One of the main reasons for sample cooler failure has been observed to be the failure of the sample coil due to stress corrosion cracking. Here is a detailed analysis of this problem :
If the chloride content of cooling water is high as per cooling water specifications (i.e. more than 35ppm), we don’t recommend use of SS316 material for sample cooler coil. The reason being, sample steam entry is at a higher temperature and pressure. In such cases sample coil may be subjected to inter-granular corrosion resulting in stress corrosion cracking. Due to high hardness of cooling water there is formation of sludge and scales on sample cooler coils causing the above problems. Scales and sludge in the sample cooler is a non conductive layer formed on the water side of sample cooler coil causing reduced heat transfer efficiency
Symptoms: Failures in the coil are characterized by a thick wall, brittle-type crack. This may be found at locations of higher external stresses, such as near attachments.
Causes: SCC is most commonly associated with austenitic (stainless steel) materials and can lead to either transgranular or intergranular crack propagation in the tube wall. It occurs where a combination of high-tensile stresses and a corrosive fluid are present. The damage results from cracks that propagate from the ID. The source of corrosive fluid may be carried over into the super heater from the steam drum or from contamination during boiler acid cleaning, if the super heater is not properly protected. Other sources include scaling, deposition, mud and impurities that can travel in the pipelines during startups
Problems faced by the user:
1. Scaling and sludge formation on the water side of sample cooler
2. Loss in sample cooler efficiency causing higher sample temperature at outlet of sample cooler
We recommend use of alloy steels like “MONEL400”/ “INCONEL 625” for sample cooler coil. We can offer “Inconnel 625 coils” in our “DHx Series” high efficiency sample coolers. We have supplied such coolers in the Middle East and coastal areas of India where chloride content in cooling water is major problem. These coolers are available with IBR approval and certification.
Use of special material like Inconnel 625 reduces formation of non conductive layer, but, it cannot avoid it 100%. Thus following are remedies to this problem:
1. D M Water is recommended for use as cooling water
2. If it is not possible, softened water with very low hardness (CaCO3 & MgCO3) can be used, to reduce scaling problems
3. Proper maintenance and periodic cleaning of sample cooler coils is must in cases where D M Water is not used for cooling purposes
To make maintenance easy:
1. All coils in our coolers are removable – so if there is damage, you don’t have to replace the entire cooler – just coil replacement is possible
2. The shell is removable without the need to dismantle the sample cooler. This facilitates cleaning of coils in a very simple way
3. For routine maintenance, a drain plug is provided at the bottom of the shell. This facilitates quick cleaning of the shell, as the mud / sediments settled at the bottom can be easily removed