After a piping system or pipeline is subjected to hydtrostatic or pneumatic testing, it may not be immediately put into service due to several factors. Until the start-up of piping/pipeline system, it needs to be temporarily preserved (mothballed) from effects of localized corrosion, pitting corrosion, crevice corrosion or other microbiologically induced damage. Lay-up refers to the temporary preservation of piping system after hydrotest to prevent corrosion damage until its start-up. Lay-up does not include preservation of piping or pipeline material during transportation or after its receipt.
In the foregoing sections, the term piping may be used to refer to both piping and pipelines. On small projects, it is recommended and may be achievable to schedule the hydrostatic testing of the piping system as close as possible to the start-up date. However, in most medium and large scale projects, the duration between completion of hydrostatic or pneumatic testing and plant start-up may be significant and hence laying-up the piping system becomes almost inevitable. Failure to properly lay-up a piping system can result in deterioration and damage to the piping sections, requiring replacement of a part or the entire system thus causing delays in start-up and putting the project objectives at risk. Hence, all piping, pipelines and equipments are required to be laid-up after hydrostatic testing unless they are immediately put in service (operation) following the test.
Lay-up methods can be broadly classified into the following:
- Lay-Up in Ambient Conditions
- Lay-Up in Wet Conditions
- Lay-Up in Dry Conditions
- Lay-Up with Inert Gas
- Lay-up with Oil
Chemical Treatment of Hydrostatic Test Water
Hydrostatic test water should be free from sand and other organic materials, which can nurture bacterial growth, and create the potential for microbiologically induced corrosion. Additionally, most engineering specifications address the limits of chloride content (30 ppm - 50 ppm) in hydrotest water used for testing stainless steel and Inconel 625 (UNS N0625) pipework. It is important to make a note that even if the above requirements are adhered to, after a stainless steel piping system is tested and drained, the evaporation of the remnant hydrotest water can result in high concentration of the chlorides far exceeding the above limits. Water tends to accumulate at the weld grooves and the high concentration of chlorides near the piping welds may cause chloride pitting and stress corrosion cracking of the welds. The importance of completely draining and drying the stainless steel pipework after hydrostatic test cannot be overemphasized. The following table provides an example of hydrotest water quality specification:
Sea Water Testing of Pipelines
In the offshore industry, testing of pipelines with treated sea water is common practice. Sea water used for testing a pipeline is required to be filtered, deoxygenated and free from debris prior to introducing any chemicals for treatment. Chemicals dosed in pipeline include corrosion inhibitors, oxygen scavengers and biocides. The type of chemicals to be used depends on the duration the pipeline will be kept filled with hydrotest fluid.
| Elements | Typical Limits |
|---|---|
| Chloride Content | Less than 30-50 ppm |
| pH | 6 to 8 (Water with pH < 5.5 is deemed corrosive |
| Sulfate Reducing Bacteria | Less than 103 per ml |
| Suspended Solids | Less than 50μm |
Examples of Chemical Used for Treating Water
Table below lists examples of chemicals used for treatment of water.
| Chemical | Trade Name | Recommended Dose |
|---|---|---|
| Oxygen Scavenger | EMO 4027 | 150 ppm |
| Biocide | Glutaraldehyde | 100 - 250 pppm |
| Biocide | Quaternary Ammonium | 100 - 250 pppm |
| Biocide | THPS Tetrakis Hydroxymethyl-Phosphonium Sulfate |
100 - 250 pppm |
| Corrosion Inhibitor | Nalco Maxicote | 2000 pppm |
The following sections discusses the various lay-up methods that can be deployed and their limitations.
Lay-up in Ambient Conditions
This method of lay-up may be used for non-critical piping systems. In this method of lay-up, the piping system after draining and dewatering is preserved as is without any additional provisions such as filling with inert gas, or complete drying of the system. After the system is drained the ends are covered to prevent the ingress of sand or water. Ambient lay-up is recommended under the following conditions:
- Drains are located at all low points (pockets) such that the complete piping system can be fully drained.
- The piping system has sufficient corrosion allowance.
- The piping system is not susceptible to corrosion by pitting.
Lay-up in Wet Conditions
Lay-up in wet conditions involves filling the piping system with treated water and maintaining it under a positive pressure of 30-50 psig. The water is treated with appropriate chemicals such as oxygen scavenger, corrosion inhibitor or biocides. The type of treatment will be determined by a corrosion specialist. The efficacy of chemicals lasts for a limited duration, which requires the water to be tested at appropriate intervals. Typical tests include verification of oxygen concentration which should be below a specified limit (e.g. 10 parts per billion), pH value of treated water or level of SRB (Sulfate Reducing Bacteria) which should typically have a count less than 103 per ml. Since the treated water contains chemicals, its disposal is subject to local statutory Environmental regulations.
Lay-up in Dry Conditions
In this method of lay-up, the system is dried by blowing dry air or inert gas immediately after dewatering. A dew point recorder is kept at the exit of the piping system and the blowing is carried out until the dew point temperature of -1deg C is achieved to ensure that no traces of moisture are left in the piping system. Once the system is confirmed to be completely dry, it is pressurised with an inert gas such as nitrogen and maintained at a positive pressure of 30-50 psig. Achieving a dew point temperature of -1deg C may take considerable time on large pipelines specially if there is remnant water at the low points. In such cases it is preferred to dewater the pipeline using foam pigs to ensure that the water has been effectively removed from the system. The system after pressurising with nitrogen is monitored at regular intervals to ensure that the positive pressure is maintained in the system and there is no ingress of oxygen in the system.
Lay-up with Inert Gas
After the piping system is fully drained and dewatered, it is filled and pressurised with inert gas such as nitrogen up to about 50 psi. A positive pressure in the pipe ensures that no oxygen ingress takes place thereby preventing any potential of corrosion. The pressure in the piping system should be monitored at regular intervals to ensure that the positive pressure is always maintained.
Lay-up with Oil
Specific systems such as lube oil system, seal oil system or hydraulic systems may be pressure tested using oil. Such systems are subject to lay-up with filled oil until the system is ready for commissioning.