8 replies to this topic

[SAprocessGD]

Hey,

I’m evaluating a PSV and flare header to a pop tank on an existing project and would like to clarify on some issues that I am having. To evaluate this flare system, I am using Flarenet and included all the necessary information.

What the current design looks like is: a 1”x1” PSV, 1” pipe, 1”x2” expander, 2” pipe, 2”x3” expander, 3” pipe to pop tank, respectively.

When I run the program I get a Mach number of 1.0 at the inlet of my 1”x2” expander. At a mach number of 1.0 my expander sees a pressure decrease across the expander, but when my mach number < 1 at the inlet of the expander, the program simulates a pressure increase across the expander. My question is: why does pressure increase across an expander when the Mach number is < 1.

Also, with regards to FLARENET, what is the difference between static pressure and total pressure?

Regards,

Edited by SAprocessGD, 01 November 2011 - 05:07 PM.

[GS81Process]

A few questions...

1. Why would you expect the pressure to increase across a pipe reducer? How is this adding energy to the system- it is not a pump, compressor, or source of heat input or chemical reaction.

2. If the mach number is 1 at the inlet to the pipe reducer, what is it at the throat of the PSV? Your choke point is likely in the wrong place.

3. A 1x1 PSV seems a bit irregular, are you sure this is properly sized?

The difference between static pressure and total pressure in Flarenet is the same as anywhere else. By definition, static pressure = total pressure - 1/2 rho v2
The 1/2 rho v2 squared term represents the fluid dynamic pressure, i.e. the energy related to fluid flow. In other words, total pressure includes static pressure and dynamic pressure. From Bernouilli's equation, the total pressure is constant along a streamline.

Edited by GS81Process, 01 November 2011 - 05:25 PM.

[S.AHMAD]

1. Have you heard phrases "garbage in garbage out"?
2. Check your input data to your computer program
3. It is very unlikely that the computer program is wrong - unless you are using the beta version

[SAprocessGD]

Hey,

Comments to GS81Process as follows

1. I understand that pressure should decrease across a reducer, but I am wondering why Flarenet has pressure increase across a reducer? Would this have anything to do with a higher mach number?

2. The mach number at the oulet of the PSV for the "current" set up is 0.509. Without any isometrics, I am assuming that the 1 x 2 reducer is as close as possible to the PSV outlet.

3. You're right, a 1x1 is unusual, and that is why I am evaluating the current set-up. From my results, it seems that I will have too large of a back pressure in the system. I resized the sytsem for a 1x2 PSV and this time the back pressure is less than 10% of the PSV set pressure. Also, corrent me if I am wrong, but all new PSVs come in standard sizes based on API 526?

As always, I really do appriciate the help and further questions you provide.

Cheers,

[GS81Process]

I assume that this is for a project and that this is not already built (i.e. this is only on paper). Is this correct?

1. Do you mean that the total pressure (not dynamic or static) is higher across the reducer? If so, there is a mistake. This is not because of mach no.

2. Confirm that the mach no. at the PSV throat is 1. This is where the choke point needs to happen. If not, the design is incorrect.

If you do not have isometrics, how have you been able to perform the fluid flow calculations? Is this in very preliminary stages where no information is yet available?

3. How do you know back pressure if you don't have isometrics necessary to calculate pressure losses? Have you also considered that a balanced bellows PSV might be used in this application to allow for higher backpressure as a % of set pressure? This is common in flare system design. PSVs typically come with standard orifice letter sizes and various flange sizes. Refer to PSV catalogue from reputable vendors like Farris, Anderson Greenwood, etc. These are available on the internet.

My biggest tip is to seek professional consultation on flare system design. It is good experience for you to learn about flares and their design to progress toward one day being an expert, but a flare system is a safety system and correct design and operation is critical.

Edited by GS81Process, 04 November 2011 - 10:24 AM.

[SAprocessGD]

Sorry for the long delay GS81Process.

1. Static Pressure is higher across the expander - I have discussed this with a FlareNet expert and understand why.

2. This 0.506 was at the outlet of the PSV, not at the orifice.

3. This project is not already built although some packages have been built (in which I do have the isometrics).

Thanks again!

[omycrone]

SAprocessGD

 

Do you mind explaining item 1 why  Static Pressure is higher across the expander - I have discussed this with a FlareNet expert and understand why.

I experience the same matter as you.

 

At a mach number of 1.0 my expander sees a pressure decrease across the expander, but when my mach number < 1 at the inlet of the expander, the program simulates a pressure increase across the expander. My question is: why does pressure increase across an expander when the Mach number is < 1.

[fallah]

omycrone,

 

[quote]...My question is: why does pressure increase across an expander when the Mach number is < 1.[unquote]
 

In fact, when M<1, the static pressure increases (recovers) across an expander due to a decrease in fluid velocity even though there would be a pressure loss due to turbulence in sudden expansion. In this situation pressure loss due to turbulence is lower than pressure recovery, then will result in static pressure rising...

 

When M=1, the pressure loss due to turbulence and noise would be higher than pressure recovery due to decrease in velocity, then static pressure would decrease...

[narendrasony]

 

 

omycrone,

 

When M=1, the pressure loss due to turbulence and noise would be higher than pressure recovery due to decrease in velocity, then static pressure would decrease...

 

Dear Fallah,

What I understand is that at Ma=1, pressure reduces but velocity is most likely to increase with expander. 

 

Regards

Narendra

Edited by narendrasony, 25 February 2013 - 07:29 AM.