HDPE Vacuum Flanges Perhaps?

That about covers it.

Further to the remarks previously provided, there are vacuum systems that utilise plastic pipe instead of lined pipes. There are a range of fittings suitable for that pipe including flanges, "T"s and bends.

What "vacuum" and temperature profile are you working with?

In our industry, we have some locations with vacuum sewer transfer at around 0.4atm (-0.6 relative to atm). Systems are up to 5km of total pipe nominal 100mm ID, maintained with constant vacuum.

Look to the reputable pipe suppliers for advice, but properly constructed "fusion" joints have not been a problem.

Fusion joints in the water network are also showing great reliability at pressures up to 80m head of water.

You may even find the plastic pipes are less expensive that your lined pipes.

Zasmine what temperatures are you dealing with ? HDPE pipe and fittings are limited in pressure above certain temperatures. At 73 degree farenheit SDR11 pipe can handle 160psi in water service. In your application the recommended pressure is half of this 80psi. This pressure ratings declines as years of service add up and temperatures increase. As PWSlack commented you can fuse the joints with butt or electrofusion to eliminate flange connections.

This is a vacuum system.

Are you considering the HDPE piping for piping the hydrochloric acid & water mixture? HDPE is susceptible to environmental stress cracking (ESC). I saw that you were concerned with the air leakage you are getting, but you also need to be aware of the chemical compatability. ESC related failures will be seen well below those predicted mechanically & thermally.

We are dealing with a vacuum of about 5 torr and about 50 deg C temperature.

Fusion weld sounds like a simple solution, I am just concerned about the ability of the pipe to withstand the vacuum with the acid being a major chunk of the hardly-condensable load.

We are familiar with ESC, especially when using HDPE pipes for scrubbers, we've had quite a few breakdowns. And in the place where we are operating the ambient temperatures range from 8 deg C in winter to more than 45 deg C in summer, pretty dry though most of the year. So, say we don't use HDPE, what kind of pipes do I look at?

I understand there are some coated pipes instead of lined with fluoropolymers. Any experience with them?

<...don't use HDPE, what kind of pipes...>

Any material that is compatible with hydrochloric acid!

http://www.coleparmer.com/Chemical-Resistance

Among the possibilities are PVC and borosilicate glass.