Criteria to choose Low Low trip on a Pressure Transmitter

[Vivaldi M. Smith]

We have a Pressure Shutdown Transmitter installed in a pipe to shut on LL pressure scenario, the old trip set point was 105 PSIG, the pipe contains Condensate of about 14MBD, the flow is still the same but we have lower pressure in the system from 150 to almost 85, but now I need to lower the low low trip of the PXT. What would be the recommended Low Low trip and what are the criteria to choose the right new Low Low trip?

 

[1503-44]

Assuming that you do not have other criteria, such as a pump NPSHL requirement or similar limitations ... its just the pipe and flow rate control you are trying to ensure ...

It is generally advisable to not let your pipeline go back and forth between liquid and gas phases, due to pressure shock, vapor lock potential and maintaining flow control at liquid rates and not possibly damaging meters and vapor locking instruments, trying to pump gas with a liquid phase pump or avoiding conditions where non liquid flow might adversely affect your system, so presumably you will want to keep it in the liquid phase.

So, unless you want to fill your system with gas bubbles, or gas pockets and enter 2 phase flow, you should set the LL to a pressure that will maintain your condensate in the liquid phase. In other words, the pressure at the lowest point in your system must be kept above the condensate's vapor pressure or bubble point. Then from the low point, work out what the corresponding pressure at the PSLL location must be (while at the highest temperature) and set the PSLL accordingly.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Vivaldi M. Smith]

I forgot to mention that the LL trip is set for a tube rupture case to protect dowsntream exchanger shell side, but there is also a Pump installed right after the exchanger

 

[1503-44]

So LL is set to shut the system down if the shell blows.
You probably have a wide range of pressure settings that would work. It should probably be above VP. Anything lower would only delay the eventual shutdown that is surely soon to follow. And gassing in the system is not likely desirable in any case anyway. You would probably want to shutdown at VP even if the shell did not rupture.

I will assume you want a 5psi safety margin above vapor pressure; could be more depending on your stream variations.

NPSHR CHECK
Assuming the shell holds, then you would want to stay 5 psi or so above NPSHR at the pump suction.
That will probably be at lowest pressure when flow rate is highest, but you may have other conditions that could be lower still. Find the condition that gives you the lowest suction pressure. That should be above vapor pressure + 5psi. So that in effect actually results in a slightly higher pump suction pressure.

PSHLL SETTING (assuming NPSH is the controlling value)
So, add the 5 psi or so to NPSHR, then subtract the (height from top of pump casing to PSHLL) x density value to see the corresponding PSHLL setting. That too should be greater than VP + 5 psi

MIN PRESSURE AT HIGH POINT
Now check the pressure at the highest point anywhere in your pipeline. That should also be at least VP + 5psi.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Mr 44, the OP said pipe not pipeline....

Is not possible from the information supplied to know why this trip is there and what it is trying to prevent.

You need to find the process description and the cause and effects together with the P&ID to try and work it out.

It's usually to do with rotating machinery or preventing vapour forming.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Pipe or pipelines generally have the same problems. I think I covered rotating equipment, vapor formation, meters and bubble collapse shock, and also my ass too with the appropriate disclaimer, "assuming you do not have other conditions,", even having anticipated a pump downstream for which I mentioned NPSHR. I believe my answers were well better than warrented, given the extremely limited information provided. The OP has not asked additional questions, so I assume he's happy. I don't see any more problems at this time. I will give it a job well done, at least for now. and Season's Greetings to one and all.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

Can you put up the process diagram where this low pressure transmitter is located. In many cases, low pressure trips are an operating nuisance, and should be removed to avoid nuisance trips. Tube rupture will remain undetected for small HX leaks, so what is the point of this low low trip ?

 

[LittleInch]

"Condensate" is also very vague.

If this is coming off a separator at high pressure then you don't want it falling below the bubble / vapour pressure maybe??

But agree, low pressure trips need to be thought about carefully but we have zero real information to help the OP....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Vivaldi M. Smith]

 

[1503-44]

I doubt it has anything to do with leak detection or vessel condition.
Again. Vapor pressure?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Vivaldi M. Smith]

the stabilizer Condensate Reid Vapor Pressure is usually around 10 psi, I have send the raw condensate to be analyzed by the lab. I don't have the raw condenste vpour pressure ath the moment.

 

[LittleInch]

SK,

You also need to understand why this trip is there as a LL trip doesn't really protect a vessel IMO.

Sometimes these things are written on P&IDs and int he process description.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

The PXT at this location is a duplicate of that on the compressor suction, and in any case is not doing anything useful. It would be better to relocate this PXT to the shellside of the HX as penultimate instrumented high pressure trip prior to overpressure relief. This high pressure trip should cut off inflow to the HX tubeside. No need to trip the compressor or the pump in this case as both these machines should go into full recycle. Am assuming there is no PSHH on HX shellside at the moment.

Also check that this HX shellside overpressure relief setpoint also protects all other devices operating in parallel on the shellside (at other HXs' etc). What do you have on this HX shellside, and what other devices are also operating in parallel ? You may need to run a mini HAZOP of sorts to make sure all overpressure relief scenarios and low temp / freezing risks are addressed when there is a tube leak at this HX.