Commissioning of the pyrolysis gas compressor

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The pyrolysis gas (feed gas) compressor must undergo mechanical operation tests in the manufacturer’s workshop in accordance with the requirements of API617 standard； If requested by the user, performance tests must also be conducted in accordance with the ASME PTC10 standard; gases such as N2 and CO2 can be used as substitute gases. Pyrolysis gas compressors are typically designed with four or five stages, using three-cylinder or two-cylinder compression. The pyrolysis gas compressors designed and manufactured by our company for a certain petrochemical company feature four stages and three cylinders, with the model designation DMCL706+2MCL706+MCL704. 1.1 Mechanical operation test: The mechanical operation test of the unit (three-cylinder or two-cylinder) can be carried out under vacuum conditions; some compressor manufacturers conduct the test using a single-cylinder closed cycle, in which case a cooler must be used, and N2 should be filled in to maintain an inlet pressure between 0.01 and 0.35 MPa. Our company carries out the process under vacuum conditions; when the shaft seal is a floating ring seal, a labyrinth seal is used as a substitute for testing purposes ； When the shaft seal is a dry gas seal, a labyrinth seal is generally used as a substitute for testing purposes; however, if specified in the contract, the product’s own dry gas seal can be installed. In such cases, the workshop must be equipped with a three-valve assembly to control the differential pressure between the buffer gas and the equalizing gas, and it is also necessary to measure the amount of gas discharged by the dry gas seal. To maintain the vacuum level inside the machine, it is necessary to calculate in advance the cushioning amount at the seal on the machine side. For the DMCL706+2MCL706+MCL704 unit, when the vacuum level inside the unit is 97.32 kPa, the total amount of buffer gas required is 225 kg/h (the buffer gas can be oil-free and water-free instrument air or nitrogen). Therefore, sufficient vacuum pumps need to be installed to ensure this, or the existing vacuum system can be modified to achieve it. The mechanical power required for vacuum extraction in a tandem unit can be determined based on the manufacturer’s experience, after which the capacity of the starting motor, hydraulic coupling, and starting coupling can be selected. Based on experience, the value is 1.2. Per ASME PTC10, the specific volume ratio, Mach number, and Reynolds number under the compressor’s design conditions and test conditions meet its requirements. The pyrolysis gas compressor belongs to Class III, and a closed-cycle test per cylinder can be employed. The gas that can be used is CO2 or N2. The test air piping can be installed and configured using either the existing closed loop in the workshop or a separately designed closed loop for testing. It should be noted that the dual-intake compressor (DMCL706) requires a pressure-stabilizing and flow-dividing air storage tank to ensure balanced intake volumes on both sides. At the same time, for pipelines in such large-scale test closed-loop systems, it is also necessary to calculate the pipeline forces and moments, as well as the amount of expansion, in order to ensure stable operation during unit testing. For the 2MCL706 centrifugal compressor, which is compressed in two stages with the impellers arranged back to back, tests can be conducted using a dual-circuit system with each stage serving as a separate circuit. The test gas is stored in gas cylinder sets (CO2, N2 cylinders) or gas storage tanks, and the diaphragm control valve is regulated by a pressure regulator of the gas supply device to feed the test gas into the test pipeline and maintain a stable inlet pressure. Since the shaft seals of pyrolysis gas compressors are usually floating ring seals, the floating ring seal of the product must be installed on the compressor. For the low-pressure cylinder, use the product’s floating ring ； For the medium and high-pressure cylinders, test floating rings are used as a substitute. The seal oil system can be supplied with oil from the seal oil station provided in the workshop; the oil is pumped into the high-level seal oil tank using an oil pump. The oil pressure at the seal rings, which prevent gas leakage from the compressor, is controlled by a differential pressure control valve and a level control valve for the high-level oil tank. The contaminated oil on the inside of the floating ring is returned to the oil tank of the workshop’s seal oil station through a seal oil return device or the level control valve of the workshop’s oil-gas separator. When the shaft seal is a dry gas seal, due to the limitations of the conditions during plant commissioning, a labyrinth seal is generally used as a substitute. Before testing, it is necessary to check the accuracy of all testing instruments, verify the airtightness of the gas pipelines, ensure the purity of the test gas in the pipelines by using gas displacement methods, and check the differential pressure between the seal oil and the reference gas. During testing, the inlet pressure must be kept at the specified value for each cylinder, and the rotational speed must also be at the specified value for the test. Adjust the water flow in the gas cooler within the control loop to ensure the air temperature at the compressor inlet. For large compressor coolers, the water volume must take into full account the cooling of the return water; otherwise, it will affect the inlet temperature of the compressor. Under stable inlet pressure conditions, the operating conditions are adjusted by regulating the outlet throttle valve. The tests include 5 test points, including the surge point, and at least two repetitions are required to confirm the surge condition. After the performance tests, the test results must be compared with the design data to ensure compliance with relevant regulations; if the difference is too large, the structural dimensions of the impeller need to be modified and testing must be repeated. In accordance with the requirements of API617 standard, the bearings, seals, and rotor shall be inspected after testing. For the pyrolysis gas compressors DMCL706+2MCL706+MCL704 designed and manufactured by our company, the comparison between design conditions and test conditions is shown in Table 1, while the comparison between design results and test results is shown in Table 2. 2 Field Tests For the field commissioning of pyrolysis gas compressors, the unit is typically operated without load using air for mechanical testing, and with load using nitrogen or natural gas, in accordance with the requirements stipulated in the contract signed by the user as well as the conditions on site. The purpose of this test is to conduct mechanical operation of the turbines, compressors, and auxiliary systems, to evaluate the performance of electrical instruments, and to assess the performance of equipment, machines, and instruments in the process flow. 2.1 Operation of the air machinery under no-load conditions: The test steps are as follows: (1) Verify that the oil system and the turbine condensate system are functioning properly ； (2) Operation status and normality of the TCC integrated system, ESD, and DCS instrument systems ； (3) Open the manholes of each suction section in the separation tank, remove the flanges at the ends of the filter screens on the suction pipelines of each section, and install 20-mesh filter screens ； (4) Elevation openings at the pipe flanges at the outlet of each section leading to the cooler (a certain opening area is required) ； (5) Open one inlet valve and four outlet valves, and open the reflux valve ； (6) The test is conducted according to the on-site acceleration curve jointly determined by the user, the design institute, and the manufacturer, with records kept. 2.2 Natural gas circulation test: The load-bearing test is determined based on the user’s on-site conditions. Some users conduct air purge load tests on site, while others carry out nitrogen load tests; the DMCL706+2MCL706+MCL704 pyrolysis gas compressors designed and manufactured by our company underwent natural gas circulation load tests. Since the molecular weight of natural gas is relatively low (about 17.34), while that of the pyrolysis gas is around 27.7, it is necessary to be careful during the experiment regarding the vibrations in the unit caused by inconsistencies in different sections due to variations in the specific volume ratio at the same rotational speed. Therefore, in this experiment, the two return valves for the “three-return-one” and “four-return-four” configurations must be opened before starting up the system. Before the test, all pipelines, including the compressor pipeline system, must be filled with natural gas at a pressure of 0.15 MPa (G) ； Commissioning of lubricating oil and seal oil systems ； In the process flow, coolers, separation tanks, towers, and reactors are all utilized. Since natural gas is relatively dirty and contains H2S, sealed buffer gas must be ethylene gas at room temperature before the feed gas is introduced. At the same time, it is necessary to closely monitor whether the degassing device of the floating ring sealing system is functioning properly, in order to maintain the viscosity and flash point of the lubricating oil. When the shaft seal is a dry gas seal, it is necessary to introduce ethylene gas as a buffer gas before starting up, and pay attention to ensure normal operation of the one-time discharge of the dry gas seal during operation. Starting up still follows the speed-up curve for operation of the air machinery under no-load conditions. At 75% of the lowest first-order critical speed of the unit (2000 r/min for this unit), the mechanical operation is monitored for a period of time; thereafter, the speed is increased rapidly beyond the first-order critical speeds of each cylinder in the unit, until it reaches near the unit’s minimum speed (6000 r/min for this unit). Once the compressor is operating properly, it can continue to run. After confirming that there are no issues with the unit and process systems, feed is introduced to the pyrolyzer, and the hydrogenation reactor begins to operate; natural gas is displaced using pyrolysis gas to initiate the process operation. 3 Conclusion (1) As long as compressor manufacturers have the necessary testing equipment, a closed-loop system, a vacuum pumping system, and a cooling system, the successful testing of pyrolysis gas compressors in the workshop is entirely guaranteed, just like with other types of compressors. (2) The manufacturer shall, together with the user, determine the test procedures for the unit’s no-load and load cycling based on the on-site conditions (including test gases, acceleration curves, etc.), and switch to cracking gas to initiate operational use under normal operating conditions. This article is from Endong Information Network: http://www.enpinfo.com. The original link is: http://enpinfo.com/Compressor/Doc/Data/200807/14817.html