Chemical and Petroleum Engineering, VoL 33, No. 5, 1997

O I L - W A T E R SEPARATOR FOR A HIGH-PRESSURE CENTRIFUGAL COMPRESSOR

V. R. Pshik

UDC 621.177:621.525

The oil system of a high-pressure centrifugal compressor for transportation of natural and petroleum gas is one of the important systems in the design of gas-pumping units of gas and petroleum and gas industry compressor stations. The basic plan of a high-pressure centrifugal compressor oil system is shown in Fig. 1. The basic functions of the oil system are delivery of oil to the annular and thrust bearings and the toothed coupling, sealing of the centrifugal compressor by delivery of oil to the end seals with a pressure somewhat higher than the gas pressure in the sealed cavities, removal of oil from the gas cavities of the centrifugal compressor under pressure with subsequent degassing of it, cooling and filtration of the oil, and elimination of leaks into the tank. To a significant degree the oil system of a centrifugal compressor determines the operating reliability of the unit as a whole. Design decisions made in the stage of development of the centrifugal compressor to a significant degree depend on the operating costs, which are influenced not only by the unplanned shutdowns of the unit but also by the irreversible losses of oil. Both the end seals and the system for removal of oil, the basic elements of which are the oil-water separators (Fig. 1) designed for removal of oil, condensate, and other liquids from the gas cavities under pressure in the whole range of operating pressures of compressors, have a substantial influence on the oil losses and operating reliability of the off system. As is known, in centrifugal compressors, oil-water separators in which the relief valve is made in the form of a needle, a ball, or a gate and connected by a system of levers with a float providing opening of it are most frequently used [1]. With an increase in operating pressure there is an increase in the force clamping the relief valve to the seat of the overflow valve, which in turn requires an increase in the lifting force of the float. The force of action of the float on the relief valve may be increased by increasing the float diameter or the amplification factor of the lever system. In this case there is an increase in the dimensions of the off-water separator and, consequently, in its metal content and also in its cost and, most importantly, a decrease in its reliability. The M. V. Frunze Sumy Machine Construction Scientific-Production Association (SMNPO) Joint-Stock Company (AO) has developed an off-water separator design (Fig. 2) providing high throughput and distinguished by simple and reliable operation at high operating pressures [2]. The oil-water separator consists of the welded housing 2 with the fittings 5 and 6, respectively, for supply of the liquid-gas mixture and removal of the gas and also the base 1, in which a channel is made for draining of the oil and a two-stage drain mechanism connected by the level system 3 to the float 4 is located. The two-stage drain mechanism (Fig. 3) includes a supplementary valve and the main slide valve located in the housing 5, in the bottom of which is the drain channel 6. The supports of the lever system 4 and 13 are fastened to plate 12, which in turn is fastened to the housing of the drain mechanism. To compensate the weight of the float and the lever system the relief device with spring 11 is located on the plate. The supplementary valve is located with a sliding fit i~ the guide sleeve 1, which is rigidly fastened to the housing 5. The main valve operating the internal cavity of the housing into the above-the-valve A and the drain B chambers is located in the holes of this housing with a sliding fit. The drain chamber is connected by holes with the inner cavity of the oil-water separator and the above-the-valve chamber by the guaranteed gaps 9 and 10, respectively, with the drain chamber and the internal cavity of the oil separator. The main valve is tightened by spring 7 and has an axial channel 8 shut off by the supplementary valve. In the general condition, that is, in the absence of oil and pressure in the cavity of the oil - w a t e r separator, under the action of the float and the lever system, spring 11 of the relief device is compressed and the supplementary valve covers the axial channel and, simultaneously compressing the spring under it, presses it to the seat of the drain channel. With an increase Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 5, pp. 39-41, September-October, 1997. 524

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Fig. 1. Basic plan of the high-pressure centrifugal compressor oil system: 1) oil tank; 2, 5) low- and high-pressure pumps; 3) cooler; 4, 6) low- and highpressure filter blocks; 7) hydraulic accumulator; 8) pressure differential regulator; 9) oil-water separator; 10) degasser. in pressure in the internal cavity of the oil -water separator the force clamping the main valve to the seat of the drain channel also increases. In this case the pressure in the above-the-valve chamber is equal to the pressure in the internal chamber of the separator since it is connected to the guaranteed gaps. With an increase in liquid level in the cavity of the separator, the liquid first reaches the float, made in the form of a sphere, and then heats it a little. With a certain depth of immersion of the float the Archimedian force equalizes the force with which the supplementary valve is clamped to the main valve. A further rise in liquid in the o i l - w a t e r separator leads to floatation of the float, as the result of which the lever system raises the supplementary valve, opening the axial channel in the main valve, through which the liquid enters into the drain channel. Since in the above-the-valve chamber the liquid enters only through the g~aaranteed gaps 9 and 10, their total hydraulic resistance is higher than the hydraulic resistance of the axial channel 8 and the pressure in the above-the-valve chamber will drop. With a reduction in oil level in the oil-water separator the float starts to drop and, acting on the levers, shifts the supplementary valve, which again covers the channel of the main valve. As a result the pressure in the above-the-valve chamber increases, which leads to closing by the main valve of the drain channel and, consequently, to stoppage of draining of oil from the separator. The level of liquid in the cavity of the oil-water separator again starts to increase and the whole operating cycle is repeated. The current value of pressure Pi in the above-the-valve chamber is determined using the education of balance off lows through the above-the-valve chamber with a stationary main valve: Qil + Qi2 < Qi3,

(1)

where Qil = qz(P - Pi); Qi2 = q2(P - Pi); and Qi3 = q3(Pi - Pd) are the flows through the gaps 9 and 10 and axial channel 8, respectively; ql, q2, and q3 are the conductivities of the throttles; and p and Pd are the pressures in the cavity of the oil--water separator and the drain channel. Having transformed Eq. (1), we obtain

pi>_P(qt+q2)+Pd, q3 qt+q2+q3

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Fig. 2. Plan of the oil-water separator. With a decrease in the pressure Pi in the above-the-valve chamber the force pressing the main valve to the seat of the drain channel also decreases and at a certain pressure the force equih'brium occurs:

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(2)

where R, r d, and r a are the radii of the main, drain, and axial channels, respectively; C and z0 are the stiffness and preliminary compression of the spring 6; and m is the weight of the main valve. From Eq. (2) it is easy to determine the limiting pressure Pi of the liquid in chamber A at which opening of the main valve starts. The equation of movement of the main discharge valve will have the form

mg- C( zo-z)+ ~( R2-r2) p i - ~ ( R 2 - z 2 ) p =md2z/dt 2,

(3)

where z is the travel of the main valve and t is time. Solution of Eq. (3) must be supplemented by the equation of the balance of flows through the above-the-valve chamber. In movement of the main valve the equation of balance of the flows will contain an additional term caused by the change in volume of the above-the-valve chamber and the compression of liquid in it [3]: QI + Q 2 - Q3+~ R 2 drJdt- W/E dpi/dt=O ,

(4)

where W is the volume of the above-the-valve chamber in the original position and E is modulus of volumetric elasticity of the liquid. Combined solution of Eqs. (3) and (4) makes it possible to investigate the dynamic stability of the main discharge valve of the oil-water separator. In design of the o i l - w a t e r separator we specify the maximum operating pressure Pmax and the maximum quantity of oil Qm delivered to the oil separator. With the minimum operating pressure the throughput Qd of the drain channel with full opening of the main valve must exceed the maximum quantity of delivered oil, that is, the condition Qd -> Qra must be fulfilled or overfilling of the o i l - w a t e r separator may occur.

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Fig. 3. The two-stage drain mechanism. For reliable operation of the oil-water separator it is necessary that with half-submersion of the float the amount of the Archimedian force pushing it from the oil corresponds to the force necessary for opening of the supplementary valve. In the absence of excess pressure in the cavity of the off-water separator opening of the supplementary valve occurs in a similar manner. In this case the main valve is opened by the spring located under it. The force of spring 7 must be sufficient for overcoming the weight of the main valve and the hydrostatic head of the column of liquid in complete filling of the oil separator. On the other hand, the residual weight of the float and the lever system must provide reliable closing of both the supplementary and main valves. Use of a discharge valve of the given design made it poss~le to modernize centrifugal compressor oil-water separators and use them at higher pressures. In addition, there was a significant reduction in irreversible losses of oil (in the majority of modernized compressors it is no more than 0.1 kg/h). At present the housings of low- and high-pressure centrifugal compressors of the prototype of a UKSP-16/500 compressor unit with a final pressure of 50 MPa in service at Timofeev Gas Condensate Deposit are equipped with such oil-water separators. The oil-water separators installed in the low-pressure housing operate at a pressure of 10-12 MPa and in the high-pressure housing at a pressure of 30-32 MPa. In addition, oil-water separators of this design have been installed in some GPA-Ts-16/76-1.45 units of Syzran' Compressor Station (Saratov Trans. Gas Production Association) and GPA-Ts6.3/125-2.2 units of Stepanov (Southern Trans. Gas Production Association) and Proletarsk (Shebelino Gas Industry Production Association) Underground Gas Storage Stations. Long production tests of oil-water separators with a two-stage drain mechanism under compressor station conditions showed their high throughput and reliability in service in a broad range of operating pressures.

REFERENCES 1.

2. 3.

K. A. Tel'nov, A. A. Fanshten, S. Z. Shabashov, et al., Automation of Gas Pumping Units with Gas Turbine Drives [in Russian], Nedra, Leningrad (1983). Patent RF No. 1726888 [in Russian]. D. N. Popov, The Dynamics and Regulation of Hydraulic and Pneumatic Systems [in Russian], Mashinostroenie, Moscow (1977).

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