Protection of vessels from temperature excersions

[MarkkraM]

Vessel design is based on worst case combination of pressure and temperature. If a scenario exists that potentially provides higher than design pressures, typically a relief device such as a PSV or bursting disk is provided. Provision/sizing/installation of these devices are well established in many standards. If design temperatures can be exceeded due to external fire there re standard fire proofing techniques can be adopted.

My question relates to satisfying pressure vessel integrity standards when protecting against temperature excursions, in this case from stagnant pressurised ethylene in molecular sieve material or from pressurisation of ethylene in a regenerated molecular sieves). A couple of standards I've skimmed through (AS1210 & ACME VIII) do NOT make allowance for the use of instrumented protective systems so I'm after any comments on whether what we have done would satisfy regulatory bodies.

1. Of course on a lower level of protection (risk mitigation) we have procedural controls in place so that ethylene is not left pressurised in the purification vessel, and that preloading takes place before pressurising a regenerated bed up. We also have warning alarms for the operators.

2. As our critical level of protection we have installed an automated blowdown system that activates when any one of the many thermocouples installed in the bed reads a high temperature (i.e. the design temperature of the vessel). The ethylene supply is cut off and depressurisation path is opened, both controlled by EIVs.

 

[Lincoln]

Regulatory bodies typically will not accept only an automated system as the critical safety protection device for pressure vessels. Since you are concerned with a temperature excursion resulting in the vessel exceeding the pressure rating, a self-initiated pressure relief device will be required.

 

[MarkkraM]

Lincoln,
I think you've missed my point (or maybe I am mistaken myself). 'Pressure' rating is not the core issue here. Integrity is based on both temperature and pressure. A blowdown system is how you stop a temperature exotherm. it is not (at this stage of the exotherm at least) about relieving "excess" pressure build up. A relief valve is installed on the vessel (for external fire) however this does not help at all in this case.

Regards,
Mark

 

[br549]

1> Design the vessel for the max temperature possible, (ideal for new system)
2> the blow down system need to be redundant, not part of the normal process control.
3> it needs to be a fail-safe system, i.e. loss of signal causes system to activate
4> it need to have controlled access, i.e. plant safety or management approval before changes are made
5> it need to be testable and inspectable, periodic testing to insure it functions correctly and that piping and controls are not plugged or frozen by build up.

 

[MarkkraM]

Thanks for the post br549.
Looks like our blow down system has everything you mentioned in place. The only other issue relates to your first point. Design vessel for maximum temperature possible

Vessel design is satisifies the following pressure temperature combinations:
1.25MPag and 360degC. (Maximum conditions during regen)
3.45Mpag and 125degC. (Maximum conditions during operation)

It does not satisfy the conditions 3.45Mpag & 360degC. Should it have been designed to satisfy both?

Regards,
Mark

 

[br549]

680F is Just above the value were ASME begins derating material strength because of temperature (600F) Ideally you would look at both pressure and temperature extremes and design for the worst case. If this is AN ASME vessel Check the CODE plate attached to the vessel you might already be designed for the Higher Temp.

 

[MarkkraM]

Checked the plate and vessel design is 3.45MPa and 120degC.

I've had a mechanical engineer check the wall thickness calculations. It appears that (exluding corrosion allowances) the required thickness at 360degC and 3.45Mpag is 0.1mm less than what we have .... mmmmm. I guess there are safety factors incorporated in these equation? Furthermore this pressure/temp combination should perhaps be considered a remote continguency and thus is subject to a certain "overpressure" allowance.