What are Check Valves and what do they do?
Check valves are one-way valves sited on the inlet and outlet of the piston chamber of an HPLC pump. As the piston moves back to draw solvent into the pump, the inlet valve opens to allow solvent in from the reservoir and the outlet valve closes to prevent high pressure solvent from the system being forced back into the pump chamber.
Then when the piston is pushed forwards by the pump motor, the outlet valve opens as the pressure in the chamber exceeds the back pressure from the system, and the inlet valve closes to prevent the solvent returning to the reservoir.Check valve housings are made of stainless steel and screw into the main pump head(s) of the pump. There will always be a pair (inlet and outlet) and sometimes the flow direction is marked on the outside by an arrow. If a pump has two pumpheads, each operating out of phase and the outputs combined, there will be a set of check valves for each pumphead.
Some pumps have two pumpheads but they operate in series. (The main piston draws enough solvent for both piston strokes, pumps half into the system and half into the second pumphead. Then when the main piston starts its refill stroke, the second piston starts and maintains the pressure on the system until the main piston is ready to pump again.) With this arrangement there will only be check valves on the main piston, because the secondary piston is essentially inside the system and operates permanently at high pressure.
Inside a check valve the working parts are a ruby ball and a ceramic seat. The ball sits in the seat and forms an excellent seal when closed. Solvent passes in the allowed direction by simply lifting the ruby ball, and flow is closed off by the system back pressure forcing the ball into its seat. There are quite a number of other spacers and washers in a check valve kit, and all must be used, in the right order and the right way up! If you do not feel confident with this we recommend you convert to a check valve cartridge. These cost only a little more than a rebuild kit, but can be fitted in seconds, and even if you are a geography teacher, the worst you can do is put it in the wrong way round, and you'll soon know if you do that!
Some check valves act more slowly than others, because there is space for the ruby ball to move sideways, and hence it takes longer to centre and close in its seat. To overcome this, some newer kits include a ball guide, which is a ceramic tube with projections from the inside surface which limit the ball to vertical movement only. In other cases, the manufacturer has designed the pump with a very fast piston stroke but an unnecessarily large ball. In order to get accurate opening and closing of the check valve an improved kit has been introduced for those pumps, and if in doubt you should ask when ordering, especially for old ACS pumps
How to tell if a check valve is not working properly.
Unlike a piston or a seal, where they either work or they do not, a check valve can be quite difficult to diagnose because it can misbehave intermittently, and to different extents.
Essentially, a check valve fails when the ruby ball does not make a good and immediate seal with the seat. Hence the manifestation of this will be a reduced or varying flow rate, with a consequent reduced or varying back pressure, and associated baseline noise. If the flow rate is affected you will also see an increase in retention times, and if the flow rate is fluctuating, retention times will be unreproducible.
It may be that the problem is simply an air bubble that has become trapped in the check valve, and to eliminate this, loosen the connecting tubing from the outlet side of the outlet check valve and check if a bubble escapes.
How to replace worn check valves.
Check valves are easily accessible from the outside of the pump. To remove them, first disconnect the inlet tubing (usually white PTFE tubing) from the inlet check valve and the steel connecting tubing from the outlet valve. Notice the different connections on each. This will help to identify which valve is which once they are removed from the pump.
Usually the PTFE inlet tube is 1/8" with a flat end, and the steel tube has a normal compression fitting. Now the inlet and outlet check valve assemblies can be removed by gripping the pump or pumphead firmly and unscrewing the valve with a suitable spanner.If you are using cartridge check valves you tip out the old one, insert the new one and reassemble. That's as hard as it gets, although do bear in mind that the monkeys sometimes put the cartridges in the wrong way round! The inlet and outlet cartridge will frequently be interchangeable and if you are just converting a pump to cartridge check valves for the first time, our cartridges will almost always fit the standard housing.
Please check when ordering, but if you have a Waters M6000 pump, there were several variants from Waters and as far as I know, none of them fit! However we have a new housing which fits perfectly!If you do get it the wrong way round there will be no flow at all from that side of the pump, because either no solvent will be drawn in (if the inlet valve is in backwards) or the pressure in the pump chamber will not allow the motor to turn (if the outlet valve is reversed.)
Note that the pressure will not trip the cut-out because the pressure transducer is fitted after the check valves. If you are unsure which way to fit a check valve, try blowing through it to establish the permitted flow direction. However you should not use your mouth if toxic solvent could be inside.
To dismantle a conventional check valve can be quite tedious, because there are a lot of components and some can be quite difficult to get out. Remove all the components, and look carefully because the last one is sometimes a clear plastic washer that can be difficult to see in the bottom of the housing. Set them out in order on a clean surface so that you know the order in which to insert the new parts.
Check that you have a kit containing the same parts as you have just removed (a previous user may have fitted another manufacturer's check valve unit to your pump) and then replace each item in turn taking great care to get them in the right order and the same way round as the old ones came out!Once the unit is rebuilt, check whether it is an inlet or outlet valve and refit to the pump. In most cases this is easy, although if you have a Waters M6000 pump you may have difficulty keeping the new parts inside the housing whilst you reconnect it to the pump. But then if you have a Waters M6000 pump you will already be something of an enthusiast! There is a distinct similarity with an old Land Rover..... they are noisy and not very nice to drive but last forever.
Once your pump is reassembled, check that the connections are leakproof and test to ensure that the problem is resolved. As a general rule, check valves should be replaced approximately once a year, although if you are happy to run them till they die they may last a lot longer.If you are totally stuck and have no spares you could try clearing check valves in an ultrasonic bath.
But do be aware that they sometimes come apart under ultrasonication and you will need to know the order in which to reassemble them! And if you get REALLY STUCK big time, collect the bits, put them in a plastic bag and send them to us. We'll sort it for you!
How not to cause check valve faults.
There is little you can do except adopt a preventative maintenance strategy and replace your check valves once a year. Always replace both the inlet and outlet together, and if possible both sides of the pump at the same time if you have a twin piston pump. Make sure your eluent is degassed and filtered, and you should have years of trouble-free motoring.