Relief Valve/ Bypass Valve or Backpressure Regulator?

I think the most economical way, is to go with the pressure relief valve.

And it's the best way to go to. They are adjustable (well most of them are)

Relieving only the excess pressure ( back pressure), and leaving inside pressure at the setting.

However you may have to order it, preset.

Have you checked out other types of vaccums pimps that only output is 5 to 8 lbs like the ones Dentist use. I use to rebuild those for a dental supply salesman years ago.

We are drawing the sample from a remote location (200 feet or more) so we need a lot of suction for minimum lag time.

Sorry I was confused I thought you were running the pump 24 / 7 for some reason and figured a fixed pressure would be best.

yes we are running the pump 24/7 to draw the natural gas from the drilling rig. We have to have enough suction so that we have fresh sample of gas all the time. In order to analyse the gas, we need only a small sample (@ 5psi) and bypass the rest.

After re-reading your question, I found the fly in the ointment. It was in the phrase were you wrote puts out. That implies positive pressure from source.

Now to answer you question.I stand by my first response. You can use a pressure relief valve to release the excess pressure, going to suction on the upstream side. reducing the pressure to suction. But that will only work if the downstream side of the relief valve has a lower pressure then the upstream side.

This sounds like you are in need of a fast loop sampling system for natural gas constituents. If this is correct, be careful about looping a relief valve back to the vacuum pump inlet as this has potential for heating issues. Any reputable instrumentation company can design and install the fast loop for you. Be carful about sticking any old regulator into your design as many economical regulators are a "bleed type" whereby you would be venting natural gas into the atmosphere around the regulator, therefore, it would require a special enclosure and venting provisions. I would recommend a high volume/low head blower as opposed to a vacuum pump with the discharge sent back to the process accross a restriction (hard to recommend without knowing the conditions) with a tap and flow regulator sent to your analyzer. If you cannot return the discharge to a process, I recommed it being vented to a poperly designed flare stack. The best reference book I can think of for this situation would be Liptak Analytical Instrumentation:

http://books.google.com/books?id=4kJxQkXo_QQC&pg=PA29&lpg=PA29&dq=fast+loop+sampling+system&source=web&ots=54uLCFwjyn&sig=YwQ0E6IHkbkwAOHhlJzmzZe8NtY&hl=en&sa=X&oi=book_result&resnum=10&ct=result

I thought you were running a pump as well. Do you need suction or are you relying on upstream pressure to push the media? And want to release back pressure to avoid pushing to fast.

Why are you assuming that putting a backpressure on the compressor will somehow cause a problem? If the machine is designed to run at 30 - 40 psi, then it will not be damaged if the backpressure is created. I would suggest a regulator such as this:

http://www.marshbellofram.com/PCD/PDFs/PCD%20Catalog-04/PCD2-Regulators.pdf

It is small and not overly expensive.

Not sure I totally understand what you are describing. It sounds like you have a process with a vacuum pump with an unknown suction or vacuum (negative pressure) on one side and 30-40 psi positive pressure at the outlet. Flow is only 0.14 cfm which I assume is air. You want to be able to use 5 psi, but flow rate is unknown.

If I understand correctly, 2 options come to mind.

First, vacuum pumps are normally operated with minimum back-pressure on the outlet side. 30-40 psi seems high, see if the pipe can be enlarged to remove the restriction, and use a backpressure valve to maintain a 5 psi output.

Second, tap into the outlet and throttle the flow to 5 psi with a low-pressure regulating valve.