7 replies to this topic

[Hossein Deldari]

dear all
I am preparing duty spec for new intrumnet air package system in a basic design project. this package is comprising compressor, dryer and reservoir vessel. I want to specify reasonable residense time for reservior vessel. is there any figure in standrads for example ISA whcih has been specified that.
regards
Hossein

[Zauberberg]

I think you would have to define normal operating and minimum operating pressures of the plant/intrument air systems first, and then to calculate required vessel volume based on envisaged/maximum air consumption and target residence time.

I remember seing figures of about 10-20 minutes residence time for air receivers, in design standards defined by Total and Shell. The same standards have stated that minimum operating pressure of an air receiver should not fall below 5.5-6.0 barg.

[ankur2061]

I think you would have to define normal operating and minimum operating pressures of the plant/intrument air systems first, and then to calculate required vessel volume based on envisaged/maximum air consumption and target residence time.

I remember seing figures of about 10-20 minutes residence time for air receivers, in design standards defined by Total and Shell. The same standards have stated that minimum operating pressure of an air receiver should not fall below 5.5-6.0 barg.

Dear Zauberberg,

Most company standards define the hold-up time (HUT) as 5 minutes for the air receiver and size it accordingly.

I don't know about 'Total' but Shell DEP calls for 5-10 minutes HUT.

Regards,
Ankur.

[Zauberberg]

Hi Ankur,

It might be that we are speaking about two different vessels. Plant air receiver, which feeds the air dryer package (and consequently nitrogen generators) is sized to provide not less than 10 minutes residence time, by taking into account pressure difference between the normal and the minimum acceptable operating pressures. Instrument air receiver, which feeds the air dryers, is sized to provide - as you say - not less than 5 minutes surge volume. I think this makes sense, because ultimatively the plant air receiver is additional buffer volume since it cascades to instrument air receiver.

I remember plant air receivers always being much bigger in size in all plants where I used to work, and this is also the case in Marathon/EG-LNG where I am based now. I have just checked the figures few hours ago: 15 minutes residence time in plant air receiver, and 5 minutes residence time in the instrument air receiver.

Regards,

[Qalander (Chem)]

Dear Zauberberg,

However I recall this air utility system Hold Up Time to be around 15~20 minutes at 4.5~5.5 barg pressure at my previous employer and

we were considering to put up an additional vessel to enhance this to around 25~30 minutes for the three refineries system.

[Zauberberg]

I'm quoting one process design standard:

"The air buffer vessel shall be of adequate size to serve:

♦ As condensate separator and draining vessel
♦ As buffer volume during compressor failure
♦ As fluctuation dampener if compressors are on load/off-load control

The buffer vessel shall be sized to maintain the air supply between the moment of compressor failure of one utility supply for the compressor(s) normally in operation, and the moment that the stand-by compressor(s) is or are operating.
The period between these moments shall be taken as the time required for starting the stand-by compressor(s) manually if automatic starting is unsuccessful, and shall be determined by plant operations in connection with mechanical engineering and utility engineering, but shall be at least 15 minutes."

[Hossein Deldari]

dears
please be informed this package will be just appropriated instrumnet air and plant air supply is not in scope of this package. furthermore, to specify instrument air reservoir in this duty spec the required instrument air flow rate and its lower and upper pressure are known. but as I said residense time is not unkown. one of my experience cooleagues says that this figure is 3 minute. but i want to know the specified figure in ISA standrad in this regards
thanx
Hossein

[Qalander (Chem)]

Dears

I seond Zauberberg, with physical witness that at times the service need that Minimum 15 minutes coverage being the usual time taken by humans responding to Alrams(Annunciators etc.) i.e manual compressor start-up and maintaining of instrumet air supply header pressure to 4.5~5.5 barg.