Cryogenic Pumps | Design, Install & Maintain
Preface
With the development of the chemical industry, particularly the rise of coal chemical industry as a substitute for petroleum in recent years, air separation equipment is trending towards larger scales, higher gas product pressure levels, more liquid products, higher product purity, lower operating costs, and stable and reliable equipment operation. In air separation processes, internal compression and hydrogen-free argon production processes commonly employ cryogenic liquid pumps (hereinafter referred to as cryogenic liquid pumps) as process pumps and production pumps. Cryogenic liquid pumps play a crucial role in air separation processes, and their operating condition directly affects whether the entire air separation process can meet design targets.
As an industry-leading supplier of cryogenic gas equipment, Zoiun Fluid & Gas Equipment Co., Ltd. has accumulated extensive experience in the design and application of cryogenic liquid pumps. We currently provide equipment that meets international standards to ensure efficient and stable operation. Due to the technological gap between domestic and foreign products, domestic cryogenic liquid pumps are typically used in high-pressure, low-flow, or low-flow, high-pressure applications, whereas air separation plants with capacities above 20,000 Nm³/h often use foreign brands of cryogenic liquid pumps.
Common foreign brands include CRYOSTAR, CRYOMEC, and ACD. Cryogenic liquid pumps are generally classified into vertical pumps and horizontal pumps based on their structure. Horizontal pumps are mainly used for small-flow-rate liquids such as argon pumps, oxygen pumps in small air separation units, and filling pumps. Vertical cryogenic liquid pumps are mainly used in internal compression air separation units with relatively large flow rates, such as large air separation process pumps. This article will focus on vertical pumps.
Engineering Design of Cryogenic Liquid Pumps
Design Principles
> Air separation processes involve liquid oxygen pumps, liquid argon pumps, liquid nitrogen pumps, and sometimes filling pumps, all designed with one in use and one on standby. For ease of installation, maintenance, and safety, each liquid pump is made with a separate cold box, placed outside the main cold box of the air separation unit. To facilitate maintenance, a partition is used to separate the two pumps, with the panel made of Q235B and reinforced with sections at appropriate locations. In the design and manufacturing process, Zoiun Fluid & Gas Equipment Co., Ltd. ensures structural strength while fully considering the convenience of operation and maintenance.
> CRYOSTAR uses a flange structure for the upper support of its pumps. The Kaifeng Air Separation Group Research Institute uses a 30mm thick steel plate to open a hole in the top of the small cold box, while ACD generally makes a separate support to meet structural requirements. The equipment supports and pipe supports used in the inner box are all made of stainless steel to ensure material strength in cryogenic environments. PTFE elastic tape is used as a cushion at the connection between the cryogenic pipeline and the pipe support. Using a support with a 30mm thick steel plate opening can better seal the pump cold box, preventing the heat exchange between external moist air and the air inside the cold box, thereby improving the cold box's insulation effect. Zoiun engineers have also adopted similar measures in cold box design to ensure our equipment operates stably under extreme low temperatures.
> The liquid discharge line at the pump inlet must be led out from the top of the pipeline. Since the pump heating gas pipeline is installed on the pump outlet pipeline, a filter must be installed. The heating gas pipeline of the pump must also be led out from the top of the pipeline. The supports for the pump inlet and outlet pipelines must be able to withstand the stress caused by the contraction of the pipelines inside the cold box and meet the pump manufacturer's support requirements. Zoiun pays special attention to the rationality of pipeline layout during equipment design to ensure equipment stability under various working conditions.
> A safety valve is added after the pump inlet valve, with the pressure level changed to 5 barg (1 bar = 10^5 Pa).
> A return gas pipeline needs to be installed for the pump, which can be directly vented or returned to the cold box. If returned to the cold box, the return gas pipeline must always maintain an upward slope. Many sites do not follow the drawings during pipeline configuration, resulting in the cryogenic liquid pump failing to start normally, even forming a liquid seal. In recent years, returning the vaporized product gas to the distillation column to reduce product waste and improve extraction rates has become a common practice. The solutions provided by Zoiun fully consider the design of the return gas pipeline to ensure smooth gas return and minimize product waste.
> The sealing gas source pressure of the pump must be at least 3 barg higher than the pump inlet pressure, but must not exceed 15 barg.
> A local pressure gauge must be installed before the sealing gas pipeline.
> An air conditioner must be installed in the inverter room, as the operating environment temperature of the inverter should be below 40°C.
> A sunshade should be added above the pump to prevent direct sunlight from causing the motor bearing temperature to rise too high.
> The motor housing must be grounded on-site to meet the motor grounding requirements.
Installation of Cryogenic Liquid Pumps
Installation Requirements
> All valves, pipes, and pipe fittings must be clean, dry, and free of oil before installation. If the manufacturer has already degreased them and they have not been contaminated, degreasing is not required during installation. If there is oil contamination, degreasing is necessary. The cleaning and degreasing procedure for aluminum alloy pipes is as follows: alkaline washing → rinsing with clean water → nitric acid passivation → rinsing with clean water → degreasing → rinsing with clean water. Before degreasing, debris and trash inside the pipeline must be removed. Tetrachloromethane solvent must not be used as a degreaser; trichloroethylene or perchloroethylene solvent must be used instead.
> The cold valves inside the cryogenic liquid pump cold box should be installed simultaneously with their corresponding supports. The valve stem of the cryogenic liquid valve should tilt upwards by 10-15 degrees. When welding the pipe and valve body, the valve should be closed first, and cooling measures should be taken to ensure the welding temperature does not exceed the required temperature. When installing the valve, the arrow direction on the valve body should align with the liquid flow direction, and attention should be paid to the correct installation of the valve.
> The parallel distance between the heating pipe and the cryogenic liquid pipe or the wall of the liquid container should be at least 300mm, and the crossing distance should be at least 200mm.
> The distance between the outer wall of the pipeline and the inner wall of the cold box steel frame should be: not less than 400mm for cryogenic liquid pipes, and not less than 300mm for cryogenic gas pipes.
> When dealing with aluminum pipes, rusty tools must not be used, and a stainless steel wire brush should be used.
> The longitudinal installation level deviation of the installed pump should not exceed 0.10/1000, and the transverse installation level deviation should not exceed 0.20/1000. The deviation should be measured on the pump's inlet and outlet flange surfaces or other horizontal surfaces.
> All fixed connections should be tight, and all display instruments, safety protection devices, and electrical control devices should be sensitive, accurate, and reliable.
Commissioning Procedures for Cryogenic Liquid Pumps
Commissioning Process
> Measure the insulation of the motor, and the value should be greater than 100 MΩ; otherwise, the cause must be identified and addressed.
> Open the pipeline in front of the sealing gas filter to blow out the sealing gas pipeline. The on-site pressure of the sealing gas should be 5-100 bar, and the dew point requirement is below -65°C. Adjust the pressure of the sealing gas gauge: the inlet pressure should be 0.2 bar higher than the reference pressure, the reference pressure should be 0.2 bar higher than the outlet pressure, and the intermediate body purging gas pressure should be 0.2 bar.
> Close the pump inlet and outlet valves, open the inlet heating valve and outlet vent valve to purge the pump body and connecting pipelines, and measure the dew point of the gas discharged from the outlet purge valve, which should be below -65°C. Dehumidification treatment of the pipeline volute should be carried out as required by the process.
> Manual rotation must be easy.
> After the dew point meets the requirement, close the inlet heating valve, open the vent valve in the three-valve group on the heating gas pipeline, open the pump inlet valve, inlet and outlet vent valves, pump body return valve, and pump body vent valve, and start cooling the pump.
> When the pump inlet and outlet vent valves discharge liquid, and the time exceeds 4 hours, manual rotation should be easy. Check whether the pump direction is correct (see the arrow on the motor end cover).
> Fully open the pump inlet valve and return valve, reconfirm that the pump inlet and outlet vent valves and the pump body vent valve discharge liquid only, with no gas-liquid mixture.
> Similarly, manual rotation should be easy at this time.
> Close the pump outlet vent valve, gradually close the pump inlet vent valve and pump body vent valve, start the pump, gradually accelerate and close the return valve.
> When approaching the pump's rated operating point, close the pump inlet purge valve (the pump inlet valve must be fully open).
> Continue to increase the speed and slowly close the pump return valve (do not fully close the pump return valve). Until the pump's speed, outlet pressure, and flow rate reach appropriate values, then close the pump body vent valve.
> It is recommended to open the pump vent valve for 10 seconds or until liquid flows out every 4 hours when the pump is not insulated.
Precautions During Daily Operation
Maintenance Recommendations
If cavitation is detected, the pump must be stopped and inspected, and restarted after reheating and cooling the pump. Signs of cavitation include:
1. Fluctuation of the reference pressure on the pump's sealing gas gauge.
2. Unstable outlet pressure when pump speed and pipeline valves remain unchanged.
3. Continuous decrease in differential pressure between inlet and outlet when pump speed does not drop.
4. No change in outlet pressure when pump speed increases and outlet valve remains unchanged.
5. Differential pressure does not increase when pump speed rises to 1000 r/min during startup.
6. Hearing a squealing sound from the pump body or significant vibration on-site.
7. It is recommended to disassemble and clean the pump inlet filter about a week after the initial operation.
8. The motor bearings should be regularly lubricated according to the motor model, with the same type of lubricant used, and mixing of different lubricants is not allowed.
9. Manual rotation should be performed before each pump startup to ensure smooth rotation without jamming.
10. If the impeller rotates and the pressure behind the pump is 0, or if the pump is repaired, reheating and purging of the pump body and pipelines is required, and the dew point must be measured below -65°C before startup.
Conclusion
This article summarizes the experiences related to the design, installation, commissioning, and operation of cryogenic liquid pumps. As the technical team of Zoiun Fluid & Gas Equipment Co., Ltd., we are committed to providing efficient and stable cryogenic liquid pump equipment for our customers and continuously accumulating experience in practical applications. We hope these recommendations can help engineering and technical personnel improve the efficiency and stability of cryogenic liquid pumps. Strict adherence to technical details and specifications in actual engineering will have a significant impact on the long-term operation and performance of the equipment.
