how to test gas pipe tightness

Depends on whether you're performing a strength test or a leak test. Strength tests (normally 1.3 to 1.5 times maximum allowable pressure) should always be performed with water or some other compatible liquid. If you have a weld fail and you've got that kind of air/gas pressure, the only thing left of your test bench (and most of the shop) will be a smoking hole in the ground. Once a strength test has proved the integrity of the system, then a leak test using air (or helium, if you can get it) can be performed.

The actual instruments vary, I've seen anything from simple bourdon-tube style pressure gauges (have to be calibrated first, or at least recently) up to 24 hr temperature & pressure chart recorders.

On the distribution lines we have built air pressure is the norm, nitrogen is sometimes used. Dist. lines are typicaly low pressure and 100psi and a chart recorder are used.
On lines that only require a short duration (1/2 - 1 hour) test, a calibrated gauge is used.
Distribution lines are tested in place so your test bench should be safe.
We typically weld pipe up to the meter and others pipe into the building, but the in building leak tests I have seen were 2 hour at 100psi, again using a calibrated guage.

JTMcC.

yuan2003 (Industrial):

I continue to recommend to all young plant engineers not to utilize pneumatic pressure testing on equipment and piping. This is just too hazardous a method as compared with hydrostatic pressure testing. If you have had a reasonable amount of years under your belt and seen the failed attempts at pneumatic testing, you'll know what I'm talking about.

Unless you have a very serious and tough reason for doing a pneumatic test, then I would advise you to seek the lead advice and assistance of an experienced engineer in carrying out this procedure.

Art Montemayor
Spring, TX

yuan 2003,

He specifically mentioned distribution lines, where the sections tested are almost always small, short and low pressure.
Of course you wouldn't test 500' of 14" to 1500psi using air, that would be almost criminal. But 25' of 3/4", at 100psi is no threat to safety. Even 4000' of 4", under 4' of dirt, at 100psi is harmless, a weld failure will not send pipe flying.
This is done thousands of times every day, a crew fabricates a 25' service from the main at the street to a newly constructed home or business, they put 100psi of air in the service, soap test the joints, wait 30 minutes, and if it holds pressure they go do a couple more.

regards,
JTMcC.