1360ch7

Chapter 7- Hydraulic System

1.- Principle of System

The hydraulic plant on third floor in the 3.6 m building supplies the hydrostatic bearing of polar - and declination axis with high pressure oil. The pressure and the flow in the supply lines are kept at such a level that they are sufficient to produce the nominal oilfilm in all the pads. The oilfilm itself is thick enough to avoid metallic contact between pad and bearing surface.

The control part of this system has a number of interlocks in order to stop the pumps when a pressure line breaks or when the level in one of the tanks becomes too high. An other interlock stops the telescope drives if there is not enough flow in one of the pads. Time relays shortcircuit these interlocks troughout the starting, otherwise it would not be possible to start the pumps.

2.- Description of System

The hydraulic plant of the 3p6 m telescope has two thanks each one having 1 cubic meter useful volume. The oil which returns from the telescope is collected in tank number 1.

2.1.- Thank 1 (figure 1)MM #7

In the middle of this tank is a separation wall which does not allow the oil to flow from the lefthand part of the tank into the righthand part unless the oil level in the lefthand part is high enough. In this lefthand part gets the water separated.

The oil gets pumped by one of the two gearpumps 11 via the filter 32 and the heat exchanger 93 into tank 2. This pump 11 gets controlled by the oil level indicators 37 of tank1 and 2. The pump is switched on when the level in tank 1 passes the maximun. The pump gets switched off, when the level in tank 2 passes the maximun or in tank 1 passes the minimun. This pump delivers 150 l/min but the telescope needs only about 80 l/min. Therefore this pump works only slightly more than half the time.

2.2.- Heat Exchanger

At present there are two valves in the water circuit of the heat exchanger. One is an electromagnetical water stop valve 89 which means it is practically always open. The other one is a remote controlled water stop valve which opens proportionally with the raising temperature of the oil before the heat exchanger. Probably there will be added a waterpump in the watercircuit which will operate in parallel with the swash plate pump1 of the declination bearings. This will make sure that always the right quantity of water passes trough the heat exchanger.

2.3.- Tank 2

Each one of the four gear pumps 16 resp. 21 which are located on the top of tank 2 suppliesthe oil to one of the fourpressure pumps, because the latter ones are not able to suck properly. The pressure switches 55 stop the presure pumps whenever the pressure in the line between gearpump and pressure pump drops. The excessive amount of oil returns to tank 2 via the non return valve 67 or 68. These valves mantain a pressure of 3 bar in the line.

2.4.- Alpha Pumps (Figure 2)MM#7

Always one of the two pressure pumps 6 supplies oil to the 6 pads which carry the horseshoe. The oil quantity which is supplied by the pump can be regulated with the hand wheel from 0 to 40 l/min. Cleaning of the oil is done by filter 34. The pressure is maintaned by the pressure limiting valve 45. The directional control valve 43 allows the backstreaming of the oil into tank 1 without pressure during the startup of the pressure pumps. The magnet of this valve gets di-energized after a fixed time delay. This pumping station has as well a pressure gauge 52 and a thermometer 39 and a pressure switch 56.

2.5.- Delta Pumps

The system with the delta pumps is built in the same way as the system for Alpha pumps. Different is the oil quantity which can be regulated with the hand wheel between 0 and 80 l/min. The pressure limiting valve 44 and the pressure gauge 51 are made for higher pressures than those used for Alpha pumps. The supply of these pumps goes to the pads of the North bearing (2 pads) and the declination bearings (12pads each).

2.6.- Oil-distribution

Each pad has on one of the 6 existing distribution blocs the following instruments:

pressure gauge 53 or 54
pressure switch 57
flow control valve 59 or 60 or 61
flow control monitor 63 or 64

The pressuer switches open if the pressure in the corresponding pad drops down which might be the case due to rupture of a pressure hose. This will stop the pressure pumps. The flow control monitor gives an electrical signal when the oil quantity drops below the minimal value which has been set in the monitor. This might happen if the pressure in the supply lines is not sufficient high. This signal switches the drives of the telescope off.

2.7.- Filling Station (figure 1)MM#7

On the ground floor of the building is pump 26 which pumps the oil from the barrels up to tank 1 on the third floor. The pump 26 on the top of tank 1 pumps the dirty oil back into the barrels. The non return valve which is in parallel to the last pump is preloaded and allows to close valve 74 on ground floor while the pump keeps running. This makes the changeover from one barrel to the next simple.

3.- Electrical Control System

The control system has a main switch on the control cabinet in the pump room on the third floor. This switch has to be operated manually. After this has been done, there are two possibilities to get the hydraulic plant into operation. One is to switch on all pumps manually by pressing on the buttons on the control cabinet in the pump room. The other possibility is to start the whole hydraulic plant with one button on the control desk on observing floor. The same two possibilities exist to stop the plant.

3.1.- Pump Selection

For all the important pumps of this installation is a spare pump available. The selection of the pump is done with a manual selector switch on the control cabinet (third floor). This allow as well to switch to the spare pump when the filter of the pump has to be cleaned.

3.2.- Automatic tank level control

The pump 11 on tank 1 is controlled automatically by the level switches in tank 1 and 2. The pump is switched on

-manually with "on" push - button
-automatically by minimum level in tank 2
-automatically by maximum level in tank 1

The pump 11 is switches off

-manually with "off" push - button
-automatically by maximum level in tank 2
-automatically by minimum level in tank 1

3.3.- Interlock System (figure 3, 4, and 5)

The signals of the pressure switches 56 and 57 which are in the pressure line just behind the pressure pumps, ond of the pressure switches 55 which are in the line between the feeding pumps 16 and 21 to the pressure pumps, and on the pressure switches 57 in front of each single pad have been incorporated into an interlock chain. As soon as one of this pressure switches open due to a sudden drop of the pressure (rupture of rubber hose) gets the interlock chain interrupted and this stop the pressure pumps.

A second interlock chain combines the information from the 32 flow control monitors 63 and 64. If one of them finds an unsufficient troughput of oil (this happens if the pressure in the supply line is not high enough) it will break this interlock chain and will stop the main drives of the telescope.

A third interlock for the pressure pumps comes from a "Mini-Alarm" level indicator in tank 2 or if the "Max-level" of tank 1 remains too long (this to prevent an overflow of tank 1, if the pump 11 does not start).

4.- Operation of System

In order to start the hydraulic plant one has to switch on the main switch on the control cabinet in the third floor. After this there are two possibilities:

4.1.- Operation from control cabinet in third floor

One has first to switch on the two feeding pumps on tanks 2. After a few seconds (time relay) one can then switch on the pressure pumps. They will start in Y and get switched after a few seconds to . Then about half a minute close the valves which allow pressure free return of the oil. As a consequence of this extinguish all the lamps which indicate "no flow" and "no pressure". A few seconds later open the relays which shortcircuited the pressure switches during the start-up of the plant. Now, the hydraulic plant is running normally. The pump 11 on tank 1 gets controlled by the oil level switches in thank 1 and 2. To stop the plant one pushes the two "off" buttons of the pressure pumps. All the pumps stop automatically.

4.2.- Operation from the LCU Control Room

One pushes the button "oil on" and has to wait for about 1 minute until the green "oil flow" light comes on, which means that the plant runs normally. To stop the plant one pushes the "off" button.

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