Desalter PSV Sizing

[Falcon03]

Hello,

Our Crude stream (main charge) will split into two parallel streams A & B. Each stream has it own preheat and desalter system.

Each Desalter has a PSV which will be discharged to the Crude tower. The PSV setting pressure is 13.45 BarG.

Since we are running at higher Crude throughout 40% over design, I was asked to check the PSV hydraulic system. The only scenario which will cause the PSV to popup is in case if we lost the booster pumps located downstream of the Desalter. Loosing these pumps will create high pressure in the desalter . The current operation pressure in both deslater is 11.0 Bar-g and operating temp. is 150 °C.

My question is, how can I calculate the PSV relieving loads if we assume Block outlet case?

What I’m intending to do is to use PROII to do a flash calculation at the operating temperature and at a pressure which is 10% above the setting pressure. (as per API 520). (adiabatic flash at 150 °C and 14.80 bar-g)

And then I will use the above results to use VisFalre program to check for PSV hydraulics.

Please confirm if the above assumption is right. Thanks for your help.

 

[SooCS]

Looks like you are almost there. Probably left out some important steps.

Using the orifice size and the discharge coeffient of the existing PSV, calculate the maximum discharge rate. Check if the flowrate is higher than the upgraded flowrate.

Then calculate the discharge back pressure to make sure it does not exceed the allowable back pressure of the PSV, otherwise you might need to derate you PSV.

The other thing you might want to check is the flare knock out drum. Presumably the relief go to flare. API recommends a 20 to 25 minutes retention time.

Refer to API RP 520 and 521.

 

[25362]

If such occurrences are expected wouldn't a low-flow or high-pressure trip be installed to shut off the stream entering the particular blocked-in desalter ? What about the other parallel loop ? How much additional oil could this other circuit absorb ?

 

[krsmani77]

I can think of five possibilities for the Desalter safety to pop:
1. Some downstream block valve ( e.g.,exchanger isolation valve) is closed accidentally.
2. The heater Flow control valves at the heater inlet are closed for handling a tube rupture or such emergencies.
3. The desalter has been kept isolated for maintenanace and somehow the hold-up has got thermally expanded(e.g.,Fire under the desalters OR Electricity on,without flow and with high water level).
4. Due to Desalter mal-operation, a slug of water has entered the heater and given a tremendous back pressure to the system, while vaporizing.
5. The primary pumps were started without opening the booster pump bypass valve OR the booster pumps were isolated before opening the bypass valve.
So the controlling case will be the primary pumps running under shut-off condition, which the PSV's have to handle without exceeding the the design pressure of the cdesalter.

 

[ukn]

i would strongly advice to check for the fire case as krsmani77 has suggested. further please note that the discharge will be a hot two phase mixture and needs a knock out drum with some residence time to flash off the gases
thanks
ukn