For gathering and transportation of associated petroleum gas, along with other compressor units, there are widely used units with screw oil-filled compressors. The engineering solutions used today allow to prevent the oil slobbering from the oil system into the inlet filter scrubber; exclude the formation of condensate in the operating cells of the compressor and its effect on the power consumption; not allow malfunction of oil circulation in the oil system during compressor start-up at negative temperatures (below minus 10 ° C). Due to what is it achieved, and what are the features of operation of units on heavy gas, with extremely low values of inlet pressure, in cold climates?

Associated petroleum gas (APG) is natural hydrocarbon gas dissolved in oil or located in the "caps" of oil fields. According to experts, the volume of APG produced in Russia exceeds 70 billion m3 today. Associated gas must be gathered, treated and fed into a transport gas pipeline. Such a technological task was solved by ENERGAS specialists at the Alekhinskoye oil field (Khanty-Mansi Autonomous District, Tyumen region).

The compressor station of the low separation stages of Alekhinskoye field was equipped with five booster compressor units (BCU) Enerproject of type EGSI-S-650 / 1500WA. Units with capacity of 7,000 m3 / h compress the low-pressure (0.01 Mpa) APG from the final separation stage and pump it into the pipeline at a pressure of 1.7 MPa.

During the commissioning of these BCUs, ENERGAS engineers encountered three problems:

• oil slobbering from oil system to inlet filter scrubber under impact of vacuum;
• condensate formation in the operating cells of the compressor when compressing APG;
• malfunction of oil circulation in compressor units at negative temperatures (below minus 10 ° C).

INLET VALVES UPGRADE

At emergency shutdown of BCU there was an oil slobbering under impact of vacuum from oil system to inlet filter-scrubber. First of all, this is due to a very large difference in the inlet and outlet pressure of the BCU, and if the input pipeline is not cut off from the main pipeline, gas pressure in the unit is discharged not only through the release flare, but also through the inlet pipeline. In this case, there is occurred oil entrainment from the oil system into the inlet scrubber. Secondly, the parameters of the specifications (on pressure), which were taken into account in the design and manufacturing of the BCU, differed from the actual operating conditions.

To ensure normal operation of compressor units at extremely low associated gas pressure, it was decided to upgrade the inlet valve system by installing high-speed valves at gas inlet (see the figure below). ENERGAS engineers and specialists of the Swiss company ENEPROJECT SA successfully solved this task.

By special order, an electromechanical drive of the inlet valve, equipped with a spring cut-off was manufactured. The difficulty was that it was necessary to mount, "fit" this drive into the existing BCU module, space of which is limited. In order to create a moment of force for instant closure in the event of an emergency of an inlet valve (damper) with a diameter of 400 mm, a force about of 3000 Nm is required. This can only be done with a special spring, but it would be unacceptably large. And then it was decided to divide this effort between two springs, which made the electric drive more compact and place it in the existing block-module.

It is important to note that the functional of the automatic control system of the BCU was expanded in parallel. ACS accurately monitors the status and essence of the incoming signals, analyzes and promptly issues the appropriate commands: to a normal shutdown or to an emergency "stop".

PREVENTION OF CONDENSATE FORMATION

Gas, which comes from the Central Processing Facility (CPF), has a density of more than 1.3 kg / m3, that is, compressor units compress so-called heavy gas. A characteristic feature of this gas is the extended solubility in oil.

In the oil separator, which operates at output pressure pc = 1.7 MPa (see Figure 1a), a large amount of gas, especially heavy fractions, dissolves in the oil. Oil from the oil separator is fed into the operating cells of the compression of oil-filled screw compressors. Pressure in the operating cells is lower than internal compression pressure p2 and gas begins to intensively escape from oil into the volume of the operating cells, which leads to an increase in pressure in them and to an increase in internal compression pressure to a value p2 '. At the same time, the cost of power for external compression and the cost of power for compressing one kilogram of gas in the compressor are reduced (in Fig. 1a the shaded area is proportional to the energy saving for compressing one kilogram of gas).

However, if the temperature of gas in the operating cells is below the condensation temperature of any gas fractions, they will begin to condense in the operating cells. The condensate occupies a smaller volume than gas, so the condensation of gas leads to a reduction in the pressure in the operating cells and reduction in internal compression pressure from p2 to p2" (see Figure 1b). This leads to an increase in power consumption for external compression and power for compression of one kilogram of gas in screw oil-filled compressors (in Fig. 1b the shaded area is proportional to the increase in the power for gas compression).

In addition, dissolving gas in the oil separator leads to a decrease in the kinematic viscosity of oil and an increase in the oil level in the oil separator.

To prevent condensation of oil in the operating cells of the compressor it was decided to expand the range of operating temperatures of oil and gas. At the same time the initial settings of the operating parameters of the compressor unit were fixed as follows: oil temperature 55 ° C, gas temperature 85 ° C. These operating temperatures did not allow the formation of condensate. Carrying out a number of calculations and experiments allowed us to justify the increase in the operating temperatures of the compressor unit: the oil temperature is up to 75 ° C, and the gas temperature is up to 105 ° C. To maintain the efficiency of compressor units, it was necessary to replace oil Mobil Glycoil MG11 with MG22 with an increased viscosity index.

Further operation of the compressor units confirmed the correctness of the engineering solution.

UPGRADE OF THE HEAT EXCHANGE SYSTEM

For normal circulation of oil at negative temperatures, there was required upgrade of the cooling system of compressor units.

Replacing MG11 oil with MG22 disrupted oil circulation in the oil system during compressor units’ start-up after prolonged standby at negative temperatures (below -10 ° C). This was due to the high viscosity of oil in the remote air-cooling apparatus. To avoid such a situation, an engineering solution was developed to heat oil in ACU during the start-up of the compressor unit.

During the start-up period, oil passes through internal ring and is heated by an electric oil preheater in the oil separator (so-called "hot start") and due to friction and resistance in the pipes. For normal operation, this heat must be removed to the ACU, but there is no oil coming in there, since a great effort is required to squeeze out thick oil from the ACU. As a result, the proposal was followed: remove heat from the oil tank and oil separator by liquid and use this liquid to heat the ACU.

To this end, there were installed additional elements of the heat exchange system (see the figure below): liquid heat exchanger in the oil tank; liquid circulation pump; liquid radiator for ACU heating; fan of forced air circulation; air duct; membrane expansion tank for liquid; connecting pipelines; control & instrumentation.

As a liquid, a solution of glycol with a concentration providing liquid freezing to -45 ° C was used. In addition to glycol, it is also possible to use antifreeze.

Thus, during the start-up period of the compressor unit (this is 20-25 minutes), heat that is evolved when gas is compressed in the compressor and transferred to oil is removed by glycol in the liquid heat exchanger (1). Heated glycol is pumped to the liquid radiator (3) by the pump (2). The radiator is forced blown by air, which is supplied by the fan (4). Heated air through the duct (5) is fed to the ACU and heats oil therein to the temperature required to start the normal operation of the compressor unit.

CONCLUSION

The experience of creating this compressor station showed that the company ENERGAS offers in each specific case individual technical solutions, developed taking into account the features of operation of the units. Individual approach allows to achieve maximum efficiency and reliability when operating booster compressor equipment.

When compressing heavy gas (high-density gas), it is proposed to raise the temperature of oil at injection into the operating cells and the temperature of gas-oil mixture on discharge in order to avoid condensation of gas in the operating cells. At the same time, it should be used a higher viscosity oil to lubricate the compressor and heat oil in the ACU during the start-up period of the compressor after a long standby at low temperatures. To work with APG, whose pressure is close to vacuum, it is necessary to install high-speed valves at gas inlet to the BCU.

Industry-specific magazine "Neftegaz.RU"