Pipe size questions

[Tryin2MakeALivin]

Hi Folks,

Can someone point me to a source or help explain my questions below on pipe sizing. I do not like to just accept things, I like to understand the details...

I have an 8" liquid pipeline with some basic block valves, elbows, ups and downs on its way to a tank. I want to add a flow meter station, so I plan to add a tap in the line to bring the liquid to the flow meter station and another tap to bring the liquid back to its main line that goes to the tank.

1)What would determine the line size for the lines going to and from the flow meter station?
Most of these lines are a size smaller than the main pipe. Why is that? Should it always be smaller?

My instinct would be that the lines to and from the meter station would be at least the same size as the main pipeline since you are basically taking flow off the large pipeline sending through a strainer, a meter of some sort, through more obstructions in the meter piping (thermowell, sample probes, etc) and then back to the main pipe. Wouldn't all these items in the flow meter piping cause pressure drops? Did I just answer my own question???
Is the branch and flow meter pipe smaller to increase the pressure(P=F/A)so that the liquid can afford the obstuctions (pressure drops) in the line and not drop too much which in the case of LPGs would cause them to vaporize?

2)When installing a basket strainer in the line, why is the line size typically increased one size to enter the strainer and then reduced back to the line size immediately after?

Thanks for your time & assistance!!!

 

[MJCronin]

NotTryin....

Q1)What would determine the line size for the lines going to and from the flow meter station?

A1)Flow meters (depending on the type you select) can be supplied in the same size of smaller than the line size you select.

Q2)When installing a basket strainer in the line, why is the line size typically increased one size to enter the strainer and then reduced back to the line size immediately after?

A2)This is a owner/piping arbitrary design choice. There are no hard and fast rules. The strainer simply requires a certain flow and will clog in a certain time. If the piping/system designer wants a minimum pressure drop, a "upsized" strainer can be installed.

I suggest that you contact the equipment vendors to get thier requirements for piping installation.

-MJC

 

[zdas04]

Another way of saying what MJCronin said is that the major determinant of line size is acceptable pressure drop due to friction. Typically, stepping down a size for 20 m or so of pipe will not significantly increase your total pressure drop, and the next size down meter station can be markedly less expensive in capital without being significantly more expensive in pressure drop.

David

 

[pennpiper]

PennPiper, (Piping)
Tryin2MakeALivin,
There is something else you must also do.
In the main line between the connection to the new Flow Meter and the connection back to the main line from the new Flow meter you must install a line size (or near line size) throttling valve (Globe or equal). This will cause the flow to go through the much longer and higher pressure drop line rather stay in the shorter lower pressure drop main line.

Now here we have a possible problem. Does your main line ever get "Pigged"? Do you know what a "Pipeline Pig" is? If the line is required to be pigged then you must install a "Full Port" valve in the main line.

When the line is to be "Pigged" then you must ghang4e the main line valve to the full open position and in effect relinquish the benefit of the "Flow Meter"

After the "Pigging" is done then you close the mainline valve enough to again cause the flow to go through the "Flow Meter"

Do you understand this?

 

[zdas04]

That mainline valve has to be a block valve, not a globe valve (which is not sold a a positive seal). For a meter station I either use a Trunnion Ball valve with the body bleed removed or put in two physical, locked shut valves with a locked open vent in between. That is the only way you can have any confidence that your fluid is going through your meter instead of around it.

David

 

[micalbrch]

A flow meter is a rather expensive unit. A flow meter of 8" size is more expensive than a 6" flow meter. That's why the flow meter line is quite often smaller. It is the same with the pressure range of a flow meter. For high pressure pipelines the flow meter is often installed on the pump's suction side to reduce the costs.

The reason for the bigger filter basket pipe is the friction loss you have through a dirty filter. The bigger the filter the less friction losses it creates and the longer it takes till it's clogged.

 

[BigInch]

MJC ?

I agree with zdas & micalbrch. It is usually acceptable to have relatively smaller sizes and a subsequent higher velocity for short lengths of pipe, valves and equipment as the pressure drop over those short lengths is typically not significant and the price of that equipment is often reduced considerably. As long as the smaller equipment will handle the capacity you need and you have the pressure to drive it, its perfectly fine practice.

Pennpiper mentins pigs. All equipment and instruments should normally be upstream of a pig launcher, or downstream of a pig receiver, if pigging is in the operation plan, except for typically full size mainline block valves, although soft pigs will pass through many reduced port valves. Instruments, such as temperture probes and densitometers, coupon holders etc. must not protrude into any pipe that will be pigged.

AS for the basket filter, a slower velocity will not force those borderline deformable bits through. The slower the velocity, the more you catch.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[MJCronin]

No arguement....Let me correct my typo:

"Flow meters (depending on the type you select) can be supplied in the same size OR smaller than the line size you select"

Its dealers choice....

It is the age-old choice between more or less operating cost versus capital cost.

-MJC

 

[cvg]

another reason that has not been addressed - if your main line has low flow rates / velocities, it may help increase meter accuracy to go with a smaller size meter. This increases the velocity through the meter and may allow more accurate flow measurement. Likewise, if you have very high flow rates in the mainline, a larger meter flowing at lower velocities may last longer.

To get accurate measurement with some flow meters, typically a straight run 10D or 20D (or perhaps more) is required before the meter, especially where there are elbows and valves. The flow straightener length is shorter when using smaller pipe.

 

[dcasto]

How accurate do you want the meter to be? Use a clamp on ultra sonic and save a bundle.

 

[sebastiancito]

is too simple.. Wich flowmeter you can use? or which fluid like to measure??.
example.. For a electromagnetic flowmeter you need 3 m/s (fluid velocity) at the max flow.