GELIIMASH NATURAL GAS EXPANSION-COMPRESSION MACHINES

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This article gives a brief account of the experience of building and operating turboexpander units in gas

distributing systems and in plants for low-temperature natural gas preparation for transportation and

examines the special features of their devices and designs.

A promising line of development of modern natural gas (NG) processing engineering and technology is buildingmodular plants for liquefying natural gas for separating and utilizing it as motor fuel as well as for building plants for NG

preparation for pipeline transportation. Another promising and priority line is developing and implementing new energy sav-

ing technologies – building utilization (recovery) type of turboexpander units (UTEU) associated with the use of differential

pressure of NG in distributing systems for generating electric power in a turboexpander-generator couple [6, 7].

In such installations, gas turbine equipment, namely, NG expander-compressors, plays a leading part. In the context

of development of new gas and gas condensate fields in northern Russia (Shtokman and Yamal) and East Siberia and because

of the problem of building high-efficiency NG cooling systems, development and manufacture of reliable and efficient gas

turbine equipment, i.e., expander and compressor, is a pressing task.

Currently, Geliimash association has been carrying out work on development, manufacture, and introduction of

modern turboexpander units (TEU) for integrated natural gas processing plants (INGPP) as well as R&D work for creating

new equipment for production, storage, and utilization of liquefied natural gas (LNG).

In 2010, a 590 kW turboexpander unit of the trademark TDA-24.6/3.6 was commissioned at the OPG-3-3-0.6 natu-

ral gas liquefier complex in Ekaterinburg for production of LNG (Fig. 1). The LNG production process at the gas distributing

station (GDS) No. 4 is based on the use of the pressure difference between the trunk pipeline and the gas distributing network.

This is the first LNG production complex in Russia. It operates on a cycle that uses an expanding machine – a turboexpander

(TE), as the cold source. The turbine module consists of a shaft on oil-lubricated bearings with a cradle-mounted working

expander and braking compressor impellers and has the following basic technical data.

Turboexpander

Pressure at turboexpander inlet p0, MPa . . . . . . . . . . . . . . . . . . . . 3.6

Pressure at turboexpander outlet p2, MPa . . . . . . . . . . . . . . . . . . . 0.76

Temperature at turboexpander inlet T 0, K . . . . . . . . . . . . . . . . . . . 218.15

Mass rate of gas flow through turboexpander G, kg/sec . . . . . . . . 6.83

Chemical and Petroleum Engineering, Vol. 47, Nos. 5–6, September, 2011 (Russian Original Nos. 5–6, May–June, 2011)

GELIIMASH NATURAL GAS

EXPANSION-COMPRESSION MACHINES

E. B. Fedorova,1

V. L. Stulov,

and G. A. Ayupov2

COMPRESSORS, PUMPS, AND PIPELINE FITTINGS

Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 5, pp. 28–32, May, 2011.

0009-2355/11/0506-0334 ©2011 Springer Science+Business Media, Inc.334

1 Gubkin Russian State University of Oil and Gas, Moscow, Russia.2 Geliimash Scientific-Production Association, Moscow, Russia.



Turbocompressor

Pressure at turbocompressor inlet pinit, MPa . . . . . . . . . . . . . . . . . 3.1

Pressure at turbocompressor outlet pfin, MPa . . . . . . . . . . . . . . . . 3.75

Temperature at turbocompressor inlet T init, K . . . . . . . . . . . . . . . . 300.15

Mass rate of gas flow through turbocompressor G, kg/sec . . . . . . 7.99

In developing TDA-24.6/3.6 expander, use was made only of tested designs that proved their worth in prolonged

operation of expander-compressors of large medium- and high-pressure air fractionating plants (AFP) as well as of long expe-

rience of designing and building TE for cooling and liquefying other gases (helium, hydrogen, nitrogen, etc.). Semi-open typeof impellers having interjacent blades made of AK-6 T1PP aluminum alloy, which are most commonly employed for the

referred design, were used in the flow section of the TE and compressor. Impellers having interjacent blades and ribs on

the blade disk help prevent flow separation in the impeller and attain acceptable efficiency values with simple manufacturing

technology. Wing-shaped blades, well proven in a wide range of subsonic, critical, and supersonic flow speeds and not infe-

rior in efficiency and technological effectiveness to other types of shapes (profiles), are used in the guide device of the TE.

The design provided for TE efficiency ηt = 75 % and 3 tons/h output of the unit.

During the start-up and tune-up operations and subsequent operation of the TDA-2.6/3.6 under conditions close to

the rated (turboexpander: p0 = 3.02 MPa, p2 = 0.64 MPa, and T 2 = 158 K; turbocompressor: pinit = 2.46 MPa, pfin = 3.2 MPa;

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Fig. 1. LNG production complex at GDS No. 4 in Ekaterinburg.

Fig. 2. Characteristic of turbine stage ηs

= ƒ( xs).



n = 26300 rpm), the output of the unit was 2.9 tons/h, which matches the value for these conditions of operation of the

expander-compressor couple. Of maximum interest, from the point of efficiency of the radial turbine stage and its operation

under various conditions, is the dependence of the expander efficiency on the reduced circumferential speed xs = u1 / cs (u1 is

the circumferential speed of the impeller, and cs is the isoentropic speed), the optimum values of which lie in the 0.6–0.7

range [2]. According to the results of processing of the obtained experimental data for this stage, the parameter xs

= 0.67 at

the obtained net turboexpander efficiencyηs = 83 % (Fig. 2). The compression ratio of the turbocompressor stage in this case

was pfin / pinit = 1.3.

In 2010, the expander-compressor DTP-95/7.0 at the plant for low-temperature separation (PLTS) of NG from Surgil

(Uzbekistan) gas condensate field was built and made ready for start-up and tune-up operations.

A 700 kW turboexpander unit (TEU) on electromagnetic bearings has the following basic parameters:

Turboexpander

Pressure at turboexpander inlet p0, MPa . . . . . . . . . . . . . . . . . . . 7

Pressure at turboexpander outlet p2, MPa . . . . . . . . . . . . . . . . . . 5

Temperature at turboexpander inlet T 0, K . . . . . . . . . . . . . . . . . . 283.15

Mass rate of gas flow through turboexpander G, kg/sec . . . . . . . 26Turbocompressor

Pressure at turbocompressor inlet pinit, MPa . . . . . . . . . . . . . . . . 4.65

Pressure at turbocompressor outlet pfin, MPa . . . . . . . . . . . . . . . 5.6

Temperature at turbocompressor inlet T init, K . . . . . . . . . . . . . . . 309.15

Mass rate of gas flow through turbocompressor G, kg/sec . . . . . 25.7

Like TDA-24.6/3.6, the turboexpander DTP-95/7.0 is a radial-axial flow (mixed flow) type of centripetal turbine.

The radial-axial flow and semiopen-type compressor impellers lie on shaft brackets and ensure maximally possible efficiency

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Fig. 3. Flow diagram of expansion cycle for producing LNG using differential pressure at

GDS: PU1, PU2 – NG purification units; HE1, HE2 – heat exchangers; TE – turboexpander;

TC – turbocompressor; EC – end cooler; TV – throttle valve; S – separator.



and reliability under the set conditions. A controllable guide device ensures efficient and steady operation of the TEU at gas

flow rates varying from +10% to –30% of the rated.

Depending on the process flow diagram, the TDA-24.6/3.6 and DTP-95/7.0 units provide for various schemes of

linking up of the expander and compressor stages of the TEU (Figs. 3 and 4). For instance, in the methane liquefaction pro-

cess diagram of OPG-3-3-0.6, the compressor stage compresses the gas flow fully before the TE, ensuring the required dif-

ferential pressure. The liquefaction factor of this technological process is 0.11. In the PLTS flow scheme, the crude gas from

the well, having passed through the separator and heat exchanger units, enters the flow section of the expanding stage of the

DTP-95/7.0, while the turbocompressor ensures the calculated (rated) pressure reduction at the expander outlet. The com-

pression ratio of the compressor stage pfin / pinit = 1.2.

Note as well the difference in the compositions of the gas for the above-described technological schemes of methane

liquefaction and low-temperature NG separation, which exerts considerable influence on the operating parameters and oper-

ating conditions of the units. This fact determined in many respects the design features of these expander-compressors.

The basic requirements of the design of an expander-compressor unit for a low-temperature NG separation plant are

the following:

1) ensuring an isoentropic efficiency of the turboexpander unit of not less than 80%;

2) reducing power loss due to friction in the bearings;

3) reducing cost of equipment and maintenance;

4) preventing process contamination risk; and

5) ensuring high TEU operation reliability.

Further, TEU operation reliability was the dominant factor.

Based on the adopted designs, the expander-compressor unit of the PLTS was built as an independent single unit, onwhose common bearing frame all the components are mounted (Fig. 5):

• expander-compressor;

• rotating guide device system;

• unit for controlling the rotor suspension and the automated process control system (APCS) of the unit as a whole;

• unit of input separator with a heating system having a set of safety valves and shutoff-control valves;

• system for delivery of buffer sealing gas;

• system of blowers;

• working gas filters;

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Fig. 4. Flow diagram of PLTS: HE1, HE2 – heat exchangers; S-0, S1, S2, S3 – separators;

TE – turboexpander; TC – turbocompressor.



• electrical local control panel;

• system for delivery of hydrate formation inhibitor (diethylene glycol);

• whole set of pipe connections, valving, sensors, instruments, cables and connectors; and

• electrical equipment, heat, sound, and anticorrosion insulators, etc.

In designs of modern expander-compressors taking part in processing of NG and its preparation for transport as well

as in utilization of the energy of the excess gas pressure at gas distributing stations (networks) and gas pickup points (ener-

gy saving turboexpander units), generally oil- and gas-lubricated bearings are used as rotor support [4, 7]. Conventional

rolling and sliding bearings have now attained a high technical level. However, the nature of the processes occurring in them

renders use of these bearings for meeting the requirements of DTP-95/7.0 practically impossible. Tests and practical experi-

ence of industrial operation of turboexpander units at gas fields have shown that failure of radial thrust bearing assembly of

the rotor often occurs in transitional regimes of operation of the equipment [5]. Also, in designing TEU, account must be

taken of the peculiarities of climatic conditions in which it operates. For many fields in CIS countries, sharply continental hot

and dry climate is typical. The temperature difference, depending on the time of the year, is quite significant. In summer, the

ground temperature attains 60–80°C, which complicates and raises cost of servicing lubricating system because of cooling of

the oil when oil-lubricated radial and axial bearings are used. Use of gas-lubricated bearings as rotor support is restricted

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Fig. 5. TEU on magnetic bearings for natural gas processing at Surgil field in Uzbekistan.

Fig. 6. DTP-95/7.0 expander-compressor for low-temperature natural gas separation:

1) impeller; 2) brake wheel of compressor stage; 3) guide device having rotatingblades; 4) rotor; 5) magnetic suspension; 6 ) expander-compressor housing.



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TABLE 1

Object Oil-lubricated bearing (OB) Active magnetic bearing (AMB)

Technicalsupport

Servicing covers stator part of bearing, lubricating and gassealing systems. Common concept of preventive servicingis applied

Maintenance covers additional ball bearings, fans and fil-ters of active magnetic bearing (AMB) cabinet. Servicing israre than for OB because the bearings operate withoutmechanical contact

Machinemonitoring

Monitoring covers vibration and temperature of bearings.Tracking of load occurs through bearing temperatureand/or axial shift

Data obtained from advanced signal pickups are processedby a digital monitoring system and stored on a hard disk,which allows one to get a complete picture of rotor behav-ior remotely. Load is measured

Machinediagnosis

For rotor diagnosis, often disassembling of machine isrequired

Some aspects of rotor diagnosis may be carried out remote-ly and without disassembling of machine

Disbalance andvibration

Disbalance is transmitted to bearing support (and, conse-quently, to entire machine) via oil film. Disbalance givesrise to vibration, which can be offset but not eliminated

Disbalance is actively restrained by magnetic bearing andnot transmitted to bearing support. Rotor is freely suspend-ed without any mechanical contact and rotates around itsinertial axis

Reverserotation

Rotation of more than 500 rpm is not recommended Reverse rotation is accomplished without problems

Speed variationand wear of

rotor

Oil film thickness is generally kept constant by oil pres-sure. In case of frequent and rapid accelerations, oil foul-ing (by metal particles or water), or oil quality variation,rotor wear increases

Rotor is constantly in levitation during operation withoutmechanical contact. In case of frequent and fast accelera-tions, there is no risk of rotor wear

Worsening of oilviscosity

Oil viscosity may deteriorate due to action of gas, mois-ture, and aging. The phenomenon intensifies if oil tank isunder high pressure

No oil is used and there is no risk of deterioration of oil vis-cosity

Leakage in gasseals

Lubricating system is under gas pressure offset by gaspressure in seals. In case of drop in buffer gas pressure, oilmay leak into process gas

No oil is used. There is no risk of fouling of pipe connec-tions and process gas and no risk of formation of locks inpipes due to oil freezing

Vibration

Turboexpander on oil-lubricated bearings must operate instringently limited vibration range in compliance with

API 617, 7th edition. Vibration is measured by specialequipment

Seventh-edition API 617 standard takes account of advan-tages of magnetic bearings and allows for higher vibrationlevels for magnetic bearings than for oil-lubricated bear-

ings. In case of use of AMB, vibration levels are not deci-sive. Sensors built into magnetic bearings are used to deter-mined rotor position. Vibration level is monitored from acontrol cabinet

Environment

Oil and its waste may cause environmental problems.Through the bleeder pipe (vent) fixed in the lubricatingsystem the leaked gas, mixed with oil, may be dischargedfrom the oil tank to the atmosphere. At low air temperature,heating of lubricating system is required, which entailsadditional expenditures

No oil, so no adverse effect on environment. Bleeder pipe(vent) is not used, as in the case of lubricating system. Heat-ing is not required

Fire fightingsystem

In view of the presence of oil, appropriate fire-fighting sys-tem is designed

Absence of oil eliminates possibility of ignition

Dimensionrequirements

and capital work

Overall dimensions of lubricating system developed incompliance with API 614 standard are generally largerthan TEU dimensions. In case of air oil-cooling system,overall dimensions of lubricating system may increaseconsiderably

Overall dimensions of turboexpander using magnetic bear-

ings can be inscribed into overall dimensions of TEU itself.Since there is no external vibration, the machine may beplaced at higher levels or on light metal supports

Transport toyard

In case of transport of large oversized machines on OB(oil-lubricated bearing), special transporting conditionsmay be necessary

Because of smaller dimensions, special transporting condi-tions are generally not required for transporting the unithaving an AMB

Initial costCost of lubricating system designed in accordance withAPI 614 standard may be the same as cost of whole unithaving magnetic bearings

For large units, additional requirements may be imposed onthe lubricating system (for example, special vessels underpressure, paint, tool kit, drive, attachments of interces-sories, etc.). Cost of such a unit, but with magnetic bear-ings, will be less





5. V. P. Malkhanov, Turboexpander Units in Natural Gas Preparation and Distribution Systems [in Russian], Neft i

Gaz, Moscow (2004).

6. E. M. Marchenko and O. V. Malkhanov, “Turboexpander units for judicious use of energy of differential pressure of

natural gas,” Energosber. Vodopodg., No. 4, 12–16 (2009).

7. A. A. Stepanets, Energy Saving Turboexpander Units [in Russian], Nedra-Biznestsentr, Moscow (1999).

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GELIIMASH NATURAL GAS EXPANSION-COMPRESSION MACHINES