Check Valve Failure

[ravenwork]

Would people please post their general trouble-shooting guidelines regarding check valve failure?

I am especially interested to hear if it is possible to tell which valve is failing on a two-piston pump.

I would post mine, but while I can usually tell when checks are failing, I cannot seem to gauge which valve. It is usually the last one I replace if I have the time to go valve-by-valve. (Is that one of Murphy's Laws?)

Evan L. Cooper, Ph.D.

[DR]

If you get a drop in flow rate w/o significant drop in pressure, it's the outlet valve. If flow and pressure both drop, it's the inlet valve. Either way, I'm inclined to use cartridge type valves and replace them in pairs, as long as I'm in there...

[skunked_once]

On the older Waters pumps, you can monitor the pump plunger on each head and compare to the pressure fluctuations to identify which of the two pump heads is having problems. That narrows it down to inlet or outlet check valv on one head. In my experience, it's almost always the outlet check valve, but DR's suggestion is a good way to check both. Right now I'm having problems with an old Waters 600 pump but only have some used check valves to swap out and none of them will hold pressure. It's a choice of ordering new check valves or getting out the big hammer! HPLC lesson: Check valves are expendable items. Keep at least one new set on hand and toss out the old ones when they no longer work. You will save lots of time and frustration.

[unmgvar]

from my experience it is mostly the inlet check valve that fails.

most of the time also the cause is salt precipitation inside of it.

the best manner of dealing with the problem is to flush the check valve with water. most of the time you also have to do manually with a syringe.

the best solution is prevention. do the least mixing with the instrument itself, and always check what is the ratio salt/organic that your method will sustain without precipitating the salt. that is where i have seen many R&D's "sin".

this salt precipitation happens when you work with 2 channels of eluents, one 100% salt/ buffer and the second 100% organic, and the mixing is performed in the instrument. or also when you have a buffer/organic solution in channel one and you perform a fast gradient to the second channel which is 100% organic.

if your instrument starts the mixing within the proportion valve chamber, then this problem will happen to you relatively rarely. in the long run you will harm your proportion valves.
intruments of this type are dionex, shimadzu, varian for example.

if your instrument has four distincted proportion valves then the mixing will start only at the mixer.
in that case you risk having your inlet check valve clogged with salt often. instrument of this type that i know is hitachi.

another problem that check valve will cause when they ware down is noise on the detector and bad reproducability of RT and or area of peaks.

[syx]

if your instrument has four distincted proportion valves then the mixing will start only at the mixer.
in that case you risk having your inlet check valve clogged with salt often. instrument of this type that i know is hitachi.

We often get trouble with Hitachi’s LC pump as told in this case. We use usually use 100% component of mobile phase per line than pre-mixed mobile phase. The problem occurs in washing step, when we change from water-organic solvent mixture to 100% organic solvent. Could you give us any advice to avoid this condition?

[DR]

Premix

[unmgvar]

I agree with DR here.

premix your starting composition and your end composition (hopefully it is not a 100% organic).
for example:
if you must have 80% buffer and 20% organic at the start of your analyses premix it to be channel A. if you wash in the end with 20% buffer 80% organic make this channel B.
this premixing will make your analyses more robust and more reproducible from instrument to instrument.

if you have to go to 100% wash organic at the end of your gradient and you still get your check valve clogged then see with your R&D and check when the saturation point of the salt in the organic solvent. meaning what is the ratio in % of salt and organic solvent when the first drop of salt sinks in the mixture.

then do try to change your method accordingly.

remember that in any case all instruments do suffer at some extent of those types of method. after them all instrument and columns should be washed with 60-100 ml H2O.

[Mark Tracy]

Another way to avoid salt precipitation at washout is to have a third bottle with water in it. At the end of the run, change to water/organic of the same percentage as your mobile phase. Run just long enough to flush the buffer salts out of the pump heads, then switch to 100% organic.