Cold Start up of Pressure Vessels
Cold Start up of Pressure Vessels
(OP)
Good Day,
I'm on the hunt for suggestions and hopefully some references on how to start up a pressure vessel from a state colder than design.
The vessel has an MDMT of -20 F, but the environmental conditions are such that parts of the vessel the vessel could be at an ambient temperature of -50 F when they go to start it. The vessel is insulated, with a heated building on one end. The areas actually exposed to ambient are large diameter nozzles, such as the manway and inlet connections.
The unit can't operate at this temperature for very long, as it's a gas/oil/water separator in a gas plant. Basically the customer is looking for assurances and recommendations to get the unit up to pressure long enough for the process to start moving, at which point the temperature of the process will keep the unit above MDMT.
Having double checked my design, all the components are good for -50 F or lower, due to UCS 66 allowances. The vessel has already been designed, registered, and built, so impact testing the welds isn't an easy option, and it appears that is the only area where we may need to be cautious.
My supervisor has already suggested restricting the design pressure as the temperature decreases, but we're hunting for references. I have been thinking about using values from Fig UCS 66.1 as a basic guideline.
If someone has a start up guideline for a non-fired pressure vessel in a similar situation, it would be a great help.
Thank you in advance for any help you can provide.
Regards,
Andrew Travers, C.E.T.
I'm on the hunt for suggestions and hopefully some references on how to start up a pressure vessel from a state colder than design.
The vessel has an MDMT of -20 F, but the environmental conditions are such that parts of the vessel the vessel could be at an ambient temperature of -50 F when they go to start it. The vessel is insulated, with a heated building on one end. The areas actually exposed to ambient are large diameter nozzles, such as the manway and inlet connections.
The unit can't operate at this temperature for very long, as it's a gas/oil/water separator in a gas plant. Basically the customer is looking for assurances and recommendations to get the unit up to pressure long enough for the process to start moving, at which point the temperature of the process will keep the unit above MDMT.
Having double checked my design, all the components are good for -50 F or lower, due to UCS 66 allowances. The vessel has already been designed, registered, and built, so impact testing the welds isn't an easy option, and it appears that is the only area where we may need to be cautious.
My supervisor has already suggested restricting the design pressure as the temperature decreases, but we're hunting for references. I have been thinking about using values from Fig UCS 66.1 as a basic guideline.
If someone has a start up guideline for a non-fired pressure vessel in a similar situation, it would be a great help.
Thank you in advance for any help you can provide.
Regards,
Andrew Travers, C.E.T.





RE: Cold Start up of Pressure Vessels
HAZOP at www.curryhydrocarbons.ca
RE: Cold Start up of Pressure Vessels
The real problem will be in the piping as it warms up and grows longer.
RE: Cold Start up of Pressure Vessels
OWG
I think the idea of hydrotesting the vessel just before start up would get discarded as being impractical, not least of which due to the temperatures involved (at least using water). However, if the liquid levels are then blown down by the process gas, the vessel wouldn't have the chance to drop much below freezing as the pressure increased.
dcasto,
We're already suggesting easing the pressure up, but we don't have a lot of resources as to what is considered a reasonable derating, other than UCS 66. In actuality, they could run it at the operating values and they wouldn't be more than 15% of the design pressure of the vessel, from the start. They would face a lot of issues though on the process side if things didn't heat up pretty quick.
I do concur that the real issue is the piping. I suspect this is why the end users are the ones who normally write these procedures, we don't have a lot of detail on that side of things from them.
RE: Cold Start up of Pressure Vessels
HAZOP at www.curryhydrocarbons.ca
RE: Cold Start up of Pressure Vessels
I'm not saying it's a bad idea, please understand. I'm just saying that it probably will be discarded as impractical, especially since they are fixated on -50 F and EG/Water mix isn't going to reach that far down. Just filling the vessel partway with EG/water may wind up being the answer (as it would facilitate heat transfer from the cold end to the hot end.
RE: Cold Start up of Pressure Vessels
Again, review the definition of MDMT. You need to analyse the stresses and temperatures achieved through a start up pressurisation and see whether you remain within the allowable envelope.
Steve Jones
Materials & Corrosion Engineer
http://www.oilandgaspeople.com/cv/11499664
http://www.linkedin.com/pub/8/83b/b04
RE: Cold Start up of Pressure Vessels
HAZOP at www.curryhydrocarbons.ca
RE: Cold Start up of Pressure Vessels
The problem lays in pressurizing a vessel leading to a specific stress level, when the wall temperature is below that of the acceptable ductility.
The problem occurs frequently and is normally caused by a misinterpretation of the design criteria (ambient temperature) and operation sequences (not continues service). Not only less or not insulated parts will get cold in sufficient time, but the entire vessel in the cold atmosphere.
If it is only for start-up, you can add heat tracing on the vessel, which will be difficult when the vessel is already insulated.
Another method is using a stream with a reduced pressure to heat-up the vessel. This should be safe guarded by sufficient measures as instrumentation etc. Heat tracing will normally be easier to imply.
Reducing the allowable working pressure as the temperature decreases can be quite difficult, pending on your position in the project. Normally a supplier will not be allowed to supply a vessel capable to less than ordered for.