Impact of vacuum on instrumentation equipment
I tried a little experiment at home !! What I did was I took some very hot water that was coming out of the steam, poured it into a plastic bottle, closed the lid tightly and kept it in the freezer. what happened? The plastic bottle bent!
But why did this happen?
The steam is in the gaseous state. And we know that gas molecules are very far apart from liquids and solids. Now what happens when the steam is completely packed in a container and you cool it? These molecules collect as vapor condensate and form a liquid. But there is no leakage or space to fill the atmosphere in this empty space.
Is this happening in our industry?
Pipes should be cleaned at regular intervals. This is done in a variety of ways. Blowing air to remove dust particles. Do not do it because the pig is stuck in it) Steam cleaning is now the third person we are interested in right now. Usually our systems (pipes) are closed in such a way that the atmosphere can not enter it and it is done while cleaning the steam. Eventually the steam cools and a vacuum is formed as discussed above. Process people also let us know when they use design pressure conditions as FV (full vacuum)
But is vacuum important to us?
Yes, this affects the instrumentation equipment in many different ways.
1. Calculation of torque in valves: –
The vacuum must also be created when calculating the torque to open the valve completely off (if a vacuum is created in the pipe, when the valve was closed after cleaning the steam) because there is additional torque required to open the valve.
2. Choosing the right seal:
The boiling point of water at ATMOSPHERIC is 100 ° C, but in complete vacuum, it starts to boil at 0 ° C. The same principle applies to other liquids, especially when choosing a diaphragm seal filler. The vendor has a temperature-pressure curve that should be used when selecting the jump fluid in vacuum cases. Otherwise, liquid will boil inside the diaphragm seal assembly
In accordance with API 551, the following is recommended for installing transmitters in a vacuum service.
Vacuum applications pose a particular problem for the use of diaphragm seals. Filled liquids usually have a separate temperature range specified for vacuum conditions. The measured vacuum can be less than the vapor pressure of another fluid: – – Pull the transmitter 1 meter (3 feet) or lower than the valve or more! The actual head pressure must be filled by multiplying the vertical distance between the end valve and the transmitter by the specific gravity of the liquid to ensure that the liquid is full of vapor pressure. As the pressure approaches a complete vacuum, it becomes difficult to reach an acceptable level of accuracy. This is despite the fact that most of the filled liquids contain microscopic amounts of trapped air and gases that tend to expand as they approach absolute zero pressure. (Attention: This is if the vacuum condition is in operating condition, not the design condition, which must be calculated accurately.) This explanation above comes from API 551. The following article explains this in detail
4. Magnetic flowmeter liner
Vacuum conditions can cause some meter liners (eg PTFE) to fall, especially in sizes larger than four inches. Steam, for starting, cleaning, etc., can cause a vacuum or overheating, which can damage a liner. (API Reference 551 Page 79)
5. Confused though
Gauge valves have ball checks as a standard feature. In the event of a breakdown gauge, they help prevent the ship’s contents from being lost. However, they should not be used in vacuum or steam services. (Reference API 551 Page 149)
6. Pegging and sealing valve:
Many shut-off or control valve closures can withstand high pressures, but only some versions allow vacuum. So check carefully whether it will withstand the vacuum or not!
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