Autoclave requalification problem

[friscoring]

Would welcome any advice / ideas as to possible cause to problem

Problem: During routine empty chamber requal of the autoclave, we are seeing a 4 degree C overshoot at start of exposure exceeding the HTM2010 limit of 3 degrees. This overshoot happened during heat up in previous requals ie it appears to have shifted by up to 30 secs. Also seeing an increased delta temp between the installed drain probes and chamber probes of 0.8C versus 0.3C during previous quals and an increased delta T of 3.08C from 1.5C across the validation probes. Also measure 6C superheat in header before the autoclave control valve per HTM2010 method versus 0 - 0.7C before.

Checked all possible caused (we could think of):

1 Measurement - recalibrated all probes to same working standard and confirmed several times - ok. We originally had a problem with the I/O card and repleced same and the CPU card was replaced because the battery voltage was slightly low - all control parameters checked to be ok. eproms not touched.

2 checked status of bleed valves and condensate bypass valve settings - ok

3 Jacket pressure switch dropped from 1.3 Bar to 0.8 Bar to eliminate Jacket superheating the chamber. Temporary change as is likely to lead to wet loads.

4 Control Valve operation - OK

5 PID values - altered the settings to get a compliant cycle but it is not the root cause and would likely lead to problems with distribution studies if left.

6 CSG functioning correctly - removed some lagging after PCV at CSG which is ~ 100m from Autoclave, to eliminate superheat & to see if it had impact on cycle - no significant impact - still failed cycle.

7 Cannot understand superheat at autoclave on CGS side of PCV as there is nothing to generate it and the temp corresponds to sat stm press in header. We use the HTM2010 method of measuring for superheat. Reconfirmed 3 times. No measurable superheat in chamber using packed towel test.

8 Need to find root cause - would really welcome any thought / suggestions

 

[MedicineEng]

Did you checked if there are possibilities of air pockets in autoclave jacket? This could justify your delta T differences.

 

[quark]

The superheat downstream of control valve results due to dryness in steam. The approximate process across control valve is isenthalpic and there is a temperature gain with the pressure reduction. You can find out the amount of superheat if you know the pressures upstream and downstream, from steam tables. If there is superheating, removing insulation may not be effective.

It is better to generate pure steam at relatively low pressures.

Secondly, as pointed out by MedicineEng, air pockets(in chamber) may be a problem.

Do you face the same problem during distribution and penetration studies as well?

 

[friscoring]

Thanks for your responses guys.

We don't have an easy way to vent the jacket - air would probably give me a cold spot in any case and this is not what I have been seeing.

I have still 4 - 6 degree C of SH in the stm header and it appears to give rise to 0.5 - 1 degree of SH in the chamber at end of heatup and start of exposure. It dissipates within 30 - 60 secs. It puzzles me as to where it is coming from as the CSG will produce ~ 95% dry steam per mfg specs and should absorb any SH generated in reducing Press form 5 bar to 3.5 bar just after the CSG. I measure SH in the header at the Autoclave 100m away and get 4 - 6 degree SH where previously we got 0 of < 1.



To get get around the problem I adjusted the PID values and requalified the Autoclave.

We introduced an empty chamber or temp distribution test in 2004 but only set a +3degree, -0.5 degree criteria in 2005. On reviewing all the val data it became clear that our process capabiltiy was not reliably able to meet the acceptance criteria particularly with a hot chamber. We comfortable met it with a cold chamber. On reviewing HTM2010 the acceptance criteria is 2 degrees and we adjusted the heatup time from 1.42 mins to 3.5 mins approx and we were able to meet this AC. Mfg recommend 4 - 6 mins.
Doing these tests on a hot chamber 116C versus a cold or cool 100C chamber makes a significant differance on the results and this is where the SH seems to kick in.
Lost two weeks production and as much sleep on this one.

Interest in any further ideas on SH. Tks

 

[MedicineEng]

You mention that according with the manufacturer you produce 95% of dry steam. Did you consider that your steam quality could have changed sue to some malfuntion in teh system?
This would explain why you have now superheated steam and before you didn't have it.
I confess that my knowledge of superheated steam is quite limited, but eventually you can have a look into your steam producing system.

 

[quark]

I suspect your steam to be drier than 95%. You said your upstream pressure is 5bar and downstream pressure is 3.5bar. You didn't mention it is gauge pressure or absolute pressure. However, considering them as absolute pressures,

Case I
Sat. temp. of steam at 3.5bar a is 138.857 deg.C
Enthalpy is 2731.96 kJ/kg

Sat. temp. of steam at 5 bar is 151.83 deg.C
Enthalpy is 2748.11 kJ/kg

Considering 100% dry saturated steam to the inlet of pressure reducing valve, as the pressure reduction process is isenthalpic, the downstream steam has a pressure of 3.5 bar a and enthalpy of 2748.11 kJ/kg. This corresponds to a downstream steam temperature of 145.97 deg.C.

So, the amount of superheat is (145.97-138.85) = 7.12 deg.C.

Case II
As per your test data, if your steam quality is 95% upstream, then 5% of it is moisture.

The enthalpy difference of upstream and downstream sides is (2748.11-2731.96) = 16.15 kJ/kg.

The latent heat of steam at 3.5 bara is 2147.7 kJ/kg. So, if you vaporize 16.15/2147.7 = 0.0075 kgs of steam, you will get saturated conditions downstream side. But you have a moisture content of 5% or 0.05 kgs/kg of steam. This shows that your moisture percentage is lower than what you observed.

I have few questions here.

1. why you are using 3.5 bara instead of 2 or 2.5 bara for sterilization. Were you maintaining 135 deg.C for reduced sterilization times?

2. Do you have a steam trap just before the inlet of pressure reducing valve and where are you checking the dryness of steam? Is it an assumption or do you check it with a throttling calorimeter?

 

[friscoring]

Thanks again. I am checking out some of the info. Pressure is guage. I'm not sure why the main pressure was set at 3.5 barg - it does serve other users and the highest pressure required is the SIP panel at 1.8 barg. It was originally installed at that pressure and probably set to ensure appropriate pressure at user points.
There is no trap immediately before the PRV which is close the CSG.
I haven't checked the dryness of the steam I have taken it from discussions from the vendor. I will get it checked by an outside agent and also get info from the vendor and revert.
Our max ster temp is 124 deg.C for 30mins. Ster band is 3.5 deg.C

I see from your calcs that we shouldn't see wet steam and that appears correct. At the PRV the pressure is 3.5 barg and the surface temp of the pipe was 147 +- 0.3 deg.C The surface temp will be lower than the stm - we measure a delta T at the Autoclave of 3 deg.C so it would indicate SH after the PRV. I'll see if I can measure the wetness and SH but it will be awkward as it will require taking the system out of service etc.