The effect of vacuum on instrumentation equipment
I tried a little experiment at home!! What I did is I took the very hot steaming water, poured it into a plastic bottle, closed it tightly and kept it in the freezer. what happened? The plastic bottle bent!
But why did this happen?
Steam is in gaseous state. And we know that gas molecules are far apart compared to liquids and solids. Now what happens when the vapor is completely packed inside a container and you cool it? These molecules condense and form a liquid. But there is no leakage or space needed to fill the atmosphere in this empty place.
Is this happening in our industry?
Pipes should be cleaned at regular intervals. This is done in different ways Blowing air to remove dust particles – The function is to pass through a “pig” (like a football) for cleaning (note the side: – Never use reduced valves for such lines Don’t because the pig will get stuck in it) steam cleaning is now the third person currently favoring us. Usually our systems (pipes) are closed so that the atmosphere cannot enter and steam cleaning is done. Finally, the vapor 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 for us?
Yes, this greatly affects instrumentation equipment in a variety of ways .
Calculation of torque in valves:-
The vacuum must also be included when calculating the torque to open the fully closed valve (if a vacuum is created in the pipe when the valve is closed after steam cleaning) because there is an additional torque required to open the valve.
Choosing the right band:
The boiling point of water at ATMOSPHERIC temperature is 100°C, but in a complete vacuum, it begins to boil at 0°C. The same principle works for other fluids, especially when choosing a diaphragm seal assembly filler fluid. The vendor has a pressure-temperature curve that should be used when selecting a jump fluid in vacuum cases. Otherwise, the liquid inside the diaphragm seal assembly will boil
In accordance with API 551, the following are recommended for transmitter installation in vacuum service.
Vacuum applications present 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 lower than the vapor pressure of another fluid: – – Move the transmitter 1 m (3 ft) or more below the valve or more! The actual head pressure should 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 to its vapor pressure. As the pressure approaches full vacuum, it becomes difficult to achieve an acceptable level of accuracy. This is because most filled liquids contain microscopic amounts of air and trapped gases that tend to expand as they approach zero absolute pressure. (Attention: (This is if the vacuum conditions are at operating conditions, not design conditions that must be calculated exactly) The above description comes from API 551 Why switch the transmitter below the bottom tap? The following article explains this in detail
Liner magnetic flowmeter
Vacuum conditions can cause some meter liners (eg, PTFE) to collapse, especially in sizes larger than four inches. Steam, for startup, cleaning, etc., can create a vacuum or excessive heat that can damage a liner. (API reference 551 page 79)
Gauge valves have ball checks as a standard feature. They help prevent the loss of the ship’s contents in the event of a meter failure. However, they should not be used in vacuum or steam service. (API reference 551 page 149)
Pegging and tying the valve:
Many check valve seals can handle high pressures, but only some versions allow vacuum. So check exactly if it will withstand the vacuum or not!
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