Fault in progressive cavity pump

[FLNZ]

Hi. I have a small progressive cavity pump that's playing up - liquid goes in, and liquid containing a stream of fine bubbles of an unknown colourless gas comes out. We're pumping MDI (methyl diphenyl diisocyanate), and the pump has been spec'd for chemical resistance against this.
To my knowledge, the pump has been bled of air properly at its intake, and the material supply is under sufficient pressure such that the inflow pressure should exceed that at the outlet (i.e. I don't think it's cavitating). We pressurise the MDI supply using dry nitrogen, and in case it's dissolved gas coming out of solution, we've also tried degassing our material supply under vacuum but that didn't seem to have any effect.
I've done some research over the last few days, but have found no reference anywhere to this problem. Has anyone here ever seen a problem like this before with a PCP, or can you tell if there is anything unusual or obviously wrong with our set-up?

 

[TenPenny]

Off the top of my head, I can think of a couple of things to look at.

Assuming that the stator materials are compatible, check with any other parts that are exposed...some of these pumps have elastomer covers over the drive shaft joints...also, is there any lubricant used that could be the culprit? I would also look carefully at the mechanical seal both for materials and for potential leakage.

 

[bosscat]

Not an answer I'm afraid but I have had a similar problem in the past.

Again, a small progressive cavity pump (20l/h) handling silicone rubber base (approx 40,000cPs) which is chemically pretty inert. Again, vacuum degassed liquid in, liquid with gas bubbles out. Discharge pressures were up around 14 - 16 Bar due to losses in a small hose. Initially we assumed high losses on the suction side were drawing air through the mechanical seal, but much work shortening/simplifying/opening out the feed made no difference. Pressurizing the supply also made no difference.

We have wasted many hours and considerable cash with this but the problem remains. I have no idea what causes it but would VERY much like to know! I've been working with PC pumps many years but this is the only time I've come across this.

The original machine for this mould filling duty was a simple piston pump like arrangement made from pneumatic cylinders which had other issues but not this one.

Nick

 

[MikeHalloran]

One disadvantage of pressurized reservoirs is that the blanket gas goes into solution in the contents... and when the contents is later depressurized, the gas comes out.

You might try running with unpressurized, vented reservoirs, or with _tall_ reservoirs, so gravity does the pressurizing.



Mike Halloran
Pembroke Pines, FL, USA

 

[jet1749]

Hi, had a similar problem pumping vacuum degassed acrylic dope at 40,000Cp using Mono PC pumps. Turned out it was the mechanical seal allowing ingress of air. Had to alter pipe work so suction became discharge and change pump rotation. Net effect was that mech seal was then at high pressure end of pump, and any leakage was then to atmosphere.

regards

 

[FLNZ]

Thanks for the input. I've just got back from Christmas break, so I haven't had time to check everything yet... I'll definitely have a go with the gravity feed idea, and when production lets up a bit I'll see if I can open the pump up and check the seals.

The most puzzling thing about this situation is that our entire system, inlet and outlet, runs above atmospheric pressure - if it was a sealing issue, I'd expect to see fluid leaks rather than air ingress.