OIL CABLE PUMPING PLANT

Abstract: In a pumping plant for oil filled power cables an air driven piston type pump (20) is used as primary pump. A PLS (26) is used to control the oil flow to the cable(s) (4). Improvements of the pump includes means for ensuring that the pump piston is operated also at low pressure and flow, and means for sealing off the piston rod to ensure maximum life of seals, to facilitate detection of possible leakages and to avoid contamin­ ation of the cable oil.

Description: The present invention relates to oil feeding systems for oil filled cables. The oil pressure in such cables has traditionally been maintained using oil reservoirs such as small cylindrical tanks containing mild steel or stainless steel cells. The reservoir could either be of the low pressure type (either gravity feed or variable pressure) or it could be of the high pressure type (either with pre-pressurized cells, or with the cells manifolded so that the gas pressure could be varied). Long submarine crossings will normally require high pressure reservoirs, particularly if the water is deep and it is required that the pressure inside the cable always is higher than that of the surrounding water. If a submarine cable should become severed (for instance by a dragging ships anchor) so much oil may be lost that the capacity of the reservoirs will not be enough during the subsequent cooling period, and water will be sucked into the cable. In order to maintain the cable free of water, even after a complete severance, it has become customary to use pumping plants rather than reservoirs for important submarine crossings. The pumping plants are generally provided with fairly large size storage tanks, and with a system for reducing the outflow of oil once the cable has been cooled down. Such systems have been designed to keep the cable free of water for periods as long as 60 days. Most pumping plants depend on electrical power supply to operate. To protect the cable even in case of a failure of the power supply, it is customary either to provide the pumping plant with a diesel engine-generator unit or to use a pump driven by compressed gas taken from bottles, as a back-up for the electrically driven pump. Pumping plants for OF cables generally use a so-called 'canned' motor-pump assembly (the unit is hermetically sealed, and the oil flows through the rotor of the motor) to avoid any possibility of vacuum leaks. These pumps are expensive and require an elaborate control system for start and stop to maintain oil pressure within preset limits. An air driven pump on the other hand will only pump when the oil pressure falls below the pressure for which the gas pressure is set, and it will stop pumping as soon as this pressure again is reached. From US-A-4.405.292 there is known a pneumatically controlled rate pump system. Upon receiving a pilot signal the piston type pump will make one stroke and then wait for the next pilot signal. The pump is provided with a counter which records the number of pump cycles and thus the volume of fluid that has been pumped. This results, however, in a rather uneven fluid flow. It is the object of this invention to provide an oil cable pumping plant making use of the many desirable features of air driven pumps and to overcome the drawbacks of existing pumping plants by improving the control of the pump. The main features of the invention are defined in the claims. One feature of the invention is that a controlled flow of oil to the cable (in case the cable has been severed) is obtained by adjusting the speed at which the pump piston is reciprocating, - rather than delaying the next pump stroke, -to match a predetermined flow program. Another feature of the invention solves the problem of operating the air driven pump at very slow speeds and low pressures. Air is applied at the necessary low pressure and flow during most of the piston stroke, but once a signal has been received indicating that the piston is at, or near, the end of its stroke both pressure and flow is increased sufficiently to operate the sliding air piston past the critical position. As soon as the piston has started to move in the opposite direction, air pressure and flow are again reduced to that required for normal pumping, or somewhat lower at the first part of the next stroke, so as to compensate for a higher oil flow during the high air pressure and flow. Above mentioned and other features and objects of the present invention will clearly appear from the following detailed description of embodiments of the invention taken in conjunction with the drawings, where Fig. 1 shows a simplified pumping plant using electrically driven pumps, Fig. 2 illustrates the novel pumping plant, and In Fig. 1 is schematically illustrated a pumping plant comprising an electrically driven canned pump 1 pumping oil 2 from a storage tank 3 to a cable 4 (not shown). A vacuum pump 5 maintains vacuum over the oil 2 in the tank 3. The pump 1 is provided with a bypass relief (safety) valve 6 and a pump relief valve 7. The oil line is also provided with three check valves 8, 10 and 17 as well as a cable relief valve 9. In Fig. 2 the canned pump 1 in Fig. 1 (with its bypass relief valve 6) have been replaced by an nitrogen or air driven pump 20. Only the cable relief valve 9 which allows oil to return to the tank when the pressure increases due to cable heating, remains. Fig. 2 illustrates an oil cable pumping plant including at least one oil tank 3 and at least one air/gas driven piston type oil pump 20 connected to the oil tank 3, at least one air/gas source 21, such as a compressor, connected to the air/gas inlet of the pump via pressure control means 22, an exit 23 for the air/gas as well as an oil exit 24 connected to at least one oil filled cable 4, and oil flow indicating means 25, such as a piston stroke counter or a flow meter. The pump is of the type which in its normal operating condition provides a predetermined oil pressure at its outlet. The plant includes oil flow control means 26 such as a PLS (Programmable Logical System) interconnected between the oil flow indicating means 25 and the air/gas pressure control means 22. The lower part of Fig. 1 is the so-called 'flow limiting' system, which will allow a high flow in the initial period after a cable severance, when the cable needs a high flow of oil to compensate for the contraction of the oil upon cooling of the cable. After a couple of hours the demand has been reduced considerably, and the upper one 11 of two electrically operated valves 11 and 12, closes, whereby the flow is limited to the sum of the flows in the lower branches. After another 6-10 hours also the second electrically operated valve 12 will close to limit the flow through a flowlimiting valve 16 to whatever is needed to keep water out of the severed end once the cable has been cooled down (6-30 liters per hour depending upon oil channel size). Flow limiting valves 14 and 15 are usually introduced in series with the said electrically operated valves 11 and 12. This type of 'flow limiting' system is not required in a pumping plant according to the present invention, as shown in Fig. 2, since a controlled flow may be obtained by monitoring the speed of the pump 20 and adjusting the driving air pressure to obtain the desired flow of oil. This task may, for instance, be performed by the control means 26, which should have a battery back-up in case of power failure. The plant may also include means 27 for detecting a predetermined pressure drop in the oil filled cable(s) 4, due e.g. to severance of the cable(s), to initiate the control means 26 to follow a predetermined flow diagram. In order to assure operation of the sliding piston, even at low pressure and flow, there may be arranged a piston position detector (not shown) which via the control means 26 will initiate a short burst of air sufficient to operate the sliding piston at the moment the piston is near or at the end of its upstroke. The air pressure and flow are adjusted by the control means 26 during the first part of the next cycle so as to compensate for the added oil flow during this burst of air. Figures 1 and 2 have been drawn to show a pumping plant for one cable only. When using the Fig. 1 technology for a number of cables, the lower part of the drawing will have to be duplicated for each cable. In the case of Fig. 2 one air driven pump 20 must be used for each cable in order to obtain the 'flow limiting' feature without reducing the oil pressure on the other cables. In Fig. 2 is indicated that the ports leading to the diagram blocks 9, 20 and 22 may be duplicated with ports 9′, 20′ and 22′ for each cable. It will, however be possible to use one control means 26, having a number of ports 20, 20′, 22, 22′, 25, 25′, 27, 27′, for controlling a number of cables and for continuously comparing their state.

Claims: Oil cable pumping plant including at least one oil tank (3) and at least one air/gas driven piston type oil pump (20) connected to the oil tank (3), at least one air/gas source (21), such as a compressor, connected to the air/gas inlet of the pump via pressure control means (22), an exit (23) for the air/gas as well as an oil exit (24) connected to at least one oil filled cable (4), and oil flow indicating means (25), such as a piston stroke counter or a flow meter, the pump being of the type which in its normal operating condition provides a predeterminated oil pressure at its outlet, characterized in that it includes oil flow control means (26) such as a PLS (Programmable Logical System) interconnected between the oil flow indicating means (25) and the air/gas pressure control means (22) adjusting the speed of the continuous movement, i.e. without waiting times between pump strokes, according to which the pump piston is reciprocating, to match a predetermined flow program. Pumping plant according to claim 1, characterized in that the oil flow control means (26) serves a number of cables by being interconnected between indicating means (25) and control means (22) of the respective cables to control, monitor and compare their state. Pumping plant according to claim 1 or 2, characterized in that it includes means (27) for detecting a predeterminated pressure drop in the oil filled cable(s) due e.g. to severence of the cable(s), to initiate the flow control means (26) to follow a predeterminated flow diagram. Pumping plant according to claim 1 or 2, characterized in that it includes a piston (43) position detector, so that operation of the sliding piston (68) is assured, even at low pressure and flow, by applying a short burst of air sufficient to operate the sliding piston (68) at the moment the piston (43) is near or at the end of its stroke, and that the air pressure and flow are adjusted during the first part of the next cycle so as to compensate for the added oil flow during this burst of air.

Inventors: AASBO, ANDERS TAPIO, EGE, SIGMUND, HEGH, FREDDY
Applicants: STANDARD TELEFON OG KABELFABRIK A/S
Application year: 1987
Main classification: F04B49
IPC codes: F04B49, H02G9, F04B9, H02G15