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Servicing Guide - Rebuilding a Rheodyne Valve
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Before a valve can be dismantled it needs to be removed from the HPLC System.

First remove the tubing from ports 2 and 3 at the back of the valve.

Port 2 is connected to the pump, and port 3 to the column.

It is not necessary to remove the loop (ports 1 & 4) or the waste pipes (ports 5 & 6).

The Rheodyne 7725i valve shown above is mounted by two screws to the black bracket (many other bracket types are available, but the two mounting screws are universal). To access the screws it is necessary to remove the rotating handle at the front

Loosen the two allen screws which mount the handle and pull it off forwards. These allen screws locate the handle onto the two flat surfaces on the shaft, shown in the picture. The valve mounting screws can then be removed. These fit into the two screw holes shown in the valve body on the left of the picture.

Next the three mounting screws should be removed from the bottom of the valve

These require an allen key, and when loosened, the stator (bottom metal plate of the valve) can be removed:

Attached to the main valve body, the stator face assembly (white ceramic disc) and stator ring (black plastic section of the valve body) will now be visible. The stator face assembly may come away still attached to the metal stator.

Check the upper surface, which may be discoloured from wear and contact with the rotor. It is unusual for this to need replacement, but it is good to clean with solvent such as methanol. It is a very hard surface, but it is important not to scratch this surface.

Stator Face assembly , side facing stator

Stator face assembly, side facing rotor seal. Clean this side!

Next remove the plastic stator ring, and note the position of the Position Sensing Switch, and the body location pin:

Now remove the rotor seal. It will be surrounded by (but not attached to) the white isolation seal. The rotor seal is the main service part in the valve, and the main purpose of disassembly is to inspect the rotor seal and replace if necessary. The pictures below show a new rotor seal, and one which has been round the block a few times.

If the wear on the seal is too great, the valve will start to leak. A temporary fix is to tighten down the valve, but although this stops the leak, it increases the rate of wear, and the only satisfactory solution is to replace the rotor seal.

Note that the rotor seal can only fit one way round:

New Rotor Seal
Used rotor seal. Note the wear.

The Needle Port Tube Assembly fits right through the valve body and into the rotor seal. So the white Teflon port on the top side of the rotor must engage with the Needle Port Tube.

When an injection is made, the syringe needle passes through this tube, right into the rotor seal so that injection is made directly into the loop. This is the “tightness” that is felt during the last 1-2mm of inserting the needle when making an injection, and is essential to ensure that there is no leakage and loss of sample into the valve.

Before re-assembly, remove the upper section of the valve body (the metal part) to expose the Thrust Bearing:

Looking from right to left, the thrust bearing is the first circular ring. Its outer surface does not move and is in contact with the valve body. But the lower surface (against the spacer washers in the picture) turns as the valve handle is turned, and contains a roller ball bearing race, just visible here. This should be lightly lubricated with a light grease. Note that the rotor seal is attached at the left hand end, as seen in the picture.

Reassembly is, as they say, the reversal of the disassembly process. Slide the valve content in the picture above into the plastic stator ring and with the locating pin and position sensing switch in place, re-attach this to the metal valve body:

Check that the white isolation seal is in place around the rotor seal. Locate the stator face assembly onto the metal stator (the pins indicate the orientation), and offer up the stator to the rest of the valve assembly. Two of the screw holes are next to a locating pin and one is not, so there is only one possible orientation. Insert the mounting screws and tighten the valve together.

Before remounting the valve, attach the handle and test the tightness of the valve. It should be a little stiff but turn easily. If it is too tight or too loose, release the handle and allow it to drop down onto the valve body, engaging with the two slots of the adjusting screw with the tabs on the valve handle. Adjust as appropriate, in steps of 1/20th of a turn, using the twenty dial markings on the body and the painted spot on the adjusting screw as guides:

Once you are satisfied with the valve tension, remount the valve on the bracket. There are two possibilities, 180o apart! Choose your preferred handle orientation, then mount the valve so that set screws on the valve handle can locate with the two flats on the rotor shaft. If necessary the position of the set screws can be changed, since there are three screw holes and only two are required. Be careful not to push the handle on too far or the screws will not align with the flats on the shaft. Do the set screws up evenly, and test the handle location. Once it is almost tight, check for any rotational free play, centre the handle with respect to this and then tighten the set screws.

Finally, reconnect the tubing to the ports at the back. The loop attaches to ports 1 & 4, the waste pipes to 5 & 6, the pump to port 2 and the column to port 3.

If you have parts left over and need an exploded diagram to see where they go, or need a copy of an instruction manual, please click here to download them from the Idex-Rheodyne website. The exploded diagram is part of the 7725 instruction sheet.
Rheodyne also have an on-line troubleshooting guide which you can access by clicking here
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