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

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.

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.

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