of the liquid column itself and the resistance of the piping system.
Gas lift technology introduces a measured amount of dry gas (typically purified air or nitrogen from downstream of the molecular sieve) into the liquid stream. This increases the vapor fraction in the gas-liquid mixture. Since gas density is far lower than liquid density, the overall density of the mixture is significantly reduced, substantially decreasing the hydrostatic resistance and enabling successful liquid transport.
- Process Implementation
The ideal implementation is to fully account for gas lift requirements during the design phase, adding a small-diameter control valve on the piping downstream of the main control valve to precisely regulate the pressure and flow rate of the dry gas entering the pipeline.
However, for ASUs already in operation, if the gas lift function is found to be missing, traditional solutions often require production shutdown, removal of insulation material, piping modifications, and valve installation - involving long schedules, high costs, and significant operational risks. Therefore, achieving on-site minimally-invasive retrofit without shutdown has become a focal point of industry attention.
III. Project Requirements and Retrofit Solution
Problem Identified: During the operation of a customer's ASU, it was found that liquid air could not consistently enter the crude argon condenser - directly impacting argon system performance and creating a bottleneck in the overall separation process.
To address the lift function deficiency in the customer's LV1 and V701 valves, CenturyVal developed a minimally-invasive retrofit solution that requires no insulation removal, no piping changes, and can be implemented directly on-site. The core concept is to retain the original valve body, trim, and actuator while replacing the bonnet assembly and adding a gas connection system to equip the valve with lift functionality.
- Key Design Elements
1.Bonnet Retrofit:Four 1/2-inch ferrule fittings routed from the bonnet into a manifold pipe, achieving multi-channel uniform gas distribution across the valve chamber.
2.Filtration & Pressure Regulation: 1-inch filter-regulator configured to precisely regulate the flow rate and pressure of air entering the valve.
3.Gas Source Connection:1-inch ferrule fitting and stainless steel tubing installed upstream of the regulator for convenient welding connection to the on-site gas supply pipeline.
4.Specialized Tools:Dedicated assembly and disassembly tools provided to ensure smooth execution of the on-site retrofit process.
- VSD DN200 Retrofit Engineering
The engineering drawings and 3D models illustrate the detailed structure of the retrofitted bonnet and gas connection system for the VSD DN200 valve, including:
•Figure 1 -VSD DN200 Retrofit Engineering Drawing
•Figure 2 & 3 -Retrofit Bonnet Assembly 3D Views
IV. Site Retrofit Record
The following sections document the complete retrofit process through on-site photographs, from pre-retrofit condition assessment through completion.
- 4.1 Pre-Retrofit Condition
The original bonnet of the LV1 valve did not have lift function interfaces. The actuator and handwheel operating mechanism adjacent to the valve remained in good condition and could continue to be used after the retrofit.
- 4.2 Retrofit Execution Process
The retrofit procedure followed a carefully planned sequence to ensure safe and efficient execution:
Step 1:Preparation -Site inspection and verification of gas supply availability. All necessary components and tools positioned for immediate access.
Step 2:Original Bonnet Removal -The original bonnet is carefully detached while preserving the valve body, trim, and actuator assembly for reuse.
Step 3:New Bonnet Installation -The replacement bonnet assembly - equipped with lift gas ports - is installed using specialized 4J36 sealing washers and torqued in three sequential passes to ensure cryogenic-tight sealing.
Step 4:Gas System Connection -The filter-regulator assembly is mounted and connected to the bonnet ports via stainless steel tubing, then connected to the plant air supply.
- 4.3 Post-Retrofit Results
Successful Completion: After completion of the retrofit, both valves (LV1 and V701) were successfully equipped with lift functionality. The post-retrofit panoramic view clearly shows the newly added gas supply connections and filter-regulators above each valve, connected to the lift ports on the bonnets via stainless steel tubing. The overall system layout is compact with properly routed piping.
Note: The views and technical information presented in this article represent professional insights and are provided for reference purposes. Specific retrofit requirements should be evaluated on a case-by-case basis.