Steam collapse - pulling vaccum in a water tank - help please

[erbru]

Hi all

I need to check the sizing of an air flow into an insulated atmospheric water storage tank for the following situation and it is causing me a headache. I will then also need to size a new relief valve for the tank based on the scenario of the air flow system injecting too much air at high pressure.

The water tank (approx 35 m3, 5 metres high by 3 metres wide) is not foreseen for vacuum service nor pressurised service. That means from about -50 mbar to + 100 mbar is the maximum range. It is a very thin walled tank.

During hot 'sanitisation', the tank is totally emptied and filled with 2 m3 of water at approx 85°C.

The pressure in the tank increases rapidly during the filling process due to the hot vapour formed above the water surface.
To protect the tank, when the tank pressure reaches above 10 mbar, an automatic valve opens and releases the pressure to vent. This process is repeated as necessary as the tank is filled with hot water.

After the tank is filled with 2 m3 hot water, the tank water is pumped from the tank bottom through a piping loop (to clean the loop with the hot water). The pumped rate is approx 22 m3/h. All the water returns directly to the tank.

The loop had been full of water at ambient temperature and this 'cool' water returns to the 'hot' tank via a sprayball, immediately causing steam condensation and subsequent collapse in pressure.

The same pressure transmitter then opens a high pressure air circuit (8 bar) to the tank to equalise the pressure (set point for opening the air inlet valve is -20 mbar).

During the recirculation process, hot water continues to be added to the tank until 6 -8 m3 level is reached. During this time air is injected many times into the vessel in small bursts.

However it seems the air injection system is not correctly sized for the condensation and loss of pressure, since just after the start of the cold water recirculation, the pressure spikes to -40 mbar. The temperature (as measured on the side of the tank, and possible not fully representative) drops from about 85 °C to 70°C during this short space of time. During the subsequent phase of continuing the filling, the tank heats back up to 85 °C gradually and there are no more sudden spikes in low pressure.

The operators report a 'trembling' of the tank during this process.

I am trying to understand the correct air flow required for the worst case, which is as the cold water enters back into the tank and causes that sudden condensation.
How can the effect of steam condensation on pressure at this temperature be estimated?
Can I use the ideal gas law?
As for steam tables, I have looked but I don't understand what is happening to the enthalpy of the system here. The cold water spray is removing enthalpy in the form of latent heat (and some sensible heat) but how do I use that calculate the resulting pressure change.

Attached an output from the plant data logger showing the parameters of tank level, pressure and tank temperature.

The process starts between 13:48 and 17:36 on the time axis.
Green : pressure in bar (-0.05 to + 0.05 bar => - 50 to +50 mbar)
Blue : tank level (0 to 25 m3)
Black: tank temperature (0 to 100°C)

I suppose I could use this data to calculate the current air flow into the tank (pressure change in a known volume)...

 

[zdas04]

Your chart didn't attach.

I've seen sudden condensation events and they happen really quickly (miliSecond time scale). It is really challenging to design air ingress adequate to manage pressure when you are losing pressure that fast. One saving grace is that while condensation is rapid after initiation, initiation can be delayed by the rate of temperature change (i.e., if you pump fast, your time period supersaturated will be very short, but if you pump slower it can be quite long). Do you have to raise and lower pressure that quickly?

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[EdStainless]

Can you circulate without using the spray balls?
Get the water stabilized before you start spraying.

use a psychometric table, at 85C and 100% RH air has a SV of 1.3 m3/kg, and at 70C it is 1.1.
The amount of air does not change, it just takes up less space.
So if you have 35m3, at 85C you have 27 kg of sat air, when you cool it it will only take up 29.6m3.
You need enough air to fill the 5.4m3 that is left.

I see people do this with low pressure steam systems, easier than using sterile air.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[erbru]

Hi
Thanks for the reply. From this PC I can't repost the chart, will try again tomorrow from another PC
Yes I have the impression that these events are very quick. I'm going to the plant tomorrow to get some more detailed data of the pressure collapse.
The pumping speed could be reduced (it is a centrifugal pump set to 50% speed during recirculation, we could reduce further to 25% I guess) although it would take some reprogramming
I was thinking that the cold water entering the tank at any flowrate would be a problem in terms of 'intiating condensation'?
I have suggested to them to empty the loop of cold water as far as possible and fill it with hot water from the tank but they seem reluctant to do this.
In fact the loop is not easy to drain. I don't want to disrupt their process too much. The air injection system 'kind of' works but I am trying to improve it (if we could eliminate the trembling tank that would be good) and I need a basis upon which to redesign it.
I am trying to understand if I can calculate a theoretical pressure reduction rate based on an assumed condensation rate. Eg, kg/h cold water entering the tank touches x% of the vapour particles causing them to condense. How to do this, is it worth bothering or should I just go on empirical data? Eg look at rate of pressure collapse on the PT with the current air flow into the tank and from this try to calculate a new flow which would cause 'less' pressure collapse.
The air pressure feed line is mostly 1 inch (increasing to 2 inch close to the tank) with a regulator of unknown orifice size, simply marked 'made in Italy' and 3/4. I have no real idea of its full flow capacity.
The current tank relief valve is 100 mbar and rated capacity of 100 Nm3/h.

 

[Compositepro]

Your explanation did not give any reason for using some of the details which are causing your problem. Why are you controlling the venting and pressurization of the tank rather than freely venting the tank? Is your "automatic valve" a conservation vent, which is just a set of weighted plates that will lift when there is too much vacuum or pressure? Why use a spray ball to return fluid to the tank rather than a solid stream below the liquid surface?

In any case this is a very complex problem, which I doubt can be handled by calculations. The biggest mitigating factor in your favor is that at 85C there is still plenty of air mixed with the steam in your tank. This air greatly interferes with the condensation of water vapor by forming an insulating air layer on any condensing surface. It also reduces the volume change due to condensation. Your problem would be far more serious if you operated at 100C.

 

[Compositepro]

I would guess that the trembling that is felt is a chattering pressure regulator trying to respond to the changes in tank pressure by opening and closing.

 

[don1980]

Steam vapor collapse can cause a tank to fail in a very short amount of time. For vessels that are not rated for full vacuum, one should be careful to ensure that the vapor space isn't filled with a condensable vapor.

To me, your procedure appears frightfully risky. I think it actively creates risks that are unnecessary and easily avoided. The procedure fills the vapor space with condensable vapor, and then sprays cold water into that vapor. That sounds like playing with fire. I would maintain a small continuous purge flow of N2 or air into the vessel during this step. All you need is a few percent air/N2 to prevent sudden condensation. This is safer than relying on your control system to open the air valve just before the tank implodes.

 

[lilliput1]

Tank should be freely vented with pipe sized equal to larger of inlet or outlet pipe. Polyethylene tanks require minimum 1" larger pipe size. Terminate vent with elbow turned down. Make sure if vent pipe is full of fluid, pressure exerted on tank would be below its design pressure.

 

[georgeverghese]

During the recirc mode, try ramping up the pump on FIC to 22m3/hr over a few minutes, say 5minutes for a start - that should reduce the temp dip rate in the tank. Am guessing the rason for this sharp drop in pressure is also due in part to a recirc return nozzle that is located in the tank vapor space - relocating this return stream to the liquid side should also help to decrease this rate of pressure collapse, but this option may involve much more work than the FIC ramp rate option.

If you dont have a FT anywhere in this recirc loop, ramp rate on pump speed may also help, but the ramp rate may need to be non linear.

 

[erbru]

I should clarify : the tank is a pharmaceutical water for injection tanks with no free vent to air permitted for hygienic reasons.
It is fitted with a hygienic vent filter but from pressure fluctuations seen on the tank we know it is insufficient for the condensaton case. It just about cover the case when water is pumped out of the tank to a user point (and sometime the dynamic air supply steps in).

The question is basically how to estimate the vent requirement for the condensation case
The filter manufacturer can calculat the air flow to compensate for pressure change due 'convective heat transfer' based on the tank geometry but in this case there is a faster heat transfer due to the cold water recirculating via spray balls

The air inlet system is not a set of weighted plates, but a pressure transmitter connected to a PLC controlling pneumatically opening air inlet & tank vent valves. The PLC reads the pressure and send the signal to open or close the vent or the air inlet. This air is filtered.

I like the idea of a small continuous purge of air.

I asked about avoiding the sprayball, but this is not possible on the loop return. The pump could be reduced but not below 15 Hz (30% speed - about 15 m3/h) as the motor would overheat. There is no secondary or alternative pump.

The site is aware that this is operating mode is not ideal and will be replacing the tanks with one which can withstand full vacuum & 3 bar overpressure but not for at least 18 months; they have asked me to look at making their system as safe as possible in the meantime, without a major & expensive overhaul. They want to 1) make sure the filtered air supply is adequate 2 ) check the vessels are protected with the corrected overpressure protection and correct underpressure protection (rupture disc)
The long and short of it is that we can't escape the dynamic venting

See this article rom Millipore:
Design Considerations and Best Practices for Tank Vent Filtration

http://www.pharmamanufacturing.com/articles/2011/149/?show=all

I am trying to do point 2 in the section on dynamic venting
2. Calculate the air flow rate necessary to replace the steam during steam collapse post-SIP.

 

[erbru]

I am trying again to attach the file on pressure fluctuations

The process starts between 13:48 and 17:36 on the time axis.
Green : pressure in bar (-0.05 to + 0.05 bar => - 50 to +50 mbar)
Blue : tank level (0 to 25 m3)
Black: tank temperature (0 to 100°C)

 

[don1980]

When a sudden vapor collapse starts, how much air must be injected to arrest the pressure decrease and prevent the tank from failing? In my opinion, that is an academic calculation that has little to no practical value. During a worst-case vapor collapse scenario (e.g. high flow rate of low temperature cold water, finely dispersed by the spray ball into the tank) there's no practical way to stop the tank from failing. For vessels that are not designed for full vacuum, the way to defend against sudden vapor collapse scenarios is to prevent them from occurring rather than attempting to stop them once they have started. That's done by preventing the vessel's vapor space from being enriched with condensable vapor. Ensure that there is always a sufficient amount of non-condensable gas present, such that this scenario can't occur. When air/N2 injection is used as a layer of defense, you should start the injection as soon as possible. That is, start at a set point that is as high as possible, but below the normal operating pressure. For example, if you normally operate a 5 psig, then start at say 1-2 psig rather than waiting until the pressure has dropped into the vacuum range. And use a quick opening valve rather than a pressure regulator. Of course, you'll also need to make sure the vessel's pressure relief device has enough capacity so that this air/N2 flow rate can't overpressure the vessel.

 

[Latexman]

I'm with don1980 on this. The only reliable, engineering solution is a FV rated pressure vessel. I understand these interim actions are necessary, but if these procedural/administrative controls are forgotten or fail at the wrong time, the tank will crumple like an empty aluminum beer can in your hand. Be sure everyone knows that!

Good luck,
Latexman

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

 

[LittleInch]

Your problem appears to be primarily due to cold water entering the tank before the warm water makes it round to the sprayball. Can you not simply insulate the pipework and add a temperature limiting electric trace heating system to raise our maintain this loop to approx 80C?? This initial cold water spray does not seem to be required and getting rid of it should solve your issues?

Other option might be to start the pump as soon as the water enters the tank before there is time to generate a steam cloud and the the loop heats up the same as the inlet water?

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

 

[georgeverghese]

So what is the red trace line ?

The temp profile is relatively flat around the time of the large pressure fluctuations - this is probably due to a TT located in the liquid space, hence it would be insulated from actual vapor space temp. In any case, thermowells normally show considerable damping and time lag in detecting changes, so you cannot tell much from these TTs with thick thermowells.

As you've suggested, a slow ramp up on recirc pumping rate is one control solution. A process safeguard would be to auto stop the recirc pump when tank pressure drops to -40mbar or so, as detected by a dedicated PT. Spraying the hot water makeup into the vapor space ( rather than plug flow in to the liquid space) may also help to minimise vapor space pressure collapse.

From an ultimate protection perspective, agree with others than a tank designed for full vacuum would be ideal. Since this may be beyond your reach at the moment, would look at a 1oo2 voting to stop recirculation with 2 press transmitters to trip out the pump / full open the pump min flow bypass line through a dedicated high integrity trip loop. Edit access to this trip loop should be password restricted to plant supervisor level or equivalent.

It may be possible to set up a vacuum safety valve which is fed from a clean high press air or N2 supply from recollection as an ultimate safeguard, but calcualting the rate for this would be difficult if the return water goes into the vapor space; it may be possible to get some values for this condensation if the recirc cold water is led back into the liquid space though, if you can find a value for liquid-vapor interface heat transfer coeff.

Could you post us a simple sketch of the tank nozzles? What are these "sprayballs"? Looks like some improvement on tank vapor space pressure controls may be required also, with the description on these controls so far.

 

[erbru]

Hi again all
Can I just say I appreciate all your input. Thanks for replying with your ideas.

The red trace line on the previous chart is actually a pressure measurement elsewhere (another (correctly rated!) hot water tank which receives hot water from the same source) so not relevant.

The hot make up water which is added (ie not the return form the loop) is added into the top of the tank (into the vapour space) but not via a sprayball.

I went to the site yesterday and had a frank discussion with them. I can see they are worried, if these tanks fail about 60-70% of their production goes down (factory employees over 200 people and makes 50 million units of product a year). They have been operating in this mode for 15 years, sanitising the tank once a week in this way. The air valves take a hammering in terms of their opening and closing, tens of thousands of cycles a year. So I have highlighted to them the sensitivity of the subject. At least the pneumatic air supply has a back up. But there is no back-up air supply to the tanks of one of these valves fails (they are all fail closed mode including the automatic vent valve which is need to vent the steam which build up during sanitisation)

They are willing to change their procedure and empty the loop and start recirculation with the hot water in the tank instead. That should help somewhat. Checking the sizing of this dynamic venting procedure will still need to be done though.

I took some close up from the plant HMI from most critical phase (the start of the cold recirculation) yesterday.

The colours have changed and I zoomed in on the scales very far to get the detail.

Black - level (m3)
Blue - temperature (degC)
Red - pressure (bar) - not the scale is zoomed to give detail at the -40 to +20 mbar level

Following the introduction of cold water, the air valve opens and closes every 2 seconds. This frequency of injection continues for about 1.5 minutes until there is some degree of stabilisation.

The temperature probe is in a thermowell but it is on the tank side near the bottom. It would be submerged with about 6-8 m3 I estimate (I don't have the exact geometry - no drawings, just out site measurements)

For info, the pumps stops recirculating if the pressure reaches -30 mbar. On the 40 second curve you can see the level plateauing out and going up in steps. I wonder if this is the pump stopping and restarting.

I think the first priority is some form of vacuum protection (I would install a vacuum rupture disc with indication of rupture, yes this would cause a stop of production while it is change and tank is resanitized but they don't produce during the half day sanitisation time) and a larger capacity overpressure protection.
The idea of changing the set points is a good one too. I think it is safer to operate between 0 mbar and +30 mbar than at all in the negative zone. Overpressurising the tank to +50 or 60 mbar worries me (slightly) less.

I will try to send some pictures of the spray balls and tank interior from another PC.
I am going this Friday to observe a sanitisation of the tank. However they will start with an empty water loop not a cold one so it is not quite the worst case we see here.

 

[erbru]

Another curve (longer time period)

 

[erbru]

One min snapshot around the start of recirculation

 

[erbru]

Forty second snapshot around the start fo recirculation

 

[erbru]

Tank during the later heat up phase (some time after the start of recirculation, level of around 4 m3) This is a much slower process than the condensation just after the recirculation starts. The injections of air occur every 10 minutes of so.