A piping system can look finished long before it is ready. The pipes are welded, the geometry is right and the pressure test has passed. Even so, it is the inside that decides whether the plant still works in twenty years, and most of that work has to be done before the system leaves the workshop.

Here we go through the three steps in the order they are carried out. First testing, then cleaning and finally sealing with nitrogen.

Why the order matters

Quality control is often seen as a check carried out at the very end. In practice each step relies on the previous one having been checked. Sealing comes last because it only has value if what is being sealed has already been tested and cleaned. Seal a system that has not been checked, and you simply lock the faults in.

First, strength and tightness are checked

Non-destructive testing is a common term for methods that examine the material without damaging it. Radiography makes it possible to look inside a weld and find porosity, slag or lack of penetration that cannot be seen from the outside. A weld can look good and still be weak.

Then come pressure testing and other strength tests, which confirm that the system can handle what it is designed for. In a workshop it is easier to carry out the tests correctly and to fix what they find. A defect found at the workbench costs far less than the same defect found in a plant room where four other trades are working at the same time.

A passed pressure test does not tell you what is inside

The pressure test shows whether the system is tight at a given pressure at a given moment. That is important to know, but it says nothing about the condition of the inside.

Metal swarf and grinding dust can be left behind, along with residue from cutting and welding and scale or early rust from the time the pipes spent in storage. A system can pass its pressure test without remarks and still contain all of this.

That is why black water is a familiar sight during commissioning. When the system is filled and the pumps start, the first water out of strainers and drain points is far from clean. At that point the issue is no longer tightness but where the particles have settled.

The pressure test can even make things worse. If the system is tested with water and the water is not drained and dried afterwards, metal, moisture and air are left together in a closed pipe. Those are exactly the conditions corrosion needs. A system left like that through weeks of transport and storage can be in worse condition inside on the day it is installed than on the day it was tested.

So the pressure test is necessary, but it measures tightness and not cleanliness. It has to be followed by cleaning and an inspection of the inside.

Then the parts you cannot see are cleaned

The particles have to come out before they start to circulate. In a water based system they settle where tolerances are tightest, such as valve seats, pump impellers and heat exchangers. The result is wear, reduced capacity and operational problems that often show up months or years after handover.

The system is flushed and cleaned, and the inside is then inspected. The inspection confirms that the cleaning actually worked. Without it, you do not know.

Finally the system is filled with nitrogen

Corrosion needs three things at the same time, namely metal, oxygen and moisture. Nothing can be done about the metal, and moisture is hard to keep out completely. The oxygen, however, can be removed.

Once the system has been tested, cleaned and inspected, it is sealed and filled with nitrogen. Nitrogen is an inert gas that does not react with the steel. When the gas fills the pipe there is no room for oxygen, and corrosion does not get started.

The seal also keeps the inside clean during transport and while the system waits on site to be connected. An open pipe end on a construction site quickly takes in dust and moisture. A sealed system is finished on the inside when it leaves the workshop.

Why this matters even more in data centres

In most buildings cooling is about comfort. In a data centre it is a condition for operation. At the same time build times are short, and the facility cannot go live until commissioning is complete.

Much of what turns up as surprises during commissioning could have been checked earlier. That applies to cleanliness, weld quality and documentation. Moving the checks into the workshop moves the uncertainty away from the phase where finding faults is most expensive. Read more about the difference between installed and ready.

The three steps

• Testing confirms that the system is strong and tight.
• Cleaning and internal inspection confirm that it is clean, which a pressure test cannot do.
• Nitrogen sealing keeps it that way until it is connected.

A prefabricated piping system should not only fit. It should also be clean, protected and documented, and that is decided before it leaves the workshop.