Liquid-air valve is the key device of the liquid air separation unit, and its operating condition directly determines the stability of the unit. The valve adopts liquid air as medium in flow direction of low inlet and high outlet, and has upstream pressure of 7.1MPaA, downstream pressure of 0.6MPaA, and operating temperature of -175°C. The corresponding saturated vapor pressure to the temperature is 0.5MPaA, which means at 0.5MPaA the medium will flash, changing from pure liquid to gas liquid two-phase flow. The original valve structure of Hualu Hengsheng is shown as Figure 1. By adopting two-stage pressure reduction structure of valve element and sleeve, it will cause the following problems in use:
1) Large vibration and noise in operation;
2) Failure to positioner, sudden closure of valve.
Figure 1 Original Valve Structure Chart
In view of above problems of the valve in operation and the importance of valve location, Hualu Hengsheng entrusts us to transform the valve.
Analysis and Response:
Vibration is the common problem of control valve, the main causes generally are that: 1) when high-speed medium impacts on the valve element, the valve element vibrates due to larger unbalanced force; 2) when medium pressure decreases below saturated vapor pressure of temperature in working condition, medium flashes and forms gas liquid two-phase flow. The poor flow stability may easily cause high-frequency vibration of valve. Except for the hardware problems and low-power actuator (by calculation, the output force of the valve actuator is enough to overcome the unbalanced force of medium), the control failure of positioner is generally associated with vibration of valve. As the positioner has the highest anti-vibration frequency, if the vibration exceeds the highest anti-vibration frequency, the control part of the positioner will fail temporarily, causing abnormal action of the valve.
For the above causes, the analogue simulation of pressure field and velocity field of medium is made by Ansys Workbench 14.0 in this article, so as to find out genesis of vibration, and make purposeful transformation to the valve.
Transformation and Effect:
According to calculation, on the premise that the overall structure of existing valve is unchanged, the original sleeve is increased by one stage; the number of pressure reduction stages is increased from 2 to 3; new trim structure is shown as Figure 2.
Figure 2A Front View Sectional Structure of Three-stage Trim after Transformation
Figure 2B Top View Sectional Structure of Three-stage Trim after Transformation
The structure after transformation eliminates flashing, reduces velocity of medium, and solves problems of large vibration and noise on the spot, so that the valve can stably operate. Since its transformation, the valve has been operating over 8 months. According to user’s feedback, online operation is stable with no vibration, no failure, reliable control of positioner and the noise is lower than 65dB.
Original sleeve structure Improved sleeve structure
Both MTG and MTO are fixed bed technologies, and the reaction system and regeneration system are executed in a cyclic manner. The reaction and regeneration reaction temperatures are above 500 ℃, and the reaction system medium is hydrocarbons, while the regeneration system medium is high-temperature air.