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Servicing Guide - Replacing a Piston Seal
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What are piston seals and what do they do?

A piston seal is spring-loaded and manufactured to very fine tolerances, to fit around the piston in an HPLC pump and prevent seepage of eluent back into the pump.

Seals are normally black or yellow (sometimes called gold!) Black seals are made of graphite-filled PTFE and are suitable for all solvent systems. Unless you have a reason for doing so always order the black seals. There are lots of blends, and the graphite can be in the form of flake, powder or fibre. Some work better than others, which is why cheap copy parts do not last as long. Unfortunately, they all look exactly the same!

Yellow or clear seals are made of ultra-high molecular weight polyethylene. In its purest form, this is clear, but normally the yellow dye is added for appearance. For aqueous-type eluents you may find that a yellow seal will give a dramatically longer seal life, but for electrochemical detection, try and use the clear seal if possible. If you use certain manufacturers equipment, their standard seals are only rated to 3000 psi, and hence we offer an improved version.

This has a longer seal and shorter back up ring (it takes the same space) and is rated to 10,000 psi. A seal kit also contains a red outer piston guide. This material is soft and abrasive, and cannot be machined accurately, so it must have a split in it. These can get sloppy, and then the seal fails. The improved kit contains a guide made of the same polymer as a Rheodyne outer seal, so it is accurate, a good guide and has a long life. Hence you get long seal life too.

If you use the pump most days you can expect to need new seals every 6-12 months, and if you are prepared to carry out preventative maintenance, you should replace the seals every 6 months. Always replace both seals together on a dual piston pump.

How to tell if a seal need replacing.

When a seal fails, some solvent will get back into the pump. The signs are a reduced flow rate, giving rise to longer retention times and a puddle at or near the front of the pump! Also should you have the pump head removed for any reason, check the appearance of the seal. If there is any sign at all that the seal material is breaking up, replace it!

How to replace a piston seal.

To get at a piston seal you need to remove the pump head, as described in the piston replacement guide. You will find the seal in the head itself. There may be a small steel ring (like a small washer over the seal, or it may be visible immediately.

To remove the seal you will need a small screwdriver or something similar to prise it out. It is quite a tight fit, but is only pressed in. You will always damage a seal in the course of removing it, so never replace the same seal even if it looks ok. As you remove it take careful note of which way round it goes (i.e. can you see the spring towards you) because the new one should go in the same way round.

Generally, the seal can be inserted by pushing it in with a flat-ended rod-shaped instrument approximately the same diameter as the seal itself. Because it is spring loaded there will always be resistance until the spring part is inside the housing. Some manufacturers provide a special tool for the insertion of piston seals, and if so it is a good idea to get one (check the pack of parts that came with the pump - there may be one in there.) In the cases of one or two manufacturers' pumps, the taper in the seal caused by the spring is so great that it is impossible to insert the spring without their special guide tool. With or without the tool, push the seal in as far as it will go, and do not forget the steel ring if appropriate.

If you bought a seal kit, you should also replace the piston guide. This is usually a reddish colour tube-shaped sleeve with a split down it's side. The piston guide will sometimes be worn, but there are rarely any outward signs. If it is worn, the piston will have more lateral movement than it should, and the new seal will wear out more quickly. Once the seal is in, replace the pumphead as described in section 1.3, and be sure to do the bolts up securely.

How not to destroy a piston seal.

Do not run the pump dry. Having solvent present offers the seal a measure of lubrication. Running dry, such as when using the pump for the first time after a period of disuse, or allowing it to run out of solvent, will cause excessive wear to the seal and should be avoided.

Do not change solvent more often than is necessary. As long as you have a black seal fitted you can use the pump with most solvents quite happily. However the seals swell slightly in certain solvents, such as tetrahydofuran (THF), and if you alternate between THF and a Methanol:Water mixture (as you might if using the pump for both HPLC and GPC, or switching back and forth from normal phase to reversed phase) the change in swelling of the seal will cause it to wear very much faster. You may feel you have little choice, but if so make sure you have a good supply of spare seals, and always the black ones. You'll need them!

Use piston backwash with aqueous buffers. With any solvent some small trace amounts will seep past the piston seal. This is not a problem because it will simply evaporate. However if you are dissolving a solid in your eluent, such as a buffer salt in an aqueous system, the small trace amounts that pass the piston seal will dry to leave the solid material behind. This forms a hard crusty layer on the back half of the piston, which will ultimately scratch away at the seal and destroy it. The answer is piston backwash. Not all pumps have the facility, but if you have a relatively modern model there will be connections at the back of the pump head to allow water to be cycled through to dissolve and wash away any deposits. Ideally this should be a continuous process, and so if you work in the ideal world, connect a peristaltic pump to the fittings at the back of the pump head and cycle water through. You should change the flushing solvent every 2 weeks. Otherwise use a syringe to flush through maybe 50ml of warm water each evening.

Don't forget. If you have problems during the repair or wish to discuss it first, please call us on 01634-294001.or email : stuart@laserchrom.co.uk

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