Hydrostatic Testing of Pilot Plant Scale Tubing: Is it Necessary?

Do I always need to hydrostatically test tubing and compression fittings in pilot plant service? Many organizations prefer that this question never be raised since many do not routinely require hydrotesting of compression fittings. Are they wrong?

Well there are no codes that address pilot plant specific piping. ASME B31.3 Process Piping, arguably the most widely used and respected code covering piping, gives its coverage in the introduction as:

B31.3 Process Piping: piping typically found in petroleum refineries; chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants; and related processing plants and terminals

And a pilot plant is not a plant on the scale of these examples. So its relevance is at least questionable. However, let’s assume you want to comply with this code if possible. The code goes on to state that:

It is the owner’s responsibility to select the Code Section that most nearly applies to a proposed piping installation. Factors to be considered by the owner include limitations of the Code Section; jurisdictional requirements; and the applicability of other codes and standards. All applicable requirements of the selected Code Section shall be met. For some installations, more than one Code Section may apply to different parts of the installation. The owner is also responsible for imposing requirements supplementary to those of the Code if necessary to assure safe piping for the proposed installation.

Certain piping within a facility may be subject to other codes and standards …

That clearly allows the owner to decide that some sections are (or are not) applicable based on their industry and experience.

Most pilot plant piping is probably likely to be classified as Normal Fluid Service. It exceeds the pressure or temperature requirements for Category D (150 psig, 366 F) piping, usually is not toxic enough for Category M piping, not pure enough for High Purity Fluid Service, and not high enough in pressure or temperature for High Pressure Fluid Service or Elevated Temperature Service.

B31.3 goes on to say that:

341.4.1 Examination — Normal Fluid Service. Piping in Normal Fluid Service shall be examined to the extent specified herein or to any greater extent specified in the engineering design. …

(4) random examination of the assembly of threaded, bolted, and other joints to satisfy the examiner that they conform to the applicable requirements of para. 335. When pneumatic testing is to be performed, all threaded, bolted, and other mechanical joints shall be examined.

Paragraph 335 talks about alignment, flanged joints, threaded joints, and tubing joints among others. It’s sections on threaded (335.3) and tubing (335.4) joints do not require much except making sure the fitting is assembled correctly per manufacturer’s instructions.

What type of examination, to use the code language is required? This is given in 314.4.1 as follows:

341.4.1 Examination — Normal Fluid Service. Piping in Normal Fluid Service shall be examined to the extent specified herein or to any greater extent specified in the engineering design.

This section goes on to require for non-welded components a visual examination of at least 5% of all the connections. Considering that in research, all pilot plants and laboratory units will have each joint leak tested certainly meats or exceeds this requirement.

The code also allows any written method to be specified for the examination:

344.1.2 Special Methods. If a method not specified herein is to be used, it and its acceptance criteria shall be specified in the engineering design in enough detail to permit qualification of the necessary procedures and examiners.

So, if the owner feels that a leak test is adequate for your non-welded components then all you need to do is provide a detailed written procedure for the examination, being careful to identify pass and fail criteria.

Is this adequate?

In the 1980’s I participated in a series of tests that were designed to see if improperly tightened compression fittings had a credible risk of blowing off having passed a leak test. Passing in our criteria was no visible leaks with soap solution at 80% of the relief device setting and having a pressure drop less than specified for the units (typically inches of water in 20 minutes or less than 1 psig per hour). The testing had some significant limitations. It only looked at our current compression fitting supplier, an industry leader. It involved a limited number of tests (less than 50). It only looked at our common fitting sizes of 1/8 to ? inch. So comprehensive it was not but it was, in my opinion, valuable.

We deliberately under tightened the compression fittings under test. We looked at 3/4- to 1 turn vs the manufacturer’s recommended 1 ? turns. At this point the fitting “felt” (and often looked) to an experienced person as too loose. We then gas leak tested the fittings to 80% of their maximum pressure rating with nitrogen. (This was, in general, much higher than we operated these fittings in actual service.) All obviously leaked and would have been caught by our standard leak test procedures. We then hydrostatically tested them, without retightening, to 100% of their maximum pressure rating. None failed. We then subjected them to being struck with a small weight remotely released to hit the fitting. None failed (defined as blowing off). We repeated the test with lower tightening until they did fail but, at that point, they were leaking water out in a steady and highly visible stream long before failure and only when well less than one turn tight. None became projectiles as the tubing on the other end effectively restrained them.

This, admittedly limited, testing along with our 20+ year history of good performance without hydrotesting confirmed (at least in our mind) that hydrostatic testing of compression fittings was not required. We never had a fitting failure in the remaining 30 years of my career that had first successfully passed a routine leak test. We did have a policy of using quality fittings. We did have a detailed, written leak test procedure. We did have a demonstrated practice of checking each individual fitting. We established a practice of making sure each fitting was confirmed as tight before starting leak testing. Gaugeable fittings made this even easier once they became available.

So no, in my opinion, hydrostatic testing of compression fittings is not required provided:

1.      You use a good quality fitting from a reputable manufacturer. I mention this because, in these days of global suppliers, you can find some very cheap and truly awful fittings that I would not use on an atmospheric drain line. I don’t think you need to use the very best fittings (although you will have a lot less leaks with quality fittings) but I do thin you need to buy at least good ones.

2.      You have a detailed leak test procedure that requires at a minimum:

a.      A visual inspection

b.      Confirmation that the fittings are tight preferably by a test gauge

c.      Leak testing at a low nominal pressure (15-30 psig)

d.       Leak testing in at least two stages (~50% of maximum operating pressure and ~80% of relief device setting). The exact break points are less important than doing it in at least two stages

3.      Appropriate safety precautions including:

a.      Keeping the area clear of unnecessary personnel

b.      Using an inert gas for pressure testing if the fluid is flammable or combustible

c.      Teaching all personnel to stand to the side (out of the line of fire) of fittings being tested for the first time

This would be, in my opinion, a “Special Method” that complies with 344.1.2. And, for compression fittings in pilot plant and laboratory service I don’t think anything more is necessary.

I would, however, require testing of any welded joint. Welding a problem fitting, or making a home made adapter by welding up parts of several fittings is not uncommon in research and these all should be appropriately hydrostatically tested before use.

I realize some personnel with plant experience will not be comfortable with this approach. The large number of fittings often scattered over a wide area make confirming each individual fitting is appropriately inspected difficult. Contractors are very concerned about potential liability and prefer to hydrostatically test to “ensure” they have installed all their fittings properly. Some safety personnel feel that hydrostatic testing is a more fail-safe practice. I understand all these concerns. However, you have to remove every relief device, every pressure gauge that is not over ranged adequately, any instrument or equipment not capable of taking the over pressure produced during the hydrostatic test. On a pilot plant or laboratory unit these are not inconsiderable relative to the size of the unit. And their re-installation leaves many connections untested anyway. I have also often seen many components valved off for the hydrostatic test leaving a small number of connections down stream of the valve untested. So, I contend that many organizations have been accepting some level of un-hydrostatically tested connections anyway. Perhaps just not recognizing it.

Something for you to consider in your pilot plant and laboratory activities and decide what makes the most sense to your organization given your construction and testing activities.