Line Pack Calculation 10

[mb73]

Hello All
I am new to this field and am looking for some guidance/suggestions in calculating the appropriate pipe size for specific conditions for natural gas..

Flow Rate 40,000 MSCFD
Length of Pipe: 15 miles
Inlet Pressure: 800 psig
Temp: 60 degF
Downstream pressure: 450 psig
Pipe 12"/stl/X65/0.5" wall
SG Nat gas- 0.6

I used Technical Toolbox and calculated the line pack to be 2954.5MSCFH.
However when using the Weymouth equation in Technical Toolboxes, I am getting a flow for the same size pipe of 4380 MSCFH.

I appreciate your feedback.

Thanks

 

[pierreick]

Hi,
Consider this resource to validate your calculation.

https://checalc.com/fluid_flow_compressible.html

I encourage you to read the document attached, you will find everything you need to support your query.
Pierre

 

[georgeverghese]

My compressible flow calcs tell me that for 35 000kg/hr equivalent to 40mmscfd, an id of 200mm is adequate; gas z = 1.0

 

[pierreick]

Hi,
Using the calculator provided you will find that 8" pipe is sufficient, similar results given by George.
Mw =17.4 g/mol; cp/cv =1.27; Z=1 (conservative value)
L=24140 m
Pierre

 

[Snickster]

These are the results of my calculations using the weymouth equation at conditions you indicated. I am not sure what line packing has to do with pressure drop/line sizing calculation that you reference, so this is just based on typical line sizing calcs:

Flow Rate 40,000 MSCFD
Length of Pipe: 15 miles
Inlet Pressure: 800 psig
Temp: 60 degF
SG Nat gas- 0.6

Compressibility Factor 0.82

Pipe 12"/stl/X65/0.5" wall

Resulting Velocity at inlet: 8.8 ft/sec
Resulting dP: 35.3 psi

Pipe 8"/stl/.322" wall

Resulting Velocity at inlet: 19.7 ft/sec
Resulting dP: 302.4 psi

Pipe 6"/stl/.28" wall

Resulting Velocity at inlet: 34.2 ft/sec
Resulting dP: 1308.5 psi

Therefore it looks like the correct line size for the available pressure drop and while still maintaining 60 ft/sec maximum velocity is 8" diameter.

The actual flow in 8" Sch. 40 pipe at 350 psi pressure drop is:

Flow:48.8 mmscfd
Inlet velocty: 24 ft/sec
Outlet velocity: 42.2 ft/sec @ 450 psig

If you want to maintain 40 mmscfd need control valve.

 

[pierreick]

Hi,
missing document to support your query.
Pierre

 

[katmar]

The difference between the old empirical equations like Weymouth, Panhandle and Spitzglass compared with the modern equations that you will find in Crane TP410 etc is that the empirical equations have an approximation of the friction factor built into the equation itself. The modern equations refer to a separate calculation of the friction factor using the Moody chart or an algebraic representation of the chart - for example the Churchill equation.

The old equations were empirically developed for specific conditions of pipe diameter and gas velocity, and their approximation of the friction factor will only be accurate for those conditions. If you are doing a calculation close to the conditions under which the old equation was developed you will find it gives close agreement with the modern equations. However, if you are using conditions that are rather different from those under which the equation was developed you will find poor agreement. Unfortunately most references for the old empirical equations do not specify the conditions where the equation applies.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[1503-44]

Very high pressure drop for 15mi pipeline. Its OK as long as you can afford that high of a pressure loss and are sure would won't need that pressure to drive larger flow rates in the future, which is almost always the case with pipelines.

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

 

[LittleInch]

mb73,

I'm just kind of curious as to why you asked two questions in 2003 and then none until 20 years later??

Anyway, your use of the term line pack is incorrect. Line pack refers to the variable volume / mass of gas you can get in a gas pipeline due to its compressibiity.

You appear to referring to line sizing??

What is your variable though?
Flowrate for the given inlet and outlet pressure?
Outlet pressure required for the fixed 40 million SCFD?
This is a large pressure drop for not a long distance so using the smallest pipe available from other calculations it would look like an 8" line which is going at a highish velocity.

In terms of calculations, Katmar has a great answer. Beware of calculations designed to be used for low pressure or low pressure drop (<5%) which don't work for high pressure drop systems.

This

https://www.lmnoeng.com/Flow/weymouth.php

has a good explaination and description of when different equations are best used.

Also

https://petrowiki.spe.org/Pressure_drop_evaluation_along_pipelines#Pressure_drop_for_gas_flow

section 5 gives guidance on when different equations are best used.

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

 

[mb73]

Thank you all for your responses. Deeply appreciated.
@littleInch: Wow!. 20 years! I believe I was starting out my career in the power industry back then. Am in the natural gas business now in an area which is new to me.
Think I will need to give some more insight on the issue I am facing.

Thanks

 

[mb73]

A better statement of the problem is as follows:
A customer wants us to deliver 40,000 MSCFD of natural gas at 450psig.
We have the ability to get gas from a transmission pipeline at 975 psig max, use an interconnect and drop it down to whatever pressure we are comfortable with ( I chose 800 psig but am flexible). At the customer, we will regulate to 450 psig. Length of main is 15 miles.
I wanted to keep the %SMYS below 20% to avoid being in the transmission realm.
What pipe diameter/ Classification/Wall thickness would you recommend?

 

[GBTorpenhow]

We have the ability to get gas from a transmission pipeline at 975 psig max, use an interconnect and drop it down to whatever pressure we are comfortable with ( I chose 800 psig but am flexible). At the customer, we will regulate to 450 psig.

Do you have a compelling reason to regulate before the 15 mile section you are designing? Usually would just design the lateral for the same MAOP as the transmission pipeline. In any case, the minimum pressure the pipeline company is guaranteed to supply should govern your line sizing.

I wanted to keep the %SMYS below 20% to avoid being in the transmission realm. What pipe diameter/ Classification/Wall thickness would you recommend?

In this size range it shouldn't be too hard to keep under 20% SMYS if you want, but 15 miles of 975 psig pipeline with one customer smells a lot more like a transmission pipeline than a main. You should plan on being able to run pigs, for example. I would recommend whatever size gives you an adequate pressure, assuming the lowest supply pressure the pipeline company could provide, after 15 miles of pressure drop, to ensure delivery of at least 450 psig downstream of the regulator station.

 

[georgeverghese]

Velocity at inlet = 8m/sec, at exit = 12m/sec. Max permissible velocity permissible for corrosion inhibitor efficiency (if used) is 25m/sec. Backpressure at line inlet is too high (nearly 1500psig) for 6inch option, so 200mm id sounds about right.
Presume 800psig is about the minimum pressure you have on this transmission line at the location of this interconnect.
Check also that the max hydrocarbon and water dewpoint temp at max operating supply pressure of 975psig is less than minimum ambient temp by at least 5degC (ie Tdew + 5degC < min ambient temp)at all times. This must be specified as a guarantee clause in the gas sales agreement (GSA) with your supplier if there are no provisions for a gas-liquid separator or slugcatcher at the 450psig end.

 

[1503-44]

You cannot avoid entering the gas transmission rhelm simply by staying below 20% SMYS. If you take from a transmission line to a customer, you're a transmission line. You need to otherwise be downstream of a "distribution" facility, usually meaning a "city gate".

You have to reduce pressure at either the take or sale point. Doing that at the take, if possible, will save 40-60% of pipe cost. Since you do not want to exceed 20% SMYS, 40-50% of that pipe steel is essentially wasted. Why not reduce pressure at the take?

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

 

[LittleInch]

A few issues here.

Basically the 975 sets your design pressure, but line sizing needs to work on the lowest guaranteed or anticipated pressure at the tie in point.

If 800 psig is just your guess, then that is quite dangerous. The interesting part comes when you try to get the transmission line to give you a pressure in writing.

It's not normally worth it to reduce the pressure at the inlet to get a lower design pressure as all the equipment you need, plus the required SIL level to be able to guarantee you won't exceed 800 psig is costly and not balanced by the small reduction in wall thickness for an 8" or 10" pipe of that length. Just connect with a simple tee and valve / pig trap arrangement and then do all your pressure let down / heating, measuring etc at the far end is what "normally" happens.

Also look at what happens to temperature. That's quite a large pressure drop = temperature drop and you don't want to arrive too cold where everything drips condensation or starts to freeze or start to get drop out.

The 20% stress thing I'll leave to you US guys to argue as I don't really understand what it gets you not being a transmission line, but seems rather extreme and the cost will be significant between 0.2 and 0.72 to balance against any other issues.

All part of the design options to study to arrive at what is the most optimal for your situation.

But get the 800 psig inlet thing sorted first. If that is only 600 psig in the worst case then you have a different size to consider.

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

 

[mb73]

@1503-44, I will use a City gate to reduce the pressure and then set another reg at the customer to 450. The reason I need that higher pressure at the inlet- in the region of 800 psig is that we are looking at potentially building out our system for additional load.
Thank you for taking your time to give me some feedback.
I appreciate it.

 

[1503-44]

I thought additional loads were probable.
That's a really high city gate outlet pressure.
If this is the beginnings of a new system, I'd suggest you get a predetermination from your state utility board if they will classify it as transmission or distribution. The lines get blurry depending on which state you are in. I think in NY anything over 175 psig, regardless of %SMYS, falls into transmission systems. Especially important if you plan on adding customers. If you fall under a state's transmission regulations, it may mean essentially becoming a PHMSA regulated pipeline in terms of what it's technical requirements will be, pig launchers and everything else that goes along with it. 15 miles might easily be considered as an extension of the type of system that you are taking from.



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

 

[LittleInch]

Can't see why you want to spend $$$ on a city gate type facility to only change between a max of 975 to 800 psi and probably in practice 850-900 down to 800.

What's so special about 800 psi?? You're still in class 600 flanges etc so this seems utterly pointless to me.

Make it 700 / class 300 and then I could understand it a bit more, but even then, any cost saving is unlikely to exceed the cost of the city gate facility which will grow arms and legs in terms of cost.

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