6 replies to this topic

[Virginia]

I have a query regarding a control scheme.
The scheme is typical oil and gas facility. Pump is pumping oil from a separator through desalters, exchangers and finally through air cooler to oil export line.
Level in the separator is controlled by a control valve provided at aircooler outlet by a cascade controller(Level-master, flow-slave).
Is this scheme okay? Would it be better to provide the control valve at pump discharge instead of aircooler outlet which is at B/L.
Which configuration is better?
Regards

[ankur2061]

Virginia,

I don't foresee any problem with the existing scheme. In fact it seems advantageous to me. By having the control valve at the air cooler outlet you are maintaining enough backpressure in the system to prevent any vapor flashing from the oil due to any fluctuations in the oil RVP since the oil is basically an unstabilized oil coming out of your separator.

Regards,
Ankur.

[Virginia]

Thanks Ankur for your reply. But why would there be any vapor formation as it is on discharge of the pump.
Had there been no pump I would worry about flashing as it is saturated liquid.
Infact I think providing a control valve would add lag in response.

[ankur2061]

Virginia,

I beleve you have also mentioned exchangers for heating in the discharge circuit. By heating the oil you are trying to stabilize the oil by bringing down it's RVP and liberating any low boiling components. The control valve maintains enough backpressure on the pump discharge circuit to prevent any vapor lock in the discharge piping and connected equipment due to the heating of the oil in the exchangers.

For a scenario where oil is flowing by gravity from the separator it would be prudent to provide the control valve close to the oil outlet connection of the separator.

Although a control lag will exist due to the far-off location of the control valve from the separtaor outlet, I think this will again be advantageous in terms of preventing any gas blow-by in the separator due to the large liquid inventory in the connected piping / system.

Regards,
Ankur.

[Virginia]

Thanks Ankur for your reply. The heat exchangers I was talking about is a cooler and not a heater. There is a heater before the separator. The liquid is flashed and vapors are removed in the separator. The liquid from separator is flowing through a pump and two coolers. First is shell and tube exchanger and second is aircooler. S & T is provided before the heater and separator.
But I agree with your point of preventing gas blow by case due to large liquid inventory.
Regards

[djack77494]

For a liquid filled system, the location of the control valve should not be very important since liquids are essentially incompressible. I have a minor preference for the CV at the location in your plant because of the reduced liklihood of vapor blowby (as you and Ankur noted) and becasue the cooled oil should have essentially zero tendency to flash. Also, with the CV downstream of the air cooler, and misadventures in performing maintenance have a lesser risk relative to what could happen in a hot location.

[S.AHMAD]

Flashing do not occur at the upstream of control valve but at the CV vena-contracta. Flashing (hence cavitation) if occur results in damages to the CV and piping just downstream of CV.

You can construct the pressure profile across the CV and determine the pressure at the vena-contracta. If the pressure is less than vapor pressure (which is the same as the separator pressure) then flashing will occur.

That is why the CV is located at the colder end.

Edited by S.AHMAD, 28 September 2011 - 11:15 PM.