NDT - Safe Distance? for Pressure Testing

Calculation of minimum safety distance for pressure testing
(LLOYDS Register 96-02) form T-0240 sections 3.3 (fluid) and 4.3 (gas).

For fluids (3.3) it is :
l = 0.15 x internal diameter x ((alpha) ^ 0.4) x ((test pressure / (cube root of density))^ 0.6

For gases (4.3) it is :
l = 30; 3.6 x cube root of (volume x ((test pressure + length) - ((test pressure + length) ^ 0.714)))

Remark(s) of the Author...

What is the safe distance for Pressure Testing?

Over the years, I have conducted hundreds of pressure tests, and I have collected a lot of documentation about this non-destructive test method. As for me, I can be very brief with my answer:

A safe distance is difficult or impossible to determine in advance.

The figure below shows a component of a pipeline, which is launched at a pneumatic pressure test. I do not know the exact circumstances but something went completely wrong.
If you had to define a safe distance for this pressure test, were you able to provide this?

Safe Distance?

Qualitative Differences

Air is compressible

  • Energy storage is large
  • Pressure change "proportional" to volume change [P1V1= P2V2]
  • Bulk modulus, K = 20.6 psi
  • Air filled balloon "pops" large, instantaneous energy release

Water is not compressible

  • Energy storage is minimal
  • Pressure changes finite amount by infinitesimal change in volume
  • Bulk modulus, K = 316,000 psi K = - Δ P / [ΔV / V]
  • Water filled balloon does not "pop"  no compressive energy


What is the stored energy in 42 NPS pipe, 36 feet length and pressurized to 7.5 psig?

Hydrostatic Test
Pneumatic Test

How To Relate to Differences?

  • 4.44 lbf -ft is a small number that can be readily grasped
  • What about 294,815 lb-ft?
  • An SUV traveling at 42 mph [68 kph] has this amount of energy
  • Typically, sudden energy release comparisons are made to a TNT equivalent 294,815 lb-ft = 0.2 lb TNT
  • TNT equivalent given as 1 kg TNT* = 4.184 x 106 J [1], or 1 lb TNT* equivalent = 1.4 x 106 lb-ft
  • *Note that some sources give 1 kg TNT = 4.63 x 106 J

Actual Practice!

Many people are unaware of the dangers of pressure testing. Daily I see practices that can and should be better. The pressure test is often regarded as a side-issue, and thus there is less attention to it.
Progression can not be made with pressure testing, but with welding and assembly, the latter is much more important for a contractor.

For pressure testing of a component that will be used in service, the pressure is usually 1.3 - 1.5 x design pressure, which keeps the material from yielding, but subjects it to more stress that it will see in service. Inspectors crawl all over the unit to look for drips.
Pneumatic testing is done at a smaller pressure 1.1 - 1.25 x design pressure out of consideration for the danger. However, inspectors still have to crawl over the unit to look for leaks.

I'm sure there are still a lot of space for improvement, in terms of pressure testing.
Personally, I have only two incidents experienced during pressure testing. Both had to do with unreliable pipe material.

My own top 5 why pressure tests fail.

  1. A wrong gasket mounted
  2. Valves which pass during the test
  3. No drain and venting options
  4. Incorrect torque applied to the bolts
  5. Dubious piping material

Personally, I believe that most accidents can be prevented during pressure testing, if a number of essential conditions, prior to the pressure test, are satisfied.

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