Blowdown System Piping - Class 150 Flanges?

[jhamil1]

All,

Please refer to the attached picture of a "T" with class 150 flanges. This piping system is connected to the discharge of a natural gas compressor. Connected to the inlets of the "T" are a relief valve and control valve. On the exit side of the "T" is piping which is connected to a blow down silencer exiting to atmosphere.

The control valve opens when the unit is shutdown to blow down the piping to atmosphere. The maximum discharge pressure of the compressor is 995 psig. The relief valve is also connected to the same piping, and if the pressure exceeds the MAOP of 995 psig, the PSV will open and begin relieving through the "T" and into the blow down piping to atmoshpere. The blow down piping is approximately 100-foot in length and is schedule 40 with 300 Class flanges. Now my question. I want to be sure that I am not missing something and would like your opinion/recommendation.

Shouldn't the "T" and relief valve connecting flanges have a class 300 rating?





In my opinion, the "T" should be replaced with class 300 flanges. Take the unit blow down situation; there would have to be a 710 psi pressure drop across the control valve to equal the pressure rating of a class 150 flange (285 psig). Based on my calculations using the critical pressure drop ratio (xt=0.42) from the valve manufacturer, the maximum pressure drop would be 577 psi with an inlet pressure of 995 psig. Thus, at the instance the valve opens, the "T" would see an estimated pressure of 450 psi. Well above the class 150 flange ratings. I believe the same holds true for the relief valve. If the "T" must be upgraded to class 300 flanges, I also have to replace the relief valve. I hope you see the predicament that I am in, or maybe I am thinking about this situation totally wrong.

Thanks for the help in advance.

 

[unclesyd]

Just looking at you piping one thing that kinda stands out is the bonnet on relief valve, it is a 4 bolt flange and the valve outlet is also 4 bolt. The branch connection on the "T" from the blowdown valve is a Class 300. It is not unusual to see the outlet of PSV to be different Class than the inlet. This could explain the Class 150 flanges. It looks like a case of just matching flanges to the valves.

Can both the blowdown and PSV be opened at the same time?

Is this the original piping arraignment?

Is you PSV available with a Class 300 outlet flange?

 

[CRG]

If B31.3 system, given owner’s permission the allowable would be 33% over 285 psig for no more than 10 hrs per event and no more than 100 hrs per year. Look at 380 psig at the T and see if it will flow to atmosphere the same mass of gas that would/could flow into the T at 380 psig. If the tail pipe can flow more with the allowable dP, 380 psig, than the valve/valves can flow to the T when it is at 380 psig, then you have no problem. Otherwise, the pressure could be greater than 380 psig at the T and you would need to consider a solution to either change the possible flow or change the flanges.

unclesyd question about both valves being open at one time is important.

 

[jhamil1]

Unclesyd,

For your first question, if the PSV opened then the unit would shutdown on high discharge pressure. Once the unit is shutdown, the valve would open but by that time the pressure should be low because the PSV is relieving at the same time.

This is the original piping configuration from the packager.

I know that lower class outlet flanges are normal, but most PSV tailpipes are rather short (3-feet) not 100-foot of line but the line is also rather large. i would like to get out of changing the PSV, but the main concern is safety. To answer your question, yes you can get a similar PSV with a class 300 outlet.

CRG

Thank you for listing the ASME B31.3 code. I hope this is applicable to this situation. I am not to familiar with B31.3 but I know it concerns process piping. My only thought is that this station is a natural gas transmission facility and I don't know if this would apply? The rest of your response I can't really follow. Maybe I am reading it wrong but could you explain/clarify.

 

[CRG]

Run a hydraulic analysis of the entire relief system, including the open valve/valves, Tee, and through the tailpipe to atmosphere. If the pressure at the Tee is greater than the code allowable then you have a problem. You need to look at more than just the critical pressure ratio. Think about the cross-section of the valve/valves (orifice size for the PSV and effective bore for the blow-down valve). You need to calculate the mass flow through the valve and determine if the resulting flow through the tail-pipe creates back-pressure greater than the flange allowable.

Depending on your geographic location your line could be B31.8 with compressor stations being designed to B31.3 or B31.8. In North America you might need to look at 49 CFR Part 192. Also the regulations are different for gathering lines and pipelines. You should know or find out the boundaries of your facilities and the relevant codes that apply.