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Thermic fluid system pressure control

Thermic fluid system pressure control

(OP)
Hi All:
So there is a small problem I'm facing and need your help..
I'm operating a thermal oil system (Therminol 72) at 320 Deg C. On the expansion tank there is a forward pressure regulator on the inlet N2 line and there was also a forward pressure regulator on the N2 vent line. The set pressure on the N2 inlet line is 0.45 Mpa and the vent line was 0.1 Mpa. The operation was fine and forward pressure regulator was held at a constant value. But we this set-up, we were losing N2 and also losing low boilers from the oil because the expansion tank is constantly being vented due to forward pressure regulator in the vent line

Upon reviewing the design I realized, this is not correct and changed the forward pressure regulator to a backpressure regulator on the vent line. Now the current set point of the N2 inlet is 0.45 Mpa and the backpressure on the vent line is set at 5 psig more than the forward pressure. Now the issue is, the forward regulator set point changes by itself and goes lower slowly to 0.4 - 0.42 Mpa in couple of days. when this happen the backpressure regulator also decreases by the same value.

When forward pressure decreases, suction pressure decreases and this is a real issue with my pump. I cannot operate the pump in good condition if suction pressure decreases

I do not understand the reason why the set point of the forward pressure regulator decreases over period of time

Thanks in advance for your help
Araza

Asif Raza

RE: Thermic fluid system pressure control

If your regulators are not instrument grade it is quite common for the pressure to drift after the initial setting. There is a dead band between when the regulator adds pressure and when it relieves pressure. Mechanical regulator valves also have a proportional band. The outlet pressure must change in order for the valve to open or close to change flow rates. The original process design may have wasted N2 but it would have overcome the dead band issue by always requiring the inlet regulator to have a small flow through it. Your current process design includes a 5 psi deadband between your 2 regulators so pressure must fluctuate in this range. But your complaint is that it is dropping too low.

To avoid this adjust the inlet regulator to your desired pressure only by increasing the set point. Your regulator was probably set by adjusting it up and then turning it down to keep the regulator from overshooting the desired pressure. When a regulator is close to its set point the flow through it will be small and it can take many minutes to final stabilize at its equilibrium state. So initially set the regulator by increasing the setting to slightly below the desired set point, then come back later and see if it has drifted up to the desired pressure. If not, make a slight upward adjustment until you have the desired pressure. If you over shoot, vent pressure to several psi under your desired setpoint and then repeat the adjustments to approach the setpoint by using only upward adjustments.

One complication is that there are other process variables that may cause the tank pressure to change, such as changes in fluid temperature and volume. You cannot compensate for these factors by turning down your inlet regulator. Whenever the pressure is too low adjust the inlet regulator up slightly, but never turn it down as long as the pressure is in the 5 psi deadband. Venting is the purpose of the back-pressure regulator.

RE: Thermic fluid system pressure control

Pilot operated type self regulated PCVs' for both feed N2 line (forward sensing) and vent N2( backpressure sensing) line may help to reduce drift ?

A 5psi gap between these 2 regulators may not be enough to cover all the operating scenarios and minimise venting of Therminal saturated N2. A much wider gap may be required.

RE: Thermic fluid system pressure control

Your description of the original set-up seems very odd - a 1 bar back pressure on the vent line sounds like a lot to me.

But as noted it would keep flow going through the valve. The mechanical valves are better at regulating flowing liquid than getting to a certain pressure then sealing.

The band between inlet set pressure and vent pressure is far too small.

YOur process conditions are not mentioned, which would be my first port of call

You state even a small loss of pressure (0.5 bar?) leads to issues with your pump - what is the requirement and why so small a margin?

You have the outlet set at 5 psi / 0,3 bar above your inlet pressure of 4.5 bar. But what is the vessel or system design pressure?

Can you create a larger margin?

As to why both regulators decrease set points over time is not possible to say without any details of the regulator. Does the temperature in the expansion tank increase? Are the regulators subject to changes of temperature which affect their performance? What are they sized for? Regulators which are too big are more likely to suffer loss of performance and set point accuracy when operating in the 0-20% capacity region than say 20-40%

You say you've reduced flow of N2 - maybe you've reduced it too much?

Maybe you should have two regulators on the vent - one to flow a certain (low) quantity of N2 continuously and a second larger unit set 1 bar higher than inlet pressure? If it worked last time then it might work again.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Thermic fluid system pressure control

asifraza0,

Therminol has some very experienced, customer focused Engineers on staff that will gladly review your unit, either "on paper" (P&ID) or in person, and recommend direct and very practical solutions to your problems. They maintain examples of best practices P&IDs backed by years of experience. And, they are "free".

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Thermic fluid system pressure control

Engineers prefer therminol 66 for a variety of reasons, one being its lower boiling point.

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