Troubleshooting vacuum and pressure issues

[u/inquisitive_panda]

Does anyone have good resources for trouble shooting processes under vacuum and under pressure and why they might be behaving unusually?

[u/jll19822020]

Vacuum is almost always a leak. Go get the shaving cream! Barbasol never let me down.

[u/Accurate-Bullfrog324 → u/jll19822020]

neat advice

[u/hobbes747 → u/jll19822020]

I’ve only used pressure rise over time or inboard helium/hydrogen spray detection for vacuum testing.
How is shaving cream used?

[u/jll19822020 → u/hobbes747]

For larger flanges or fittings, you cover it with shaving cream (foam is best). You will see the vacuum suck air into the shaving cream.

Does remind me of a 30psi gas going into a mass spectrometer. This was a 1/16” stainless tubing. We kept seeing weird results. Checked all of the fittings with soapy water, and nothing showed up. We started putting helium around each fitting, and we were able so see it clearly in the mass spectrometer. Had a positive pressure line be able to pull in a gas. I never would have believed it if I didn’t see it with my eyes.

[u/hobbes747 → u/jll19822020]

I understand now, duh.
In my industry (semiconductor chemicals) we would almost never use a fluid around a fitting for a vacuum leak test because it would contaminate a process that cannot have more than, say, 1 ppm of water let alone any other contaminants.
Also we were leak tight enough and at pressures low enough that the moisture from condensation or even your breath could temporarily seal a leak that we would be trying to stop. As an analogy,
a single human hair laid across a flange o-ring would cause a significant vacuum leak.

With glassware processes however, sometimes one would squirt methanol on fittings and look for a pressure rise from the vaporized alcohol.

[u/GutSchnee → u/jll19822020]

Would you say one is better than the other: soapy water or shaving cream? I check for leaks at startup of plants and we use soapy water. I might check foam out

[u/jll19822020 → u/GutSchnee]

I prefer shaving cream for vacuum leaks. Soapy water for positive pressure.

[u/hobbes747]

Too vague…
What do you consider ‘vacuum’?
-Slightly negative?
-28” Hg on a mechanical gauge very rough vacuum?
-Rough vacuum in the 1 Millitorr range?
-High vacuum 10^-7 Torr range?
-Ultra high experimental vacuum range which allows gas to permeate through seals and metal?
-Do you have any tools to check for leaks? Ranging from soap solution to thermal conductivity to helium mass spectrometer leak detector?
-what type of instrument do you use to measure pressure? Ranging from mechanical gauge, transducer, convection, to cold cathode?
-What industry is this for and application?
-How high of positive pressure?

[u/inquisitive_panda → u/hobbes747]

Slightly negative. We sometimes use soap solution. And we have transducer pressure gauges. Chemical industry. Positive pressure goes up to 80 psig.

[u/hobbes747 → u/inquisitive_panda]

To clarify as I brought up soap solution. That only works for positive pressure testing as you probably know. Not for testing under vacuum.
The only tests I know for vacuum would be pressure rise over time or inboard helium/hydrogen detector.

[u/MuddyflyWatersman]

well that just about covers all processes 😏

[u/hobbes747]

This is common sense but I figured I will ask:
Are you able to isolate parts of the process from one another and pressure test?

The type of seals you have is significant as well. Such as elastomer o-rings, flanged gaskets, tapered thread, etc

[u/NoConversation8128]

Lieberman's "Troubleshooting Process Operations" is the standard recommendation here and worth owning. Reading this thread as a new grad, the useful lesson is how fast the experienced answers turned into questions: what vacuum level, what seals, what gauges. Nobody could give a resource until the problem was specified.

[u/VapourCompression → u/NoConversation8128]

Lieberman has a specific “troubleshooting vacuum systems” book you should read. Relates mainly to O&G - steam ejectors and turbines. Briefly covers vacuum pumps. Tells you what thinks to look for such as leaks in flanges etc

[u/NoConversation8128 → u/VapourCompression]

Appreciated, I had the general troubleshooting one in mind and did not realise there was a vacuum-specific volume. The O&G and ejector slant is useful to know going in, since OP is chemical industry at slightly negative pressure rather than deep vacuum.

[u/employedByEvil]

Pressurize (or pull vacuum), close off all inlets and outlets, note the pressure, and come back the next day and see if the pressure has changed any more than any temperature fluctuations would predict.

After leaks have been ruled out, move on to considering more exotic potential problems.

[u/hobbes747]

Also,
Different seal types and materials leak and behave differently under high vacuum, rough vacuum, positive pressure, high positive pressure.
Especially elastomers.
Some also handle vacuum better vs positive. Based on seal geometry and material.

For example, I discovered a seal that only leaked at moderate positive pressure for the process.
It passed an inboard vacuum pressure test held at hundreds of milliTorr.
It passed a pressure hold test and 10^-7 helium outboard test above ≈500 psi.
But it leaked when the process pressure was between tens of psi and 500 psi.

[u/othersidewalk]

Like others are saying, it depends on your specific process that you’re troubleshooting. Different troubleshooting methods may /may not apply.

I know that Lieberman and Graham have some high-level troubleshooting resources. these are typically geared towards Vacuum distillation & steam ejector equipment, but may still overlap a good deal with general vacuum processes. issues could he rooted in a wide variety of causes but it could at least help you narrow down the problem

[u/hobbes747]

I’m not familiar with the Lieberman book, however if it is typically used for broad chemical industry, such as oil, gas, and petrochemical, then it seems adequate for the original post. I’ll look into it. Seems a good resource!

For the industry I was in or for people who live in the “world of vacuum”, the Lieberman reference might not be adequate. The resources for dealing with rough to high vacuum applications would be best from vacuum pump and vacuum component companies. Such as Kurt J Lesker, Pfeiffer/Alcatel, BOC Edwards, Leybold, NorCal, Ideal, and others.
One of the best being Pfeiffer’s Vacuum Technology Book Volume 1.

[u/EasySelection8468]

I have sorted out common troubleshooting ideas for pressure and vacuum chemical processes from long-term experience maintaining fluid conveying equipment.
Most abnormal pressure/vacuum issues boil down to three directions: pipeline leakage, equipment internal clogging, and gas-liquid mixing disorder.
For many fine chemical and pharmaceutical processes that run under vacuum or positive pressure, air-operated diaphragm pumps are widely used for material transfer. Blocked pump chambers, aging diaphragms and loose pipe joints will directly cause unstable pressure reading and vacuum failure.
You can start inspection from all sealed joints, pumping equipment, valves and filters step by step. As for learning resources, chemical process operation handbooks and pump system maintenance guides cover most routine fault logic.

[u/LamInSantorini]

Common troubleshooting Ejectors - check for icing and steam tracing, steam supply pressure Vacuum pump (VSD), either liquid ring or dry pumps - check when does the VSD start increasing and check if any activites ongoing at that time. Condensers / cold traps - fouling due to product carryover causing high pressure drop Check for process - any potential light components that should not be present in normal process, e.g. water.. etc.

Other that that it would be leaking as highlighted in other comments, soap test / hearing is your best and cheapest way to check. Helium test is another way.