Control suggestion for a hyperbaric chamber
Control suggestion for a hyperbaric chamber
(OP)
Hello, guys.
I was designated to think about a solution to control the pressure inside a hyperbaric chamber we have here at the workshop that is used to perform pressure tests on equipment designed for subsea operation.
The point is today the technicians have to manually control the water ingress inside the hyperbaric chamber to control the pressure inside it until it reaches the test pressure. Pressurized water is provided by a pneumatic pump that is mounted in the hydraulic circuit outside the chamber. There is a pressure transmitter installed in the chamber inlet to monitor its pressure. We start to pressurize the chamber after completely filling it with water.
Recently we were informed of a requirement that has not been considered since then. We need to reach the test pressure in 30 minutes +/-1 minute. Therefore, it is quite difficult to meet this requirement manually controlling the pressure inside the chamber.
Do you have any suggestion on how I can implement this control? We have a small PLC available that can be used as part of the hardware needed for the solution. However, I don't know if it would be best to control the air pressure in the pneumatic pump pilot (and this way controlling the flow rate in its outlet), or if a control valve should be used to control the water flow based on the pressure transmitter readings.
Any guess? I appreciate any help.
Thank you,
Marcelo
I was designated to think about a solution to control the pressure inside a hyperbaric chamber we have here at the workshop that is used to perform pressure tests on equipment designed for subsea operation.
The point is today the technicians have to manually control the water ingress inside the hyperbaric chamber to control the pressure inside it until it reaches the test pressure. Pressurized water is provided by a pneumatic pump that is mounted in the hydraulic circuit outside the chamber. There is a pressure transmitter installed in the chamber inlet to monitor its pressure. We start to pressurize the chamber after completely filling it with water.
Recently we were informed of a requirement that has not been considered since then. We need to reach the test pressure in 30 minutes +/-1 minute. Therefore, it is quite difficult to meet this requirement manually controlling the pressure inside the chamber.
Do you have any suggestion on how I can implement this control? We have a small PLC available that can be used as part of the hardware needed for the solution. However, I don't know if it would be best to control the air pressure in the pneumatic pump pilot (and this way controlling the flow rate in its outlet), or if a control valve should be used to control the water flow based on the pressure transmitter readings.
Any guess? I appreciate any help.
Thank you,
Marcelo
RE: Control suggestion for a hyperbaric chamber
Brad Waybright
It's all okay as long as it's okay.
RE: Control suggestion for a hyperbaric chamber
No, it does not include fill time. My doubt is more related to what I should control to meet the requirement.
The system itself is capable of achieving the test pressure in less time, indeed. The main difficulty is making it gets from 0 psi to, let's say, 3,900 psi in 30 minutes. We have already lost some tests because the technicians were not able to manually keep this rise time and the test pressure was reached only after 40-45 minutes (or, on the other hand, on 20-25 minutes).
I believe a PI control would be appropriate for this process. However, I still do not know what I should control.
Thanks once again,
Marcelo
RE: Control suggestion for a hyperbaric chamber
Brad Waybright
It's all okay as long as it's okay.
RE: Control suggestion for a hyperbaric chamber
This hyperbaric chamber has overpressure protection devices already installed (relief valves). Although we are not in North America, indeed it is an ASME code pressure vessel plus our local regulations.
RE: Control suggestion for a hyperbaric chamber
Brad Waybright
It's all okay as long as it's okay.
RE: Control suggestion for a hyperbaric chamber
If you have a system that fixes the amount of air then this would be trivial. For instance, a dribble port. You fill the tank from the bottom until the water overflows out the port then close the port. This easily traps the same amount of air every cycle. If that's your system, you use the PLC to run the pump at a specific rate completely dead-reckoned. A few trials would nail the pumping rate needed.
If you have no way to provide the repeated air amount then you add to the dead-reckoned method. A monitoring subroutine watches the first minute of pumping at a dead-reconed rate and at the end of the first minute assesses the rate at which pressure is building then sets the dead-reckoned speed constant for the next 29 minutes.
I would use what's called an I to P to control the constant air normally provided to the pump. This would allow full speed control of the pump.
I/Ps take a 4-20mA signal and convert it to 0-Xpsi. The more air pressure out the back the faster your pump will cycle. You could also use a dumber pneumatic valve that will work the same way.
Sounds like a lot of fun.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Control suggestion for a hyperbaric chamber
I would also consider a PID controller with a ramp and soak function.
Set the terminal pressure and set the ramp time to 30 Minutes.
You could use an accumulator rather than leaving air in the vessel.
The PID with ramp and soak would also piggyback onto your trapped air scheme.
Here is a Ramp and Soak process controller from Omega for under $200.Link
Bill
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