Thursday, October 30, 2008
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Multi-stages compressor with intermediate cooling and liquid knock-out is commonly applied in compressing gas from low pressure (say 6 barg) to very high pressure (say 60 barg). This may required 2-stages compression using centrifugal compressor. Intermediate pressure at interstage compressor section could be one-third or half of discharge pressure of final compression stage. Thus in many event, the final stage compression section and intermediate section will have the higher design pressure whilst suction section will have lower design pressure.
Check valve is normally used to install between the two section with different design pressure. Check valve failure (stuck open or leakage) is one of the credible scenario which shall be considered. One of the question is in this check valve failure case, how much back flow shall be considered and how to reduce back-flow and minimize suction Pressure relief valve (PRV) relief capacity ?
Following are some strategies can be considered :
a) Single Check Valve
If single check valve is installed, one shall consider the check valve possibly stuck open. This results 100% back flow through the check valve.
b) Single Check Valve plus Single Shut-down valve
If single check valve and single shutdown valve are installed, one shall consider the check valve possibly stuck open. Although Shutdown valve is available for shut-close purpose, the reliability of single shutdown valve is still insufficient to act as ultimate safeguarding purpose. Thus, this results 100% back flow through the check valve.
c) Double Check Valve
If additional check valve with different technology and manufacturer, one may consider one check valve is stuck open and second check valve is still in place to perform it duty. Nevertheless, one shall not consider the second check valve is 100% reliable and leakage is still possible. Thus, one of the recommendation is to consider 1st check valve full open second check valve leak with 10% check valve flow area.
d) Single Control Valve, Single Check Valve Plus Single Shutdown Valve
If single control valve, single check calve plus single shutdown valve in series are installed and there are at lease two trips are available to trigger back-flow occurrence, this arrangement may be considered acceptable as ultimate safeguard. However, a proper Instrumentation Protective Functional (IPF) studies shall be conducted to confirm.
Import and Debatable Remarks
Fluid Cleanliness Nature - Having say that, above has considered the fluid is clean (no fouling and solid) and will not cause check valve unable to prevent reverse flow. The cleanliness of fluid is one of the critical factor shall be analyzed to derive the safeguarding strategy.
Operation Experiences - Besides cleanliness of fluid, any possible corrosion and surging of system may lead to damage of check valve internal. Some operation experiences shown that there is possibility of check valve internal damaged without operator awareness. Thus, well maintenance procedure and testing and checking procedure is also part of the factors to be considered.
Above strategies may be considered to minimize or eliminate back flow scenario, however a details analysis shall be conducted.