4 replies to this topic

[harini]

Dear All,

This post is about the use of SIS in the place of PSV in a low pressure storage tank. The systen consists of a 3-phase separator that receives oil from well. The produced water is sent to low pressure storage tanks that operate around 100 mbar g. While evaluating the over pressure scenario for the tanks, gas blowby from the separator to the tank was identified as a cause for overpressure, but was ruled out saying that there is a SIL3 protection for low level shut down in the tank.In addition there is also a high pressure shutdown in the tank inlet.
My concerns are,
1. The low level shut down in the separator and the high pressure shut down in the tank are not 2 independent layers, as the logic solver for both the loop is the same ESD system.
2.Also an ESD system is not a HIPS system (triple redundant). Please correct me if i am wrong.
3.According to API 521. each vessel/equipment should be self protected mechanically. But SIS can be used as an alternative like in the case of HIPPS.
4.But according to API 14C,
Refer to API RP 14C including sections 4.2.1.4, A.4.2.2.2, A.4.3.2 and the associated tables listed.
A low level signal can interlock a shutdown valve on the liquid outlet (or inlet) as the primary protection from gas blowby. Safety devices downstream provide secondary protection for the downstream equipment.
This is a totally opposite of API 521, or thats what i understand.May be I am wrong.

I really appreciate the valuable input from the forum members.
I hve attached a rough sketch of the system with the protections.
Many Thanks in advance

Attached Files

•  diagram.pdf   349.84KB   59 downloads

[demank]

3.According to API 521. each vessel/equipment should be self protected mechanically. But SIS can be used as an alternative like in the case of HIPPS.

Hello,

remember, API 521 is use in pressurized vessel, so for storage tank with atmospheric pressure not compatible. I recommend for atmosperic storage tank you refer to API 2000

[harini]

Hello,

remember, API 521 is use in pressurized vessel, so for storage tank with atmospheric pressure not compatible. I recommend for atmosperic storage tank you refer to API 2000
[/quote]

Thank you for the reply. But however API 2000 addresses only about the calculation and scenarios for over pressure and doent talk about Instrumrnted protection system.

[skearse]

I tend to agree with demank-since these aren't pressure vessels I'm not sure how much of 521 you may be able to apply to this system-inclduing HIPS/SIS. I'm not that familiar with API 14C, but it seems to apply to off shore platforms, so is it really applicable in this case (I'm assuming your tanks are on shore, seperator is on a platform)? How large are these vessels?

My thoughts would be that you would have to size a normal vent for the routine inbreathing/outbreathing and emergency vent per API 2000. What would be your gas breakthrough flow rate-it may be possible that your emergency vent would be large enough to handle this contingency as well.

The other thing would be to check with your insurance carrier and or the local authority, and see what they would be willing accept. If you detail a LOPA analysis for them using the HIPS/SIS, they may allow you to take credit for it.

[harini]

Hi,

I agree to the fact that API 521 cannot be applied to the Storage tanks. But API 2000 doesnt cover any protection system for tanks.
I am detailing the flowrate as below,
PSV on the tank - Sized for the Blanketting Valve failure 2100 kg/hr (Installed Cv - Full open)
Manway in the tank - Sized for Fire Case

Gas Blow-by Flowrate corresponds to 21500 kg/hr, very huge when compared to the blanketting valve failure.

A LOPA study was carried out in the FEED phase and the risk level for this consequence was assessed as high and the required Integrity level for this risk was classified as SIL3.

Hope this details the conditions more.

Thanks
Harini