Controlling head-pressure

[desA]

To my mind, the million $ question is... what happened at the 80 minute mark that changed everything?

Wish it were so easy. Actually, the simulations show that, given a random disturbance on the pressure signal, the system instability can stop on its own, given the 'correct' instantaneous value - strange as it may seem.

If the random number generator of the white noise (pressure disturbance) is changed, the correction point can move.

So, what could very well be happening is that the system is being disturbed (noise) on the pressure signal. This then, via perhaps a poorly-tuned PID controller, coupled with saturation & system local transport lags, ends up setting the system into a limit cycle for a period, whereafter it, then moves onto a stable trajectory. This is an uncontrolled system - in chaos.

This is why I ask the question as to whether, in this case, a temperature sensor may not be less noisy than a pressure sensor for such a system?

[Gary]

The object of PID is to minimize the offset. In the Pr chart I would expect to see a large offset decreasing in amplitude. Instead, the offset was limited compared to the later offset (after 80 minutes).

To my mind, it seems unlikely that the oscillations were caused by the water flow/pump control.

[desA]

Fair comments. So we are sure that pump control is not the cause of the above disturbance?

The interesting thing about this head-pressure problem is that it occurred on two tests - around 10 days apart - where the ambient air reached around 10'C, & was not seen on other tests with higher ambients.

There could be a few other issues - but, these did not show at other ambients, only at the 10'C ambient tests:
1. Reversing valve stiction, or malfuncton;
2. Cavitation in the water pipes.

[Gary]

Fair comments. So we are sure that pump control is not the cause of the above disturbance?

Possibly others will see the situation differently.

Lacking hands-on, I'm not sure of anything... lol

The interesting thing about this head-pressure problem is that it occurred on two tests - around 10 days apart - where the ambient air reached around 10'C, & was not seen on other tests with higher ambients.

There could be a few other issues - but, these did not show at other ambients, only at the 10'C ambient tests:
1. Reversing valve stiction, or malfuncton;
2. Cavitation in the water pipes.

Or could be just about anything.

[desA]

Or could be just about anything.

What 'anything' would cause the head pressure to bounce around like this? This is the question.

With a high side so responsive, it would pay to understand what the root cause/s could be.

[Gary]

Speculating about what might be wrong is not what I do, and in fact I am opposed to the practice. I don't like guessing. Its right up there with parts changing. I want to know what is wrong.

That said, if you don't have access to the machine, then all we can do is play guessing games.

Anything could include instrument error, pump cavitation, air bubbles... maybe even intermittant compressor flooding... and who knows what else.

[desA]

Thanks Gary. Let's see how the discussion progresses. Thanks for your help, so far.

With a PID controller, as I remember things, there is a reference junction. When this is either incorrectly configured, or even faulty, the controller may exhibit limit cycle phenomena, amongst other things e.g. drift.

This thought came off the back of Tesla's mention of referencing to chilled water temperature.

[Peter_1]

Also keep in mind that a pressure sensor (transducer) is much more expensive than a temperature sensor.

Or use a temperature sensor somewhere in the 2-phase region at LP and convert it to pressure. Mitsubishi Electric did his on their VRF's

[desA]

^ Thanks Peter. That is an elegant way to do it.

I would think that the temperature sensor may perhaps have a slower response than the pressure sensor & better internal damping - from the pipe wall and thermal mass. This could turn out to be a good thing.