Agilent 1100 pump problem

[randy]

I'm having trouble with one of our 1100 systems. It has two pumps with column switching. Both pumps are having problems, albeit different ones. Usually the problems with 1100 pumps are fairly straightforward, but not these.

The mystifying problem is with Pump 1. The analyst complained of a noisy baseline, and it was indeed noisy. When I began attempting to diagnose the problem, one of the first things I did was to purge the pump to eliminate any air bubbles. With the purge valve open and the flow set to 4 mL/min, the pressure profile displayed by Chemstation definitely showed something wrong: The pressure was spiking at a regular frequency, basically oscillating between 4 and 20 bar. I replaced the purge valve frit, but that didn't solve the problem. I replaced the outlet check valve and then the active inlet valve, but neither solved the problem. I replaced the pump seals. The pressure still spiked. I used the Chemstation diagnostics to perform the pressure and leak tests to help locate the problem. Both failed the final pressure bleed-down test. I had eliminated just about everything Agilent had told me could be wrong except for the pressure damper. There was no way to isolate this as the cause, so I ordered one and installed it. The pressure was still oscillating. The degasser was bypassed and the gradient proportioning valve switched out. Neither fixed the problem. Fast forward, and at this point I have replaced everything that could be replaced in the flow path except for the pump block itself, the motor drive, and the two stainless lines going to and from the pressure damper. If I open the purge valve and set the flow to 1, 2, or 4 mL/min, the pressure will oscillate between 4 and another number depending on the flow. The oscillation seems to occur with the frequency of a pump stroke, but the pressure profile isn't displayed quickly enough by Chemstation to determine whether it's the intake or delivery stroke. I am officially out of ideas on this problem.

The problem with the other pump is similar but converse. With the purge valve open, the pressure profile is nice and smooth. Once the valve is closed and the system comes up to operating pressure, a sawtooth pressure profile emerges. I've replaced the piston seals, outlet check valve and purge valve frit, but none have eliminated the sawtooth pattern.

[khw]

... The oscillation seems to occur with the frequency of a pump stroke, but the pressure profile isn't displayed quickly enough by Chemstation to determine whether it's the intake or delivery stroke. I am officially out of ideas on this problem.

If you do the pressure/leak test, you can get a diagnostic window that does not only show the pressure, but also the stroke of the pumps.

... The problem with the other pump is similar but converse. With the purge valve open, the pressure profile is nice and smooth. Once the valve is closed and the system comes up to operating pressure, a sawtooth pressure profile emerges. I've replaced the piston seals, outlet check valve and purge valve frit, but none have eliminated the sawtooth pattern.

What amplitude of the saw tooth are you observing? We have ripples with about 0.5% of the actual pressure.

[Bruce Hamilton]

Could be insulting you, it's unintentional...

Are you sure the degasser is working OK, and the solvent inlet filter is flowing OK. I assume you've checked and the pumps are delivering the correct volumes at various flows and pressures.

For the first pump, if it has been used for a lot of buffers without seal washes, the pistons or heads could be worn, and new seals may not fix it - but they should have helped, unless the pump head was seriously worn. Ensure the mobile phase components were soluble and miscible. My first thought would be the active inlet had failed, but you're replaced the complete assembly, not just the cartridge...

I'd suspect the same things for the second plus the outlet check valve, focusing on the solvent degassing then the active inlet valve and outlet check valve.

Please keep having fun,

Bruce Hamilton

[LCbob]

Hi Randy

The second problem sounds to me like a valve problem, you didn't mention if you had changed the active inlet check valve and / or cartridge. I would change these and try the leak test.

The first problem sounds interesting, first off I have found a pressure of 20bar at 4ml/min with the purge valve open and a new frit is high! Can i suggest you take some peek tubing and connect this pump to the other ones damper/pressure sensor and see what you get then. This will tell us if the problem is damper/sensor / board.
Something to check is the tubing before the damper there may be a partial blockage causing motor to maybe slow, I have never seen this problem on a G1311A though.

As Bruce says blocked inlet filters do cause pressure fluctuations especially at high flow rates, this always catches people out.

[randy]

I thought the problem could be a blocked inlet frit, so I removed that and pumped at 4mL/min and the problem still occurred.

One of the last things I replaced were the pistons, and those along with another new pair of seals didn't help.

The pumps have been used to pump an 8mM/1mM carbonate/bicarbonate buffer exclusively. There is no seal wash. Just curious, how would a worn-out head cause the pressure to fluctuate?

I will replace the ss tubing to see if there are any blockages.

Also, how would I verify that the degasser is working correctly? I did bypass it once but got the same result.

[randy]

I found the problem. The two stainless steel lines going to and from the damper were an aftermarket replacement with IDs much smaller than the factory lines. Once I switched out the line from the damper to the delivery head, the pressure stopped fluctuating so wildly at 4mL/min.

So the line was either blocked, or the ID was too small.