9 replies to this topic

[ayan_dg]

For sizing of Depressurizing lines (inlet & outlet) , should we have the same approach as used in sizing PSV inlet & outlet lines

[ARAZA]

Hi There,

Sizing of the depressurizing lines can be treated as Transfer lines from high pressure to low pressure, this is entirely different then the PSV inlet and outler sizing. Sizing of DP lines is a function of available pressure at source, choke velocity, noise and virbation, while PSV lines is a function of max. pressure drop which is limited to 3 % at inlet and 10 % for outlet for conventional relief valves.

Sizing of DP lines is usually not a function of presure drop, providing you have enough pressure available at source. For sizing these lines, I use a thumb rule of a velocity of 100 ft/sec.

Hope this helps.

ARAZA

For sizing of Depressurizing lines (inlet & outlet) , should we have the same approach as used in sizing PSV inlet & outlet lines

[gvdlans]

Araza is right, sizing of DP lines is different from sizing of PSV inlet and outlet lines.

The 3% and 10% pressure drop (for conventional PSVs) are resp. related to prevention of PSV chattering and prevention of incomplete PSV opening caused by the backpressure acting on the PSV disk. Chattering and the backpressure problems are no issues for a depressurizing valve.

[katmar]

I agree with Araza's approach of using velocity rather than pressure drop as the criterion. If the line turns out to be a rather large diameter it may be worth going for a smaller pipe with a velocity higher than 100 ft/s and using a silencer. I've seen recommendations of up to Mach 0.3. It's all about the cost.

If you use a velocity of around 100 ft/s the pressure drop in the line becomes almost negligible and the pressure drop problem is shifted to the valve, which in all likelihood will be choked.

[ARAZA]

Hi All,

I agree with Katmar's commets that the velocity could be higher than 100 ft/sec. it all depends what the system is all about, what is the flow-rate, what is the source pressure and stuff like that. This velocity criteria is only a guideline, the system as a whole needs careful analysis.

Hope this helps,

ARAZA

I agree with Araza's approach of using velocity rather than pressure drop as the criterion. If the line turns out to be a rather large diameter it may be worth going for a smaller pipe with a velocity higher than 100 ft/s and using a silencer. I've seen recommendations of up to Mach 0.3. It's all about the cost.

If you use a velocity of around 100 ft/s the pressure drop in the line becomes almost negligible and the pressure drop problem is shifted to the valve, which in all likelihood will be choked.

[Austro]

"If you use a velocity of around 100 ft/s the pressure drop in the line becomes almost negligible and the pressure drop problem is shifted to the valve, which in all likelihood will be choked."

Why is the pressure drop in the line a problem at all? Unsatisfactorily low flow rate?

What do you mean the valve will be choked? By choked do you mean smaller in diameter and (thus where most of your pressure drop will be) in the main line or you will have air speed = Mach 1 choked flow at the valve?

[katmar]

Why is the pressure drop in the line a problem at all? Unsatisfactorily low flow rate?

Normally the pressure drop through a line is one of the most important factors in determining the line size, but once you specify a relatively low velocity the pressure drop in the pipe becomes almost irrelevant.

What do you mean the valve will be choked? By choked do you mean smaller in diameter and (thus where most of your pressure drop will be) in the main line or you will have air speed = Mach 1 choked flow at the valve?

I had the "air speed = Mach 1" concept in mind when I wrote that, but as the pressure in the vessel gets lower and you no longer have this choking condition (or if the pressure was too low for Mach 1 to start with) then it will be true that most of the pressure drop is across the valve.

[pjh33hae]

Dear Ayan,

Hi.

You look like to use typical PSV for vessel depressuring. I confuse it with atomic venting pipe.

If you mean the former case, it is almost the same as general PSV inlet/outlet sizing to flare header.

I recommend 3 % rule for PSV inlet and about below 10 % rule assuming it's conventional type. And I recommend you to check Mach number @ outlet line to atm or flare header. Maybe below 1.0 would be good for noise problem.

Normally we do not use PSV as emergency depressuring facility. Instead we use control valve with DCS pressure controller. If you use control valve, pipe sizing is performed as velocity guideline.

Bye..

[fallah]

Instead we use control valve with DCS pressure controller.

Normally,pressure control valve (PV) connected to flare system,would be used as pressure controller of upstream vessel not as emergency depressuring facility.

[ARAZA]

Dear,

Please send me a brief description of the system with flow-rate and process conditions, this will be best way for analyzing the system otherwise, we can talk whole day without getting the problem solved.

ARZA

For sizing of Depressurizing lines (inlet & outlet) , should we have the same approach as used in sizing PSV inlet & outlet lines