A comparative analysis has been performed of the suitability of using turbo-and piston (reciprocating) expanders in low-consumption units of natural gas liquefaction plants operating at an initial pressure p i = 3-5 MPa. Two versions have been investigated: 1) monoblock (single-block) expander-compressor unit (ECU) consisting of a turboexpander and a turbocompressor and 2) expander-generator unit (EGU) consisting of a piston expander and a generating engine. The merits and demerits of both versions have been indicated. It has been shown that in the liquefied natural gas (LNG) output range m LNG ≤ 1.0-1.5 ton/h use of EGU is preferable because it ensures partial recovery of power expended for compressing and movement of natural gas in trunk pipelines. Based on an analysis of domestic opposite compressors pressed into production, it has been concluded that it is possible to build efficient EGU on Y-and E-like bases for plants producing up to 5 ton/h of LNG with a power recovery of up to 1 MW.Nowadays, a major source of energy is natural gas (NG). The latter is transported from the field to the consumption point under pressures of up to 7.5 MPa by compressors, for driving of which considerable amounts of energy resources are expended.For industrial and household utilities, NG is delivered at a lower pressure. The pressure of the compressed natural gas (CNG) is generally reduced with the help of two-stage throttling. In the throttling process, the energy expended for driving the compressors is lost irrecoverably.In the 1940s, a pilot plant was built for efficient utilization of the energy of the CNG. In conformance with this plant, at gas distributing stations (GDS) expansion machines (expanders) are connected to throttling devices in parallel. Reduction of gas pressure in the expander is accompanied by a fall in gas temperature and transmission to the shaft of the unit of mechanical energy that can be used if the expander is connected to a generator for power generation. Thus, the power expended for driving trunk-line compressors is partially balanced out on account of utilization of the energy of the CNG.For maximum utilization of the CNG energy, the most appropriate is to employ expander-generator units (EGU), which operate under gas engine conditions without burning fuel and ensure that at an expander inlet-outlet pressure ratio π ≈ 11-12 the temperature falls to a level essential for getting liquefied natural gas (LNG) and generation of electric power of up to 0.05-0.06 kWh/kg of the gas processed.Use of plants with an EGU in NG transporting and processing systems conforms to current energy resource saving requirements. The benefit from their use increases with increase in EGU power, which predetermined the creation of high-unit-power plants provided with turboexpanders connected to the generator shaft through a reducer, at the outlet of which a standard shaft rotation speed n = 3000 rpm was maintained.The unit power of similar type of units built of late is 1.0-10.5 MW at an NG consumption of 40000-200000 m 3 /...

Efficiency, reliability, and operating life of piston compressors depend on the perfection of designs of their constituent units, distinctive amongst which are seals of pistons and glands operating with high and time-variable pressure differentials.Sealing units must meet two basic requirements:• leak-tightness, which is defined by the permissible (safe) degree of gas leakage through the sealing unit under the compressor stage operating conditions set at the design stage; and • operating life, which is characterized by the continuation of operation of the sealing unit with no change in the technical parameters of the compressor stage (efficiency, power consumed, temperature, etc.) at the level as demanded by the operating conditions. The leak-tightness of the seal with conventional split or ganged piston rings at a known maximum pressure differential ∆p on the sealing unit depends on a host of design parameters: number of rings z r and their cross-sectional area b r h r , interring spaces that depend on the dimensions of the pieces connecting the rings or on the ring spacing t r , radial clearance in the piston-cylinder pair δ p-c , running clearance in the ring lock ∆ l , presence or absence of lubricant, etc. To a first approximation, the required number of rings in the sealing unit of the piston can be determined by the proposed empirical relationship √  5∆p ≤ (z r -1) ≤ √  10∆p.(1)The operating life of the sealing unit depends on the materials of the piston-cylinder friction pair, speed of motion of the piston, roughness of the contact surfaces of the friction elements, cooling conditions, overall pressure differential ∆p in the seal as well as on the pressure distribution across the elements of the sealing unit. In medium-and high-pressure stages, the last of the referred factors determine to a large measure the rate of wear of the piston rings and the scheduled time for their replacement.In special-purpose compressors with one-way acting (unidirectional) pistons, the overall pressure differential in the sealing unit of high-pressure stages attains 20-40 MPa, which, in accord with Eq. (1), increases the number of sealing rings to be installed on the piston to 11 ≤ z r ≤ 21 and, consequently, increases the axial dimensions of the cylinders.Experience of designing, computational analysis, experimental studies [1-3], and industrial use of sealing units of domestic piston compressors shows that excessive increase in the number of sealing rings does not reduce external leaking much and does not facilitate uniform distribution of pressure across the rings of the sealing unit.

The design of nonmetallic plates formed from a glass-fiber-base laminate is analyzed for potential use in intake valves of the "dry version" of rapidly reciprocating expanders, and results are cited for five types of plates. Promising use of nonmetallic plates as component parts of expander stages is noted.Provision for high efficiency and reliability of gas-distribution elements is one of the basic problems encountered in the development of reciprocating expanders. Considering the need to separate the "hot" and "cold" zones of the working chamber, preference is usually given to a direct-flow expander stage with the intake valve located in the face of the cylinder. In that case, the role of exhaust valve is played by a sliding device with exhaust ports (windows) of circular or arbitrary shape, which are located on the lateral surface of the cylinder in the zone of bottom dead center (BDC). With this gas-distribution system, maximum possible channel sections are achieved in the seat of the exhaust valve; with a fixed section in the slot, this makes it possible to reduce the displacement of the valve plates h va , their impact velocities with the seat w s and limiter w l , and the stresses σ in the valve plates to the maximum extent possible.For this design of the expander stage, it is expedient to replace the metallic valve plates with nonmetallic ones (formed from a pure, or copper-coated glass-fiber-base laminate), which offer a number of advantages:• low noise level during operation of the intake valves;• high heat resistance of the plates; this makes it possible to use them in expander-generator units with a gas temperature falling within the range from 150 to 300°C at the inlet; • airtightness of the closed valves, and absence of seat and limiter wear, as well as light weight of the nonmetallic plates predetermine the expediency of their use in lubricant-free stages in units with increased rotational speed of the shaft and moderate piston speed; • a very high level of operational safety, since breakdown of the valve plates will not result in even greater damages when components of the driving mechanism, the cylinder-piston groups (CPG), and expensive greaseless piston and rod seals malfunction; and • the simplicity with which the nonmetallic valve plates are fabricated, and also their maintainability not only enhance the reliability and longevity of the working components of the CPG, but also make it possible to lower operating expenditures considerably.

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