Questions regarding whether tank equipment operating at micro-high pressure is considered a pressure vessel and whether it is equipped with safety valves
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I come across many pressure vessels at work, but I have some questions, mainly regarding: 1. Regular sewage discharge tanks； 2. Atmospheric deaerator ； 3. Expansion tank of the heat transfer oil system. The actual operating pressure of these is not higher than 0.1 Mpa(g), but many of them seem to be classified as pressure vessels. Another strange point is that even though the regular sewage tanks are pressure vessels, safety valves are basically not installed on them ； Atmospheric deaerators can also be equipped with either safety valves or without them. So my question is: 1. What is the rationale for classifying these devices as pressure vessels? 2. Is a safety valve necessary?
These tanks all share one common issue, namely the connection to the boiler. Then in the past, there were various interpretations. Now, with proper parameter design for atmospheric deaerators (105°C), classification is no longer necessary, but there are still very rare cases where boiler inspection agencies require classification.
It means that local approaches are taken as the standard, with no uniformity?
It is determined based on the actual operating parameters; if changes are necessary due to constraints, modifying the diagram should not result in significant alterations. For atmospheric deaerators (105°C), no specific classification is required; whether to install a safety valve is left to your discretion. For instance, if you add one in order to maintain uniformity in the design, this will increase costs for the customer. Alternatively, using it as a spare port will raise the company’s own costs. The open expansion tank is not classified, but there should be no problems when it is used as a closed type; the design does not require significant changes. Fixed displacement systems are generally pressure vessels, with pressures above 0.1 MPa.
Is it because of special reasons that safety valves are generally not installed in regularly discharged tanks? Are some pressure vessels not equipped with safety valves?
Whether to set it depends on how the designer interprets the scheduled arrangement. The continuous blowdown system is not a constantly open device; the opening time and duration of continuous blowdown are determined by factors such as water quality and the process conditions. If the user’s scheduling is unreasonable, the furnace may emit more or less waste per month, or even emit more than once in a month. The water fed in at regular intervals is saturated water at furnace pressure; it undergoes pressure reduction, volume expansion, and cooling to separate the steam.
Based on your reply, I understand that this issue is relatively variable. Unlike medium-pressure deaerators and small boilers, whose definitions are more standardized?
All of these can be matched according to the boiler auxiliary equipment. Take a simple example: for split/collection cylinders, if you design them separately, then they are containers. It must meet container standards. If you design it according to the auxiliary equipment standards, then meeting the boiler standards will suffice. Furthermore, the installation of safety valves is a matter related to the system’s design considerations, and it cannot be determined by looking at just one component alone. With the system interconnected, and provided that the process permits it, it is sufficient to install a safety valve at the highest point. Then, consider under safe operating conditions which node components require a separate safety valve. Your summary is quite accurate: relatively variable.
It’s not difficult to classify items according to the requirements of the pressure vessel regulations. Atmospheric deaeration equipment has a very low set pressure for its safety valve; many use water seals. Installing a safety valve on the deaerator is a requirement of industry standards.
So, does the atmospheric deaerator equipped with a water seal that you mentioned need to be fitted with a safety valve?
It is generally not necessary; it depends on the size of your deaerator – add more if it is large.