4 replies to this topic

[Chemcato]

Dear all,

I'm currently looking at the overpressure protection for equipment of a chem plant. One of these equipment is a tank with no overpressure protection at all (in my opinion at least) but the client is unwilling to install one because they think that they can take credit for an atmospheric vent at an adjacent tank.

I disagreed but on thinking about this and reading more on pressure regulators, now i'm not so sure. Interested to know what everyone here thinks.

So here goes:
I've attached a diagram and am talking about tank T1000. The design pressure is at 20 kPag (although the client tells me the actual MAWP is close to 350 kPag), rated for full vacuum and it doesn't have any PSV or PVV or even an open vent to atmosphere. There is continuous liquid coming in (ammonia solution) and continuous liquid coming out. There isn't a vapor outlet but there is a N2 blanketing system. The pressure regulator at the inlet to the tank will start to open at about 5 kPag and the pressure regulator at the outlet will start to open at 15 kPag. I don't have the operating pressure of the tank but i'm guessing it's about 10 kPag.

So I'm mostly worried about when the inlet pressure regulator fails open (let's put aside fire first).

My first question:
Can a pressure regulator actually fail open? Reading about pressure regulators, i've learned its a mechanical device unlike a control valve and that it works very much depending on the line pressure. And if my downstream pressure increases, there is no way it would fully open. Does anyone know more about this to help me?

The people i'm talking to certainly think this is the case and furthermore, they argue that there is a relief path via P2000 to T2000 that is open to atmosphere. To me, that's taking credit for positive action and that's not allowed since the relief device is supposed to be my last line of defense. But then i'm thinking again that it's not a control valve but a pressure regulator so maybe there is a difference here. They also want to make P2000 high integrity or safety critical so it's highly reliable and can be used for overpressure protection. I'm not sure this matters if pressure regulators can fail much like control valves. To me, unless it's a HIPPS system, it's still not reliable enough. And even if it was HIPPS, you can't take credit for HIPPS in a fire.

I also did have another silly question. At this point, it's not clear to me what code this tank is built to. I've been assuming it's either API 650 or API 620 and following the rules of API 2000. But my question is: Is it mandatory to follow these APIs much like we do ASME code? With the exception of fire, the highest pressure this tank could be exposed to is 450 kPag in case of the N2 pressure regulator failure. If indeed the MAWP of the tank is 350 kPag and we can prove that the tank's probability of failure is low, perhaps no relief device is actually needed? And as for fire, well the tank would be damaged anyway. Although for both cases, we'd have to additionally prove the unlikelihood of people getting hurt.

That's it. My thoughts are currently all over the place and hoping to hear what the folks here think.

Thank you!

Attached Files

•  20231221_200711.jpg   82.9KB   5 downloads

[latexman]

Regulators can fail open. It is RAGAGEP to size reliefs for regulator fails wide open scenario, if credible. See API 520 and 521.

With a manual valve between T1000 and T2000, that could be closed inadvertently, the open vent on T2000 cannot be the relief for T2000. There are Code requirements for intervening valves. As a minimum, that valve would have to be locked or tagged open and administratively controlled.

Lastly, a regulator is probably not a Code certified relief, if required, and I doubt what you have can handle fire scenario, if it is credible.

[breizh]

Hi,

To add to Latexman's comments, consider a LS (level switch) LSH (dry contact) to actuate the on/off valve, not the LAH.

In case of failure of the LT or LS actions will be taken, alarm to operator or valve closure.

 

Breizh  

[Chemcato]

latexman and Breizh,

Thank you both for the input. The clients are still insisting we can take credit for the pressure regulator and the open vent on the adjacent tank so this will probably need more discussion. I don't agree and this thread convinced me i'm on the right path.

Breizh,
If i understand correctly, you are recommending to change because if it is an LAH, failure in the control loop might not necessarily result in any action taken?

Thank you again both for the help!
And happy new year!!

[breizh]

Hi,

yes right! LAH is an alarm to support board operator and LSH is a safety interlock to shut the ON/OFF valve.

2 different sensors.  LAH threshold is lower than LSH threshold.

 

Note: There is no overflow on tank.

Probably good to see the altimetry of all the tanks and the interconnection between them. Can you share some information about the product, ammonia solution?

At the end of the day, it's the client's decision. You should request a written document from them stating that they don't want additional protection on their tanks.

 

I've attached for reference a handbook for ammonia solution. You will find a typical design for storage tank.

Good luck 

Breizh 

Attached Files

•  Ammonia_Handbook.pdf   2.75MB   18 downloads