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Calculating settle-out pressure in compressor loops 11.05.2006 | Heydari Gorji, A., Sazeh Consultants , Tehran, Iran; Kalat Jari, H., Sazeh Consultants , Tehran, Iran
Simple procedure can be used to help determine piping and equipment design pressures into process loops Keywords: Ehen a compressor compresses gas from one system at P 1 and T 1 to another system at P 2 and T 2 and stops during the maximum pressure drop case, a differential pressure is developed. After a compressor shutdown, the gas is trapped between the upstream and downstream discharge check valve and the pressure is equalized out. This equalized pressure throughout compressor loops is called settle-out pressure. The maximum settle-out pressure is calculated from coincident high-trip pressures on both suction and discharge sides of the compressor. In some situations, a single pressure safety valve is desirable to protect equipment in a process loop. In this regard, and also to establish the design pressure for piping and equipment installed in the process loop, the settle-out pressure is required. Therefore, it is desired to reduce the settle-out pressure as much as possible. Settle-out pressure will decrease if the discharge-side volume of centrifugal compressors is minimized, for instance gas precooling is preferable to compressor discharge cooling. A simple procedure has been developed to determine the settle-out pressure into compressor loops (for instance, a reaction recycle-gas loop in which a compressor continuously recycles the process gas) or centrifugal compressor stations including knock-out drums, inter coolers and antisurge lines. Calculation procedure. To calculate the settle-out pressure, isobar sections of the system should be identified. The system volume and confined masses need to be determined and the lumped mass and enthalpy should be distributed in the settle-out situation to calculate the final pressure. At settle-out pressure, there is still the same mass of gas and system total volume equals the initial mass of gas. If some system components are filled with liquid and their volume doesn't change, then liquid volume is not considered. The calculation procedure is summarized as: . Estimate the equipment volume at each pressure level. Operating pressure and temperature are used to estimate the mass. In this regard, the following assumptions can be considered as a rule of thumb if there are not enough data. Assume 60% occupation of catalyst/ adsorbent inside reactors Assume 40% and 90% vapor phase inside separator and knock-out drums, respectively Assume 50% vapor phase inside piping, heaters and exchangers for two-phase services The compressor internal volume normally wouldn't be considered. . Settle-out temperature should be calculated based on the volume of each isothermal section, since it will change at more locations than pressure. In this regard, all gas masses in the system at the various pressures and temperatures are added to estimate the total system mass and finally, equalized temperature. . Then, settle-out pressure is calculated based on:
where: PS = Settle-out pressure, bara VS = Total actual volume, m3 TS = Settle-out temperature, °K PN = Atmospheric pressure, bara VN = Total normal volume, Nm3 TN = 273, °K To determine the settle-out pressure of the compressor trains for which there are not enough data to calculate the system mass or the temperature difference throughout compressor loop is not significant, a rough estimation procedure is proposed. In this procedure, the settle-out pressure is calculated based on the average value of system volumes and operating pressures. This estimation method is further explained in case study 2. Settle-out pressure in compressor trains is used as a basis for determining equipment and piping design pressures. In this regard, design pressure of the lowestpressure part should be calculated as 1.05 times the settle-out pressure to minimize unnecessary flaring during compressor trips or shutdowns. This will provide an adequate basis for calculating design pressure of higher-pressure parts. Thus, design pressure of other equipment is obtained by summation of this base design pressure plus operating pressure difference between each piece of equipment and the lowest operating pressure in the system. Case study 1: Reaction recycle-gas loop. A sample reaction loop (Fig. 1) is considered for calculating settle-out pressure. Reactor liquid feed, after combination with the recycled gas feed, is preheated by the reactor feed/ effluent exchanger and further heated by the fired heater up to the reaction temperature and supplied to the reactor. The reactor effluent gas is cooled by a feed/ effluent exchanger and two coolers. The cooled effluent enters the product separator and separated gas is recycled to the reactor by the recycle gas compressor.
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Calculating settle-out pressure in compressor loops | Hydrocarbon Proc... http://www.hydrocarbonprocessing.com/Article/2598353/Calculating-se...
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Fig. 1
Sample reaction recycle-gas loop.
Settle-out pressure should be calculated when the compressor is in maximum pressure drop shutdowns. The system should be assumed to be at normal operating pressure before compressor stoppage with all purge gas lines closed. The example with typical conditions in Table 1 illustrates a settle-out pressure calculation for the reaction loop to explain the procedure more clearly.
TABLE 1
Settle-out pressure calculation for a reaction recycle-gas loop Click image for enlarged view
Case study 2: two-stage compressor station. A sample two-stage compressor (Fig. 2) is considered for calculating settle-out pressure. The example in Table 2 illustrates a settle-out pressure calculation for a two-stage centrifugal compressor station to explain the rough estimation procedure more clearly. HP
Fig. 2
Sample two-stage compressor station.
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Calculating settle-out pressure in compressor loops | Hydrocarbon Proc... http://www.hydrocarbonprocessing.com/Article/2598353/Calculating-se...
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LITERATURE CITED 1
Guide for Pressure-Relieving and Depressuring Systems, 5th Edition, Standard 521, American Petroleum Institute (API), Washington, DC, 1997. ACKNOWLEDGEMENT The authors would like to thank the board of directors and process division director of Sazeh Consultants Company for their support.
* Corresponding author: E-mail:
[email protected]; Tel.: +98 21 88511256; fax: +98 21 88737540
Aliakbar Heydari Gorji is a senior process engineer at SAZEH Consultants in Tehran, Iran. He is a PhD student of chemical engineering and holds BS and MS degrees in chemical engineering, all from the Amirkabir University of Technology (Tehran Polytechnic). His area of specialization has included gas separation with membranes and his industrial experience has focused on simulation, basic and detail design of gas and petrochemical plants. He can be reached at e-mail:
[email protected].
Hamid Reza Kalat Jari is a senior process engineer in the process department of SAZEH Consultants in Tehran, Iran. Mr. Kalat Jari has been with the Sazeh Process Department for four years. Previously, he worked for four years with Total Fina Elf and eight years with National Iranian Gas Company as a process engineer and process control engineer. He holds a BS degree in chemical engineering from the Sharif University of Technology and an MS degree in chemical engineering from Tarbiat Modarres University. He can be reached at e-mail:
[email protected].
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