Flare Header Network Piping Connector

[Iomcube]

I will be designing an orifice plate with a known upstream pressure & gas. While the downstream conditions are unknown. But can be known; after all at the outlet (after a length piping) KO drum is placed vented to atmosphere. I have annotated the attached PDF.

My question is in practice how do you join a 3" line into a 18" header because a standard reducer here gives me choking (using ABZ DesigNET)

 

[mk3223]

One option could be to have the larger pipe size than 3", so the flow won't be choked.

 

[Iomcube]

Sir the 3" piping has a large distance to cover before it ends up into 18" header, I think client had decided this upon economical constraints. But we change it!

The upstream of orifice is 1250psia & regardless of downstream dia I think this flow will be choked at about 0.55*1250 psia. Now the 3" line length is more than 70m would you recommend using staged orifices). If yes I cannot find any mathematical treatise covering this approach

 

[mk3223]

Is a flow control valve a better solution, instead of staged orifices design, for this application since the downstream condition is not well constant?

For the staged orifice design, suggest to do a google search for the topics or check out the following links:

thread124-119130

thread124-247112

 

[Iomcube]

mk3223, For x3 days I am reading decade old threads at eng-tips to get around my problem. Based on my study & different threads here I must write further clarifications

Distance b/2 multiple Restriction Orifices (RO)
This article refernced multiple times is only for liquids but is a great read

The Tung and Mikasinovic article is from Chemical Engineering, December 12, 1983, pgs 69-71.

For metering orifices (not RO) my standards suggest 15 times the internal pipe dia (15D) but this is metering orifice so if am using RO in a 2" SC.80 line where ID is 49.2mm RO will be 0.8m apart; this can be relaxed to say 0.5m

Iterative solution or not:
Because I know upstream pressure (1250psia) & mass flow (3000kg/hr of N2) as well as process conditions so the solution will not be iterative rather tuning will be a proper word!

https://www.eng-tips.com/viewthread.cfm?qid=247112

sshep (Chemical) 23 Jun 09 08:59
For sizing a system to give a specified flow, I think you just chose a low noise pressure profile (dP/P for each plate) first, and then size each orifice sequentially. No iteration is required.
Iteration should only be required for a flow calculation- i.e. to find the flow through a system of known orifice sizes and boundary pressure conditions. This can be done by choosing flows (start low and work up) until the outlet pressure from the last orifice equals the boundary condition.

The downstream line attaches into a flare header which ultimately dumps into an KO drum operating at near atmospheric condition (upto 2barg allowed at KO inlet). This is something very similar to

https://www.eng-tips.com/viewthread.cfm?qid=119130

CatBoy (Chemical) 22 Mar 05 15:53
I've been asked to check the design of a line with three restriction orifices in series. That line is connecting the discharge of a recycled H2 compressor discharge line at 2350 psia to a 14.7 psia gas seal drain pot. The line is 0.434in ID and each RO has a bore of 0.1in.

Same bore RO or not:
Per my understanding for staged pressure letdown while avoiding choke & keeping noise level down from 85dB(A) I will start with small bore & increase it downstream? This increasing bore will avoid choking tendency of higher volumetric flowrate downstream

How many stages:
Via fine tuning in ABZ DesigNET I calculated 7-8 stages. See attachment
The process I followed is fine-tuning such that the last orifice is nearly choked (Beta for RO increasing NOT same)

Further query:

https://www.eng-tips.com/viewthread.cfm?qid=119130

EricKvaalen (Chemical) 9 Apr 05 03:55
First a comment to sshep--adiabatic does not mean constant enthalpy. If the gas does some work or accelerates, then its enthalpy decreases even if no heat is transferred out of the gas. (I think this is what hacksaw was saying.)

Now some comments on the calculation of a series of orifices. When an orifice is in critical flow (or "choked"), then the flow is related simply to the upstream conditions, as sailoday has stated:

W = A const. (Pb/Tb^.5)

(I use Tb and Pb for the stagnation temperature and pressure before the orifice.)

This can be solved for any unknown if the others are known.

But when the flow is not critical, the situation is more complicated. The flow is

W = A rho v = A v M Pa / (R To) = A v M Pa / (R Tb) (Pb/Pa)^(R/Cp)

(M is molecular weight, Pa is pressure after the orifice (and in the orifice), To is temperature in orifice.)
The velocity is related to the pressures by

M v^2 / 2 = Cp Tb [1 - (Pa/Pb)^(R/Cp)]

Combining the above, we have

W^2 = 2 A^2 M Pa^2 (Cp/R) [(Pb/Pa)^(R/Cp) - 1] / (R Tb)

This cannot be solved for Pa explicitly, but it can be solved for other things. For instance,

Pb = Pa [1 + W^2 R Tb (R/Cp) / (2 A^2 M Pa^2)]^(Cp/R)

In the two-orifice case, we can calculate the flow W either from P0 and P1 (that is, before and after the first orifice), or from P1 alone using the simpler equation for the second orifice (assuming it is critical). Equating these two expressions for W gives an equation for P1 in terms of P0, A, etc. This can be solved for P1, but requires an iterative method (it is not a quadratic equation).

In the case of three orifices, one can also use an iterative method, as I wrote on March 25. We know P0 and P3, but not P1 or P2. We can assume a W, find P2 (simple equation assuming third orifice is critical), then find P1 (complicated equation), then find the P0 which would give our assumed W. We compare this with the desired P0 and iterate.

If the last orifice is not critical for the assumed W, then we just have to use the complicated formula to find P2 instead of the simple formula. It doesn't change anything essential.

There are other variants, such as assuming P1, finding W from this and P0, then finding P2 from W (using choked orifice 3), then finding P1 from W and P2, and comparing with the assumed P1.

But if you do it wisely, there is only one iteration to do--no nesting or solving for more than one variable simultaneously.

This was a very informative post! Can I get refernces of these forumals I will try to make SMATH sheet if I can understand the algorithm of calculation

 

[MortenA]

Your flow rate in the 3" line is too high. You will (i think) just move the choking place to somewhere else. Normally you woulndt increase your pipe size just at the tee. You should limit the velocity to max 2/3 of the sonic velocity (assuming that you have a fixed flow rate by increasing the diameter). Its also a noise concern.

If your calculation is the other way around - you assume unlimited flow and a fixed N2 pressure of 1250 psig then you will always get choking at the downstream point (and a very large flow). So if you task is the calculate a RO to limit the purge rate follow these steps: 1) Determine your N2 flow rate required for purging (e.g. velocity at flare tip) 2) assume choked flow in RO and use a formula similar to API520 PSV (basically an RO) but your factor maybe different (usually not). If you wish back calculate the pressure at your RO at normal purging condition and check that the flow is choked.

Best regards, Morten

 

[Iomcube]

MortenA, So by your logic one RO is required; that surely will be a noise problem! Provided you are decreasing 1250psia to ~40psia

Also the main purpose of this orifice is pressure reduction to an acceptable amount at the inlet of KO drum. This 3" line ultimately exits to 18" headers which ends into a KO drum

 

[MortenA]

Yes but i will assume that the flare drum is open to atm? So the flow will determine the back pressure. But are you sure thats how you want to operate? Wouldnt the operational point be a flow rate to ensure purging and no O2 ingress?

 

[MortenA]

BTW i dont think noise will be a big issue with a gas RO under these circumstances

 

[Iomcube]

MortenA, yes the flare header will ultimately open to KO drum atmospheric pressure). Secondly flowrate is fixed (25000kg/hr) & not the other way around (that is constrained)

The objective of this project is to provide purge lines from Y-1 and Y-2 platforms to Y-1B and Y-2B flowstations respectively. These purge lines will serve as depressurization lines to depressurize gas injection pipelines individually.

The above is by client who instructed us that whatever line size you choose; per provided line table extract max. velocity & thereby assume flowrate. I had increased line size to 4" & after last orifice to 6" this ultimately dumps into 18" header to KO drum

For such an arrangement I think staged orifices will do the trick? Single orifice will not do the trick!

@

http://www.piping-engineering.com/restriction-orifice-ro-flow-control-in

Restriction Orifice (RO) at the downstream of blowdown valves.
These are used to ensure controlled flow rate in a blowdown piping or blowdown
header.
When the blowdown valve (which is usually a FB or RB ball valve) opens to release
the high pressure on its upstream, the restriction orifice plate at its downstream
ensures that the flow is not excessive to overload the flare header. Usually the
pressure drop in blowdown circuit across the restriction orifice could be very high
say, typically 80-100 bar. If high pressure drop is achieved through a single stage
device or by a device with not too many stages, there will be fall in temperature
during the blowdown event due to Joule Thompson effect. Thus the design of the RO
needs to take care of the low temperature.

 

[Iomcube]

BTW I have used this methodology in calculating Orifice Betas / Bore

First did the forward calculation using Mass flow after having acceptable answers did back calculations using final downstream pressure & calculated mass flow

@

https://www.eng-tips.com/viewthread.cfm?qid=51260

denniskb (Mechanical)
12 May 03 21:09
The solution method for the blowdown system with many components is simple so long as the flow does not go critical at any point. Simply guess a flow and work forward or backward calculating the pressures at each step. When you get to the end if the calculated pressure does not match that required then simply adjust the flow and run it again until the calculated pressure matches the actual pressure.

It fact the flow can go critical at many points and often at more than one at a time....