Pressure testing is the practice of subjecting pipes or pipeline systems to pressure far above operating pressure to confirm the integrity of pipe, fittings, and joints. The concept is to locate pipe weaknesses before something goes wrong. Depending on the situation, testing may be done using pressured air, gas, or water. Pressure testing may occur either before the pipe is put into service or on existing lines that have been in service for many years.
Pressure testing of new installations
It is now standard for all new transmission line, distribution line, or service line installations to be pressure tested before being put in service. Practices are different depending on the type of line. For instance, transmission lines are often tested using pressurized water because any loss of pressure is sudden and leaks are easy to find (you simply walk the line and look for a pool of water). An example of pressure testing a transmission line is to fill it with water (before it is connected to the rest of the transmission system), pressure the water to 1.5 times the Maximum Allowable Operating Pressure (MAOP), and then leave the line pressurized for eight hours to ensure that pressure holds.
On the other end of the spectrum, a two-inch service line might be tested using gas pressurized to 90 pounds. If the line can hold the pressure for 10 minutes, it is considered good. By pressure testing prior to placing a line in service, it can be assured that strength is satisfactory throughout the line. Of course, it is important as a last step to record the results of the test and place this information into the gas company's records system.
Pressure testing of existing lines
Pressure testing of existing lines is more complex. Customers are being served by the lines, there is already gas in the lines, and existing lines may be more difficult to access. But pressure testing is an important way to ensure that existing pipe in the ground is capable of safely carrying the rated line pressure. This is especially true for older lines because regulations once allowed lines to be installed without pressure testing, and maximum operating pressures in the early 1970s were established based on the highest pressure the line had experienced during actual operations rather than through testing. And even for lines that have been tested upon installation, lines may have been damaged or experienced deterioration in the years since.
Testing of all existing lines is a long process since many gas companies may have thousands of miles of pipe in the ground. Companies typically prioritize pipes based on highest potential pipe failure and the pressure of the pipe. Typically, companies focus first on transmission pressure pipe (above 60 pounds per square inch). Hydrotesting is performed as described above but is made more difficult since gas must first be removed from the line. Then:
- the line must be accessed and isolated by cutting out a portion of the pipe;
- the pipe is tested;
- if any issues are found, the pipe must be replaced;
- the pipe must be cleaned before being reinstalled and put back into service.
Meanwhile, any customers downstream of the test are out of service unless the gas company has a means of providing supplemental downstream supply. This is sometimes done using compressed natural gas (CNG) or liquefied natural gas (LNG) trucks connected into the downstream pipe.
Although time-consuming and sometimes inconvenient for downstream customers, pressure testing existing lines is an important part of the over pipeline integrity management process. Testing can identify existing flaws in pipe material, detect cracking or other damage that may have occurred, point to where corrosion has weakened a pipe, or expose locations where initial installation procedures were faulty. This allows the faulty asset to be replaced prior to an incident.